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Cellone, S.A., and Buzzoni, A.:
"The low-luminosity galaxy population in the NGC 5044 group",

2007, a contributed paper to the intl. conference "Groups of Galaxies in the Nearby Universe" (Santiago de Chile, 5-9/Dec/2005), ESO Astrophys. Symp., eds. I. Saviane, V. Ivanov and J. Borissova, (Springer Verlag: Heidelberg), p. 91

To track back this project see also link 1,
link 2, and link 3 locally, as well as

Sergio Cellone's Web pages @UNLP, Argentina

  Pick up the paper at Astro-ph/0603697

  Local link to the PDF version (280Kb)

Detailed surface photometry for 79 (mostly dwarf) galaxies in the NGC 5044 Group area is analysed, revealing the existence of different morphologies among objects originally classified as early-type dwarfs. Particularly, a significant fraction of bright dwarf "ellipticals" show a distinct bulge+disc structure; we thus re-classify these objects as dwarf lenticulars (dS0). Our finding points at a possible scenario where these systems are the remnants of "harassed" disc galaxies. This is emphasized by the discovery of a few objects with hints for very low-surface brightness spiral-like structure. The colours, structure, and spatial distribution of the different galaxy types suggest that our classification may indeed be separating objects with different origins and/or evolutionary paths.

    Introduction and observational material

Due to their low luminosities and sizes, a detailed classification of dwarf (MB> ≥ −18) galaxies is not easy, thus leading to a deceptively simple picture: those objects showing conspicuous signatures of present/recent star formation and interstellar material are called dwarf irregulars (dI), while the remaining smooth-looking, gas-poor objects fall within the dwarf elliptical (dE) designation(*). However, there is growing evidence for a morphological diversity among dwarfs; in particular, embedded disc structure and/or rotation were discovered within a fraction of dEs (Jerjen et al. 2000; Pedraz et al. 2002; Simien & Prugniel 2002; De Rijcke et al. 2003; Graham et al. 2003}, which seems to favor the idea that a fraction of the dEs may be remnants of "harassed" disc galaxies (Moore et al. 1998). Whether these objects are related to the still poorly known class of dwarf lenticulars (dS0) or not, is still matter of debate (Aguerri et al. 2005; Lisker et al. 2005). Aiming at a comprehensive study of the low-luminosity galaxy population in the NGC 5044 Group (m-M = 31.9 mag) (Ferguson & Sandage 1990) we have gathered multicolour surface photometry for a representative sample, comprising 40 galaxies with Gunn system griz imaging data, observed with the ESO 3.6m telescope (1999-2000), and 57 galaxies observed at CASLEO, Argentina (1996-1999) in V and B or RC with a 2.1m telescope. There are 18 objects in common between both subsamples, hence we have a total of 79 different galaxies on the NGC 5044 Group area, observed at least in one photometric band. Of these, 74 galaxies have at least one colour information. A subsample of 13 galaxies was also observed spectroscopically at ESO. First results, involving nearly 50% of these data, have been already presented (Cellone 1999; Cellone & Buzzoni 2001; Cellone & Buzzoni 2005).

(*) Blue compact dwarfs (BCD), at least in groups and clusters, are rare objects, and will not be treated here.

Figure 1 -- Left panel: Surface brightness profile for N79 (dS0) showing Sèrsic fits to the whole profile and to bulge and disc components (dashed lines); the solid line is the B+D fit. The lower panel shows residuals from a Sèrsic fit to the whole profile (squares) and from the B+D fit (triangles).
Right panel: Contour plot for N79.

(click on the figures to enlarge)


Background objects were identified by means of morphological criteria when a redshift was not available (see Cellone & Buzzoni 2005). Group members were classified mostly relying on the behaviour of their surface brightness profiles (SBP), with the aid of colour information and morphological appearance. We were able to assign any individual Group member into one of the following classes:

  • [dE:] A Sèrsic law (Sèrsic 1968) gives very good fits to their SBPs. Generally, these galaxies show no isophote twisting.
  • [dE/dS0:] These objects are well fit by bulge+disc (B+D) models. They usually show isophote twisting, ellipticity gradients, and/or colour gradients.
  • [dI/dE:] Very low surface brightness (LSB) objects, with very extended and nearly exponential SBPs (Sèrsic index n ~ 1).
  • [Im:] "Magellanic" irregulars.
  • [dSph:] Objects (mostly new ones) with Mg ≥ −12 and central surface brightnesses μ0 ≥ 24 mag/arcsec2.

    As an example, Fig. 1 shows the SBP and contour plot for an object (N79) we re-classify as dS0. Trying a single Sèrsic fit to the whole useful profile leaves both positive and negative systematic residuals ("wave pattern", see Balcells et al. 2003). The contour plot (right) clearly shows this galaxy's isophote twisting. The dI/dE class, in turn, includes a few objects with LSB outer spiral arms (Cellone & Buzzoni 2005).

  • (click on the figures to enlarge)
    Figure 2 --Left panel: Galaxy colour distribution for the whole sample (top) and for each morphological type. The integrated (g−r) colour refers to g and r galaxy luminosity collected within a μ (g) = 27 mag/arcsec2 isophotal aperture. The 15 Gyr template galaxy models from Buzzoni (2002, 2005) are compared for reference (top triangle markers, as labeled in the upper panel).
    Right panel: Same as left for the "compactness" parameter log(α/reff), defined as the ratio between Sèrsic pseudo scale-length and galaxy effective radius; compactness increases from right to left. Note the outstanding case of galaxy N139, likely a background cD at z~0.4.

        Photometric properties

    The galaxy colours in our sample show the usual trend with morphological type, with later types having bluer mean colours (Fig. 2, left). The blueing from dE's to dSph's, in turn, is most probably due to a luminosity--metallicity relation. Dwarf spheroidals display a flat and mildly broad distribution, due in part to photometric errors (worse for these faint objects), but probably also reflecting an intrinsic scatter in their origins and star formation histories, as is known for their Local Group counterparts (see e.g. Grebel et al. 2003). Also the latest types have a broad color distribution, although in this case the cause may be internal reddening. Structural differences between morphological types may be tested by means of a compactness parameter defined as α/reff, where α is the pseudo scale-length in Sèrsic's formula, and reff is the effective radius. Fig. 2 (right) shows the distributions of log(α/reff) for each morphological type. The dSph galaxy class is characterized by a shallow SBP, clearly distinct from the dE one (but see Cellone & Buzzoni 2005 for possible selection effects). Among the latter, N139 (the most compact object labelled on Fig. 2) stands out for its extremely "spiky" SBP; our photometric redshift estimate locates this (likely cD) galaxy in the far background at z ~ 0.4.

    (click on the figures to enlarge)
    Figure 3 --Left panel: Projected number-density map, considering only definite and likely member galaxies from Ferguson & Sandage (1990). Superposed is the galaxy distribution from our sample: E+dE (circles); S0+dE/dS0 (squares); S--Im+dI/dE (diamonds); dSph (triangles). The big central dot is NGC 5044.
    Right panels: Galaxy distribution for different morphological types against distance to the nearest bright (BT < 15 mag) galaxy. Note the striking spatial segregation of dwarf spheroidals, preferentially located around bright group members.

        Projected spatial distribution

    There is evidence for a morphology -- density relation within the NGC 5044 Group, as shown in Fig. 3. While the S0+dE/dS0 galaxies are intermediate between E+dE (more concentrated) and late-type objects (less concentrated), dSph's seem to prefer the highest density regions in the Group. Differences between galaxy types become marginally more significant when the distance to the nearest bright (BT < 15 mag) neighbour is considered instead of local number density (see right panels in Fig. 3). In fact, dSph's are only found in the (projected) vicinity of brighter member galaxies.

    SC would like to thank the conference organizing committee. This project received partial financial support from CONICET (Argentina) and the Italian INAF, under grant PRIN/05.


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