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

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 high-precision V , B − V color-magnitude diagrams (CMDs) for the classic second parameter globular clusters M3 and M13 from wide-field deep CCD photometry. The data for the two clusters were obtained during the same photometric nights with the same instrument, allowing us to determine accurate relative ages. Based on a differential comparison of the CMDs using the ∆(B − V ) method, an age difference of 1.7 ± 0.7 Gyr is obtained between these two clusters. We compare this result with our updated horizontal-branch (HB) population models, which confirm that the observed age difference can produce the difference in HB morphology between the clusters. This provides further evidence that age is the dominant second parameter that influences HB morphology. Subject headings: color-magnitude diagrams -globular clusters: individual (M3, M13) -stars: evolution -stars: horizontal-branch 1 Data were obtained using the 2.4 m Hiltner Telescope of the Michigan-Dartmouth-M.I.T. (MDM) Observatory. 2 Visiting Astronomer, MDM Observatory.

To investigate AGN outflows as a tracer of AGN feedback on star-formation, we perform integral-field spectroscopy of 20 type 2 AGNs at z<0.1, which are luminous AGNs with the [O III] luminosity >10 41.5 erg s −1 , and exhibit strong outflow signatures in the [O III] kinematics. By decomposing the emission-line profile, we obtain the maps of the narrow and broad components of [O III] and Hα lines, respectively. The broad components in both [O III] and Hα represent the non-gravitational kinematics, i.e., gas outflows, while the narrow components, especially in Hα, represent the gravitational kinematics, i.e., rotational disk. By using the integrated spectra within the flux-weighted size of the narrow-line region, we estimate the energetics of the gas outflows. The ionized gas mass is 1.0-38.5×105 M , and the mean mass outflow rate is 4.6±4.3 M yr −1 , which is a factor of ∼260 higher than the mean mass accretion rate 0.02±0.01 M yr −1 . The mean energy injection rate of the sample is 0.8±0.6% of the AGN bolometric luminosity, while the momentum flux is (5.4±3.6)×L bol /c on average, except for two most kinematically energetic AGNs with low L bol , which are possibly due to the dynamical timescale of the outflows. The estimated outflow energetics are consistent with the theoretical expectations for energy-conserving outflows from AGNs, yet we find no supporting evidence of instantaneous quenching of star formation due to the outflows.

Deep images of 10 early‐type galaxies in low‐density environments have been obtained with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope. The global properties of the globular cluster (GC) systems of the galaxies have been derived in order to investigate the role of the environment in galaxy formation and evolution. Using the ACS Virgo Cluster Survey as a high‐density counterpart, the similarities and differences between the GC properties in high‐ and low‐density environments are presented. We find a strong correlation of the GC mean colours and the degree of colour bimodality with the host galaxy luminosity in low‐density environments, in good agreement with high‐density environments. In contrast, the GC mean colours at a given host luminosity are somewhat bluer [Δ(g−z) ∼ 0.05] than those for cluster galaxies, indicating more metal poor (Δ[Fe/H] ∼ 0.10 − 0.15) and/or younger (Δage > 2 Gyr) GC systems than those in dense environments. Furthermore, with decreasing host luminosity, the colour bimodality disappears faster, when compared to galaxies in cluster environments. Our results suggest that: (1) in both high‐ and low‐density environments, the mass of the host galaxy has the dominant effect on GC system properties; (2) the local environment has only a secondary effect on the history of GC system formation; and (3) GC formation must be governed by common physical processes across a range of environments.

There is a growing body of evidence for the presence of multiple stellar populations in some globular clusters, including NGC 1851. For most of these peculiar globular clusters, however, the evidence for the multiple red giant-branches (RGBs) having different heavy elemental abundances as observed in ω Centauri is hitherto lacking, although spreads in some lighter elements are reported. It is therefore not clear whether they also share the suggested dwarf galaxy origin of ω Cen or not. Here we show from the CTIO 4m U V I photometry of the globular cluster NGC 1851 that its RGB is clearly split into two in the U − I color. The two distinct RGB populations are also clearly separated in the abundance of heavy elements as traced by Calcium, suggesting that the type II supernovae enrichment is also responsible, in addition to the pollutions of lighter elements by intermediate mass asymptotic giant branch stars or fast-rotating massive stars. The RGB split, however, is not shown in the V − I color, as indicated by previous observations. Our stellar population models show that this and the presence of bimodal horizontal-branch distribution in NGC 1851 can be naturally reproduced if the metal-rich second generation stars are also enhanced in helium.