1.65mic (H-band) surface photometry of galaxies. VI: The history of star formation in normal late-type galaxies

We have collected a large body of NIR (H band), UV (2000 A) and Halpha measurements of late-type galaxies. These are used, jointly with spectral evolutionary synthesis models, to study the initial mass function (IMF) in the mass range m > 2 Mo. For spirals (Sa-Sd), Magellanic irregulars (Im) and blue compact dwarfs (BCD), our determination is consistent with a Salpeter IMF with an upper mass cutoff M_up = 80 Mo. The history of star formation and the amount of total gas (per unit mass) of galaxies are found to depend primarily on their total masses (as traced by the H band luminosities) and only secondarily on morphological type. The present star formation activity of massive spirals is up to 100 times smaller than that average over their lifetime, while in low mass galaxies it is comparable to or higher than that at earlier epochs. Dwarf galaxies have presently larger gas reservoirs per unit mass than massive spirals. The efficiency in transforming gas into stars and the time scale for gas depletion (10 Gyrs) are independent of the luminosity and/or of the morphological type. These evidences are consistent with the idea that galaxies are coeval systems,that they evolved as closed-boxes forming stars following a simple, universal star formation law whose characteristic time scale is small (1 Gyr) in massive spirals and large (10 Gyr) in low mass galaxies. A similar conclusion was drawn by Gavazzi and Scodeggio (1996) to explain the colour-magnitude relation of late-type galaxies. The consequences of this interpretation on the evolution of the star formation rate and of the gas density per comoving volume of the Universe with look-back time are discussed.Comment: LaTex, 24 pages, 12 figures, accepted for publication on Astronomical Journa

An Ultraviolet-Selected Galaxy Redshift Survey - II: The Physical Nature of Star Formation in an Enlarged Sample

We present further spectroscopic observations for a sample of galaxies selected in the vacuum ultraviolet (UV) at 2000 \AA from the FOCA balloon-borne imaging camera of Milliard et al. (1992). This work represents an extension of the initial study of Treyer et al. (1998). Our enlarged catalogue contains 433 sources; 273 of these are galaxies, nearly all with redshifts z=0-0.4. Nebular emission line measurements are available for 216 galaxies, allowing us to address issues of reddening and metallicity. The UV and Halpha luminosity functions strengthen our earlier assertions that the local volume-averaged star formation rate is higher than indicated from earlier surveys. Moreover, internally within our sample, we do not find a steep rise in the UV luminosity density with redshift over 0<z<0.4. Our data is more consistent with a modest evolutionary trend as suggested by recent redshift survey results. We find no evidence for a significant number of AGN in our sample. We find the UV flux indicates a consistently higher mean star formation rate than that implied by the Halpha luminosity for typical constant or declining star formation histories. Following Glazebrook et al. (1999), we interpret this discrepancy in terms of a starburst model for our UV-luminous sources. Whilst we can explain most of our observations in this way, there remains a small population with extreme UV-optical colours which cannot be understood.Comment: 27 pages, 19 figures, accepted for publication in MNRA

The far-ultraviolet emission of early-type galaxies

We have assembled a UV-flux selected sample of 82 early-type galaxies and collected additional information at other wavelengths. These data confirm a large spread of the UV-V color in the range 2 to 5. The spread in UV-V is accompanied by a spread in B-V that is mainly attributed to the range of morphological types and luminosities. A large fraction of the objects have red colors, UV-V = 4 +-0.4, corresponding to a weak UV-upturn as observed with IUE. If the current interpretation for the UV emission from early-type galaxies is applicable to our sample, the PAGB (Post-Asymptotic Giant Branch) tracks are the most common evolution path for the low-mass stars responsible for the UV emission. A small number of very blue (UV-V < 1.4) objects have been found that can be reasonably interpreted as harbouring some low level of star formation. In contrast with a previous sample based on IUE observations, no correlation is found between the UV-V color and the Mg_2 spectral line index; possible explanations are reviewed. The potential of a more extended UV survey like GALEX is briefly presented.Comment: accepted for publication on Astronomy and Astrophysic

Evolution of the UV Excess in Early-Type Galaxies

We examine the UV emission from luminous early-type galaxies as a function of redshift. We perform a stacking analysis using Galaxy Evolution Explorer (GALEX) images of galaxies in the NOAO Deep Wide Field Survey (NDWFS) Bo\"otes field and examine the evolution in the UV colors of the average galaxy. Our sample, selected to have minimal ongoing star formation based on the optical to mid-IR SEDs of the galaxies, includes 1843 galaxies spanning the redshift range $0.05\leq z\leq0.65$. We find evidence that the strength of the UV excess decreases, on average, with redshift, and our measurements also show moderate disagreement with previous models of the UV excess. Our results show little evolution in the shape of the UV continuum with redshift, consistent either with the binary model for the formation of Extreme Horizontal Branch (EHB) stars or with no evolution in EHB morphology with look-back time. However, the binary formation model predicts that the strength of the UV excess should also be relatively constant, in contradiction with our measured results. Finally, we see no significant influence of a galaxy's environment on the strength of its UV excess.Comment: 30 pages, 10 figures; accepted by ApJ. Modified from original version to reflect referee's comment

Star Formation in Galaxies Along the Hubble Sequence

Observations of star formation rates (SFRs) in galaxies provide vital clues to the physical nature of the Hubble sequence, and are key probes of the evolutionary properties of galaxies. The focus of this review is on the broad patterns in the star formation properties of galaxies along the Hubble sequence, and their implications for understanding galaxy evolution and the physical processes that drive the evolution. Star formation in the disks and nuclear regions of galaxies are reviewed separately, then discussed within a common interpretive framework. The diagnostic methods used to measure SFRs are also reviewed, and a self-consistent set of SFR calibrations is presented as an aid to workers in the field.Comment: 41 pages, with 9 figures. To appear in Volume 36 of the Annual Review of Astronomy and Astrophysic

UV and FIR selected samples of galaxies in the local Universe. Dust extinction and star formation rates

We have built two samples of galaxies selected at 0.2micron (hereafter UV) and 60micron (hereafter FIR) covering a sky area of 35.36 deg^2. The UV selected sample contains 25 galaxies brighter than AB_0.2=17mag. All of them, but one elliptical, are detected at 60micron with a flux density larger or equal to 0.2Jy. The UV counts are significantly lower than the euclidean extrapolation towards brighter fluxes of previous determinations. The FIR selected sample contains 42 galaxies brighter than f_60=0.6Jy. Excepting four galaxies, all of them have a UV counterpart at the limiting magnitude AB_0.2=20.3mag. The mean extinction derived from the analysis of the FIR to UV flux ratio is ~1mag for the UV selected sample and ~2mag for the FIR selected one. For each sample we compare several indicators of the recent star formation rate (SFR) based on the FIR and/or the UV emissions and we find linear relationships with slopes close to unity, meaning that no trend with the SFR exists when converting between each other. Various absolute calibrations for both samples are discussed in this paper. A positive correlation between extinction and SFR is found when both samples are considered together although with a considerable scatter. A similar result is obtained when using the SFR normalized to the optical surface of the galaxies.Comment: 34 pages, 17 figures. Accepted for publication in A&

The Clustering of Extragalactic Extremely Red Objects

We have measured the angular and spatial clustering of 671 K5 Extremely Red Objects (EROs) from a 0.98 square degree sub-region of the NOAO Deep Wide-Field Survey (NDWFS). Our study covers nearly 5 times the area and has twice the sample size of any previous ERO clustering study. The wide field of view and BwRIK passbands of the NDWFS allow us to place improved constraints on the clustering of z=1 EROs. We find the angular clustering of EROs is slightly weaker than in previous measurements, and w(1')=0.25+/-0.05 for K<18.40 EROs. We find no significant correlation of ERO spatial clustering with redshift, apparent color or absolute magnitude, although given the uncertainties, such correlations remain plausible. We find the spatial clustering of K5 EROs is well approximated by a power-law, with r_0=9.7+/-1.1 Mpc/h in comoving coordinates. This is comparable to the clustering of 4L* early-type galaxies at z<1, and is consistent with the brightest EROs being the progenitors of the most massive ellipticals. There is evidence of the angular clustering of EROs decreasing with increasing apparent magnitude, when NDWFS measurements of ERO clustering are combined with those from the literature. Unless the redshift distribution of K>20 EROs is very broad, the spatial clustering of EROs decreases from r_0=9.7+/-1.1 Mpc/h for K20 EROs.Comment: Accepted for publication in the ApJ. 29 pages with 10 figures. The NOAO Deep Wide-Field Survey Bootes data release is available online at http://www.noao.edu/noao/noaodeep