Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/~marel/abstracts/psdir/mergers4.ps Äàòà èçìåíåíèÿ: Thu Oct 14 21:27:53 1999 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 11:31:08 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: ngc catalog

Galaxy Dynamics: from the Early Universe to the Present
ASP Conference Series, Vol. XX, 2000
F. Combes, G. A. Mamon and V. Charmandaris, eds.
Nuclear Brightness Profiles of Merger Remnants:
Constraints on the Formation of Ellipticals by Mergers
Roeland P. van der Marel and David Zurek
Space Telescope Science Institute, Baltimore, USA
Abstract. We present preliminary results of an HST/NICMOS pro­
gram to image merger remnants in the J, H and K bands. The nuclear
brightness profiles for most sample galaxies are similar to those typical
for elliptical galaxies, but some (including the well­studied NGC 3921
and 7252) have an unusually high luminosity density at small radii. This
is consistent with the prediction of N­body simulations that gas flows to
the center during a merger and forms new stars.
1. Introduction
The possibility that many elliptical galaxies formed from mergers of disk galaxies
is a topic of continuing interest. That mergers form elliptical­like remnants has
been demonstrated through numerical simulations, and ground­based imaging
has shown that many merger remnants have r 1/4 luminosity profiles. These ar­
guments, along with the detection of shells, ripples and kinematically decoupled
cores in elliptical galaxies, support this `merger hypothesis' (e.g., Kennicutt,
Schweizer, & Barnes 1998; hereafter KSB98).
Theoretical arguments indicate that it is in the nuclei of remnants where
the merger hypothesis may face its most stringent test. If dynamical relaxation
is the dominant physical process in mergers, then remnant nuclei will be very
di#use with large cores (Hernquist 1992), unless the progenitor nuclei were dense
to begin with. If both merging galaxies contain a central black hole, then the
stellar density of the merger remnant will be lower than that of the progenitor
galaxies (Quinlan & Hernquist 1997). Alternatively, if mergers are accompanied
by strong gaseous dissipation and central starbursts, then the remnant may have
a high stellar density and steep luminosity profile (Mihos & Hernquist 1994).
A comparison between the observed nuclear properties of merger remnants
and elliptical galaxies can shed more light on the viability of the merger hypoth­
esis and on the physical processes that govern the structure of merger remnants.
The nuclear brightness profiles of elliptical galaxies have been mapped in great
detail with HST. Faber et al. (1997; hereafter F97) studied a large sample of nor­
mal ellipticals. Carollo et al. (1997; hereafter C97) studied a sample of elliptical
galaxies with kinematically decoupled cores (presumably old merger remnants),
and found few di#erences as compared to the sample of F97. To complement
this work, we initiated an HST study of a sample of younger merger remnants
(van der Marel, Zurek, Mihos, Heckman & Hernquist 2000, in preparation), and
we present here some of the preliminary results.
1

2 van der Marel & Zurek
Figure 1. Solid dots show the observed H­band surface brightness
profile (left) and J -H color (right) for NGC 3921. Open dots are for
NGC 7727, shifted horizontally to the distance of NGC 3921. NGC
7727 has a fairly typical `power­law' (F97) profile with I(R) # R -0.8
at the HST resolution limit. By contrast, NGC 3921 is unusually blue
and bright at small radii, with I(R) # R -1.9 .
2. Sample Selection
A well­known compilation of nearby interacting galaxies and mergers is Toomre's
list of 11 galaxies selected from the NGC Catalog (see KSB98). The two latest­
stage mergers in the list are NGC 3921 and NGC 7252, which have tidal tails
but show no remaining signs of two galaxies with a separate identity. To cre­
ate a sample for our study, we sought galaxies with morphological properties
similar to NGC 3921 and 7252 from the Catalogs of Arp (1966) and Vorontsov­
Velyaminov (1977), and from the imaging survey of (UV­bright) Markarian
galaxies by Mazzarella & Boroson (1993). This yielded a sample of 19 galax­
ies with cz < 10000 km s -1 , of which we imaged 14 galaxies (including NGC
3921 and 7252). The remaining five galaxies are classified as ultra­luminous IR
galaxies, and were imaged with HST by other teams.
3. Observations
To minimize any influence of dust on the observed brightness profiles we ob­
served the galaxies in the near­IR with the HST/NICMOS instrument (Cycle 7
project GO­7268). Images were obtained with the NIC2 camera (pixel size 0.076 ##
square) using the filters F110W, F160W and F205W, corresponding roughly to
J, H and K, and with the NIC1 camera (pixel size 0.043 ## square) only in F110W.
Each image was subjected to basic reduction steps followed by Lucy deconvo­
lution with an appropriate PSF. Azimuthally averaged brightness profiles were
extracted for all camera/filter combinations. Example results for two galaxies
are shown in Figure 1.
Each brightness profile was fit by a `nuker' law (Lauer et al. 1995), which
was deprojected to obtain the three­dimensional luminosity density. Figure 2
shows this density in the H­band at a fiducial radius r = 50 pc, as function
of galaxy luminosity (assuming H 0 = 80 km s -1 Mpc -1 ), both for the galaxies
in our sample and for those in the samples of F97 and C97 (transformed to
the H­band under the assumption of a proto­typical V - H = 3.0 for elliptical

Nuclear Brightness Profiles of Merger Remnants 3
Figure 2. Three­dimensional H­band luminosity density at r = 50pc
as function of M V . Solid dots are the galaxies in our HST/NICMOS
sample. Open dots and triangles are galaxies from F97 and C97 re­
spectively. NGC numbers are indicated for selected galaxies.
galaxies; Peletier, Valentijn & Jameson 1990; Silva & Bothun 1998). While most
of the galaxies in our sample follow the same approximate correlation as normal
ellipticals, there are three galaxies that strongly stand out because of their high
luminosity density: NGC 34, 3921 and 7252. One other galaxy that stands out
for the same reason is NGC 1316 from the F97 sample, which is also a merger
remnant (Schweizer 1980).
The J - H and H - K color profiles of NGC 3921 and 7252 show that
they become bluer towards the center, presumably due to recent star formation.
This is consistent with the detection of strong Balmer absorption lines in ground­
based spectra of these galaxies (KSB98). NGC 34 becomes redder towards the
center, probably as a result of dust absorption. NGC 34 is the most IR­luminous
galaxy of those that we observed, suggesting the presence of ongoing or recent
star formation in this galaxy as well.
4. Discussion and Conclusions
The high luminosity densities observed in NGC 34, 3921 and 7252 are probably
a direct consequence of recent star formation triggered by a merger. Stellar
populations fade with time, and these galaxies will therefore become more similar
to normal ellipticals as time passes. Dynamical and spectral evidence suggest
that the mergers happened 0.5--1.5 Gyr ago (KSB98). The models of Bruzual
& Charlot (e.g., 1993) indicate that a single­burst population fades by a factor
of # 10 between 0.5 and 10 Gyr. Figure 2 therefore suggests that these galaxies
may become similar to normal ellipticals within a Hubble time. Most galaxies in
our sample are already now similar to normal ellipticals in terms of their nuclear

4 van der Marel & Zurek
luminosity density, although some fall on the high end of the range occupied by
normal ellipticals. If these galaxies are the remnants of disk­disk mergers, then
either the merger ages must be large so that the newly formed stars have mostly
faded, or they never formed many new stars, e.g., because the progenitors were
gas poor or the star formation e#ciency was low.
Results such as those for NGC 3921 in Figure 1 indicate that its star for­
mation was limited mostly to the central region, r < 0.5 ## # 200 pc. This is con­
sistent with predictions of dissipative N­body simulations of disk­disk mergers,
in which the gas quickly falls to the central few­hundred pc (Mihos & Hernquist
1994). The `excess' light in the central arcsec of NGC 3921 (as compared to
NGC 7727; see Figure 1) represents # 4% of the total galaxy luminosity, and
probably a smaller fraction in terms of mass. CO complexes observed in the
central kpc of merger remnants support the view that gas flows to the center in
galaxy interactions, but even if all the CO observed in NGC 3921 and 7252 were
soon turned into stars, the light from recently formed stars would still provide
only a small fraction of the total galaxy luminosity (Hibbard & Yun 1999).
To summarize, we have detected the luminosity spikes predicted by dissi­
pative simulations of disk­disk mergers, but only in some of our galaxies. In
general, it appears that the light from young stars does not provide a major
contribution to the total galaxy luminosity. This is consistent with work by
Silva & Bothun (1998), who found that the near­IR colors of most morpho­
logically disturbed ellipticals are inconsistent with intermediate age (2--5 Gyr)
stars providing much of the luminosity. This raises the question whether these
galaxies were ever similar to ultra­luminous infrared galaxies, in which massive
starbursts are known to occur as a result of galaxy interactions.
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