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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Title/Abstract Page: Total Magnitudes of Virgo
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Bright-end consistency checks

It is reassuring to find that for the 3 objects in common between Tables 2 and 3, the new total-magnitude values are in good agreement. For NGC 4352, NGC 4431 and NGC 4436, the differences (Table 2 minus Table 3) are -0.01, +0.04 and +0.18 mag. respectively.

**Table 4:** B_T values adopted from Michard (1982) in order to provide a consistency check on the improved values listed in Table 3
Designation	VCC type/	B_T	$B_{A_{{\rm max}}}-B_T$
	membership	(mag.)	(mag.)
NGC 4124	S0/M	12.10	0.42
NGC 4168	E/M	12.25	0.45
NGC 4262	SB0/M	12.43	0.16
NGC 4267	SB0/M	11.81	0.47
NGC 4339	S0/M	12.37	0.44
NGC 4365	E/M	10.64	0.44
NGC 4374	E/M	10.26	0.40
NGC 4382	S0(pec)/M	10.03	0.33
NGC 4387	E/M	12.97	0.22
NGC 4406	S0/M	10.08	0.59
NGC 4417	S0/M	12.02	0.27
NGC 4425	SBa/M	12.75	0.18
NGC 4429	S0-Sa(pec)/M	11.11	0.49
NGC 4435	SB0/M	11.68	0.37
NGC 4442	SB0/M	11.32	0.28
NGC 4458	E/M	12.85	0.44
NGC 4459	S0/M	11.39	0.43
NGC 4461	Sa/M	12.08	0.31
NGC 4472	E-S0/M	9.34	0.74
NGC 4473	E/M	11.07	0.56
NGC 4474	S0/M	12.57	0.29
NGC 4476	S0/M	13.17	0.29
NGC 4477	SB0-SBa/M	11.45	0.49
NGC 4486	E/M	9.65	0.72
NGC 4503	Sa/M	12.13	0.38
NGC 4526	S0/M	10.70	0.15
NGC 4550	E-S0/M	12.46	0.19
NGC 4564	E/M	11.96	0.32
NGC 4570	S0-E/M	11.79	0.24
NGC 4596	SBa/M	11.50	0.44
NGC 4638	S0/M	12.22	0.20
NGC 4649	S0/M	9.88	0.80

Notes: (1) The
$B_{A_{{\rm max}}}-B_T$ extrapolation values listed here are based on the maximum aperture for which a reliable measurement was obtained. (2) Membership assignments are based on radial velocities: M (for member) if less than 3500 km ${\rm s}^{-1}$ . (3) NGC 4124 is not listed in the VCC. Its type was taken from the RC3 instead.

**Figure 2:** A comparison between the new B_t values derived from Caon et al.'s surface-brightness profiles (Table 3) and Michard's B_T values (Table 4) for the 18 galaxies in common between Tables 3 and 4. The datapoints represent individual galaxies and the equality line is shown for reference. The error bars only represent uncertainties in the extrapolation terms, and have been set to one quarter of the extrapolation terms in mag. Larger ` $\bullet$ ' symbols have been used to distinguish the 6 objects for which n<0.25. These objects are listed in Table 5. If these objects are excluded, the mean B_T(Table 4) - B_t(Table 3) offset for the remaining 12 objects is only +0.03( $\pm 0.04$ ) mag.
$\begin{figure} \begin{center} \psfig{file=p1f2.ps,height=7.5cm,angle=-90} \end{center} \end{figure}$

For bright Virgo galaxies, another dataset was identified as likely to be of the highest quality-that of Michard (1982) whose total magnitudes, which were derived from photoelectric aperture photometry measurements, have been adopted here without modification. Note however, that whilst we have excluded all objects whose photometry Michard flagged as unreliable, we have not excluded objects when his only concern was that the extrapolation terms were large. One aspect of Michard's work that sets it apart from most other authors' is that it took into account systematic departures from the standard growth curves relevant to each galaxy's morphological type. This was done by applying the best fitting standard growth curve from the Second Reference Catalog of Bright Galaxies (RC2) of de Vaucouleurs et al. (1976) in each case, not the one listed as appropriate to the galaxy's morphogical type. A list of the values adopted is given in Table 4. As is evident from Fig. 2, there is no evidence for any scale error between the new total magnitudes derived from Caon et al.'s original photometry and Michard's values, provided one excludes the six bright objects for which the profile-shape parameter n was found to be less than 0.25. This would appear to confirm that zero-point differences between the photometry of Caon et al. and Michard are small¹. Note that the error terms on Michard's magnitudes are generally larger than those on our values due to larger extrapolation terms² and fewer measurements per luminosity profile. Unfortunately there is only one object in common between Michard's galaxy sample and those objects listed in Table 2, namely NGC 4468. As Michard flagged most of the aperture measurements for this galaxy as unreliable, it has not been listed in Table 4. However, our value of B_t=13.82 is in excellent agreement with Michard's value of B_T=13.79.

**Table 5:** A comparison between the total magnitude values derived by Michard, Caon et al. and the author for galaxies listed in Table 3 that were found to possess luminosity profiles of n<0.25
Designation	Table 3	Michard	Caon et al.	Caon et al.	Caon et al.	Table 3
	n	B_T	B_T (1990) Table I	B_T (1990) Table VI	B_T (1994)	B_t
		(mag.)	(mag.)	(mag.)	(mag.)	(mag.)
NGC 4261	0.19	-	-	-	11.03	11.00
NGC 4269	0.15	-	-	-	13.32	13.38
$\bullet$ NGC 4365	0.19	10.64	-	-	10.35	10.28
$\bullet$ NGC 4374	0.15	10.26	10.26	9.71	-	9.71
$\bullet$ NGC 4406	0.15	10.08	10.06	9.63	-	9.15
$\bullet$ NGC 4472	0.20	9.34	-	-	8.87	8.93
$\bullet$ NGC 4473	0.22	11.07	11.10	11.02	-	11.07
$\bullet$ NGC 4486	0.23	9.65	9.58	9.48	-	9.43
NGC 4552	0.15	(10.91)	10.78	10.38	-	10.48
NGC 4621	0.15	(10.83)	10.76	10.30	-	10.30
NGC 4636	0.21	-	-	-	10.01	9.79

Notes: (1) Three objects were not observed by Michard: NGC 4261, 4269 and 4636; whilst Michard's B_T values for a further two are listed in parentheses because they were flagged by him as unreliable. Only the six objects for which Michard obtained reliable B_T measurements appear on Fig. 2. These objects are flagged with ` $\bullet$ ' symbols.

In the cases of the six bright objects shown in Fig. 2 for which n<0.25 then (see also Table 5), the large systematic differences between our new B_t values and Michard's B_T ones cannot be the result of zero-point differences, but must primarily be due to the different extrapolations applied. The RC2 set of standard growth curves available to Michard did not include any curve specific to galaxies more centrally concentrated than n=0.25 objects, and so Michard necessarily applied the growth curve specific to the n=0.25 case to his most centrally concentrated objects. Consistency checks on our extrapolation terms for all of the objects listed in Table 3, including the most centrally concentrated ones, can be provided by comparisons with Caon et al.'s total magnitudes. It was found that for objects of $n \geq 0.25$ (with a scatter of 0.24 mag. about the mean offset):

$\begin{displaymath} {\rm Extrapolated} \hspace{1mm} B_{T}{\rm (Caon \hspace{1mm... ...hspace{1mm} 3)} = -0.06 (\pm 0.05) \hspace{1mm} {\rm mag.} \end{displaymath}$

(1)

and (with a scatter of 0.14 mag.):

$\begin{displaymath} {\rm Integrated\/} \hspace{1mm} B_{T}{\rm (Caon \hspace{1mm... ...space{1mm} 3)} = -0.06 (\pm 0.02) \hspace{1mm} {\rm mag.;} \end{displaymath}$

(2)

where `extrapolated' values are from Caon et al. (1990) Table I and `integrated' values are from Caon et al. (1990) Table VI or Caon et al. (1994). For objects of n < 0.25 on the other hand, it was found that (with a scatter of 0.31 mag.):

$\begin{displaymath} {\rm Extrapolated\/} \hspace{1mm} B_{T}{\rm (Caon \hspace{1... ...hspace{1mm} 3)} = +0.40 (\pm 0.12) \hspace{1mm} {\rm mag.} \end{displaymath}$

(3)

and (with a scatter of 0.17 mag.):

$\begin{displaymath} {\rm Integrated\/} \hspace{1mm} B_{T}{\rm (Caon \hspace{1mm... ...\hspace{1mm} 3)} = +0.05 (\pm 0.05) \hspace{1mm} {\rm mag.} \end{displaymath}$

(4)

In the latter case, if NGC 4406 were excluded³, we find that (with a scatter of 0.09 mag.):

$\begin{displaymath} {\rm Integrated\/} \hspace{1mm} B_{T}{\rm (Caon \hspace{1mm... ...\hspace{1mm} 3)} = +0.01 (\pm 0.03) \hspace{1mm} {\rm mag.} \end{displaymath}$

(5)

The agreement with Caon et al.'s integrated magnitudes is therefore very good and much better than with Caon et al.'s extrapolated values. This is reassuring because integrations to large radii should yield relatively unbiased (even if often relatively noisy) estimates of total magnitude⁴. The disagreements with Caon et al.'s extrapolated magnitudes and Michard's values at the bright end are therefore no cause for concern. Both of the latter sources of magnitude estimates are strongly affected by the

$r^{\frac{1}{4}}$ -law extrapolations applied-especially in the cases of the most centrally concentrated objects. As far as our profile fits and parameterisations for the most centrally concentrated objects are concerned, the almost perfect agreement between the B_t(systemic) and B_t(integrated to $r_{{\rm max}}$ & extrapolated) values listed in Table 3 ⁵, would appear to confirm the superiority of our fits with respect to ones rigidly assuming n=0.25. Furthermore, our finding that the profiles of the brightest early-type Virgo galaxies are best fitted by Sérsic profiles of n<0.25 is in full agreement with Graham et al.'s (1996) conclusion that brightest cluster galaxies typically have n<0.25⁶.

Next Section: The scope of this
Title/Abstract Page: Total Magnitudes of Virgo
Previous Section: Bright galaxy sample (14th-8th
Contents Page: Volume 18, Number 2

ASKAP

Public

Australia Telescope National Facility

Bright-end consistency checks