A&A 429, 819-823 (2005)
DOI: 10.1051/0004-6361:20041455

Peculiarities and populations in elliptical galaxies

II. Visual-near IR colours as population indices[*],[*]

R. Michard

Observatoire de Paris, LERMA, 77 Av. Denfert-Rochereau, 75015 Paris, France

Received 11 June 2004 / Accepted 6 August 2004

As a complement to the data collected and discussed in Paper I of this series, 2MASS near-IR images have been used, in connection with available V light aperture photometry, to derive the colours V-J, V-K, J-H and J-K within the effective aperture $A_{\rm e}$: nearly the same complete sample of 110 E-type galaxies is treated. In Paper I these were classified, based on morphological criteria, into the "peculiar'' (or Pec) and "normal'' (or Nop) subsamples.

For the Nop subsample, the derived colour indices are tightly related to the galaxy masses, as measured by the central velocity dispersion $\sigma _0$, although with rather small slopes as regards J-H and J-K. For the Pec subsample, the V-J and V-K colours behave as UBV and line-indices: part of the objects show blue residuals from the appropriate colour-$\sigma _0$ regression, which is evidence of a younger population mixed with the "normal'' one traced by the Nop regressions; the other shows no deviations from the Nop subsample. The distinction among Pec objects between the YP family (NGC 2865 type), and the NP one (NGC 3923 type), is statistically supported, and generally confirmed in specific cases.

Key words: galaxies: elliptical and lenticulars, cD - galaxies: photometry

1 Introduction

The present paper introduces a complementary dataset into our previous discussion in Paper I (Michard & Prugniel 2004) of the relations between morphological peculiarities and stellar population indices in a nearly complete sample of local E-type galaxies. Indeed the publication of the Large Galaxies Atlas from 2MASS observations (Jarrett et al. 2003) was crucial to make possible the derivation of broad-band colours for many local galaxies of large apparent diameters.

According to classical calculations of the colours of stellar populations (Worthey 1994; Bressan et al. 1994), or to more recent such contributions (Bruzual & Charlot 2003), the near-infrared indices J-H and J-K accessible from 2MASS are not very sensitive to the properties of such populations as encountered in ellipticals. This is not the case however for indices measured across the broad near-IR maximum of the energy distribution of old stellar populations, typically V-J or V-K, which provide good metallicity and/or age criteria. We have therefore derived these two colours, not neglecting of course J-H and J-K, for 110 galaxies of the sample of Paper I.

In Sect. 1 the definition of the sample and of its previously determined morphological properties are briefly summarized. Then, the derivation of the new data, i.e. the four above-quoted colour indices, is described. Section 2 contains a discussion of these data which follows closely the steps of Paper I. We first derive the characteristic correlations between the galaxy mass, as measured by the central velocity dispersion $\sigma _0$, and the various population indices. Then the residuals from these regressions are calculated for the objects of the Pec subsample. For J-H and J-K these quantities are not much dependent on morphological peculiarities, as is the case for the $\langle{\rm Fe}\rangle$ index. For V-J or V-K on the other hand, they behave like the other population sensitive colours and line-indices in Paper I.

It was shown there that the effects of morphological peculiarities on stellar populations could not be reduced to the "classical'' picture (Schweizer et al. 1990; Schweizer & Seitzer 1992). The Pec subsample must be separated in two groups, the YP (or NGC 2865) family and the NP (or NGC 3923) family. Objects in the YP one show evidence of a younger population added to the standard old star mixture of ellipticals, while NP objects have quasi-normal populations.

2 The sample and the new data

2.1 Sampled objects

In Paper I we treated a nearly complete sample of confirmed ellipticals bound in distance to a modulus of 33.52 and brighter than $B_{\rm T}= -18.8$. The sample is extracted from a catalogue by Prugniel & Simien (1996) with a few objects added: it contains 114 galaxies. In the present paper, this figure is reduced to 110:3 objects could not be measured in 2MASS frames and one was forgotten.

For all objects in the Paper I sample, such morphological peculiarities as isophote asymmetries, shells, jets or similar features, were studied and qualified by an ad hoc $\Sigma _2$ index. This is not a purely qualitative scale. It incorporates: i) actual measurements of the amplitude and radial range of asymmetries of the isophotal contours; ii) the careful counting of distinct "features'' such as "shells'' (Malin & Carter 1983), "ripples'' (Schweizer & Ford 1984), "jets'' or "fans''. See Paper I for details.

The objects with $\Sigma_2<1$ are designed as "non peculiar'' or "normal'' and form the Nop subsample. The other galaxies are assumed "peculiar'' and members of the Pec subsample; their $\Sigma _2$ reaches 10.6 for NGC 2865 and 10.3 for NGC 3923. In the present slightly modified sample, there are only 35 Pec objects instead of 37, as NGC 1344 and 3640 could not be measured in 2MASS.

2.2 Measurements

2.2.1 V-light data
Several authors, besides the well known Reference Catalogue of Bright Galaxies (RC2 and RC3), have fitted aperture photometry data to ad hoc "growth curves'' in order to derive by extrapolation an asymptotic total magnitude, hence the effective aperture $A_{\rm e}$ and the corresponding integrated magnitude. An exploratory comparison of several sources (RC3; Michard & Marchal 1994; Prugniel & Simien 1996; Prugniel & Héraudeau 1998) shows puzzling accidental and systematic differences between them. For this reason the use of the original photometric data was preferred to the use of these catalogues in order to get an accurate $V_{\rm e}$ magnitude at the selected $A_{\rm e}$. The choice of $A_{\rm e}$ is not critical for our present purpose, because the integrated colours change quite slowly with the limiting aperture. We used either the $A_{\rm e}$ values of Paper I, or those derived in Poulain (1988) and Poulain & Nieto (1994). Two techniques were used to obtain $V(A_{\rm e})$:

2.2.2 Near IR data
It is quite straightforward to get JHK photometry precisely at the selected $A_{\rm e}$, by integration of the 2MASS frames, either from the Large Galaxy Atlas or the Extended Source Catalogue. The colours V-J, V-K, J-H and J-K are readily derived.

2.2.3 Corrections
Tables of K-corrections have been recently published by Poggianti (1997) and Mannucci et al. (2001). The first source has been used because it agrees better, for optical colours, with the system used in Paper I.

The values of galactic extinction for each object have been taken from the RC3, while the relative coefficients for each colour are from Rieke & Lebofsky (1985). The galactic reddening for V-J and V-K is much larger than for the currently used optical colours. For instance the colour excess E(V-K) is 2.74 E(B-V), and important errors may be induced in the colours of objects of relatively low galactic latitude. An interesting possibility might be to use these "dust sensitive indices'' for a search of the effects of internal dust on colours and colour gradients. Witt et al. (1992) compare the effects of internal dust in the V and K bands. Michard (in preparation) tries to ascertain dust effects in V-J and V-K radial gradients.

2.2.4 Errors

3 The relations of the VJHK colours with the velocity dispersion

\includegraphics[width=8.4cm,clip]{1455fig1.eps} \end{figure} Figure 1: The colour-$\sigma _0$ relations for Nop galaxies (circles and fitted lines), and Pec objects (stars). The calculated regressions for the Nop subsample are shown. In V-J and V-K, part of the "peculiar'' objects tend to be bluer than "normal''. This effect is lessened in the high mass range, and much reduced in J-H and J-K.
Open with DEXTER

The properties of the colour indices collected here may be described as follows:

Graphs. The correlations between the galaxy mass, as represented by the central velocity dispersion $\log \sigma_0$, and the newly derived colours V-J, V-K, J-H and J-K are plotted in Fig. 1, with distinct symbols for the Nop and Pec subsamples. For V-J, V-K the diagrams are similar to B-V or U-V (see Paper I), with part of the Pec galaxies distinctly bluer than implied by the relatively tight correlations of the Nop subsample. Part of the "peculiar'', however, have normal colours, or nearly so, especially in the higher $\log \sigma_0$ range. The J-H and J-K colours appear generally not sensitive to morphological peculiarities.

A check has been made of an eventual effect of large galactic extinction errors, by plotting the diagrams of Fig. 1, for V-J and V-K, with distinct symbols for the objects with large extinction, $A_{\rm g} > 0.5$ in the RC3 notation. These however appear well mixed with the others.

Table 1: Populations of the "normal'' Es ( Nop subsample). The correlations between the central velocity dispersion and the various colour-indices are given in the form y=px+y0, where $x=\log \sigma_0$ (1) Index; (2) Number of objects (not taking into account the 1 to 3 rejected by the least squares routine); (3) Mean value $C_{\rm m}$ (4) Slope p; (5) Intercept y0; (6) Standard deviation $\sigma $; (7) Coefficient of correlation $\rho $.

Table 2: Residuals of the VJHK colours of ellipticals in the Pec subsample, against the standard relations calculated for the Nop subsample between Indices and the central velocity dispersion. (1) Name; (2) $\Sigma _2$ index of peculiarity; (3)  $\Delta V-J,~V-J$ residuals; (4)  $\Delta V-K,~V-K$ residuals; (5)  $\Delta J-H,~J-H$ residuals; (6)  $\Delta J-K,~J-K$ residuals; (7) "Mean Index of Younger Population'' or MIYP (see definition in text) in mag; (8) Character of population: YP, Young Population present, NP, probably normal population, : doubtful.

Reference correlations for the Nop sample. In Table 1 are given the correlations between the colours collected here and $\log \sigma_0$. The coefficients of correlation and the slopes for V-J, V-K are as large as for the classical luminosity indicator U-V, while the $\sigma $ values are not much larger. The J-K from 2MASS on the other hand, might be as useful a luminosity indicator as B-V for "normal'' ellipticals. The slope becomes quite small for H-K and the $\sigma $-value of 0.013 was taken above as an upper limit of probable errors in the 2MASS derived colours.
Colour residuals of Pec galaxies from the reference $\log \sigma_0$ relations. Table 2 gives the O-C residuals between the observed colours of the 35 Pec galaxies and the ones predicted from the above reference relations: these are noted $\Delta$V-J and similarly for other colours. The "Mean Index of Younger Population'' (MIYP), defined in Paper I as a weighted mean of the $\Delta$U-B, $\Delta$B-V, $\Delta$Mg2, $\Delta$Mgb, $\Delta$H$\beta$ residuals, has been added to the table for comparison. Also added is our classification of Pec objects into the YP family (type NGC 2865) with evidence of mixing of a young population, and the NP one (type NGC 3923) with no such evidence. Table 3 summarizes the statistics of the O-C residuals in Table 2.
Comparison with Paper I results. It appears from Table 2 that the $\Delta$V-J and $\Delta$V-K residuals are generally compatible with our classification of objects into the YP and NP families, i.e they show large negative values for the first one and remain near zero for the other. Notable exceptions are NGC 2768 and 4406, too blue for their NP: classification (doubtful), IC 3370 too red for the YP family and NGC 7454 found much too red. The V-J residuals are correlated with the ad hoc index MIYP: we find $\Delta$ $V-J\simeq 2\times MIYP$ with a dispersion of 0.060 about this regression, or 0.054 rejecting the outlyer NGC 7454. This may be compared with the distribution of the $\Delta$V-J in Table 3.

We have also plotted the correlation diagram between V-J and Mg2, to check for large colour errors perhaps due to inaccurate galactic extinction corrections. NGC 4976 ( $A_{\rm g}=0.96$) is too blue for its Mg2, possibly due to too large an adopted $A_{\rm g}$ value. The reverse is true for NGC 7454 ( $A_{\rm g}=0.25$), again much too red in V-J for its spectral index.

Table 3: Mean deviations of the VJHK colour indices of morphologically peculiar Es ( Pec subsample) against the standard relations calculated for the Nop ("normal'') subsample. Ind. index studied; N number of objects; mRes mean residuals; $\sigma $ standard deviation.

4 Concluding discussion

The V-J, V-K, J-H and J-K colours within $A_{\rm e}$ have been obtained for 110 ellipticals already studied in Paper I, using 2MASS data and published results of aperture photometry. The photometric accuracy of the 2MASS calibration has been verified to be quite good: a probable error of 0.013 magnitude is evaluated for the two near-IR colours. The probable error is found to be 0.025 for the two visible-IR colours. The complete results of our measurements are collected in Table 4, to be made available at the CDS, Strasbourg.

The correlations between the velocity dispersion $\log \sigma_0$ and the colours of the 75 "non peculiar'' objects, forming the Nop subsample, have been found to be very good (Fig. 1 and Table 1). Their dispersion, i.e. 0.06 for V-J and 0.07 for V-K, are distinctly larger than expected from known errors in the variables. There is a "cosmic dispersion'', with two probable physical causes. On the one hand, there are small fluctuations in the parameters (say mean age and mean metallicity) describing the stellar population of each object; on the other, there are also fluctuations in the small amount of residual dust in ellipticals, producing a variable reddening, changing from galaxy to galaxy and from colour to colour. It might be feasible to disentangle these two sources of cosmic dispersion in the colour distributions of ellipticals.

The colours of morphologically peculiar objects, i.e. the Pec subsample of 35 galaxies, have been discussed as in Paper I: the O-C residuals between the observed colours and those expected from the calculated regressions of the Nop subsample have been evaluated (Table 2). These are generally not large and of limited significance for J-H and J-K. For V-J, V-K, as for other colours and line-indices used in Paper I, there is a striking dichotomy between a family of objects with large negative residuals and another with near zero residuals: these have been termed the YP (type NGC 2865) and the NP family (type NGC 3923). The residuals in V-J and V-K correlate with MIYP, the Mean Index of Young Population introduced in Paper I to describe the various populations by reference to the standard one of the Nop sample. They also agree, but for a few conflicting cases (4 out of 35), with the classification in the YP and NP groups introduced there.

This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
The kind attention of Dr. Jarrett is gratefully acknowledged.



Copyright ESO 2005