We have studied ∼ 2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range 0 < z < 0.11. Combining GALEX U V photometry with corollary optical data from the SDSS, we find that, at a 95 percent confidence level, at least ∼ 30 percent of galaxies in this sample have U V to optical colours consistent with some recent star formation within the last Gyr. In particular, galaxies with a N U V − r colour less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to N U V flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semi-analytical ΛCDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have ∼ 1 to 3 percent of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ∼ 300 to 500 Myrs. We also find that 'monolithically' evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colours (N U V − r < 5.5) seen in a significant fraction (∼ 30 percent) of our observed sample.

The discovery of the Sagittarius dwarf galaxy 1 , which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars 2,3 , and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is ω Centauri, and here we report that ω Centauri also has several distinct stellar populations, as traced by red-giant-branch stars. The most metal-rich red-giant-branch stars are about 2 Gyr younger than the dominant metal-poor component, indicating that ω Centauri was enriched over this timescale. The presence of more than one epoch of star formation in a globular cluster is quite surprising, and suggests that ω Centauri was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were much more frequent in the early history of the Galaxy and ω Centauri appears to be a relict of this era.As part of our investigation of the luminosity-metallicity relation of the RR Lyrae stars in the globular cluster ω Cen, we have obtained 2K BV CCD frames with the CTIO 0.9-m b 0.0 0.4 0.8

We use GALEX (Galaxy Evolution Explorer) near-UV (NUV) photometry of a sample of earlytype galaxies selected in SDSS (Sloan Digital Sky Survey) to study the UV color-magnitude relation (CMR). N U V − r color is an excellent tracer of even small amounts (∼ 1% mass fraction) of recent ( 1 Gyr) star formation and so the N U V − r CMR allows us to study the effect of environment on the recent star formation history. We analyze a volume-limited sample of 839 visually-inspected early-type galaxies in the redshift range 0.05 < z < 0.10 brighter than M r of −21.5 with any possible emission-line or radio-selected AGN removed to avoid contamination. We find that contamination by AGN candidates and late-type interlopers highly bias any study of recent star formation in early-type galaxies and that, after removing those, our lower limit to the fraction of massive early-type galaxies showing signs of recent star formation is roughly 30 ± 3% This suggests that residual star formation is common even amongst the present day early-type galaxy population.We find that the fraction of UV-bright early-type galaxies is 25% higher in low-density environments. However, the density effect is clear only in the lowest density bin. The blue galaxy fraction for the subsample of the brightest early-type galaxies however shows a very strong density dependence, in the sense that the blue galaxy fraction is lower in a higher density region.

We present the first measurement of the galaxy luminosity function (LF) at 1500 in the range A 0 . 2 ≤ z ≤ based on Galaxy Evolution Explorer VIMOS-VLT Deep Survey observations (∼1000 spectroscopic redshifts 1.2 for galaxies with ) and at higher z using existing data sets. Our main results are summarized as NUV ≤ 24.5 follows: (1) Luminosity evolution is observed with mag between and andmag between and . This confirms that the star formation activity was significantly higher in Ϫ1.0 z p 1 z p 3 the past. (2) The LF slopes vary in the range , with a marginally significant hint of increase Ϫ1.2 ≥ a ≥ Ϫ1.65 at higher z. (3) We split the sample in three rest-frame intervals, providing an approximate spectral type (B Ϫ I) classification: Sb-Sd, Sd-Irr, and unobscured starbursts. We find that the bluest class evolves less strongly in luminosity than the two other classes. On the other hand, their number density increases sharply with z (∼15% in the local universe to ∼55% at ), while that of the reddest classes decreases. z ∼ 1

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