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Register a new userThe GALEX/S4G UV-IR color-color diagram: Catching spiral galaxies away from the Blue Sequence
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We obtained GALEX FUV, NUV, and Spitzer/IRAC 3.6$mu$m photometry for > 2000 galaxies, available for 90% of the S4G sample. We find a very tight GALEX Blue Sequence (GBS) in the (FUV-NUV) versus (NUV-[3.6]) color-color diagram which is populated by irregular and spiral galaxies, and is mainly driven by changes in the formation timescale ($tau$) and a degeneracy between $tau$ and dust reddening. The tightness of the GBS provides an unprecedented way of identifying star-forming galaxies and objects that are just evolving to (or from) what we call the GALEX Green Valley (GGV). At the red end of the GBS, at (NUV-[3.6]) > 5, we find a wider GALEX Red Sequence (GRS) mostly populated by E/S0 galaxies that has a perpendicular slope to that of the GBS and of the optical red sequence. We find no such dichotomy in terms of stellar mass (measured by $rm{M}_{[3.6]}$), since both massive ($M_{star} > 10^{11} M_{odot}$) blue and red sequence galaxies are identified. The type that is proportionally more often found in the GGV are the S0-Sas and most of these are located in high-density environments. We discuss evolutionary models of galaxies that show a rapid transition from the blue to the red sequence on timescale of $10^{8}$years.
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We present GALEX data for 44 Galactic globular clusters obtained during 3 GALEX observing cycles between 2004 and 2008. This is the largest homogeneous data set on the UV photometric properties of Galactic globular clusters ever collected. The sample selection and photometric analysis are discussed, and color-magnitude diagrams are presented. The blue and intermediate-blue horizontal branch is the dominant feature of the UV color-magnitude diagrams of old Galactic globular clusters. Our sample is large enough to display the remarkable variety of horizontal branch shapes found in old stellar populations. Other stellar types that are obviously detected are blue stragglers and post core-He burning stars. The main features of UV color-magnitude diagrams of Galactic globular clusters are briefly discussed. We establish the locus of post-core He burning stars in the UV color-magnitude diagram and present a catalog of candidate AGB-manqu e, post early-AGB, and post-AGB stars within our cluster sample.
We present our new, spatially-resolved, photometry in FUV and NUV from images obtained by GALEX, and IRAC1 (3.6 $mu$m) photometry obtained by the Spitzer Space Telescope. We analyzed the surface brightness profiles $mu_{rm{FUV}}$, $mu_{rm{NUV}}$, $mu_{[3.6]}$, as well as the radial evolution of the (FUV-NUV), (FUV - [3.6]), and (NUV - [3.6]) colors in the Spitzer Survey of Stellar Structures in Galaxies (S$^{4}$G) galaxies (d$<$40 Mpc) sample. We defined the GALEX Blue Sequence (GBS) and GALEX Red Sequence (GBR) from the (FUV - NUV) versus (NUV - [3.6]) color-color diagram, populated by late-type star forming galaxies and quiescent early-type galaxies respectively. While most disk becomes radially bluer for GBS galaxies, and stay constant for GRS galaxies, a large fraction ($>$50%) of intermediary GALEX Green Valley (GGV) galaxies outer disks are becoming redder. An outside-in quenching mechanism such as environmentally-related mechanisms such as starvation or ram-pressure-stripping could explain our results.
We introduce a new quantity, the mass flux density of galaxies evolving from the blue sequence to the red sequence. We propose a simple technique for constraining this mass flux using the volume corrected number density in the extinction-corrected UV-optical color magnitude distribution, the stellar age indexes H-delta-a and D_n(4000), and a simple prescription for spectral evolution using a quenched star formation history. We exploit the excellent separation of red and blue sequences in the NUV-r band color-magnitude diagram. The final value we measure, 0.033 M_sun yr^-1 Mpc^-3, is strictly speaking an upper limit due to the possible contributions of bursting, composite, and extincted galaxies. However, it compares favorably with estimates of the average mass flux that we make based on the red luminosity function evolution derived from the DEEPII and COMBO-17 surveys (Bell et al 2004; Faber et al. 2005), 0.034 M_sun yr^-1 Mpc^-3. We find that the blue sequence mass has remained roughly constant since z=1 (mass flux 0.01 M_sun yr^-1 Mpc^-3) but the average on-going star formation of 0.037$ M_sun yr^-1 Mpc^-3 over 0<z<1 is balanced by mass flux off the blue sequence. We explore the nature of the galaxies in the transition zone with particular attention to the frequency and impact of AGNs. The AGN fraction peaks in the transition zone. We find circumstantial, albeit weak evidence that the quench rates are higher in higher luminosity AGNs.
Globular Clusters (GCs) in the Milky Way are the primary laboratories for establishing the ages of the oldest stellar populations and for measuring the color-magnitude relation of stars. In infrared (IR) color-magnitude diagrams (CMDs), the stellar main sequence (MS) exhibits a kink, due to opacity effects in M dwarfs, such that lower mass and cooler dwarfs become bluer in the IR color baseline. This diagnostic offers a new opportunity to model GC CMDs and to reduce uncertainties on cluster properties (e.g., their derived ages). In this context, we analyzed Hubble Space Telescope Wide Field Camera 3 IR archival observations of four GCs - 47Tuc, M4, NGC2808, and NGC6752 - for which the data are deep enough to fully sample the low-mass MS, reaching at least ~ 2 mag below the kink. We derived the fiducial lines for each cluster and compared them with a grid of isochrones over a large range of parameter space, allowing age, metallicity, distance, and reddening to vary within reasonable selected ranges. The derived ages for the four clusters are respectively 11.6, 11.5, 11.2, and 12.1 Gyr and their random uncertainties are sigma ~ 0.7 - 1.1 Gyr. Our results suggest that the near-IR MS kink, combined with the MS turn-off, provides a valuable tool to measure GC ages and offers a promising opportunity to push the absolute age of GCs to sub-Gyr accuracy with the next generation IR telescopes such as the James Webb Space Telescope and the Wide-Field Infrared Survey Telescope.
We present GALEX UV observations of a sample of Low Surface Brightness (LSB) galaxies for which HI data are available, allowing us to estimate their star formation efficiency. We find that the UV light extends to larger radii than the optical light (some galaxies, but not all, look similar to the recently discovered XUV-disk galaxies). Using a standard calibration to convert the UV light into star formation rates, we obtain lower star formation efficiencies in LSB galaxies than in high surface brightness galaxies by about one order of magnitude. We show however that standard calibrations may not apply to these galaxies, as the FUV-NUV color obtained from the two GALEX bands (FUV and NUV; lambda_eff= 1516 and 2267 A, respectively) is redder than expected for star forming galaxies. This color can be interpreted as a result of internal extinction, modified Initial Mass Function or by star formation histories characterized by bursts followed by quiescent phases. Our analysis favors this latter hypothesis.
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