931 research outputs found
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
The Massive Stellar Content in the Starburst NGC3049: A Test for Hot-Star Mode
We have obtained high-spatial resolution ultraviolet and optical STIS
spectroscopy and imaging of the metal-rich nuclear starburst in NGC3049. The
stellar continuum and the absorption line spectrum in the ultraviolet are used
to constrain the massive stellar population. The strong, blueshifted stellar
lines of CIV and SiIV detected in the UV spectra indicate a metal-rich,
compact, massive (1E6 Msol) cluster of age 3--4 Myr emitting the UV-optical
continuum. We find strong evidence against a depletion of massive stars in this
metal-rich cluster. The derived age and the upper mass-limit cut-off of the
initial mass function are also consistent with the detection of Wolf-Rayet (WR)
features at optical wavelengths. As a second independentconstraint on the
massive stellar content, the nebular emission-line spectrum is modeled with
photoionization codes using stellar spectra from evolutionary synthesis models.
However, the nebular lines are badly reproduced by 3--4 Myr instantaneous
bursts, as required by the UV line spectrum, when unblanketed WR and/or Kurucz
stellar atmospheres are used. The corresponding number of photons above 24 and
54 eV in the synthetic models is too high in comparison with values suggested
by the observed line ratios. Since the ionizing spectrum in this regime is
dominated by emission from WR stars, this discrepancy between observations and
models is most likely the result of incorrect assumptions about the WR stars.
Thus we conclude that the nebular spectrum of high-metallicity starbursts is
poorly reproduced by models for WR dominated populations. However, the new
model set of Smith et al. (2002) with blanketed WR and O atmospheres and
adjusted WR temperatures predicts a softer far-UV radiation field, providing a
better match to the data.Comment: To be published in ApJ, Dec. issue 17 figures, 3 in gif forma
The Primordial Helium Abundance: Towards Understanding and Removing the Cosmic Scatter in the dY/dZ Relation
We present results from photoionization models of low-metallicity HII
regions. These nebulae form the basis for measuring the primordial helium
abundance. Our models show that the helium ionization correction factor (ICF)
can be non-negligible for nebulae excited by stars with effective temperatures
larger than 40,000 K. Furthermore, we find that when the effective temperature
rises to above 45,000 K, the ICF can be significantly negative. This result is
independent of the choice of stellar atmosphere. However, if an HII region has
an [O III] 5007/[O I] 6300 ratio greater than 300, then our models show that,
regardless of its metallicity, it will have a negligibly small ICF. A similar,
but metallicity dependent, result was found using the [O III] 5007/H
ratio. These two results can be used as selection criteria to remove nebulae
with potentially non-negligible ICFs. Using our metallicity independent
criterion on the data of Izotov & Thuan (1998) results in a 20% reduction of
the rms scatter about the best fit line. A fit to the selected data
results in a slight increase of the value of the primordial helium abundance.Comment: 10 pages, 5 figures, accepted by the Ap
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
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
The Spitzer View of Low-Metallicity Star Formation: II. Mrk 996, a Blue Compact Dwarf Galaxy with an Extremely Dense Nucleus
(abridged) We present new Spitzer, UKIRT and MMT observations of the blue
compact dwarf galaxy (BCD) Mrk 996, with an oxygen abundance of
12+log(O/H)=8.0. This galaxy has the peculiarity of possessing an
extraordinarily dense nuclear star-forming region, with a central density of
~10^6 cm^{-3}. The nuclear region of Mrk 996 is characterized by several
unusual properties: a very red color J-K = 1.8, broad and narrow emission-line
components, and ionizing radiation as hard as 54.9 eV, as implied by the
presence of the OIV 25.89 micron line. The nucleus is located within an
exponential disk with colors consistent with a single stellar population of age
>1 Gyr. The infrared morphology of Mrk 996 changes with wavelength. The IRS
spectrum shows strong narrow Polycyclic Aromatic Hydrocarbon (PAH) emission,
with narrow line widths and equivalent widths that are high for the metallicity
of Mrk 996. Gaseous nebular fine-structure lines are also seen. A CLOUDY model
requires that they originate in two distinct HII regions: a very dense HII
region of radius ~580 pc with densities declining from ~10^6 at the center to a
few hundreds cm^{-3} at the outer radius, where most of the optical lines
arise; and a HII region with a density of ~300 cm^{-3} that is hidden in the
optical but seen in the MIR. We suggest that the infrared lines arise mainly in
the optically obscured HII region while they are strongly suppressed by
collisional deexcitation in the optically visible one. The hard ionizing
radiation needed to account for the OIV 25.89 micron line is most likely due to
fast radiative shocks propagating in an interstellar medium. A hidden
population of Wolf-Rayet stars of type WNE-w or a hidden AGN as sources of hard
ionizing radiation are less likely possibilities.Comment: 48 pages, 13 figures, accepted for publication in the Astrophysical
Journa
The Brightest Ly Emitter: Pop III or Black Hole?
CR7 is the brightest emitter (LAE) known to date,
and spectroscopic follow-up by Sobral et al. (2015) suggests that CR7 might
host Population (Pop) III stars. We examine this interpretation using
cosmological hydrodynamical simulations. Several simulated galaxies show the
same "Pop III wave" pattern observed in CR7. However, to reproduce the extreme
CR7 /HeII1640 line luminosities () a
top-heavy IMF and a massive () PopIII burst with age
Myr are required. Assuming that the observed properties of and HeII emission are typical for Pop III, we predict that in the
COSMOS/UDS/SA22 fields, 14 out of the 30 LAEs at with should also host Pop III stars producing an
observable . As an alternate
explanation, we explore the possibility that CR7 is instead powered by
accretion onto a Direct Collapse Black Hole (DCBH). Our model predicts
, , and X-ray luminosities that are in agreement
with the observations. In any case, the observed properties of CR7 indicate
that this galaxy is most likely powered by sources formed from pristine gas. We
propose that further X-ray observations can distinguish between the two above
scenarios.Comment: 6 pages, 4 figure
HST's view of the youngest massive stars in the Magellanic Clouds
Accurate physical parameters of newborn massive stars are essential
ingredients to shed light on their formation, which is still an unsolved
problem. The rare class of compact H II regions in the Magellanic Clouds (MCs),
termed ``high-excitation blobs'' (HEBs), presents a unique opportunity to
acquire this information. These objects (~ 4" to 10", ~ 1 to 3 pc, in diameter)
harbor the youngest massive stars of the OB association/molecular cloud
complexes in the MCs accessible through high-resolution near-IR and optical
techniques. We present a brief overview of the results obtained with HST mainly
on two HEBs, one in the LMC (N159-5) and the other in the SMC (N81).Comment: 5 pages, to appear in the Proceedings of the 41st ESLAB Symposium
"The Impact of HST on European Astronomy", 29 May to 1 June 2007, ESTEC,
Noordwijk, Netherlands; eds. Guido De Marchi and Duccio Macchett
- âŠ