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The low-luminosity galaxy population in the NGC 5044 Group (Conference proceeding)

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 نشر من قبل Sergio A. Cellone
 تاريخ النشر 2006
  مجال البحث فيزياء
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 تأليف Sergio A. Cellone




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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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Low-luminosity galaxies are known to outnumber the bright galaxy population in poor groups and clusters of galaxies. Yet, the investigation of low-luminosity galaxy populations outside the Local Group remains rare and the dependence on different grou p environments is still poorly understood. Previous investigations revealed photometric scaling relations for early-type dwarfs and a strong dependence of morphology with environment. The present study aims to analyse the photometric and spectroscopic properties of the low-luminosity galaxy population in the nearby, well-evolved and early-type dominated NGC 5846 group of galaxies. It is the third most massive aggregate of early-type galaxies after the Virgo and Fornax clusters in the local universe. Photometric scaling relations and the distribution of morphological types as well as the characteristics of emission-line galaxies are investigated. Spectroscopically selected low-luminosity group members from the Sloan Digital Sky Survey with cz<3000 km/s within a radius of 2 deg=0.91 Mpc around NGC 5846 are analysed. Surface brightness profiles of early-type galaxies are fit by a Sersic model r^(1/n). Star formation rates, oxygen abundances and emission characteristics are determined for emission-line galaxies. [abridged]
Context. Ultra-compact dwarfs (UCDs) are stellar systems displaying colours and metallicities between those of globular clusters (GCs) and early-type dwarf galaxies, as well as sizes of Reff <= 100 pc and luminosities in the range -13.5 < MV < -11 ma g. Although their origin is still subject of debate, the most popular scenarios suggest that they are massive star clusters or the nuclei of tidally stripped dwarf galaxies. Aims. NGC 5044 is the central massive elliptical galaxy of the NGC 5044 group. Its GC/UCD system is completely unexplored. Methods. In Gemini+GMOS deep images of several fields around NGC 5044 and in spectroscopic multi-object data of one of these fields, we detected an unresolved source with g~20.6 mag, compatible with being an UCD. Its radial velocity was obtained with FXCOR and the penalized pixel-fitting (pPXF) code. To study its stellar population content, we measured the Lick/IDS indices and compared them with predictions of single stellar population models, and we used the full spectral fitting technique. Results. The spectroscopic analysis of the UCD revealed a radial velocity that agrees with the velocity of the elliptical galaxy NGC 5044. From the Lick/IDS indices, we have obtained a luminosity-weighted age and metallicity of 11.7+/-1.4 Gyr and [Z/H] = -0.79 +/- 0.04 dex, respectively, as well as [alpha/Fe] = 0.30 +/- 0.06. From the full spectral fitting technique, we measured a lower age (8.52 Gyr) and a similar total metallicity ([Z/H] = -0.86 dex). Conclusions. Our results indicate that NGC 5044-UCD1 is most likely an extreme GC (MV ~ -12.5 mag) belonging to the GC system of the elliptical galaxy NGC 5044.
119 - A. Buzzoni 2011
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