901 research outputs found
The BHK Color Diagram: a New Tool to Study Young Stellar Populations
A new method to derive age differences between the various super star
clusters observed in starburst galaxies using the two color diagram (B-H) vs
(H-K) is presented. This method offers a quick and easy way to differentiate
very young and intermediate age stellar populations even if data on extinction
are unavailable. In this case, discrimination of regions younger and older than
4 Myr is feasible. With the availability of data on extinction, the time
resolution can be improved significantly. The application of the method to the
starbursting system Arp 299 is presented. The validity of the method is
confirmed by comparing the equivalent width of the H-alpha line with the
chronological map of the northern part of NGC 3690.Comment: 32 pages, 7 figures, 1 table, AJ accepte
Insights on star formation histories and physical properties of Herschel-detected galaxies
We test the impact of using variable star forming histories (SFHs) and the
use of the IR luminosity (LIR) as a constrain on the physical parameters of
high redshift dusty star-forming galaxies. We explore in particular the stellar
properties of galaxies in relation with their location on the SFR-M* diagram.
We perform SED fitting of the UV-NIR and FIR emissions of a large sample of
GOODS-Herschel galaxies, for which rich multi-wavelength observations are
available. We test different SFHs and imposing energy conservation in the SED
fitting process, to face issues like the age-extinction degeneracy and produce
SEDs consistent with observations. Our models work well for the majority of the
sample, with the notable exception of the high LIR end, for which we have
indications that our simple energy conservation approach cannot hold true. We
find trends in the SFHs fitting our sources depending on stellar mass M* and z.
Trends also emerge in the characteristic timescales of the SED models depending
on the location on the SFR-M* diagram. We show that whilst using the same
available observational data, we can produce galaxies less star-forming than
usually inferred, if we allow declining SFHs, while properly reproducing their
observables. These sources can be post-starbursts undergoing quenching, and
their SFRs are potentially overestimated if inferred from their LIR. Fitting
without the IR constrain leads to a strong preference for declining SFHs, while
its inclusion increases the preference of rising SFHs, more so at high z, in
tentative agreement with the cosmic star formation history. Keeping in mind
that the sample is biased towards high LIR, the evolution shaped by our model
appears as both bursty (initially) and steady-lasting (later on). The global
SFH of the sample follows the cosmic SFH with a small scatter, and is
compatible with the "downsizing" scenario of galaxy evolution.Comment: 28 pages, 26 figures, one appendix, Accepted for publication in
Astronomy & Astrophysic
Very High-Redshift Lensed Galaxies
We review in this paper the main results recently obtained on the
identification and study of very high-z galaxies usinglensing clusters as
natural gravitational telescopes. We present in detail our pilot survey with
ISAAC/VLT, aimed at the detection of z>7 sources. Evolutionary synthesis models
for extremely metal-poor and PopIII starbursts have been used to derive the
observational properties expected for these high-z galaxies, such as expected
magnitudes and colors, line fluxes for the main emission lines, etc. These
models have allowed to define fairly robust selection criteria to find z~7-10
galaxies based on broad-band near-IR photometry in combination with the
traditional Lyman drop-out technique. The first results issued from our
photometric and spectroscopic survey are discussed, in particular the
preliminary confirmation rate, and the global properties of our high-z
candidates, including the latest results on the possible z=10.0 candidate
A1835-1916. The search efficiency should be significantly improved by the
future near-IR multi-object ground-based and space facilities. However, strong
lensing clusters remain a factor of ~5-10 more efficient than blank fields in
this redshift domain, within the FOV of a few arcminutes around the cluster
core, for the typical depth required for this survey project.Comment: 14 pages, 7 figures, Proceedings of IAU Symposium No. 225: The Impact
of Gravitational Lensing on Cosmology, Y. Mellier and G. Meylan, Ed
ISAAC/VLT observations of a lensed galaxy at z=10.0
We report the first likely spectroscopic confirmation of a z 10.0 galaxy from
our ongoing search for distant galaxies with ISAAC/VLT. Galaxy candidates at z
>~ 7 are selected from ultra-deep JHKs images in the core of gravitational
lensing clusters for which deep optical imaging is also available, including
HST data. The object reported here, found behind Abell 1835, exhibits a faint
emission line detected in the J band, leading to z=10.0 when identified as
Ly-a, in excellent agreement with the photometric redshift determination.
Redshifts z < 7 are very unlikely for various reasons we discuss. The object is
located on the critical lines corresponding to z=9 to 11. The magnification
factor \mu ranges from 25 to 100. For this object we estimate SFR(Ly-a)
(0.8-2.2) Msun/yr and SFR(UV) (47-75) Msun/yr, both uncorrected for lensing.
The steep UV slope indicates a young object with negligible dust extinction.
SED fits with young low-metallicity stellar population models yield (adopting
mu=25) a lensing corrected stellar mass of M*~8.e+6 Msun, and luminosities of
2.e+10 Lsun, corresponding to a dark matter halo of a mass of typically M_tot>~
5.e+8 Msun. In general our observations show that under excellent conditions
and using strong gravitational lensing direct observations of galaxies close to
the ``dark ages'' are feasible with ground-based 8-10m class telescopes.Comment: To be published in A&A, Vol. 416, p. L35. Press release information,
additional figures and information available at http://obswww.unige.ch/sfr
and http://webast.ast.obs-mip.fr/galaxie
Emission-line Helium Abundances in Highly Obscured Nebulae
This paper outlines a way to determine the ICF using only infrared data. We
identify four line pairs, [NeIII] 36\micron/[NeII] 12.8\micron,
[NeIII]~15.6\micron /[NeII] 12.8\micron, [ArIII] 9\micron/[ArII]
6.9\micron, and [ArIII] 21\micron/[ArII] 6.9\micron, that are sensitive
to the He ICF. This happens because the ions cover a wide range of ionization,
the line pairs are not sensitive to electron temperature, they have similar
critical densities, and are formed within the He/H region of the
nebula. We compute a very wide range of photoionization models appropriate for
galactic HII regions. The models cover a wide range of densities, ionization
parameters, stellar temperatures, and use continua from four very different
stellar atmospheres.
The results show that each line pair has a critical intensity ratio above
which the He ICF is always small. Below these values the ICF depends very
strongly on details of the models for three of the ratios, and so other
information would be needed to determine the helium abundance. The [Ar III]
9\micron/[ArII] 6.9\micron ratio can indicate the ICF directly due to the
near exact match in the critical densities of the two lines. Finally, continua
predicted by the latest generation of stellar atmospheres are sufficiently hard
that they routinely produce significantly negative ICFs.Comment: Accepted by PASP. Scheduled for the October 1999 issue. 11 pages, 5
figure
Grids of stellar models. VIII. From 0.4 to 1.0 Msun at Z=0.020 and Z=0.001, with the MHD equation of state
We present stellar evolutionary models covering the mass range from 0.4 to 1
Msun calculated for metallicities Z=0.020 and 0.001 with the MHD equation of
state (Hummer & Mihalas, 1988; Mihalas et al. 1988; D\"appen et al. 1988). A
parallel calculation using the OPAL (Rogers et al. 1996) equation of state has
been made to demonstrate the adequacy of the MHD equation of state in the range
of 1.0 to 0.8 Msun (the lower end of the OPAL tables). Below, down to 0.4 Msun,
we have justified the use of the MHD equation of state by theoretical arguments
and the findings of Chabrier & Baraffe (1997).
We use the radiative opacities by Iglesias & Rogers (1996), completed with
the atomic and molecular opacities by Alexander & Fergusson (1994). We follow
the evolution from the Hayashi fully convective configuration up to the red
giant tip for the most massive stars, and up to an age of 20 Gyr for the less
massive ones. We compare our solar-metallicity models with recent models
computed by other groups and with observations.
The present stellar models complete the set of grids computed with the same
up-to-date input physics by the Geneva group [Z=0.020 and 0.001, Schaller et
al. (1992), Bernasconi (1996), and Charbonnel et al. (1996); Z=0.008, Schaerer
et al. (1992); Z=0.004, Charbonnel et al. (1993); Z=0.040, Schaerer et al.
(1993); Z=0.10, Mowlavi et al. (1998); enhanced mass loss rate evolutionary
tracks, Meynet et al. (1994)].Comment: Accepted for publication in A&A Supplement Serie
Near-Infrared Microlensing of Stars by the Super-Massive Black Hole in the Galactic Center
We investigate microlensing amplification of faint stars in the dense stellar
cluster in the Galactic Center (GC) by the super-massive black hole (BH). Such
events would appear very close to the position of the radio source SgrA*, which
is thought to coincide with the BH, and could be observed during the monitoring
of stellar motions in the GC. We use the observed K-band (2.2 um) luminosity
function (KLF) in the GC and in Baade's Window, as well as stellar population
synthesis computations, to construct KLF models for the inner 300 pc of the
Galaxy. These, and the observed dynamical properties of this region, are used
to compute the rates of microlensing events, which amplify stars above
specified detection thresholds. We present computations of the lensing rates
and amplifications as functions of the event durations (weeks to years), for a
range of detection thresholds. We find that short events dominate the total
rate and that long events tend to have large amplifications. For the current
detection limit of K=17 mag, the total microlensing rate is 0.003 1/yr, and the
rate of events with durations >1 yr is 0.001 1/yr. Recent GC proper motion
studies have revealed the possible presence of one or two variable K-band
sources very close to SgrA* (Genzel et al 97; Ghez et al 98). These sources may
have attained peak brightnesses of K~15 mag, about 1.5-2 mag above the
observational detection limits, and appear to have varied on a timescale of ~1
yr. This behavior is consistent with long-duration microlensing of faint stars
by the BH. However, we estimate that the probability that such an event could
have been detected during the course of the recent proper motion studies is
\~0.5%. A ten-fold improvement in the detection limit and 10 yr of monthly
monitoring would increase the total detection probability to ~20%. (Abridged)Comment: 29 p. with 5 figs. To appear in ApJ. Changed to reflect published
version. Short discussions of solar metallicity luminosity function and
star-star microlensing adde
- âŠ