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 ir
regular 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.
We extend our initial study of the connection between the UV colour of galaxies and both the inferred stellar mass-to-light ratio, $Upsilon_*$, and a mass-to-light ratio referenced to Salpeter initial mass function (IMF) models of the same age and me
tallicity, $Upsilon_*/Upsilon_{Sal}$, using new UV magnitude measurements for a much larger sample of early-type galaxies, ETGs, with dynamically determined mass-to-light ratios. We confirm the principal empirical finding of our first study, a strong correlation between the GALEX FUV-NUV colour and $Upsilon_*$. We show that this finding is not the result of spectral distortions limited to a single passband (eg. metallicity-dependent line-blanketing in the NUV band), or of the analysis methodology used to measure $Upsilon_*$, or of the inclusion or exclusion of the correction for stellar population effects as accounted for using $Upsilon_*/Upsilon_{Sal}$. The sense of the correlation is that galaxies with larger $Upsilon_*$, or larger $Upsilon_*/Upsilon_{Sal}$, are bluer in the UV. We conjecture that differences in the low mass end of the stellar initial mass function, IMF, are related to the nature of the extreme horizontal branch stars generally responsible for the UV flux in ETGs. If so, then UV color can be used to identify ETGs with particular IMF properties and to estimate $Upsilon_*$. We also demonstrate that UV colour can be used to decrease the scatter about the Fundamental Plane and Manifold, and to select peculiar galaxies for follow-up with which to further explore the cause of variations in $Upsilon_*$ and UV colour.
The Calar Alto Legacy Integral Field Area survey (CALIFA survey) is providing Integral Field Spectroscopy (IFS) data in the entire optical window for a diameter-limited sample of 600 objects in the Local Universe (0.005$<$z$<$0.03). One of the main g
oals of this survey is to explore the spatial distribution of the star formation in nearby galaxies free from the limitations associated to either UV (dust attenuation) or narrow-band H$alpha$ imaging (underlying H$beta$ absorption). These are limitations that have prevented (until now) carrying out a detailed study of the evolution of the SFR by components (nuclei, bulges, disks), even locally. This kind of studies are key, for example, for understanding how galaxies really evolve from the Blue Cloud to the Red Sequence. We will first discuss in detail the validity of the assumption that the SFR given by the extincion-corrected H$alpha$ is a good measure of the total SFR by means of cross-comparing this with other SFR estimators, namely the integrated UV+22$mu$m, UV+TIR, H$alpha_{rm{obs}}$+22$mu$m, or H$alpha_{rm{obs}}$+TIR. Only once these effects are properly accounted for we can obtain preliminary results from the spatially-resolved analysis of the contribution of disks to the total SFR in the Local Universe, as a local benchmark for future studies of disks at high redshift. Our analysis shows that at least in the Local Universe the H$alpha$ luminosity derived from observations of the CALIFA IFS survey can be used to trace the SFR and that the disk to total (disk + bulge) SFR ratio is on average $sim$88 $%$.
We present the analysis of the full bi-dimensional optical spectral cube of the nearby spiral galaxy NGC 5668, observed with the PPAK IFU at the Calar Alto observatory 3.5m telescope. We make use of broad-band imaging to provide further constraints o
n the evolutionary history of the galaxy. This dataset will allow us to improve our understanding of the mechanisms that drive the evolution of disks. We investigated the properties of 62 H II regions and concentric rings in NGC 5668 and derived maps in ionized-gas attenuation and chemical (oxygen) abundances. We find that, while inwards of r,$sim,36,sim$,4.4kpc,$sim$,0.36,$(frac {D_{25}}{2})$ the derived O/H ratio follows the radial gradient typical of spiral galaxies, the abundance gradient beyond r$sim36$ flattens out. The analysis of the multi-wavelength surface brightness profiles of NGC 5668 is performed by fitting these profiles with those predicted by chemo-spectrophotometric evolutionary models of galaxy disks. From this, we infer a spin and circular velocity of $lambda$=0.053 and v$_{c}$=167,km,s$^{-1}$, respectively. The metallicity gradient and rotation curve predicted by this best-fitting galaxy model nicely match the values derived from the IFU observations, especially within r,$sim36arcsec$. The same is true for the colors despite of some small offsets and a reddening in the bluest colors beyond that radius. On the other hand, deviations of some of these properties in the outer disk indicate that a secondary mechanism, possibly gas transfer induced by the presence of a young bar, must have played a role in shaping the recent chemical and star formation histories of NGC 5668.
We present recent results from several on-going studies: The first addresses the question of gas-density thresholds for star formation, as probed by the outer disks of normal nearby galaxies. The second concerns the observational evidence for the exi
stence of gravitating non-luminous (GNL) galaxies, as predicted by most recent simulations of galaxy formation in Lambda-CDM cosmologies. We find that (1) If star formation is traced by far-ultraviolet light, then there is no evidence for a threshold to star formation at any gas density so far probed, and (2) there is no evidence for GNL galaxies gravitationally interacting with known optical systems based on the observations (a) that there are no ring galaxies without plausible optically visible intruders, (b) all peculiar galaxies in the Arp Atlas that are bodily distorted have nearby plausibly interacting companions, and (c) there are no convincingly distorted/peculiar galaxies within Karachentsevs sample of more than 1,000 apparently/optically isolated galaxies.
