431 research outputs found
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
. 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 UV luminosity function of nearby clusters of galaxies
We present the UV composite luminosity function for galaxies in the Virgo,
Coma and Abell 1367 clusters. The luminosity function (LF) is well fitted by a
Schechter function with M*(UV} - 5*log h(75) = -20.75 +/- 0.40 and alpha =
-1.50 +/- 0.10 and does not differ significantly from the local UV luminosity
function of the field. This result is in agreement with recent studies carried
out in the Halpha and B-bands which find no difference between the LFs of star
forming galaxies in clusters and in the field. This indicates that, whatever
mechanisms are responsible for quenching the star formation in clusters, they
influence similarly the giant and the dwarf populations, leaving the shape of
the LF unchanged and only modifying its normalization.Comment: 5 pages, 4 figures, 2 tables.Accepted for publication on A&A. Data
avaliable at http://goldmine.mib.infn.it/papers/LF_UV.htm
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