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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 |
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 Pick up the paper at Astro-ph/0603697 Local link to the PDF version (280Kb) |
| Summary: | ||||
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.
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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. | ||||
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Classification
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:
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). | ||||
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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. | ||||
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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.
Acknowledgements
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Bibliography
1. J.A.L. Aguerri, J.Iglesias-Pàramo, J.M. Vìlchez, C.Muñoz-Tuñòn, R.Sànchez-Janssen: AJ 130, | ||||
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