Australia Telescope National Facility

Total Magnitudes of Virgo Galaxies. I.
Construction of a Self-consistent Reference Dataset
Spanning 8th to 18th mag.
Christopher Ke-shih Young
, PASA, 18 (2), in press.

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Faint galaxy sample (18th-14th mag.)

At present, the most reliable souce of blue total magnitudes for large numbers of faint Virgo galaxies is Young & Currie's (1998a) Virgo Photometry Catalogue (VPC), which presents U, B_J and R_C photographic photometry for over one thousand galaxies in the direction of the Virgo Cluster's core. Curiously, the VPC is actually the first independently calibrated general catalogue of galaxies to cover Virgo since Volumes I and II of Zwicky et al.'s (1961 & 1963) Catalog of Galaxies and Clusters of Galaxies. It is based on numerically integrated plate-scan data obtained from U.K.-Schmidt plates using the Royal Observatory Edinburgh's COSMOS microdensitometer. All total magnitudes listed in the VPC are derived according to the t system of Young et al. (1998). Unfortunately, the VPC does not present B_J-band photometry for any galaxy brighter than 14th mag., on account of saturation effects. As its B-band total magnitudes are transformed values based on the original B_J-band values and B_J-R_C colours, the VPC lists B-band values only for objects for which B_J-R_C colours could be obtained and whose B_J-band photometry was unsaturated. Comparisons between the VPC's B_t magnitude scale and the total-magnitude scales of other works, specifically Young (1994 & 1997) and Young & Currie (1998b), have therefore previously been restricted to faint Virgo galaxies exclusively. However, as the VPC's U-band photometry was slightly shallower than its B_J-band photometry, there are actually 48 objects for which new U-band photometry is presented in the VPC, even though their B_J-band surface-brightness profiles are saturated.

**Table 1:** VPC objects for which alternative
${B_t}^{\prime }=U_{t}-(U-B)$ values (bold type) could be derived from a combination of VPC U_t values and non-VPC (U-B) colours based on aperture-photometry measurements
Designation	type	U_t	U₂₅-U_t	(U-B)	$\sigma_{(U-B)}$	aperture	(U-B)	${B_t}^{\prime}$	B_t	B_J₂₅-B_{J_t}
VPC/other		(mag.)	(mag.)	(mag.)	(mag.)	(arcsec)	source	(mag.)	(mag.)	(mag.)
35/VCC 334	dI	15.99	0.16	-0.21	0.04	19.0	GH	16.20	16.49	0.25
256/IC 3239	dI	16.32	0.12	-0.28	0.05	40.0	GH	16.60	16.47	0.22
342/VCC 729	dE	15.77	0.25	0.10	0.04	35.6	NC	15.67	15.78	0.38
408/VCC 793	dI	17.14	0.28	-0.02	0.08	19.0	GH	17.16	17.17	0.50
420/VCC 810	dE	17.18	0.69	0.27	0.10	35.6	NC	16.91	17.12	0.69
502/IC 3355	dI	15.74	0.14	-0.20	0.04	60.0	GH	15.94	15.35	0.34
670/IC 3416	dI	15.37	0.25	-0.19	0.05	19.0	GH	15.56	15.51	0.24
801/VCC 1348	dE	15.72	0.73	0.34	0.05	39.0	NC	15.38	15.66	0.57
808/VCC 1352	dE	17.81	0.85	0.12	0.15	39.0	NC	17.69	17.60	0.66
829/VCC 1377	dI	16.61	1.23	0.11	0.11	40.0	GH	16.50	16.11	0.87
834/VCC 1386	dE	15.69	0.48	0.19	0.05	35.6	NC	15.50	15.26	0.52
835/VCC 1389	dE	16.65	0.42	0.21	0.08	35.6	NC	16.44	16.61	0.38
843/VCC 1407	dE	15.83	0.25	0.26	0.04	35.6	NC	15.57	15.61	0.40
856/VCC 1420	dE	17.47	0.40	0.24	0.08	35.6	NC	17.23	16.95	0.36
897/IC 3483	dI	15.22	0.28	0.04	0.04	19.0	GH	15.18	15.08	0.47
937/VCC 1539	dE	16.60	0.66	0.15	0.09	35.6	NC	16.45	15.92	0.76

Notes: (1) The sources of (U-B) aperture-photometry measurements are NC: Caldwell (1983) and GH: Gallagher & Hunter (1986). Except for
${B_t}^{\prime}$ , the other quantities are from the VPC. (2) The extrapolation terms U₂₅-U_t and

B_J₂₅-B_{J_t} are the bases of the error budgets shown in Fig. 1. (3) The new

${B_t}^{\prime}$ values are probably less accurate for galaxies of type dI than for objects of type dE, as the former are more likely to possess significant colour gradients.

**Figure 1:** A comparison between B_t values derived from the VPC's U and B_J-band photometry (see Table 1). The error bars only represent uncertainties in the extrapolation terms beyond the limiting U and B_J isophotes, and have been set to one quarter of the
B_J₂₅-B_t and U₂₅-U_t extrapolation terms (from Table 1) in mag. Datapoints represent individual galaxies and the equality line is shown for reference.
$\begin{figure} \begin{center} \psfig{file=p1f1.ps,height=7.5cm,angle=-90} \end{center} \end{figure}$

In order to investigate the possibility of extrapolating the VPC's B_t magnitude scale brightward of 15th mag. by combining U_t values from the VPC with published (U-B) measurements from other sources, a literature search was conducted for (U-B) measurements of faint Virgo galaxies. It was found that Caldwell (1983) and Gallagher & Hunter (1986) had observed a total of 16 objects for which the VPC quotes new B_t values. These objects are listed in Table 1. A comparison was then made between the B_t values quoted in the VPC and alternative

${B_t}^{\prime }=U_{t}-(U-B)$ values. The mean

${B_t}^{\prime} - B_t$ offset between the two sets of values was found to be only +0.08( $\pm$ 0.07) mag. >From Fig. 1, there is no evidence for any scale error either, and the mean offset collapses to +0.01( $\pm$ 0.05) mag. if the two outliers, IC 3355 and VPC 937 (=VCC 1539), are excluded. As the (U-B) aperture photometry measurements are susceptible to errors arising from colour gradients (when present) not being taken into account and/or centering problems, particularly when small apertures are used, the

${B_t}^{\prime}$ values are probably on average of similar accuracy to the VPC's B_t values (for which transformation from B_J was necessary). They can however, only be derived for a small number of objects on account of the scarcity of published (U-B) colours for faint objects. In the case of VPC 937, the cause of the 0.53 mag. discrepancy between its

${B_t}^{\prime}$ and B_t is unclear. However, in the case of IC 3355, the 0.59 mag. discrepancy may well be due to the proximity of several giant galaxies (see Section 8 of Young & Currie 1998a).

Next Section: Intermediate galaxy sample (14th-12th
Title/Abstract Page: Total Magnitudes of Virgo
Previous Section: Introduction
Contents Page: Volume 18, Number 2

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