177 research outputs found
Dependence of Dust Obscuration on Star Formation Rates in Galaxies
Many investigations of star formation rates (SFRs) in galaxies have explored
details of dust obscuration, with a number of recent analyses suggesting that
obscuration appears to increase in systems with high rates of star formation.
To date these analyses have been primarily based on nearby (z < 0.03) or UV
selected samples. Using 1.4 GHz imaging and optical spectroscopic data from the
Phoenix Deep Survey, the SFR-dependent obscuration is explored. The use of a
radio selected sample shows that previous studies exploring SFR-dependent
obscurations have been biased against obscured galaxies. The observed relation
between obscuration and SFR is found to be unsuitable to be used as an
obscuration measure for individual galaxies. Nevertheless, it is shown to be
successful as a first order correction for large samples of galaxies where no
other measure of obscuration is available, out to intermediate redshifts (z ~
0.8).Comment: 9 pages (including 5 encapsulated postscript figures), aastex, uses
emulateapj5.sty. Accepted for publication in Ap
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 ultraviolet extragalactic background light: dust extinction and the evolution of the cosmic star formation rate from z=0 to 0.6
We show that the accumulated light of galaxies in the ultraviolet can be
evaluated from their luminosity density as a function of the evolution of the
cosmic star formation rate and dust extinction properties. Constraints on the
evolution rate are expected in future. Data available at the moment are
consistent with an evolution rate at low z steeper than (1+z)^(3.5). A
shallower rate remains possible if the luminosity-weighted dust extinction at
2000 A, as suggested by some data, is lower than about 1.2.Comment: 4 pages, 1 figure, accepted for publication in MNRA
Constraints on the Lyman continuum radiation from galaxies: first results with FUSE on Mrk 54
We present Far Ultraviolet Spectroscopic Explorer observations of the
star-forming galaxy Mrk 54 at z = 0.0448. The Lyman continuum radiation is not
detected above the HI absorption edge in our Galaxy. An upper limit is
evaluated by comparison with the background measured in regions of the detector
adjacent to the observed spectrum. A spectral window of 16 A, reasonably free
of additional HI Lyman series line absorption is used. No correction is needed
for molecular hydrogen absorption in our Galaxy but a foreground extinction of
0.29 mag is accounted for. An upper limit of 6.15 10^{-16} erg/cm^2/s/A is
obtained for the flux at ~ 900 A in the rest frame of Mrk 54. By comparison
with the number of ionizing photons derived from the H-alpha flux, this limit
translates into an upper limit of f_esc < 0.062 for the fraction of Lyman
continuum photons that escape the galaxy without being absorbed by interstellar
material. This limit compares with the limits obtained in three other nearby
galaxies and is compatible with the escape fractions predicted by models.
The upper limits obtained in nearby galaxies contrasts with the detection of
Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z ~
3.4. The difficulties and implications of a comparison are discussed.Comment: 9 pages, 3 figures, accepted for publication in A&A include aa.cls
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Two-stage opening of the northwestern basin and range in eastern Oregon : evidence from the Miocene Crane basin
New data are presented that provide evidence for the onset of extensional deformation in the Northwestern Basin and Range within 1 million years after the eruption of the Steens Basalt at 16.5 Ma. New geologic mapping (1:24,000), stratigraphic sections, and ⁴⁰Ar/³⁹Ar dating of the Crane Basin rocks provide a control of the structural and depositional history of the basin. Extension-related horst and graben formation began at 15.5 Ma and continued until ~7 Ma. The Crane Basin, is one of a series of these half-grabens located to the east of the Harney Basin and Burns, Oregon on the footwall block of the Steens Mountain normal fault. Basin fill consists of volcaniclastic sedimentary rocks, tuffs, and lava flows. Key intra-basin marker beds include the 9.63 Ma Devine Canyon tuff and 11.5 Ma Visher Creek lava, in addition to a newly-discovered air fall ash with a ~12.5 Ma age, the 7.25 Ma Drinkwater basalt, and the 1.93 Ma Voltage basalt age. Crane Basin strata unconformably onlap tilted Steens Basalt. Angular unconformities exist in the Crane Basin strata, between the basal sedimentary rocks, overlying Devine Canyon tuff and capping Drinkwater basalt. Fault histories in the Crane Basin record similar rates of displacement from ~15.5 Ma to 9.63 Ma, divergent and heightened rates from 9.63 to 7.25 Ma and extremely slow rates after 7.25Ma. Together, these observations indicate that broad extensional deformation initiated following the eruption of the 16.5 Ma Steens Basalt and continuing until ~9.63 Ma (STAGE 1). After ~9.63 Ma deformation became heterogeneous and localized, migrating east to west, eventually shifting from the Crane Basin to the Harney Basin. The field relationships signal that the second deformational event was short-lived, ceased by ~7Ma (STAGE 2). Post 7.25 Ma the displacement on Crane Basin faults and the Harney Basin depositional record demonstrates that only minor fault deformation occurred. The findings of this study provide a previously undocumented resolution of the development stages of the margin of a continental extensional province. Moreover, these new data demonstrate a "temporal" link between magmatic and tectonic activity in Southeastern Oregon, suggesting that both deformation mechanisms jointly accommodated extensional strain at the margin of the developing Basin and Range Province
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