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Our galaxy samples and CGCG completeness

Total Magnitudes of Virgo Galaxies. II.
An Investigation into the mp Scale of Volume I of Zwicky et al.'s

Catalog of Galaxies and Clusters of Galaxies

Christopher Ke-shih Young and Zheng-yi Shao, PASA, 18 (2), in press.

Next Section: Isophotal magnitudes
Title/Abstract Page: Total Magnitudes of Virgo
Previous Section: Introduction
Contents Page: Volume 18, Number 2

Our galaxy samples and CGCG completeness


Table 1: Completeness of the CGCG within the VPC survey area as a function of Bt, see text for definitions of Galaxy-samples CI and CII
á  Volumeá I á  Volumeá II
Bt range

NCGCGCI(Bt)

NVPCCI(Bt)

completeness á 

NCGCGCII(Bt)

NVPCCII(Bt)

completeness
(mag.) (objects)1 (objects)2 (%) á  (objects)1 (objects)2 (%)
Bt < 14.0 57 57 100 á  11 11 100

$14.0 \leq B_t < 14.5$

16 16 100 á  3 4 75

$14.5 \leq B_t < 15.0$

9 12 75 á  4 4 100

$15.0 \leq B_t < 15.5$

10 14 71 á  2 4 50

$15.5 \leq B_t < 16.0$

17 25 68 á  0 0 N/A

$16.0 \leq B_t < 16.5$

10 41 24 á  0 8 0

$16.5 \leq B_t < 17.0$

2 51 4 á  0 9 0
á  á  á  á  á  á  á  á 
overall total 121 216 56 á  20 40 50
  • Notes: (1) The number of CGCG-listed objects in Galaxy-sample CI or CII (where a subscript I denotes Volumeá I and a subscript II denotes Volumeá II). (2) The total number of VPC-listed objects in Galaxy-sample CI or CII including objects omitted from the CGCG (where a subscript I denotes the Volumeá I survey area and a subscript II denotes the Volumeá II survey area). This quantity is also the galaxy-sample size on which the corresponding completeness ratio is based.

The Virgo Photometry Catalogue (VPC) of Young & Currie (1998), which is complete to

BJ25=18.0 mag. over the whole of its survey area of 23á deg.2, can be used in order to investigate the completeness of Volumeá I of the CGCG to Bt = 17.0 as a function of Bt. It is however, only of limited use in investigating the completeness of Volumeá II of the CGCG, because most of the VPC survey area lies within the area of sky covered by Volumeá I. Note that no parts of the VPC survey area lie exterior to those [much larger] areas of sky covered by Volumesá I and II of the CGCG and that all CGCG-listed objects lying within the VPC survey area are also listed in the VPC. For the purpose of studying the CGCG's completeness as a function of Bt, we required a large and complete galaxy sample including every object down to a specified limiting Bt value (that was considerably fainter than the faint-end limit of the CGCG) within a clearly defined area of the sky. Precise Bt values were not essential for every galaxy, and especially at the bright end (where the CGCG could be expected to be 100% complete) some Bt estimates of lower accuracy could be tolerated. However, for the purposes of investigating the CGCG's magnitude scale, only CGCG-listed objects with reliable photometry could be invoked. In order to make optimum use of the data available, we therefore defined two overlapping samples of galaxies for the area of sky covered by Volumeá I and a further two for the area of sky covered by Volumeá II: hereunder CI & MI and CII & MII respectively. Each Sampleá C was defined as the set of all galaxies to be used for the CGCG-completeness study, whilst each sampleá M was defined as the set of all galaxies to be used for the CGCG magnitude-scale studies. Each Sample C comprised all galaxies listed in the VPC (regardless of whether they are listed in the CGCG or not) which we found to be brighter than Bt=17.0; approximate Bt values having been derived for those objects lacking Bt measurements in the VPC or in Tablesá 2 and / or 3 of Paperá I. For 10 Volumeá I objects; VPCá 64, 99, 159, 171, 242, 379, 505 863, 878 & 953; BJt values are listed in the VPC but Bt values are not1, and so the approximation:

\begin{displaymath} B_{t} \sim B_{J_t} + 0.35(B-V) \end{displaymath} (1)

(with (B-V) set to 0.8 mag.) was used. For the outstanding high surface-brightness VPC objects for whom Young & Currie's (1998) BJ-band photometry was saturated, and for whom the VPC therefore does not list Bt or BJt values, rougher approximations were necessary. These were based on the BJ25 (BJ-band isophotal magnitude measured to the

$\mu_{B_J} = 25.0$á mag.arcsec-2 isophote) values listed in the VPC, but most of which were for the bright objects in question based on various sources of magnitude measurements external to the VPC (that are not necessarily of high accuracy and will be investigated in future papers in this series). For these galaxies, we obtained estimates of Bt from Equationá 1 (also with (B-V) set to 0.8 mag.) and:

\begin{displaymath} B_{J_t} \sim B_{J_{25}}-0.35 {\rm\hspace{1.5mm}mag.} \end{displaymath} (2)

(this latter offset being the VPC's mean BJ-band extrapolation). This was of course equivalent to:

\begin{displaymath} B_{t} \sim B_{J_{25}} - 0.07 {\rm\hspace{1.5mm}mag.} \end{displaymath} (3)

and enabled the CGCG's completeness to be quantified as a function of Bt, as documented in Tableá 1. The galaxy VPCXá 50 was also taken into consideration using Equationá 3. The total number of VPC galaxies brighter than

$B_{t} \sim 17.0$ was found to be 256. Of these, 216 were found to lie within the area of the sky covered by Volumeá I of the CGCG, whilst only 40 were found to lie within the area covered by Volumeá II. Galaxies not listed in the CGCG were flagged and divided into would-be Volumeá I and would-be Volumeá II objects. The brightest sample object found to be omitted from Volumeá I is VPCá 800 (

$B_t \sim 14.7$), whilst the brightest object omitted from Volumeá II is VPCá 283 (

$B_{t} \sim 14.4$). Each Sample M on the other hand, comprised only of CGCG objects for which reliable Bt measurements are listed in the VPC or Tablesá 2 and / or 3 of Paperá I. Note that not all of the objects listed in Tableá 3 of Paperá I lie within the VPC survey area, but that all of these objects lie within the areas of sky covered by Volumesá I and II of the CGCG. At the faint end, it was found that 34 galaxies for which Bt values are listed in the VPC correspond to CGCG objects. As all of the 58 objects listed in Tablesá 2 & 3 of Paperá I are listed in the CGCG, the combined sample comprised of 92 galaxies in total, of which 90 were Volumeá I objects whilst only 2 were Volumeá II ones. For any galaxy of Bt < 17.0 then, the following rules determined which subset of each Samples C and /or M it belonged to; the overlap region between each Sample C and M being denoted $C \cap M$, and subsets of C and M not overlapping with M and C respectively being denoted

$C \backslash M$ and

$M \backslash C$ respectively.
$\bullet$ Reliable Bt available from VPC or Tablesá 2 and / or 3 of Paperá I?
NO: Member of

$C \backslash M$.
YES: $\bullet$ Galaxy listed in CGCG?
NO: Member of

$C \backslash M$.
YES: $\bullet$Bt available from Tableá 3 of Paperá I?
NO: Member of $C \cap M$.
YES: $\bullet$ Galaxy outside VPC survey area?
NO: Member of $C \cap M$.
YES: Member of

$M \backslash C$. Bearing in mind that the brightest object found to be omitted from Volumeá I of the CGCG is of

$B_t \sim 14.7$, on the basis of Tableá 1 we can say that Volumeá I of the CGCG is only about 70% complete over the range

$14.75 \leq B_{t} < 15.75$. Furthermore, we find that late-type galaxies are heavily over-represented faintward of the CGCG's completeless limit. In fact, out of the 43 VPC galaxies for which

$ 14.5 \leq B_{t} < 16.5$ but which were omitted from the CGCG, only 4 are listed as irregulars, probable irregulars or spirals in the VPC and one as a BCD. Apart from a further 6 whose types are unknown, the remaining 32 are all early-type objects (29 ellipticals, 2 lenticulars and 1 probable lenticular). We take this to be evidence that Zwicky et al.'s selection criteria were heavily morphology dependent at the faint end, at least for Volumeá I. Unfortunately, our galaxy sample is not extensive enough to be able to confirm this finding with confidence for Volumeá II.


Next Section: Isophotal magnitudes
Title/Abstract Page: Total Magnitudes of Virgo
Previous Section: Introduction
Contents Page: Volume 18, Number 2

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