61 research outputs found
Pre-encounter observations of 951 Gaspra
Photometry and colorimetry of 951 Gaspra were obtained on nine nights during the 1990 opposition. A composite lightcurve constructed using data from eight of those nights yielded a synodic rotational period of 7.04346 +/- 0.00006 hours, a mean absolute V magnitude of 11.8026 +/- 0.0025, and a slope parameter of 0.285 +/- 0.005. The apparent discrepancy can be easily resolved by realizing that their determination is based primarily on data obtained after opposition. Different phase functions pre- and post-opposition are a natural consequence of a changing aspect during an opposition. If the sub-Earth latitude on Gaspra is at a less equatorial aspect after opposition than it was before opposition, then we would expect to see a shallower phase function (corresponding to a larger numerical value of the slope parameter). Adding weight to this hypothesis is the last observation of the opposition, made in May after Gaspra had passed post opposition quadrature, which is displaced toward brighter absolute magnitudes relative to the rest of our data, indicating an even more poleward sub-Earth latitude than earlier in the opposition. Because the orbits of Earth and Gaspra are nearly coplanar, a substantial change in sub-Earth latitude during the opposition would not have been possible unless the obliquity of the asteroid's rotational axis is not small
Multiwavelength study of the starburst galaxy NGC7714. I: Ultraviolet-Optical spectroscopy
We have studied the physical conditions in the central 300 pc of the
proto-typical starburst galaxy NGC 7714. Our analysis is based on ultraviolet
spectroscopy with the HST+GHRS and ground-based optical observations.The data
are interpreted using evolutionary models optimized for young starburst
regions. The massive stellar population is derived in a self-consistent way
using the continuum and stellar absorption lines in the ultraviolet and the
nebular emission line optical spectrum.
The central starburst has an age of about 4.5 Myr, with little evidence for
an age spread. Wolf-Rayet features at the ultraviolet indicates a stellar
population of 2000 Wolf-Rayet stars. The overall properties of the newly
formed stars are quite similar to those derived, e.g., in 30 Doradus. A
standard Salpeter IMF is consistent with all observational constraints. We find
evidence for spatial structure within the central 300 pc sampled. Therefore it
is unlikely that the nucleus of NGC 7714 hosts a single star cluster exceeding
the properties of other known clusters. Contrary to previous suggestions, we
find no evidence for a nuclear supernova rate that would significantly exceed
the total disk-integrated rate. About one supernova event per century is
predicted.Comment: 19 pages, 9 figures in a tar file. Accepted for publication in ApJ,
1999, March, issue 51
Star-Forming or Starbursting? The Ultraviolet Conundrum
Compared to starburst galaxies, normal star forming galaxies have been shown
to display a much larger dispersion of the dust attenuation at fixed reddening
through studies of the IRX-beta diagram (the IR/UV ratio "IRX" versus the UV
color "beta"). To investigate the causes of this larger dispersion and attempt
to isolate second parameters, we have used GALEX UV, ground-based optical, and
Spitzer infrared imaging of 8 nearby galaxies, and examined the properties of
individual UV and 24 micron selected star forming regions. We concentrated on
star-forming regions, in order to isolate simpler star formation histories than
those that characterize whole galaxies. We find that 1) the dispersion is not
correlated with the mean age of the stellar populations, 2) a range of dust
geometries and dust extinction curves are the most likely causes for the
observed dispersion in the IRX-beta diagram 3) together with some potential
dilution of the most recent star-forming population by older unrelated bursts,
at least in the case of star-forming regions within galaxies, 4) we also
recover some general characteristics of the regions, including a tight positive
correlation between the amount of dust attenuation and the metal content.
Although generalizing our results to whole galaxies may not be immediate, the
possibility of a range of dust extinction laws and geometries should be
accounted for in the latter systems as well.Comment: 18 pages, 17 figures, accepted for publication in Ap
Star Formation and Dust Attenuation Properties in Galaxies from a Statistical UV-to-FIR Analysis
We study two galaxy samples selected in ultraviolet (UV) and in far-infrared
(FIR) for which the spectral energy distributions (SEDs) from the far UV (FUV)
to the FIR are available. We compare the observed SEDs to modelled SEDs with
several star formation histories (SFHs; decaying star formation rate plus
burst) and attenuation laws (power law + 2175 Angstroem bump). The Bayesian
method allows to estimate statistically the best parameters by comparing each
observed SED to the full set of 82800 models. We reach the conclusion that the
UV dust attenuation cannot be estimated correctly from an SED analysis if the
FIR information is not used. The deduced dispersion is larger than with the FIR
data and the distribution is not symetrically distributed about zero: there is
an over-estimation for UV-selected galaxies and an under-estimation for
FIR-selected galaxies. The output from the analysis process suggests that
UV-selected galaxies have attenuation laws in average similar to the LMC
extinction while FIR-selected galaxy attenuation laws more resemble the MW
extinction law. The dispersion about the average relation in the
Log(Fdust/Ffuv) vs. FUV-NUV diagram (once the main relation with FUV-NUV is
accounted for) is explained by two other parameters: the slope of the
attenuation law and the instantaneous birthrate parameter b_0 for UV-selected
galaxies and the same ones plus the strength of the bump for the FIR-selected
galaxies. We propose a recipe to estimate the UV dust attenuation for
UV-galaxies only (that should be used whenever the FIR information is not
available because the resulting Afuv is poorly defined with an uncertainty of
about 0.32): A_{FUV} = 1.4168 (FUV-NUV)^2 + 0.3298 (NUV-I)^2 + 2.1207 (FUV-NUV)
+ 2.7465 (NUV-I) + 5.8408Comment: Accepted for publication in MNRAS, Main Journa
The Spitzer Local Volume Legacy: Survey Description and Infrared Photometry
The survey description and the near-, mid-, and far-infrared flux properties
are presented for the 258 galaxies in the Local Volume Legacy (LVL). LVL is a
Spitzer Space Telescope legacy program that surveys the local universe out to
11 Mpc, built upon a foundation of ultraviolet, H-alpha, and HST imaging from
11HUGS (11 Mpc H-alpha and Ultraviolet Galaxy Survey) and ANGST (ACS Nearby
Galaxy Survey Treasury). LVL covers an unbiased, representative, and
statistically robust sample of nearby star-forming galaxies, exploiting the
highest extragalactic spatial resolution achievable with Spitzer. As a result
of its approximately volume-limited nature, LVL augments previous Spitzer
observations of present-day galaxies with improved sampling of the
low-luminosity galaxy population. The collection of LVL galaxies shows a large
spread in mid-infrared colors, likely due to the conspicuous deficiency of 8um
PAH emission from low-metallicity, low-luminosity galaxies. Conversely, the
far-infrared emission tightly tracks the total infrared emission, with a
dispersion in their flux ratio of only 0.1 dex. In terms of the relation
between infrared-to-ultraviolet ratio and ultraviolet spectral slope, the LVL
sample shows redder colors and/or lower infrared-to-ultraviolet ratios than
starburst galaxies, suggesting that reprocessing by dust is less important in
the lower mass systems that dominate the LVL sample. Comparisons with
theoretical models suggest that the amplitude of deviations from the relation
found for starburst galaxies correlates with the age of the stellar populations
that dominate the ultraviolet/optical luminosities.Comment: Accepted for publication in ApJ; Figures 1,8,9 provided as jpeg
The star formation history and chemical evolution of star forming galaxies in the nearby universe
We have determined the O/H and N/O of a sample of 122751 SFGs from the DR7 of
the SDSS. For all these galaxies we have also determined their morphology and
their SFH using the code STARLIGHT. The comparison of the chemical abundance
with the SFH allows us to describe the chemical evolution in the nearby
universe (z < 0.25) in a manner which is consistent with the formation of their
stellar populations and morphologies. A 45% of the SFGs in our sample show an
excess of abundance in nitrogen relative to their metallicity. We also find
this excess to be accompanied by a deficiency of oxygen, which suggests that
this could be the result of effective starburst winds. However, we find no
difference in the mode of star formation of the nitrogen rich and nitrogen poor
SFGs. Our analysis suggests they all form their stars through a succession of
bursts of star formation extended over a few Gyr period. What produces the
chemical differences between these galaxies seems therefore to be the intensity
of the bursts: the galaxies with an excess of nitrogen are those that are
presently experiencing more intense bursts, or have experienced more intense
bursts in their past. We also find evidence relating the chemical evolution
process to the formation of the galaxies: the galaxies with an excess of
nitrogen are more massive, have more massive bulges and earlier morphologies
than those showing no excess. As a possible explanation we propose that the
lost of metals consistent with starburst winds took place during the formation
of the galaxies, when their potential wells were still building up, and
consequently were weaker than today, making starburst winds more efficient and
independent of the final mass of the galaxies. In good agreement with this
interpretation, we also find evidence consistent with downsizing, according to
which the more massive SFGs formed before the less massive ones.Comment: 69 pages, 27 figures, accepted for publication in Ap
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