101 research outputs found
Stellar Populations Found in the Central kpc of Four Luminous Compact Blue Galaxies at Intermediate Redshift
We investigate the star formation history of the central regions of four
Luminous Compact Blue Galaxies (LCBGs). LCBGs are blue (B-V<0.6), compact
(MU_B<21.5 mag arcsec^-2) galaxies with absolute magnitudes M_B brighter than
-17.5. The LCBGs analyzed here are located at 0.436<z<0.525. They are among the
most luminous (M_B < -20.5), blue (B-V < 0.4) and high surface brightness (MU_B
< 19.0 mag arcsec^-2) of this population. The observational data used were
obtained with the HST/STIS spectrograph, the HST/WF/PC-2 camera and the
HST/NICMOS first camera. We find evidence for multiple stellar populations. One
of them is identified as the ionizing population, and the other one corresponds
to the underlying stellar generation.
The estimated masses of the inferred populations are compatible with the
dynamical masses, which are typically 2--10x 10^9 M_sun. Our models also
indicate that the first episodes of star formation the presented LCBGs
underwent happened between 5 and 7 Gyr ago.
We compare the stellar populations found in LCBGs with the stellar
populations present in bright, local HII galaxies, nearby spheroidal systems
and Blue Compact Dwarf Galaxies. It turns out that the underlying stellar
populations of LCBGs are similar yet bluer to those of local HII galaxies. It
is also the case that the passive color evolution of the LCBGs could convert
them into local Spheroidal galaxies if no further episode of star formation
takes place. Our results help to impose constraints on evolutionary scenarios
for the population of LCBGs found commonly at intermediate redshifts.Comment: 35 pages, 10 Figures. Accepted for publication in AJ. Compile with
pdflatex. Contains png figure
A spectroscopic study of component C and the extended emission around I Zw 18
Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are
used to investigate the stellar population and the evolutionary status of I Zw
18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18.
Hydrogen H and H emission lines are detected in the spectra of I
Zw 18C, implying that ionizing massive stars are present. High signal-to-noise
Keck II spectra of different regions in I Zw 18C reveal H, H
and higher order hydrogen lines in absorption. Several techniques are used to
constrain the age of the stellar population in I Zw 18C. Ages derived from two
different methods, one based on the equivalent widths of the H,
H emission lines and the other on H, H absorption lines
are consistent with a 15 Myr instantaneous burst model. We find that a small
extinction in the range = 0.20 -- 0.65 mag is needed to fit the observed
spectral energy distribution of I Zw 18C with that model. In the case of
constant star formation, all observed properties are consistent with stars
forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available
observational constraints for I Zw 18C, including those obtained from Hubble
Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18
should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized
gas emission around I Zw 18. H emission is detected as far as 30" from
it. To a B surface brightness limit of ~ 27 mag arcsec we find no
observational evidence for extended stellar emission in the outermost regions,
at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the
Astrophysical Journa
Starburst99: Synthesis Models for Galaxies with Active Star Formation
Starburst99 is a comprehensive set of model predictions for
spectrophotometric and related properties of galaxies with active star
formation. The models are an improved and extended version of the data set
previously published by Leitherer & Heckman (1995). We have upgraded our code
by implementing the latest set of stellar evolution models of the Geneva group
and the model atmosphere grid compiled by Lejeune et al. (1997). Several
predictions which were not included in the previous publication are shown here
for the first time. The models are presented in a homogeneous way for five
metallicities between Z = 0.040 and 0.001 and three choices of the initial mass
function. The age coverage is 10^6 to 10^9 yr. We also show the spectral energy
distributions which are used to compute colors and other quantities. The full
data set is available for retrieval at
http://www.stsci.edu/science/starburst99/. This website allows users to run
specific models with non-standard parameters as well. We also make the source
code available to the community.Comment: 32 pages, LaTeX. All the Figures and the summary Table are located at
http://www.stsci.edu/science/starburst99/, ApJ accepte
New Models for Wolf-Rayet and O Star Populations in Young Starbursts
Using the latest stellar evolution models, theoretical stellar spectra, and a
compilation of observed emission line strengths from Wolf-Rayet (WR) stars, we
construct evolutionary synthesis models for young starbursts. We explicitly
distinguish between the various WR subtypes (WN, WC, WO), and we treat O and Of
stars separately. We provide detailed predictions of UV and optical emission
line strengths for both the WR stellar lines and the major nebular hydrogen and
helium emission lines, as a function of several input parameters related to the
starburst episode. We also derive the theoretical frequency of WR-rich
starbursts. We then discuss: nebular HeII 4686 emission, the contribution of WR
stars to broad Balmer line emission, techniques used to derive the WR and O
star content from integrated spectra, and explore the implications of the
formation of WR stars through mass transfer in close binary systems in
instantaneous bursts. The observational features predicted by our models allow
a detailed quantitative determination of the massive star population in a
starburst region (particularly in so-called "WR galaxies") from its integrated
spectrum and provide a means of deriving the burst properties (e.g., duration,
age) and the parameters of the initial mass function of young starbursts.
(Abridged abstract)Comment: Accepted by ApJ Supplements. LaTeX using aasmp4, psfigs macros. 49
pages including 23 figures. Paper (full, or text/figures separated) and
detailed model results available at
http://www.stsci.edu/ftp/science/starburst/sv97.htm
A Spectroscopic Study of a Large Sample of Wolf-Rayet Galaxies
We analyze long-slit spectral observations of 39 Wolf-Rayet (WR) galaxies
with heavy element mass fraction ranging over 2 orders of magnitude, from
Zsun/50 to 2Zsun. Nearly all galaxies in our sample show broad WR emission in
the blue region of the spectrum (the blue bump) consisting of an unresolved
blend of N III 4640, C III 4650, C IV 4658 and He II 4686 emission lines. Broad
C IV 5808 emission (the red bump) is detected in 30 galaxies. Additionally,
weaker WR emission lines are identified, most often the N III 4512 and Si III
4565 lines, which have very rarely or never been seen and discussed before in
WR galaxies. These emission features are characteristic of WN7-WN8 and WN9-WN11
stars respectively.
We derive the numbers of early WC (WCE) and late WN (WNL) stars from the
luminosities of the red and blue bumps, and the number of O stars from the
luminosity of the Hbeta emission line. Additionally, we propose a new technique
for deriving the numbers of WNL stars from the N III 4512 and Si III 4565
emission lines. This technique is potentially more precise than the blue bump
method because it does not suffer from contamination of WCE and early WN (WNE)
stars and nebular gaseous emission.
The N(WR)/N(O+WR) ratio decreases with decreasing metallicity, in agreement
with predictions of evolutionary synthesis models. The N(WC)/N(WN) ratios and
the equivalent widths of the blue bump EW(4650) and of the red bump EW(5808)
derived from observations are also in satisfactory agreement with theoretical
predictions.Comment: 49 pages, 9 figures, to appear in Astrophys.
A Keck/HIRES Study of Kinematics of the Cold Interstellar Medium in Dwarf Starburst Galaxies
We have obtained high resolution Echelle spectra (R = 30,000-50,000) of the
Na D absorption doublet (5890A, 5896A) for six dwarf starburst galaxies and two
more luminous starbursts: M82 and NGC 1614. The absorption features were
separated into multiple components and separated into stellar and interstellar
parts based on kinematics. We find that three of the dwarfs show outflows, with
an average blueshift of 27 km/s. This is small compared to the highest velocity
components in NGC 1614 and M82 (blueshifted by 150 km/s and 91 km/s,
respectively); these two brighter galaxies also show more complex absorption
profiles than the dwarfs. None of the outflow speeds clearly exceed the escape
velocity of the host galaxy. Sightlines in NGC 2363 and NGC4214 apparently
intersect expanding shells. We compare the shocked gas velocity (v_NaD) to the
ionized gas velocity (v_Halpha) and interpret the velocity difference as either
a trapped ionization front (NGC 4214) or a leaky HII region (NGC 2363). The
dwarfs show N(NaD) = 10^(11.8-13.7) cm^-2, while the Na D columns in M82 and
NGC 1614 are 10^13.7 cm^-2 and 10^14.0 cm^-2, respectively. The mass of
expelled gas is highly sensitive to outflow geometry, dust depletion, and
ionization fraction, but with a simple shell model we estimate neutral outflow
gas masses from ~10^6 M_solar to ~10^10 M_solar.Comment: 36 pages, including 9 figures. Accepted for publication in Astrophys.
Multiwavelength Study of the Starburst Galaxy NGC 7714. II: The Balance between Young, Intermediate Age and Old Stars
We combine existing multiwavelength data (incl. an HST/GHRS UV spectrum, an
optical spectrum, far-IR, Xray and radio fluxes) with new HST/WFPC2 images,
near-IR photometry and K band spectroscopy. We use these data to constrain the
young, the intermediate age and the old stellar populations in the central 330
pc of the starburst galaxy NGC 7714. [...]
We find that the young burst responsible for the UV light is only a small
part of an extended episode of enhanced star formation (SF) [...]. The mass of
young and intermediate age stars thus formed equals at least 10% of the mass
locked in pre-existing stars of the underlying galaxy nucleus [...]. The
spectrophotometric SF timescale is long compared to the ~110 Myr elapsed since
closest contact with NGC 7715. The trigger of the starburst remains elusive.
NGC 7714 owes its brightness in the UV to a few low extinction lines of sight
towards young stars. [...] The different extinction estimates obtained from
different indicators result naturally from the coexistence of populations with
various ages and obscurations. The near-IR continuum image looks smoothest, as
a consequence of lower sensitivity to extinction and of a larger contribution
of old stars. We compare the nuclear properties of NGC 7714 with results from
studies in larger apertures. We emphasize that the global properties of
starburst galaxies are the result of the averaging over many lines of sight
with diverse properties in terms of obscuration and stellar ages.Comment: 29 pages (+20 figures and tables), Latex2e (figs. included), uses
aastex.cls. To be published in ApJ (May 2001 issue
Application of a spring-dashpot system to clinical lung tumor motion data
A spring-dashpot system based on the Voigt model was developed to model the
correlation between abdominal respiratory motion and tumor motion during lung
radiotherapy. The model was applied to clinical data comprising 52 treatment
beams from 10 patients, treated on the Mitsubishi Real-Time Radiation Therapy
system, Sapporo, Japan. In Stage 1, model parameters were optimized for
individual patients and beams to determine reference values and to investigate
how well the model can describe the data. In Stage 2, for each patient the
optimal parameters determined for a single beam were applied to data from other
beams to investigate whether a beam-specific set of model parameters is
sufficient to model tumor motion over a course of treatment.
In Stage 1 the baseline root mean square (RMS) residual error for all
individually-optimized beam data was 0.90 plus or minus 0.40 mm. In Stage 2,
patient-specific model parameters based on a single beam were found to model
the tumor position closely, even for irregular beam data, with a mean increase
with respect to Stage 1 values in RMS error of 0.37 mm. On average the obtained
model output for the tumor position was 95% of the time within an absolute
bound of 2.0 mm and 2.6 mm in Stage 1 and 2, respectively.
The model was capable of dealing with baseline, amplitude and frequency
variations of the input data, as well as phase shifts between the input tumor
and output abdominal signals. These results indicate that it may be feasible to
collect patient-specific model parameters during or prior to the first
treatment, and then retain these for the rest of the treatment period. The
model has potential for clinical application during radiotherapy treatment of
lung tumors
Gene Set Enrichment in eQTL Data Identifies Novel Annotations and Pathway Regulators
Genome-wide gene expression profiling has been extensively used to generate biological hypotheses based on differential expression. Recently, many studies have used microarrays to measure gene expression levels across genetic mapping populations. These gene expression phenotypes have been used for genome-wide association analyses, an analysis referred to as expression QTL (eQTL) mapping. Here, eQTL analysis was performed in adipose tissue from 28 inbred strains of mice. We focused our analysis on âtrans-eQTL bandsâ, defined as instances in which the expression patterns of many genes were all associated to a common genetic locus. Genes comprising trans-eQTL bands were screened for enrichments in functional gene sets representing known biological pathways, and genes located at associated trans-eQTL band loci were considered candidate transcriptional modulators. We demonstrate that these patterns were enriched for previously characterized relationships between known upstream transcriptional regulators and their downstream target genes. Moreover, we used this strategy to identify both novel regulators and novel members of known pathways. Finally, based on a putative regulatory relationship identified in our analysis, we identified and validated a previously uncharacterized role for cyclin H in the regulation of oxidative phosphorylation. We believe that the specific molecular hypotheses generated in this study will reveal many additional pathway members and regulators, and that the analysis approaches described herein will be broadly applicable to other eQTL data sets
Stellar population synthesis at the resolution of 2003
We present a new model for computing the spectral evolution of stellar
populations at ages between 100,000 yr and 20 Gyr at a resolution of 3 A across
the whole wavelength range from 3200 to 9500 A for a wide range of
metallicities. These predictions are based on a newly available library of
observed stellar spectra. We also compute the spectral evolution across a
larger wavelength range, from 91 A to 160 micron, at lower resolution. The
model incorporates recent progress in stellar evolution theory and an
observationally motivated prescription for thermally-pulsing stars on the
asymptotic giant branch. The latter is supported by observations of surface
brightness fluctuations in nearby stellar populations. We show that this model
reproduces well the observed optical and near-infrared colour-magnitude
diagrams of Galactic star clusters of various ages and metallicities.
Stochastic fluctuations in the numbers of stars in different evolutionary
phases can account for the full range of observed integrated colours of star
clusters in the Magellanic Clouds. The model reproduces in detail typical
galaxy spectra from the Early Data Release (EDR) of the Sloan Digital Sky
Survey (SDSS). We exemplify how this type of spectral fit can constrain
physical parameters such as the star formation history, metallicity and dust
content of galaxies. Our model is the first to enable accurate studies of
absorption-line strengths in galaxies containing stars over the full range of
ages. Using the highest-quality spectra of the SDSS EDR, we show that this
model can reproduce simultaneously the observed strengths of those Lick indices
that do not depend strongly on element abundance ratios [abridged].Comment: 35 pages, 22 figures, to appear in MNRAS; version with full
resolution figures available at http://www.iap.fr/~charlot/bc2003/pape
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