Journal Article

Evolutionary properties of the low-luminosity galaxy population in the NGC 5044 Group

A. Buzzoni, S. A. Cellone, P. Saracco and E. Zucca

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 420, issue 4, pages 3427-3450
Published in print March 2012 | ISSN: 0035-8711
Published online March 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.20267.x
Evolutionary properties of the low-luminosity galaxy population in the NGC 5044 Group

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In this third paper of a series we present Johnson–Gunn B, g, V, r, i, z multicolour photometry for 79 objects, including a significant fraction of the faintest galaxies around NGC 5044, assessing group membership on the basis of apparent morphology (through accurate Sérsic-profile fitting) and low-resolution (R= 500–1000) optical spectroscopy to estimate the redshift for 21 objects.

Early- and late-type systems are found to be clearly separate in Sérsic parameter space, with the well-known luminosity versus shape relation being mostly traced by different morphological types spanning different ranges in the shape parameter n. A significantly blue colour is confirmed for Magellanic irregulars (Sm/Ims), while a drift toward bluer integrated colours is also an issue for dwarf ellipticals (dEs). Both features point to moderate but pervasive star-formation activity even among nominally ‘quiescent’ stellar systems. Together, dEs and Ims provide the bulk of the galaxy luminosity function, around M(g) ≃−18.0 ± 1.5, while the S0 and dwarf spheroidal (dSph) components dominate the bright and faint-end tails of the distribution respectively. This special mix places the NGC 5044 Group just ‘midway’ between the high-density cosmic aggregation scale typical of galaxy clusters and the low-density environment of looser galaxy clumps like our Local Group. The bright mass of the 136 member galaxies with available photometry and morphological classification, as inferred from appropriate M/L model fitting, amounts to a total of 2.3 × 1012 M. This is one seventh of the total dynamical mass of the group, according to its X-ray emission. The current star-formation rate within the group turns to be about 23 M yr−1, a figure that may however be slightly increased as a result of the evident activity among dwarf ellipticals, as shown by enhanced Hβ emission in their spectra.

Lick narrow-band indices have been computed for 17 galaxies, probing all the relevant atomic and molecular features in the 4300–5800 Å wavelength range. Dwarf ellipticals are found to share a subsolar metallicity (−1.0 ≲ [Fe/H] ≲− 0.5), with a clear decoupling between iron and α elements, as already established for high-mass systems. Both dEs and dS0s are consistent with a high age, about one Hubble time, although a possible bias towards higher values of age may be induced by the gas emission affecting the Hβ strength.

Keywords: galaxies: clusters: individual: NGC 5044 Group; galaxies: dwarf; galaxies: elliptical and lenticular, cD; galaxies: fundamental parameters; galaxies: photometry

Journal Article.  15175 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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