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.

Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster ω Centauri have shown that it has a striking double main sequence (MS), with a minority population of bluer and fainter MS well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y 2 isochrones, that this special feature can only be reproduced by assuming a large variation (∆Y = 0.15) of primordial helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal-branch (HB) stars observed in ω Cen, which are hotter than normal HB stars. Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, are explained by large internal variations of helium abundance. Supporting evidence for the helium-rich population is also provided by the far-UV (FUV) observations of extreme HB stars in these clusters, where the enhancement of helium can naturally explain the observed fainter FUV luminosity for these stars. The presence of super helium-rich populations in some globular clusters suggests that the third parameter, other than metallicity and age, also influences CMD morphology of these clusters.

The colors of globular clusters in most of large elliptical galaxies are bimodal. This is generally taken as evidence for the presence of two cluster subpopulations that have different geneses. Here we find however that, due to the non-linear nature of the metallicity-to-color transformation, a coeval group of old clusters with a unimodal metallicity spread can exhibit color bimodality. The models of cluster colors indicate that the horizontal-branch stars are the main drivers behind the empirical non-linearity. We show that the scenario gives remarkably simple and cohesive explanations for all the key observations, and could simplify theories of elliptical galaxy formation.One of the most outstanding discoveries from observations of elliptical galaxies over the last decade is the bimodal color distribution of globular clusters − gravitationally bound collections of millions of stars (1-8). The phenomenon is widely interpreted as evidence of two cluster sub-systems with distinct geneses within individual galaxies (9). However, given many ways of forming clusters in elliptical galaxies, it is quite surprising that the cluster color distributions behave in an orderly way.For instance, the numbers of blue and red clusters in large galaxies are roughly comparable (1-8);blue and red clusters are old (> 10 billion years) and coeval (9), and differ systematically in spatial distribution and kinematics (5,10-16); and their relative fractions and peak colors strongly correlate with host galaxy properties (2-8). Here, we propose a simpler solution that does not necessarily invoke distinct cluster sub-systems and has a sound basis both on the empirical and theoretical relations between metallicity and colors.A recent observation (8) reveals that the g− z color (17) of clusters correlates with their [Fe/H] (18) (Fig 1A). The observed relation is tight enough to show a significant departure from linearity with a slope rapidly changing at [Fe/H] ≈ −1.0. A closer inspection suggests that they might follow an inverted S-shape "wavy" curve with a quasi-inflection point at [Fe/H] ≈ −0.8. To examine this 1