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arXiv:astroíph/0402556v1
24
Feb
2004
Astronomy & Astrophysics manuscript no. n1068íusero January 25, 2007
(DOI: will be inserted by hand later)
Molecular Gas Chemistry in AGN
I. The IRAM 30m Survey of NGC 1068
A. Usero 1,2 , S. GarcÒÐaíBurillo 1 , A. Fuente 1 , and J. MartÒÐníPintado 3 , N. J. RodrÒÐguezíFernÒandez 4
1 Observatorio AstronÒomico Nacional (OAN), C/ Alfonso XII 3, 28014 Madrid, Spain
2 Instituto de MatemÒaticas y FÒÐsica Fundamental, CSIC, C/ Serrano 113bis, 28006 Madrid, Spain
3 Instituto de Estructura de la Materia, DAMIRíCSIC, C/ Serrano 121, 28006 Madrid, Spain
4 LERMA (UMR 8112), Observatoire de Paris, 61, Av. de l'Observatoire, 75014 Paris, France
Received 1 December 2003 / Accepted 5 February 2004
Abstract. There is observational evidence that nuclear winds and Xírays can heavily influence the physical conditions and
chemical abundances of molecular gas in the circumnuclear disks (CND) of Active Galactic Nuclei (AGN). In this paper we
probe the chemical status of molecular gas in the CND of NGC 1068, a prototypical Seyfert 2 galaxy. Precedent claims that the
chemistry of molecular gas in the nucleus of NGC 1068 is abnormal by galactic standards were based on the high HCN/CO
luminosity ratio measured in the CND. Results from new observations obtained in this survey have served to derive abundances
of molecular species such as SiO, CN, HCO + , HOC + , H 13 CO + and HCO. These estimates are complemented by a reíevaluation
of molecular abundances for HCN, CS and CO, based on previously published singleídish and interferometer observations
of NGC 1068. We report on the first detection of SiO emission in the CND of NGC 1068. The estimated large abundance
of SiO in the CND, X(SiO)#(5í10)½10 -9 , cannot be attributed to shocks related to star formation, as there is little evidence
of a recent starburst in the nucleus of NGC 1068. Alternatively, we propose that silicon chemistry is driven by intense Xíray
processing of molecular gas. We also report on the first extragalactic detection of the reactive ion HOC + . Most remarkably, the
estimated HCO + /HOC + abundance ratio in the nucleus of NGC 1068, #30--80, is the smallest ever measured in molecular gas.
The abundances derived for all molecules that have been the subject of this survey are compared with the predictions of models
invoking either oxygenídepletion or Xíray chemistry in molecular gas. Our conclusions favour an overall scenario where the
CND of NGC 1068 has become a giant Xíray Dominated Region (XDR).
Key words. Galaxies:individual:NGC 1068 -- Galaxies: Seyfert -- Galaxies: nuclei -- Galaxies: ISM -- ISM: abundances -- Radio
lines: galaxies
1. Introduction
Active Galactic Nuclei (AGN) are able to inject vast amounts
of energy into their host galaxies, carried by strong radiation
fields and rapidly moving jets. It is predictable that AGN should
have a disruptive influence on the gas reservoir near their cení
tral engines. There is multiíwavelength observational evidence
that the general properties of neutral interstellar matter in AGN
di#er from those of quiescent staríforming disks and starburst
galaxies (Genzel et al. 1998; Laurent et al. 2000). In particular,
molecular gas close to the central engines of active galaxies
can be exposed to a strong Xíray irradiation. While the accreí
tion disks of AGN are strong UV emitters, the bulk of the UV
flux can be attenuated by neutral gas column densities of only
N(H)#10 21 cm -2 . Hard Xíray photons (2--10 keV) can peneí
trate neutral gas column densities out to N(H)#10 23 í10 24 cm -2 ,
however. Therefore, Xíray dominated regions (XDR) could beí
Send o#print requests to: A. Usero,
eímail: antonio.u@imaff.cfmac.csic.es
come the dominant sources of emission for molecular gas in the
harsh environment of circumnuclear disks (CND) of AGN, as
originally argued by Maloney et al. (1996).
First observational evidence that the physical and chemical
properties of molecular gas in the CND of AGN depart from
`normality' came from the singleídish and interferometer obí
servations of HCN and CO emission in NGC 1068 (Tacconi
et al. 1994; Sternberg et al. 1994). This prototypical Seyfert
2 galaxy hosts a circumnuclear starburst ring of #2.5í3 kpc--
diameter (see Fig. 1); the ring delimits a 2.3 kpc stellar bar deí
tected by Scoville et al. (1988) in the NIR. The strong emission
detected in the 1--0 and 2--1 CO lines coming from the starburst
ring corroborates that massive star formation is fed by a signifí
icant gas reservoir (Planesas et al. 1989, 1991; Helfer & Blitz
1995; Schinnerer et al. 2000). Significant CO emission arises
also from a 200 pc CND of M(H 2 )#5½10 7 M # (inferred using a
N(H 2 )/I(CO) conversion factor of 2.2½10 20 cm -2 (K km s -1 ) -1 ,
from Solomon & Barrett 1991). The CND, partly resolved into
two knots, surrounds the position of the active nucleus identií

2 A. Usero et al.: Molecular Gas Chemistry in AGN
fied as the compact radioísource S1 in the map of Gallimore et
al. (1996a). Most remarkably, the CND is prominent in HCN
emission (Tacconi et al. 1994). According to the analysis of
Sternberg et al. (1994), the high HCN/CO intensity ratio meaí
sured by Tacconi et al. (1994) (#1--10) leads to an abnormally
high HCN/CO abundance ratio in the nucleus of NGC 1068:
N(HCN)/N(CO)#a few 10 -3 í10 -2 , i.e., the highest ratio ever
found in the centre of any galaxy.
Di#erent explanations have been advanced to quantify the
possible link between the anomalous HCN chemistry and the
presence of an active nucleus in NGC 1068. The selective deí
pletion of gasíphase oxygen in the dense molecular clouds
would explain the high HCNítoíCO abundance ratio (Sternberg
et al. 1994; Shalabiea & Greenberg 1996). The same oxygen
depletion scheme predicts a loweríthanínormal abundance of
all oxygeníbearing species. Alternatively, an increased Xíray
ionization of molecular clouds near the AGN could enhance the
abundance of HCN (Lepp & Dalgarno 1996). Furthermore, Xí
rays could evaporate small (#10 Š) silicate grains, increasing
the fraction in gas phase of all refractory elements and subseí
quently enhancing the abundance of some molecules (e.g., SiO)
in Xíray irradiated molecular gas (Voit 1991; MartÒÐníPintado
et al. 2000). While the aforementioned scenarios succeed to
reproduce the measured enhancement of HCN relative to CO
in NGC 1068, their predictions about the abundances of other
molecular species di#er significantly. The lack of tight observaí
tional constraints for these models, prompted by the first mmí
observations made in NGC 1068, has hampered thus far the
choice of an optimum scenario, however.
In this paper we discuss the results of a molecular surí
vey made in NGC 1068 with the IRAM 30m mmítelescope.
NGC 1068 is the optimum target to quantify the feedíback
of activity and star formation on the chemistry of molecuí
lar gas. Furthermore, the spatial resolution of the 30m teleí
scope is well suited to discern between the emission coming
from the star forming ring and that coming from the CND. We
discuss the results obtained from new mmíobservations of 6
molecular species. The list includes: SiO(v=0, J=2--1 and J=3--
2), HCO(J=3/2--1/2, F=2--1), H 13 CO + (J=1--0), HCO + (J=1--0),
HOC + (J=1--0) and CN(N=2--1). For comparison purposes, we
include in our analysis the results from previous singleídish
and interferometer observations of CO (J=1--0 and 2--1 from
Schinnerer et al. 2000; J=4--3 from Tacconi et al. 1994), HCN
(J=1--0 and 4--3) (Tacconi et al. 1994) and CS (J=2--1) (Tacconi
et al. 1997). This data base has served for estimating the abuní
dances of eight molecular species in the CND of NGC 1068 usí
ing LVGmodel calculations. The inferred abundances are comí
pared with the predictions of models invoking either oxygení
depletion or Xíray chemistry in molecular gas. We present
in Section 2 the 30m observations made for this survey as
well as the data compiled from previous works on NGC 1068.
Section 3.1 presents the results obtained from our SiO study
and their implications for the CND chemistry. Section 3.2 is deí
voted to discuss the chemistry of the HOC + /HCO + active ions.
The molecular gas inventory of the CND is globally presented
and discussed in Section 4. We discuss in Section 5 the interí
pretation of these results in the framework of di#erent chemí
istry models and summarize the main conclusions of this work
in Section 6.
2. Observations
The observations have been carried out in four sessions from
January 2000 to August 2002 with the IRAM 30m radioteleí
scope at Pico Veleta (Spain). We used 3 SIS receivers tuned
in singleísideband mode in the 1 mm, 2 mm and 3 mm bands
to observe several transitions of the molecular species shown
in Tab. 1, which summarizes the relevant parameters of these
observations. We have obtained singleípointed spectra toward
the nucleus of NGC 1068 for all the molecules with the excepí
tion of SiO, HCO and H 13 CO + , for which we obtained partial
maps by observing three additional positions on the starburst
ring (see Sect. 3.1). The line temperature scale used by default
throughout the paper is TMB , i.e., main brightness temperature.
TMB is related to antenna temperature, T #
A , by T #
A =TMB ½ # B ;
the values assumed for # B are listed in Tab. 1. When explicí
itly stated, TMB temperatures are corrected by a source couí
pling factor, f S 1 ; this factor accounts for the estimated diluí
tion of the source within the beam. To improve the stability of
spectral baselines, the observations have been carried out in
beamíswitching mode, with an azimuthal switch of ‘4 # with a
frequency of 0.5 Hz. Only linear polynomials were used in the
baseline correction.
In this paper we also use the data from previously pubí
lished HCN, CS and CO observations of NGC 1068 made with
the IRAM Plateau de Bure InterferometeríPdBI (HCN(1--0):
Tacconi et al. 1994; CS(2--1): Tacconi et al. 1997; CO(1--0)
and CO(2--1): Schinnerer et al. 2000). Complementary obserí
vations of high J transitions (J=4--3) of CO and HCN, taken at
James Clerk Maxwell TelescopeíJCMT (Tacconi et al. 1994),
are also included. The main parameters of these observations
are listed in Tab. 1.
Hereafter, we will assume a distance to NGC 1068
of 14.4 Mpc (BlandíHawthorn et al. 1997). This implies
1 ## =72 pc. The assumed heliocentric systemic velocity is
v sys =1137 km s -1 (from NASA/IPAC Extragalactic Database
(NED)).
3. The IRAM 30m Survey of NGC 1068
3.1. SiO Emission in NGC 1068
NGC 1068 was originally part of a larger extragalactic survey
searching for SiO emission in starbursts (Usero et al. 2003 in
prep). Di#erent mechanisms have been found thus far to exí
plain the enhancement of SiO abundances in molecular gas
in galaxies: either related to recent star formation (NGC 253:
Garc Ò iaíBurillo et al. 2000), to the disruption of galaxy disks
by largeíscale shocks (M 82: Garc Ò iaíBurillo et al. 2001) or to
the Xíray irradiation of molecular clouds (Milky Way: Mart Ò iní
Pintado et al. 2000).
1 Correction for dilution: T# f S T with f S
=# beam
/# S ,
where# beam
is the beam area
and# S the area of the emitting region estimated from
the CO(1í0) interferometer map.

A. Usero et al.: Molecular Gas Chemistry in AGN 3
Table 1. Main parameters of the new 30m observations (top). Typical receiver and system temperatures are shown as T rec and T sys , respectively.
We also show the relevant parameters for previous observations used in this work (bottom). See original references for details.
New Observations
Line Freq.(GHz) Obs. dates Beam ( ## ) # B T rec /T sys (K)
H 12 CO(3/2í1/2,2í1) 86.670 Jun00/Aug02 28 0.82 70/130
H 13 CO + (1--0) 86.754 Jun00/Aug02 28 0.82 70/130
SiO(2--1) 86.847 Jun00/Aug02 28 0.82 70/130
H 12 CO + (1--0) 89.189 Jan01/May01 27 0.81 60/120
HO 12 C + (1--0) 89.487 Jan01/May01 27 0.81 60/120
SiO(3--2) 130.269 Jun00/Aug02 19 0.77 125/225
CN(2--1) 226.875 Aug02 11 0.58 120/390
Previous Data
Line Freq.(GHz) Telescope Reference paper
HCN(4--3) 354.505 JCMT Tacconi et al. (1994)
CO(4--3) 461.041 JCMT Tacconi et al. (1994)
HCN(1--0) 89.088 IRAM PdBI Tacconi et al. (1994)
CS(2--1) 97.981 IRAM PdBI Tacconi et al. (1997)
CO(1--0) 115.271 IRAM PdBI Schinnerer et al. (2000)
CO(2--1) 230.538 IRAM PdBI Schinnerer et al. (2000)
Table 2. Parameters of gaussian fits to the SiO/H 13 CO + /HCO lines observed in NGC 1068. Errors (in brackets) are 1í#. For the nonídetection
of SiO(3--2) in the N position we give a 3í# upper limit.
Position Line I(K km/s) T peak (mK) vív sys (km/s) #v(km/s)
CND SiO(2í1) 0.56 (0.05) 2.8 í26 (10) 189 (22)
(0 ## ,0 ## ) SiO(3í2) 0.60 (0.06) 3.0 í36 (10) 190 (19)
H 13 CO + (1í0) 0.57 (0.07) 2.1 9 (13) 254 (38)
S SiO(2í1) 0.32 (0.04) 1.5 í48 (9) 200 (13)
(0 ## ,í16 ## ) SiO(3í2) 0.17 (0.04) 2.2 í57 (9) 69 (18)
H 13 CO + (1í0) 0.27 (0.04) 1.3 í48 (13) 200 (13)
HCO(3/2í1/2,2í1) 0.11 (0.03) 1.1 í107 (13) 100 (13)
N SiO(2í1) 0.39 (0.08) 1.4 í53 (29) 261 (52)
(0 ## ,+16 ## ) SiO(3í2) <0.25 ... ... ...
H 13 CO + (1í0) 0.44 (0.07) 2.8 í3 (11) 145 (27)
HCO(3/2í1/2,2í1) 0.20 (0.07) 1.4 26 (23) 138 (60)
E SiO(2í1) 0.28 (0.04) 1.8 í60 (19) 150 (13)
(+16 ## ,0 ## ) SiO(3í2) 0.11 (0.04) 1.1 1 (19) 103 (36)
H 13 CO + (1í0) 0.21 (0.03) 2.2 í122 (9) 90 (13)
HCO(3/2í1/2,2í1) 0.18 (0.04) 1.6 í130 (20) 110 (13)
We show in Fig. 1 the 4 positions over the NGC 1068
disk where we searched for SiO emission. To better constrain
the physical conditions of the gas, we have observed simulí
taneously the J=2--1 and J=3--2 rotational transitions of SiO.
SiO(2--1) emission is detected at every o#set, while SiO(3--2),
very prominent in the CND, is detected in 2 out of the 3 posií
tions mapped over the ring. The observing grid was chosen to
discriminate between SiO emission coming from the starburst
ring (N[0 ## ,+16 ## ], E[16 ## ,0 ## ] and S[0 ## ,--16 ## ]) and that coming
from the circumnuclear disk (CND[0 ## ,0 ## ]). Parameters of the
gaussian fits to the lines detected are listed in Tab. 2.
3.1.1. Emission in the Starburst Ring
These observations show that SiO emission is widespread in
the starburst ring of NGC 1068. Where detected over the ring,
SiO(3--2) lines are narrower than SiO(2--1) lines. This result
can be explained if, contrary to the compactness of SiO emisí
sion in the CND (see below), the emission of SiO on the ring

4 A. Usero et al.: Molecular Gas Chemistry in AGN
Fig. 1. Emission spectra of the 2--1 and 3--2 lines of SiO detected in
the inner 3 kpc of NGC 1068. Four starred markers, overlaid on the
CO(1--0) integrated intensity map of Schinnerer et al. (2000), highlight
the central positions of the beams in the disk where we searched for
SiO emission: the central o#set (0 ## ,0 ## ) coincides with the position of
the AGN, given by the S1 compact radioísource of Gallimore et al.
(1996b) (# 2000 =02 h 42 m 40 s .71,# 2000 = --00 # 00 # 47.9 ## ), while o#sets N,
S and E probe the SiO emission over the starburst ring. Emission in
the H 13 CO + (1--0) and HCO(1--0) lines is detected in the CND and over
the starburst ring. The circles represent the beam sizes at 130.3 GHz
(19 ## ) and 86.8 GHz (28 ## ).
extends significantly beyond a single SiO(3--2) beam. Within
the errors, the I(SiO(3--2))/I(SiO(2--1)) integrated intensity raí
tios are #0.5 in the two positions with detection of the 2mm
line. These ratios are slightly lowered to 0.4‘0.1 if we apply
a correction due to the di#erent coupling factors of the 3--2
and 2--1 beams with the source (correcting for dilution of the
nearly oneídimensional elongated arm inside the beams, i.e.,
by a factor #19 ## /28 ## ).
There are two precedents for the detection of largeí
scale SiO emission associated with ongoing star formation:
NGC 253 (Garc Ò iaíBurillo et al. 2000) and M 82 (Garc Ò iaíBurillo
et al. 2001). The derived enhancement of SiO abundances
(X(SiO)#a few 10 -10 --10 -9 ) takes place on scales of several
hundred pc in these starbursts and has been interpreted as a sigí
nature of shocks driven by YSO, SN explosions and/or density
waves. In the starburst ring of NGC 1068, a significant frací
tion of the stellar population (#40% of the total optical light;
GonzÒ alezíDelgado et al. 2001) has typical ages #10 7 yr. This
supports that a recent short burst of star formation has occurred
coevally throughout the ring on a timeíscale of#10 6 yr (Davies
et al. 1998).
Beside the detection of the 1--0 line of H 13 CO + (see Fig. 1),
which is 93 MHz redshifted with respect to the SiO(2--1) line,
we have detected the emission of the strongest hyperfine comí
ponent (F=2--1) of the J=3/2--1/2 line of HCO over the starburst
ring. Observations of HCO in galactic clouds suggest that the
abundance of this molecule is enhanced in Photon Dominated
Regions (PDR). More recently, Garc Ò iaíBurillo et al. (2002)
have reported on the detection of widespread HCO emission
in the nuclear starburst of M 82, where it traces the propagaí
tion of PDR chemistry in the disk. Based on studies of HCO
emission in Galactic PDR (Schenewerk, Snyder, & Hjalmarson
1986; Schenewerk et al. 1988), it is plausible to suppose that
the HCO lines should be optically thin also in the starburst
ring of NGC 1068. For H 13 CO + we also consider optically
thin emission and the same excitation temperature as that así
sumed for HCO. These are reasonable guesses, especially for
T ex , as the two molecules have similar critical densities for the
examined transitions. In this case, the calculation of the HCOí
toíH 13 CO + column density ratio is straightforward using the
expression (Schenewerk et al. 1988):
N(HCO)
N(H 13 CO + ) #
12
5
I HCO A -1
HCO
I H 13 CO +A
-1
H 13 CO +
(1)
where N is the total column density, I is the integrated iní
tensity, and A is the Einstein coe#cient of the transition. We iní
fer an average value for N(HCO)/N(H 13 CO + ) of #8. Adopting
an average fractional abundance for H 13 CO + of 10 -10 (GarcÒÐaí
Burillo et al. 2000, 2001), we derive X(HCO)#8½10 -10 . The
estimated N(HCO)/(H 13 CO + ) abundance ratios in prototypií
cal PDR range from 30, in the Orion Bar, to 3, in NGC 7023
(Schilke et al. 2001).
Altogether, the detection of widespread SiO and HCO
emission in the starburst ring of NGC 1068 can be naturally
explained by the chemical processing of molecular gas after a
recent episode of star formation.

A. Usero et al.: Molecular Gas Chemistry in AGN 5
3.1.2. SiO Emission in the Circumnuclear Disk (CND)
As is shown in Fig. 1, the spatial resolution of the 30m in
the 3--2 line (19 ## ) guarantees that the SiO(3--2) emission deí
tected toward the CND has little if any contamination from the
starburst ring (of #30 ## diameter). The similar lineíwidths of
the 2--1 and 3--2 SiO spectra at (0 ## ,0 ## ) provide further evií
dence that the bulk of the central SiO(3--2) emission comes
from the CND. Furthermore, the linewidth of both SiO lines
(FWZP=350 km s -1 ) coincides with the total line width of the
CO(1--0) emission integrated within the CND, as derived from
the interferometer map of Schinnerer et al. (2000). While the
SiO(3--2) line at (0 ## ,0 ## ) has no significant contribution from
the starburst ring, the situation is less clear in the case of the
SiO(2--1) spectrum: the 28 ## 30m beam at half power may pick
up emission coming mostly from the southern ridge of the star
forming ring (see Fig. 1). Taking into account that the SiO(2--
1) line temperatures measured over the ring are a factor of 2
lower than in the CND, the derived upper limit for the `alien'
contribution to the SiO(2--1) CND spectrum is #25%, at most.
The I(SiO(3--2))/I(SiO(2--1)) ratio in the CND is of
0.7‘0.1, once corrected for the contribution of the starburst
ring to the 2--1 CND line (½1/0.75) and for the twoídimensional
beam dilution of the CND (½(28 ## /19 ## ) 2 ). Simultaneously, we
have evaluated the contribution of the CND to the SiO(2--1)
spectra in the ring to be, at most, #25%. When we correct
for this e#ect, the I(SiO(3--2))/I(SiO(2--1)) average ratio on the
ring derived in Sect. 3.1.1 is raised to 0.5‘0.1, i.e., a factor 1.5
smaller than the ratio in the CND. Although the di#erence is
only marginal, it suggests that the excitation of SiO lines in the
CND is di#erent from that of the ring. In particular gas densií
ties in the CND could be larger by a factor of #4 compared to
the starburst ring.
A relevant contribution from the molecular bar to the SiO
emission detected at (0 ## ,0 ## ) is also very unlikely for several
reasons. First, the bar hardly stands out in the HCN and CS
interferometer maps of NCG 1068 (Tacconi et al. 1997): this
is a relevant result, as the critical densities of HCN(1--0) and
CS(2--1) lines are similar to that of SiO(1--0). Second, while
weak CO emission is detected along the bar, it is significant
only at v files, roughly symmetric on both sides around v sys .
Most remarkably, there is no evidence for significant recent
star formation in the CND itself. Several multiwavelength crií
teria have classified the nucleus of NGC 1068 as a pure Seyfert
nucleus, with little contribution from a nuclear starburst (MIR:
Laurent et al. 2000; NIR: Imanishi 2002; Optical/NearíUV:
CidíFernandes et al. 2001); the compact starburst emits # 1%
of the total IR luminosity (Marco & Brooks 2003). The cirí
cumnuclear stellar population is concentrated in a 50 pc core
of `postístarburst' intermediate age stars (age #5í16½10 8 yr)
(Thatte et al. 1997).
We can exclude star formation either inside or outside the
CND as the mechanism explaining the emission of SiO deí
tected at (0 ## ,0 ## ). This poses the problem of the origin of SiO
emission in the CND. The energy budget inside the CND seems
to be largely dominated by the AGN itself; thus the chemistry
of molecular gas, in particular the silicon chemistry, could be
driven by nonístellar processes. We discuss in Section 5.1 how
the high abundances derived for SiO in the CND might be
linked to the onset of XDR chemistry.
3.2. Emission of Reactive Ions in NGC 1068: the
HOC + /HCO + Isomers
Detailed chemical models of XDR predict enhanced abuní
dances of some reactive ions (e.g., H +
3 , HCO + , SO + CO + and
HCNH + ) as well as related neutral species (such as CN and
HCN) (Maloney et al. 1996; Black 1998a, 1998b; Lepp &
Dalgarno 1996). The tentative detection of CO + in the radio
galaxy Cygnus A (Fuente et al. 2000) suggests that reactive
ions may be used as an e#cient diagnostic tool to study XDR
chemistry in AGN. As part of this multiíspecies survey of
NGC 1068, we have observed the 1--0 line of HCO + toward
the CND. Most importantly, we have also searched for emisí
sion of its metastable isomer, HOC + . There is recent observaí
tional evidence that X(HCO + )/X(HOC + ) ratios, usually rangí
ing from 300í6000 for dense molecular clouds in our Galaxy
(Apponi et al. 1997, 1999), can reach values as low as 50í100
in UVíirradiated clouds (e.g., the prototypical PDR NGC 7023:
Fuente et al. 2003). These results urged us to estimate the
HCO + --to--HOC + ratio in the Xíray bathed environment of an
AGN.
Fig. 2 shows the J=1--0 30m spectra of HCO + and HOC +
observed toward the CND of NGC 1068. The emission of both
species is detected. The interferometer HCO + map of Kohno
et al. (2001) shows that the CND largely dominates the emisí
sion of HCO + in the inner 3 kpc of NGC 1068. Moreover,
we can estimate a conservative upper limit for the contribuí
tion of the starburst ring to the HCO + emission detected at
(0 ## ,0 ## ). Following the same procedure used in Section 3.1.2,
here adapted to H 13 CO + , we estimate that <30% of the (0 ## ,0 ## )
H 13 CO + emission can be attributed to the starburst ring. We can
reasonably extrapolate this estimate to HCO + . Additional eví
idence, similar to the one discussed in Section 3.1.2, supports
that the 30m HCO + spectrum is heavily dominated by emisí
sion coming from the CND.
The most remarkable result is the tentative detection of
the HOC + (1--0) line, the first thus far obtained in an external
galaxy. HOC + (1--0) emission is detected over 2# levels in a
215 km s -1 velocity range (#[--65 km s -1 ,+150 km s -1 ]). The
emission integrated within this velocity window reaches a 8.5#
significance level. The line profile of HOC + is noticeably asymí
metrical with respect to v sys : HOC + emission is mostly detected
at red velocities. As is shown in Fig. 2, HCO + --to--HOC + iní
tensity ratios for v#v sys range from #40 to 100. These surprisí
ingly low values rival the lowest values thus far derived in
PDR. The low HCO + ítoíHOC + intensity ratio measured in the
CND of NGC 1068 suggests that the chemistry of molecular
gas could be driven by the pervading X/UV irradiation comí
ing from the Seyfert 2 nucleus. Most remarkably, the asymmeí
try of the HOC + line profile suggests that whatever causes the
enhancement of this active ion, the process responsible seems
to be unevenly e#cient inside the CND. As it is discussed in
Section 5.2, Xíray driven chemistry in the CND may satisfací

6 A. Usero et al.: Molecular Gas Chemistry in AGN
torily explain a dramatic change in the HCO + ítoíHOC + abuní
dance ratio.
Fig. 2. top and middle: HCO + (1--0) and HOC + (1--0) spectra of the
CND of NGC 1068. bottom: HCO + (1--0)--to--HOC + (1--0) temperature
ratio profile derived for channels fulfilling T[HOC + (1--0)]> 2#. Error
bars are ‘#.
3.3. CN Emission in NGC 1068
CN is a highídipole radical typically found in dense regions
(#10 5 cm -3 ). The abundance of CN is strongly linked to that
of HCN. Theoretical models (Lepp & Dalgarno 1996) predict
large CNítoíHCN abundance ratios (>1) in XDR. The comparí
ison of the CN and HCN emission may thus provide a suitable
diagnostic of the relevance of Xírays in the chemistry of the
CND.
The CN(2--1) transition is split up into 18 hyperfine
lines that appear blended into three groups at frequencies
#226.9 GHz, #226.7 GHz and #226.4 GHz. We were able
to observe the two most intense groups of the transition (the
226.9 GHz and 226.7 GHz groups, hereafter referred to as
high frequency and low frequency respectively), although the
low frequency group was only partially covered by the spectral
bandwidth. The beam size at this frequency (11 ## ) guarantees
that the detected CN(2--1) emission must be coming from the
CND.
The CN(2--1) spectrum is shown in Fig. 4 (mainí
beam temperature scaled to the CND; see Section 4.2).
The measured highífrequencyítoílowífrequency intensity raí
tio is below the expected value for the optically thin limit
(high/low#1.64‘0.14 instead of 1.80). However, this estimate
is hampered by the insu#cient baseline coverage in the specí
trum.
4. Molecular Gas Inventory of the CND
Understanding the peculiar chemistry of molecular gas reí
vealed in the CND of NGC 1068 requires a global analysis of
its molecular inventory. Furthermore, higher spatial resolution
is key to extracting the maximum information from the 30m
spectra of SiO, HCO + and HOC + discussed above.
With this aim we have included in our analysis the informaí
tion provided by published interferometer maps of NGC 1068
obtained for CO, CS and HCN (Schinnerer et al. 2000, Tacconi
et al. 1994, 1997). These maps can help to improve our knowlí
edge on the molecular abundances for species such as HCN or
CS in the CND; due to their high spatial resolution, these obí
servations are not hampered by source confusion between the
CND itself and the starburst ring. In particular, the CO interferí
ometer map allows us to estimate the molecular hydrogen colí
umn densities in the CND. Moreover, the spatioíkinematical iní
formation of the CO interferometer map is used for calculating
the size and the location inside the CND of the gas components
emitting at di#erent velocities. Altogether, this information is
employed in Sect. 4.3 to estimate via LVG models the abuní
dances of several molecular species in the CND, separately, for
the relevant velocity components.
4.1. Morphology of the CND: the Interferometer
CO(1--0) Map
Fig. 3 represents the CO(1--0) spatially integrated spectrum
of the CND of NGC 1068. The line emission profile has been
obtained by integrating the CO(1--0) interferometer data of
Schinnerer et al. (2000) inside a 6 ## ½4 ## --rectangular region
which contains the bulk of the CO emission in the CND.
According to Schinnerer et al. (2000)'s estimates, we expect
little zeroíspacing flux missing in the CND integrated specí
trum/map. Molecular gas in the CND is not evenly distributed
around the AGN: two conspicuous knots (denoted as E[1 ## ,0 ## ]
and W[í1.5 ## ,0 ## ] knots) form an asymmetrical ring around the
AGN (See Fig. 1 of Schinnerer et al. 2000 and Fig. 1 in this
work). The asymmetrical distribution of molecular gas in the
CND is reflected by the profile of Fig. 3: the CO(1--0) emisí
sion integrated for v #2 times that measured for v>v sys (hereafter, called red comí
ponent). As expected for a disk rotating around the AGN, the

A. Usero et al.: Molecular Gas Chemistry in AGN 7
Fig. 3. top panel: Integrated intensity maps of CO(1--0) toward the
CND of NGC 1068 obtained for the blue (thin contours:from 6# by
steps of 3#; #=0.67 K km s -1 ) and red (thick contours: same leví
els with #=0.47 K km s -1 ) emission components as defined in text
(see also bottom panel). The maps have been derived from the data of
Schinnerer et al. (2000). The starred marker highlights the AGN locus.
bottom panel: Integrated spectrum of CO(1--0) emission in the CND.
The W and E knots in the CO map correspond, respectively, to the red
and blue components in the spectrum.
emission coming from the E and W knots roughly correspond,
respectively, to the blue and red components defined above.
This is illustrated in Fig. 3.
The full sizes of the E and W knots, deconvolved by the
1.8 ## ½1.8 ## beam, are alike: FWZP#2.2 ## . Therefore we deduce
similar areas for the blue and red emitting
regions:# source #
# ½ 1.1 2 arcsec 2 =3.8 arcsec 2 .
4.2. Molecular Line Profiles of the CND
Fig. 4 displays all the molecular lines observed in this work
toward the CND of NGC 1068. This includes the 30m specí
tra of SiO, HOC + , HCO + , H 13 CO + and CN (panels 4--9 in
Fig. 4). Temperatures have been rescaled assuming that the
emission comes from the 6 ## ½4 ## --rectangular region containing
the CND. Furthermore, we represent in panels 1--3 of Fig. 4,
the CND spectra of CO, CS and HCN obtained from pubí
lished interferometer maps. Similarly to CO (see Sect. 4.1),
these CND spectra have been obtained by integrating the HCN
and CS emission inside the 6 ## ½ 4 ## rectangular region which
contains the bulk of the CND flux in both interferometer maps.
We can redefine more precisely what we call red and blue veí
locities, ascribed, as argued above, to the W and E knots, reí
spectively: based on the observed molecular profiles of Fig. 4,
most of the molecular emission detected at red (blue) velocities
for all species arises within the interval 0 (-185 km s -1 and red components of the spectra are listed in Tab. 3.
There are noticeable di#erences between the line shapes of
the CND spectra shown in Fig. 4. We find line profiles domií
nated by emission at blue velocities for CO and CS, while line
profiles of HCO + , H 13 CO + and HOC + are dominated by red
emission. As argued in Sect. 3.2, HOC + represents an extreme
case as the bulk of the HOC + emission is detected at red velocí
ities. HCN profiles are rather symmetrical with respect to v sys .
Finally, the SiO line profiles represent a case somewhat interí
mediate between HCN and CO. These di#erences are quantií
fied in Figure 4 and Tab. 3, which show the blue--to--red (east--
to--west) average brightness temperature ratio (R E/W ) for all the
CND spectra (except for CN(2--1), for which the determinaí
tion of the blue and red components is hampered by the partial
blending of the lines). R E/W ranges from 2.2‘0.4 (CS(2--1))
to 0.6‘0.2 (HOC + (1--0)), i.e., from one extreme to the other,
this ratio changes by a significant factor (#4) among the obí
served molecules. Fig. 5 also illustrates this result: the HCN(1--
0)/CO(1--0) temperature ratio is a factor of 2--3 larger for the
red component than for the blue component. Furthermore, the
CO(2--1)/CO(1--0) ratio profile, shown in Fig. 5, is also asymí
metrical with respect to v sys : the (2--1)ítoí(1--0) ratio reaches
higheríthaníone values within a 70 kms -1 interval at red velocí
ities, while it oscillates between 0.6 and 0.8 for the blue comí
ponent.
Taken together, these results suggest that there is a chemical
di#erentiation between the E and W knots of the CND.
4.3. Molecular Gas Abundances in the CND
4.3.1. LVG Models
We have used singleícomponent Large Velocity Gradient
(LVG) models to estimate the column densities of the observed
molecular species under certain assumptions which are the baí
sis of all our calculations. First, we assume that the kinetic temí
perature (T K ) of molecular gas in the CND is 50 K. This value
was derived by Sternberg et al. (1994) from the LVG analyí
sis of several CO emissionílines observed toward the CND.

8 A. Usero et al.: Molecular Gas Chemistry in AGN
Fig. 4. Molecular lines in the CND. Subípanels are labeled with the name of the line displayed. Subípanels 1 to 3 are derived from interferometer
data (Tacconi et al. 1994, 1997 and Schinnerer et al. 2000); panels 4 to 9 show singleídish spectra observed towards the nucleus (temperatures
corrected by dilution e#ects assuming that the emission is coming from the CND). Two vertical pointídashed lines at vív sys =í185 km s -1 and
155 km s -1 , delimit the blue and red kinematical components. For each line, the blueítoíred (eastítoíwest) average brightness temperature ratio
(R E/W ) is indicated.

A. Usero et al.: Molecular Gas Chemistry in AGN 9
Fig. 5. Temperature ratio profiles derived from spectra of Tab. 4. The left panel shows the CO(2--1)--to--CO(1--0) ratio, and the right panel the
HCN(1--0)--to--CO(1--0) ratio. Error bars are ‘3#.
Table 3. Integrated intensities of the spectra of Fig. 4 in the blue
(col.2) and red (col.3) components. Col.4=blueítoíred (eastítoíwest)
ratio of mean temperatures. Errors (in brackets) are 1í#.
Transition I blue (K km/s) I red (K km/s) R E/W
CO(1--0) 127.1 (0.9) 60.1 (0.9) 1.77 (0.03)
CO(2--1) 95.5 (0.4) 51.2 (0.3) 1.56 (0.01)
CS(2--1) 8.7 (0.6) 3.2 (0.5) 2.24 (0.40)
HCN(1--0) 121.5 (1.7) 91.1 (1.5) 1.12 (0.02)
SiO(2--1) 8.5 (0.8) 6.2 (0.7) 1.15 (0.18)
SiO(3--2) 4.7 (0.5) 2.5 (0.4) 1.55 (0.32)
H 13 CO + (1--0) 6.3 (0.8) 6.9 (0.7) 0.77 (0.13)
HCO + (1--0) 167.2 (2.2) 158.4 (2.1) 0.88 (0.02)
HOC + (1--0) 2.1 (0.5) 2.8 (0.4) 0.61 (0.17)
Table 4. Mean temperatures in the East and West knots of the CND
after correction for dilution: col. 1 = name of the line; col. 2 = mean
temperature in the Eastíknot; col. 3 = idem in the Westíknot.
Transition #T# E (K) #T# W (K)
CO(1--0) 4.34 2.44
CO(2--1) 3.26 2.08
CO(4--3) 7.58 6.50
HCN(1--0) 4.15 3.71
HCN(4--3) 1.24 0.62
SiO(2--1) 0.29 0.25
SiO(3--2) 0.16 0.10
T peak (K)
CN(2--1, high freq.) 1.51
CN(2--1, low freq.) 0.92
HCO + (1--0) 5.73 6.50
H 13 CO + (1--0) 0.21 0.28
HOC + (1--0) 0.07 0.12
CS(2--1) 0.30 0.13
Therefore this value can be taken as a conservative lower limit
for TK . Furthermore, we adopt in our calculations an isotopic
ratio of 12 C/ 13 C=40 (Wannier 1980).
It has been previously reported that LVG models of CO
emission in PDRítype environments can lead to inconsistení
cies related to spatial fine structure, density and kinetic temí
perature (see the case of M 82 in Mao et al. 2000). However,
high Jínumber transitions (out to CO(J=7--6)), not available
for NGC 1068, are required to constrain LVGíparameters sufí
ficiently to search for inconsistencies.
As argued in Sect. 4.1, the interferometric CO maps reveal
two distinct knots (E--W) in the CND. These knots have simií
lar sizes
(# source #3.8 arcsec 2 ) and can be identified with two
adjacent velocity components of emission in the spectra. As
discussed in Sect. 4.2, the relative intensity ratio between these
components depends on the molecular species. In our calculaí
tions we thus give our estimates of abundance ratios separately
for the E/blue and W/red components. All source brightness
temperatures (T S (E/W)), listed in Tab. 4, have been derived
from the CND temperature scale used in Fig. 4, corrected by
a dilution factor f
=# CND
/# source .
The range of LVG solutions (n(H 2 ), N/#v) are determined
straightforwardly for SiO, CO, HCN and CN from the observed
line ratios and the source brightness temperatures. In the case of
SiO, we fit the (3--2)ítoí(2--1) ratio and the 2--1 line source temí
perature. Correction for contamination from the ring is taken
into account for SiO(2--1) (also for HCO + (1--0); see below).
For CO and HCN we use the (4--3)ítoí(1--0) line ratios and the
1--0 line source temperatures; 4--3 line temperatures of CO and
HCN are derived from singleídish data published by Tacconi
et al. (1994). In the case of CN, we fit the ratio of the two fine
structure lines and the lowífrequency line source temperature.
However, and due to partial blending of the two fine groups,
LVG solutions refer to global abundances with no distinction
between red and blue velocity components. The LVG solution
for H 12 CO + is obtained by fitting both the H 12 CO + ítoíH 13 CO +
temperature ratio measured for the 1--0 line and the H 12 CO + (1--
0) source temperature. We have implicitly assumed that the deí

10 A. Usero et al.: Molecular Gas Chemistry in AGN
Fig. 6. LVG estimates for oxygenated species in the E/W knots of the CND. a1: for CO, continuous (pointed) curves are contours of constant
1--0 line temperature ((4--3)ítoí(1--0) line ratio). a2: for SiO, same for 2--1 line temperature ((3--2)ítoí(2--1) line ratio). a3: for H 12 CO + , same
for 1--0 line temperature ([H 12 CO + ]ítoíH 13 CO + ] 1--0 line ratio). a4: for HOC + , same for 1--0 line temperature. Squared (starred) markers show
solutions for the East (West) knot.
rived density solution n(H 2 ) can be considered as common for
both H 12 CO + and H 13 CO + . In the case of HOC + , LVG estií
mates are only obtained in the W knot, since the signalítoínoise
ratio of the integrated emission at blue velocities (E knot) is
too low (<5). The estimate of N/#v for HOC + , only observed
in the 1--0 line, rests on the assumption of a value for n(H 2 ),
here taken from H 12 CO + . This approach is justified as HOC +
and HCO + are known to be formed/destroyed in chemical reací
tions taking place in the same gas clouds. Similarly, N/#v valí
ues for CS are derived assuming for this molecule the same gas
density inferred from HCN in order to fit the CS(2--1) source
brightness temperature.
Figures 6 and 7, and Tab. 5 summarize the results of LVG
calculations for CO, HCN, CS, CN, SiO, HCO + and HOC + .
Normalized with respect to N(CO), the column densities of
SiO, HCO + and HOC + (i.e., N(SiO)/N(CO), N(HCO + )/N(CO)
and N(HOC + )/N(CO)) are #2--3 larger in the W knot than in the
E knot. In contrast, N(HCN)/N(CO) and N(CS)/N(CO) column
density ratios are similar in the two knots within a 25% uncerí
tainty. These abundance ratios are reflecting the asymmetries
of the spectra discussed in Sect. 4.2, suggestive of an uneven
processing of molecular gas in the CND.
As a byproduct of LVG models for CO, we have estií
mated the X#N(H 2 )/I(CO) conversion factor for the molecí
ular gas in the CND. Assuming a range of abundance raí
tios [CO]/[H 2 ]#5½10 -5 --10 -4 , we infer a X value of 3í
6½10 19 cm -2 /(K km s -1 ), i.e., #4--8 times smaller than
the canonical value X=2.2½10 20 (Solomon & Barrett 1991).
Unless CO is underabundant by a similar factor (a scenario iní
voked by Sternberg et al. 1994 in the oxygen depletion models
clearly invalidated by the results of our work; see section 5) we
conclude that the X conversion factor is lower in the CND of
NGC 1068. Similar deviations have been previously reported
in other galactic central regions (Dahmen et al. 1998 and refí
erences therein). This might reflect the failure of some of the
basic hypothesis that support the canonical value. In particular,
the strong gravitational forces near galactic nuclei may prevent
molecular clouds from reaching virialization.

A. Usero et al.: Molecular Gas Chemistry in AGN 11
Fig. 7. LVG estimates for noníoxygenated species in the E/W knots
of the CND. b1: for HCN, continuous (pointed) lines are contours of
constant 1--0 line temperature ((4--3)ítoí(1--0) line ratio). b2: for CS,
same for 2--1 line temperature. b3: for CN, same for 2--1/high freq.
line temperature ((2--1/high freq.)ítoí(2--1/low freq.) ratio); a range
of possible solutions found is highlighted in bold face: we impose
n(H 2 )<10 6 cm -3 for consistency with the results from others species
and allow for a ‘2# uncertainty in the measured ratio. Markers are as
is Fig. 6.
Table 5. LVG results: col. 1= chemical species; col. 2 = Parameters
determined from the LVG models (n: molecular gas densities in cm -3 ;
N/#v: column densities per velocity interval in cm -2 km -1 s; X: chemí
ical abundances relative to H 2 ; we assume X(CO)=8½10 -5 and comí
pute the rest of abundances accordingly from column density ratios
relative to CO); col. 3= solutions for the Eastíknot; col. 4 = same for
the Westíknot; col 5 = eastítoíwest ratio of abundances.
species LVGísol. E/blue W/red N E/W
CO n 1.3 ½ 10 4 2.5 ½ 10 4
N/#v 6.3 ½ 10 15 4.0 ½ 10 15 1.0
X 8.0 ½ 10 -5 8.0 ½ 10 -5
SiO n 1.6 ½ 10 5 6.0 ½ 10 4
N/#v 4.0 ½ 10 11 5.0 ½ 10 11 0.5
X 5.1 ½ 10 -9 1.0 ½ 10 -8
HCO + n 4.0 ½ 10 4 2.5 ½ 10 4
N/#v 5.0 ½ 10 12 8.0 ½ 10 12 0.4
X 6.3 ½ 10 -8 1.6 ½ 10 -7
HOC + n --- 2.5 ½ 10 4
N/#v --- 2.0 ½ 10 11 ---
X --- 4.0 ½ 10 -9
HCN n 6.3 ½ 10 5 4.0 ½ 10 5
N/#v 6.3 ½ 10 12 6.3 ½ 10 12 0.8
X 8.0 ½ 10 -8 1.0 ½ 10 -7
CS n 6.3 ½ 10 5 4.0 ½ 10 5
N/#v 1.6 ½ 10 12 5.0 ½ 10 11 1.2
X 2.0 ½ 10 -8 1.6 ½ 10 -8
CN (global values)
n >1.6 ½ 10 5
N/#v 1-5½10 13 ---
X 9 ½ 10 -8 -5 ½ 10 -7
5. Chemistry of Molecular Gas in the CND of
NGC 1068
To give further insight into the chemistry of molecular gas in
the CND we have compared, for a common set of abundance
ratios, the values measured in NGC 1068 with those observed
in a reference galactic region. Here we take as `zeroípoint' ení
vironment the ``Extended Ridge'' of OMCí1 (OER) (Blake et
al. 1987), a relatively quiescent molecular region whose chemí
istry has been described as intermediate between the one typií
cal of cold dark clouds and that of warm cores (Sutton et al.
1995). The choice of the OER as a reference region is also
motivated by the similarity of physical parameters of molecí
ular gas density (n(H 2 )#10 4 --10 5 cm -3 ) and kinetic temperaí
ture (T K#50 K) in the CND and in the OER. Therefore, sigí
nificant di#erences in the abundance ratios of `critical' tracers
between the CND and the OER can be mostly attributed to difí
ferent chemistries being at work in these regions. We will also
use the OER as the `zero point' basis to extrapolate the abuní
dance ratios in the case of oxygen depletion models (Ru#e et
al. 1998).
We list in Table 6 the following set of abundance
ratios: N(HCN)/N(CO), N(CS)/N(CO), N(HCN)/N(HCO + ),

12 A. Usero et al.: Molecular Gas Chemistry in AGN
Table 6. Abundance ratios predicted/observed in di#erent molecular regions: the Eíknot of the CND of NGC 1068, same for the Westíknot;
a prototypical XDR (Lepp & Dalgarno 1996; Yan & Dalgarno 1997), the Orion Extended Region (OER) (Blake et al. 1987), and the OER
corrected with oxygen depletion (Ru#e et al. 1998).
Abundance Ratios CND(E/blue) CND(W/red) XDR OER OER+Oxygen Depletion
(Gas phase model)
HCN/CO 1.0½10 -3 1.2½10 -3 5½10 -4 1.0½10 -4 1½10 -3
CS/CO 2.5½10 -4 1.6½10 -4 1í5½10 -4 5.0½10 -5 7½10 -5
HCN/HCO + 1.3 0.6 0.5--1 2.2 25
CN/HCN 1--5 (global value) 3 0.7 20
SiO/CO 6.4½10 -5 1.2½10 -4 ---- <6.6½10 -6 ----
HCO + /HOC + ---- 50 ---- ---- ----
N(CN)/N(HCN), N(SiO)/N(CO) and N(HCO + )/N(HOC + ).
These abundance ratios can be significantly di#erent dependí
ing on the chemical environment. As argued below, an evaluaí
tion of these ratios allows us to compare the chemical status of
the CND and the OER with the predictions of models invoking
either oxygenídepletion or Xíray driven chemistry:
-- Our observations provide new constraints for oxygení
depletion models first proposed by Sternberg et al. (1994)
as an explanation for the high HCN/CO ratio measured
in the CND of NGC 1068. This scenario is supported
by Xíray and ultraviolet observations of the hotíionized
gas in the narrowíline region of NGC 1068 (Marshall et
al. 1993, Ogle et al. 2003). With the inclusion of dustí
grain chemistry in timeídependent models, Shalabiea &
Greenberg (1996) were able to fit at `early times' (t#10 6 yr)
HCN/CO#a few 10 -3 with values less restrictive for the
oxygen depletion. The overall consequences of selective
oxygenídepletion in the chemistry of molecular clouds
have been more extensively studied in the framework of
gasíphase (Terzieva et al. 1998; Ru#e et al. 1998) and gasí
grain chemical models (Shalabiea 2001). The primary efí
fect of an oxygen underabundance is a reduced formation of
CO. The fraction of carbon not consumed in the CO syntheí
sis is then increased and it can thus enhance the abundances
of some carbonated species, such as HCN, CS or CN; on
the contrary, abundances of oxygeníbearing species are
expected to be lower. This decrease is less important for
HCO + , as in this case a lower oxygen abundance is mostly
balanced by the increase of available carbon.
As shown in Table 6, the measured HCN/CO ratio in the
CND of NGC 1068 (#a few 10 -3 ) is 1 order of magnií
tude larger than that derived for the OER. Oxygen depleí
tion models can fit the HCNítoíCO ratio of the CND with
an oxygen depletion of [O] CND /[O] OER #1/2. However, this
value of oxygen depletion would lead to large HCN/HCO +
ratios (#25) which are at odds with the low ratios (#1)
of the CND. Furthermore, these models predict a signifií
cant enhancement of CN due to the reduction of O # which
is an important source of CN destruction (Bachiller et al.
1997). Here also the CNítoíHCN ratio in the oxygen deí
pletion models solution (#20) is nearly one order of magí
nitude larger than the CND ratio (#1--5, i.e., slightly above
the OER standards). Finally, the predicted variation for the
CS/CO ratio is marginal (½1.4) in the adopted oxygení
depletion solution, leading to values similar to that reported
for the CND: CS/CO#2½10 -4 .
-- Lepp & Dalgarno (1996) proposed an alternative explanaí
tion of the high HCN/CO ratio measured in the CND of
NGC 1068: Xírays coming from the central engine may
significantly enhance the abundance of HCN in the neighí
bouring molecular gas. Thus, the HCN/CO ratio measured
in NGC 1068 can be easily accounted for. In a XDR chemí
istry some diatomic species, such as CN and OH are parí
ticularly robust (Lepp & Dalgarno 1996). Moreover, large
abundances of OH favour the formation of CO + and H 2 O
(Sternberg et al. 1996); these species take part directly in
the production of large quantities of HCO + . The abuní
dances of HCN, CN and HCO + simultaneously reach their
peak values at similar depths inside XDR (Yan & Dalgarno
1997). The XDR model of Yan & Dalgarno (1997) preí
dicts an average CS/CO abundance ratio of 1--5#10 -4 for
the range of depths inside the XDR that are expected to
dominate the emission of molecular gas. As summarized in
Table 6, the HCN/CO, HCN/HCO + , CN/HCN and CS/CO
abundance ratios predicted by XDR models (see Lepp &
Dalgarno 1996 for the three first ratios; the CS/CO ratio has
been estimated from Yan & Dalgarno 1997) are in close
agreement with the corresponding values estimated for the
CND of NGC 1068.
In summary, while oxygen depletion models are able to fit
the HCN/CO ratio measured in the CND of NGC 1068, the
adopted solution leads to HCN/HCO + and CN/HCN abundance
ratios which are excessively large compared to that actually
measured for the CND. In contrast, the models invoking XDR
chemistry explain naturally the ratios measured in NGC 1068;
these values depart significantly from the standard reference
values of the OER. In the following sections we discuss how
the detection of high abundances of SiO and HCO + in the CND
of NGC 1068 add supporting evidence to the XDR chemistry
scenario.
5.1. SiO in XDR
As is shown in Tab. 6, the SiOítoíCO abundance ratio meaí
sured toward the CND of NGC 1068 is high by normal galací
tic standards: N(SiO)/N(CO)#6½10 -5 í1.2½10 -4 . The normalí

A. Usero et al.: Molecular Gas Chemistry in AGN 13
ized SiO column densities toward the CND are at least one
order of magntitude larger than the upper limit derived for
the OER (<7½10 -6 ). Assuming an absolute abundance for CO
of X(CO)=8½10 -5 , this implies X(SiO)=5½10 -9 í1.0½10 -8 . As
discussed in Sect. 3.1.1, a significant enhancement of SiO in
molecular gas has been attributed to heavy shock processing
of grains in starburst galaxies where values of X(SiO) up to
a few 10 -10 have been reported on scales of several hundred
pc (Garc Ò iaíBurillo et al. 2000, 2001). The CND abundances
of SiO estimated here are significantly larger than those reí
ported for starbursts, however; this is further evidence that silí
icon chemistry in the CND is not being driven by star formaí
tion. In contrast, the estimated SiO abundances in the starburst
ring of NGC 1068 (X(SiO)#2í3½10 -10 ) are in close agreement
with SiO abundances measured in starbursts on similar spatial
scales.
Alternatively, it has been suggested that Xíray irradiated
dust grains can enhance silicon chemistry in gas phase. Xírays
are able to heat very small silicate grains (10 Š), subsequently
leading to their evaporation and to an enlargement of the Si
gasíphase fraction (Voit 1991). Most remarkably, the nucleus
of NGC 1068 shows a strong Fe K# line (Ogle et al. 2003 and
references therein). The bulk of the 6.4 keV line of Fe # most
likely comes from fluorescence in the Comptoníthick molecuí
lar gas torus of NGC 1068. The detection of strong Fe K# line
emission is therefore an indication that large column densities
of molecular gas are being processed by Xírays. In a preceí
dent study, MartÒÐníPintado et al. (2000) found a correlation beí
tween the intensity of the Fe 6.4 keV line and the derived abuní
dance of SiO in the Sgr A and Sgr B molecular complexes at
the Galactic Center.
5.2. HOC + in XDR
According to the estimates of Sect. 4.3, HOC + abundances
derived for the CND of NGC 1068 are the largest ever
measured in interstellar medium: X(HCO + )/X(HOC + )#30--80.
These low ratios are in direct contrast with those typically meaí
sured in galactic dense molecular clouds where values from
#6000 to #300 have been reported thus far (Apponi et al. 1997,
1999). Most interestingly, the lowest value found by Apponi et
al. (1999) corresponds to the Orion bar, a prototypical PDR.
Very low ratios (#50í120) have been recently found in the proí
totypical PDR NGC 7023 (Fuente et al. 2003). As argued beí
low, we propose that low R#X(HCO + )/X(HOC + ) ratios can be
explained for molecular clouds with high ionization degrees,
either in XDR or in PDR.
The fast hydrogenícatalyzed isomerization of HOC + into
HCO + usually shifts the equilibrium between both species toí
wards significantly lower abundances of HOC + . However, as
suggested by Smith et al. (2002), the isomerization process
converting HOC + into HCO + could be compensated by the
destruction of HCO + due to interaction with electrons. This
process is likely to be enhanced at high electron densities
(X(e - )# 10 -5 ). The latter could explain why the lowest R valí
ues have been measured in galactic PDR (Apponi et al. 1999;
Fuente et al. 2003). Furthermore, the X(HCO + )/X(HOC + ) ratio
Fig. 8. Top panel: steady state HCO + ítoíHOC + abundance ratio as a
function of the ionization degree of molecular gas. Curves for single
formation paths are plotted; the thick line shows the predicted ratio
for a XDR chemistry. Bottom panel: fraction of HCO + and HOC +
molecules formed along each chemical path in a XDR chemistry.
at equilibrium is also sensitive to the dominant mechanism of
HCO + /HOC + formation: the more e#cient is the relative proí
duction of HOC + , the lower is the ionization degree required
to reach a certain R ratio. Typical paths for the formation of
HCO + /HOC + are (Apponi et al. 1997 and references therein):
H +
3 + CO -# HOC +
/HCO +
+ H 2 (2)
CO + + H 2 -# HOC + /HCO + + H (3)
H 2 O + C +
-# HOC +
/HCO + + H (4)
The branching ratio for the net production of nascent
HOC + , hereafter denoted by #, depends on the particular forí
mation pathway. The value of # is 0.06 for reaction 2, 0.48 for
3 and 0.8 for 4. In a real case scenario the three reactions coexí
ist, and thus the equivalent branching ratio, # e# , is an average of

14 A. Usero et al.: Molecular Gas Chemistry in AGN
the individual #, weighted by the fraction of HCO + and HOC +
particles that are actually formed following a certain pathway.
We have derived how R depends on the ionization degree of
molecular gas, separately, for the di#erent reactions, assuming
the rate coe#cients given by Smith et al. (2002) (Fig. 8, top
panel). We find that in order to obtain values of R#50í300 high
ionization degrees are needed: X(e - )#10 -6 í10 -4 . These high
electronic abundances are typically reached in XDR (Lepp &
Dalgarno 1996, Maloney et al. 1996). Fig. 8 illustrates also the
relation between the dominant formation path and the isomer
ratio: less extreme ionization degrees are needed to reach low
R ratios if the predominant reaction has a large #. On the other
hand, much higher ratios #300í6000, like that typically meaí
sured in molecular clouds, can be easily accounted for if elecí
tronic abundances approach standard levels <10 -6 --10 -7 .
We have also derived the dependence of # e# on the ioní
ization degree for an adopted XDR model (see the curve for
R in the top panel of Fig. 8). The abundances of all molecular
species, contributing to (2), (3) and (4), have been taken from
Maloney et al. (1996), except for CO + , whose abundance curve
is taken from the PDR model of Sternberg et al. (1995). Values
of R#30--80, like that measured in the CND of NGC 1068, can
be easily accounted for assuming an average ionization degree
of X(e - )#10 -5 for the bulk of molecular gas.
The relative weight of the 3 formation paths of HOC + in
a typical XDR is also represented, as a function of X(e - ), in
Fig. 8 (bottom panel). While reaction (2) clearly dominates
the balance for X(e - )<10 -5 , reactions involving H 2 O (4) and
CO + (3) are predominant for X(e - )>10 -4 . Recent observations
of galactic PDR (Fuente et al. 2003; Rizzo et al. 2003) have
confirmed that low HCO + /HOC + ratios are indeed correlated
with large abundances of CO + and/or H 2 O.
5.3. Anisotropic Xíray Illumination of the CND?
The results of this work strongly favour an overall scenario
where the CND of NGC 1068 has become a giant XDR. It
is tempting to speculate if Xíray driven chemistry can also
explain the mild but systematic di#erences in the molecular
abundances of SiO, HCO + , HOC + and HCN between the E
and the W knots of the CND. Figure 9 shows the distribuí
tion of Xíray emission inside the 6í8 keV band in the CND
of NGC 1068 (adapted from Ogle et al. 2003). Xíray emisí
sion in this energy band is dominated by the Fe # K# line
at 6.4 keV in NGC 1068. There is a 10 ## extended emission
(not shown in Figure 9) which corresponds to the ionization
cones. The strongest component, however (shown in Figure 9),
should be tracing the illuminated inner wall of the CND torus.
Most interestingly, this figure shows tantalizing evidence of a
di#erent degree of penetration of Xírays into the E/W knots:
the western side of the molecular torus, corresponding to the
inner wall of W knot, seems to be more illuminated than its
eastern counterpart. This would be in agreement with the reí
ported chemical di#erentiation seen between the E/W molecuí
lar knots. A di#erence in the attenuating column densities, estií
mated from CO, exists between the two CND knots: on 100 pc
scales, N(H 2 )| East /N(H 2 )| West #2. However, we do not know if
these or even greater di#erences hold at smaller scales which
are probably more relevant to probe Xíray absorption by neuí
tral gas in the torus. In this context, it is however suggestive to
note that the strongest H 2 O megamasers, which are collisioní
ally excited in the warmest region of the CND illuminated by
X rays (Neufeld et al. 1994), are mostly located in the western
side of the molecular torus (Greenhill & Gwinn 1997).
Highíresolution interferometer observations will give a
sharp view of molecular abundance changes inside the CND
at small scales for critical tracers such as SiO, CN and HOC + .
A detailed comparison of these maps with the Chandra images
of the CND may help to constrain this scenario.
6. Conclusions
We summarize the main results obtained in this work as folí
lows:
-- We report on the detection of significant SiO(3--2) and
SiO(2--1) emission in the 200 pc circumnuclear disk of
NGC 1068. The large overall abundance of SiO in the CND
(#(5í10)½10 9 ) cannot be explained by shocks driven by star
formation on molecular gas as there is counteríevidence of
a recent starburst in the nucleus of NGC 1068. While SiO
emission is also detected over the starburst ring, we estií
mate that SiO abundances there are 10 times lower than
those measured in the CND. These lower abundances of
SiO are in close agreement with that measured in starbursts
on similar spatial scales, however.
-- We also report on the first extragalactic detection of
the reactive ion HOC + . Most remarkably, the estimated
HCO + /HOC + abundance ratio in the nucleus of NGC 1068,
#30--80, is the smallest ever measured in molecular gas.
The line profile of HOC + is markedly asymmetrical with
respect to v sys : HOC + emission is mostly detected at red
velocities. Whatever process is responsible for the enhanceí
ment of this reactive ion, it seems to be unevenly e#cient
inside the CND.
-- Results from additional mmíobservations have served for
estimating abundances of CN, HCO + , HOC + , H 13 CO +
and HCO. These estimates are complemented by a reí
evaluation of molecular abundances for HCN, CS and CO,
based on previously published singleídish and interferomeí
ter observations of NGC 1068. While models invoking oxyí
gen depletion in molecular gas successfully fit the HCN/CO
ratio measured in the CND, they fail to account for our
estimates of the HCN/HCO + and CN/HCN abundance raí
tios. On the contrary, XDR models can simultaneously exí
plain these ratios. The detection of high abundances of SiO
and HOC + in the CND of NGC 1068 gives further support
to the XDR chemistry scenario. The processing of 10 Š
dust grains by Xírays, as a mechanism to enhance silicon
chemistry in gas phase, would explain the large SiO abuní
dances of the CND. Finally, we have shown that the low
HCO + /HOC + ratios measured in the CND can be explained
if molecular clouds have the high ionization degrees typií
cal of XDR (X(e - )# 10 -6 í10 -4 ). An examination of the
di#erent formation paths of HOC + suggests that reactions

A. Usero et al.: Molecular Gas Chemistry in AGN 15
Fig. 9.Xíray emission and molecular gas in
the CND: overlay of the distribution of hard
Xíray emission in the 6í8 keV band (gray
scale adapted from Ogle et al. 2003: whiter
shades stand for stronger emission) and the
CO(1--0) integrated emission as in Fig. 3.
The AGN locus is highlighted by the starred
marker.
involving H 2 O and/or CO + would be the predominant preí
cursors of HOC + in XDR.
-- The XDR scenario could also provide an explanation for
the di#erent abundances of SiO, HCO + and, especially, of
HOC + measured in the E and W knots. The Chandra imí
ages of the CND in the 6í8 keV band, dominated by the
emission of the Fe # K# line, show tantalizing evidence of
a di#erent degree of penetration of hard Xírays into the E
and W knots. This suggests that larger columns of molecuí
lar gas are being processed by Xírays in the W knot.
Acknowledgements. We acknowledge the IRAM sta# from Pico
Veleta and Granada for help provided during the observations.
We wish to thank A. RodrÒÐguezíFranco for his support during
the observations. We also wish to thank E. Schinnerer and L. J.
Tacconi for providing their interferometer data. This research has
made use of NASA's Astrophysics Data System (ADS) and the
NASA/IPAC Extragalactic Database (NED). This paper has been parí
tially funded by the Spanish MCyT under projects DGES/AYA2000í
0927, ESP2001í4519íPE, ESP2002í01693 , PB1998í0684, ESP2002í
01627 and AYA2002í10113E.
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