62 research outputs found
Reply to comment by Fries on Ăą Cometary origin of atmospheric methane variations on Mars unlikelyĂą
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137383/1/jgre20652_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137383/2/jgre20652.pd
Constraining the solutions of an inverse method of stellar population synthesis
In three previous papers (Pelat 1997, 1998 and Moultaka & Pelat 2000), we set
out an inverse stellar population synthesis method which uses a database of
stellar spectra. Unlike other methods, this one provides a full knowledge of
all possible solutions as well as a good estimation of their stability;
moreover, it provides the unique approximate solution, when the problem is
overdetermined, using a rigorous minimization procedure. In Boisson et al.
(2000), this method has been applied to 10 active and 2 normal galaxies. In
this paper we analyse the results of the method after constraining the
solutions. Adding {\it a priori} physical conditions on the solutions
constitutes a good way to regularize the synthesis problem. As an illustration
we introduce physical constraints on the relative number of stars taking into
account our present knowledge of the initial mass function in galaxies. In
order to avoid biases on the solutions due to such constraints, we use
constraints involving only inequalities between the number of stars, after
dividing the H-R diagram into various groups of stellar masses. We discuss the
results for a well-known globular cluster of the galaxy M31 and discuss some of
the galaxies studied in Boisson et al. (2000). We find that, given the spectral
resolution and the spectral domain, the method is very stable according to such
constraints (i.e. the constrained solutions are almost the same as the
unconstrained one). However, an additional information can be derived about the
evolutionary stage of the last burst of star formation, but the precise age of
this particular burst seems to be questionable.Comment: Accepted in A&A. 15 pages, 5 figures and 6 table
The intermediate-redshift galaxy cluster CL 0048-2942. Stellar populations
We present a detailed study of the cluster CL 0048-2942, located at z~0.64,
based on a photometric and spectroscopic catalogue of 54 galaxies in a 5 x 5
square arcmin region centred in that cluster. Of these, 23 galaxies were found
to belong to the cluster. Based on this sample, the line-of-sight velocity
dispersion of the cluster is approximately 680 +- 140 km/s. We have performed
stellar population synthesis in the cluster members as well as in the field
galaxies of the sample and found that there are population gradients in the
cluster with central galaxies hosting mainly intermediate/old populations
whereas galaxies in the cluster outskirts show clearly an increase of younger
populations, meaning that star formation is predominantly taking place in the
outer regions of the cluster. In a general way, field galaxies seem to host
less evolved stellar populations than cluster members. In fact, in terms of
ages, young supergiant stars dominate the spectra of field galaxies whereas
cluster galaxies display a dominant number of old and intermediate age stars.
Following the work of other authors (e.g. Dressler et al. 1999) we have
estimated the percentage of K+A galaxies in our sample and found around 13% in
the cluster and 10% in the field. These values were estimated through means of
a new method, based on stellar population synthesis results, that takes into
account all possible absorption features in the spectrum and thus makes optimal
use of the data.Comment: Accepted by Astronomy & Astrophysics. 24 pages, 10 figures, 10 tables
(figures 3, 4, 5 and tables 1, 3, 4, 5, 6, 7, 8 will be available in
electronic format only in the A&A published version
Prospects for population synthesis in the H band: NeMo grids of stellar atmospheres compared to observations
For applications in population synthesis, libraries of theoretical stellar
spectra are often considered an alternative to template libraries of observed
spectra, because they allow a complete sampling of stellar parameters. Most
attention in published theoretical spectral libraries has been devoted to the
visual wavelength range. We present a detailed comparison of theoretical
spectra in the range 1.57-1.67m, for spectral types from A to early M and
for giants and dwarf stars, with observed stellar spectra at resolutions around
3000, which would be sufficient to disentangle the different groups of late
type stars. We have selected the NeMo grids of stellar atmospheres to perform
such a comparison. We first demonstrate that after combining atomic and
molecular line lists, it is possible to match observed spectral flux
distributions with theoretical ones very well for almost the entire parameter
range covered by the NeMo grids at moderate resolution in the visual range. In
the infrared range, although the overall shape of the observed flux
distributions is still matched reasonably well, the individual spectral
features are reproduced by the theoretical spectra only for stars earlier than
mid F type. For later spectral types the differences increase and theoretical
spectra of K type stars have systematically weaker line features than those
found in observations. These discrepancies are traced back to stem primarily
from incomplete data on neutral atomic lines, although some of them are also
related to molecules. Improving atomic data in the near infrared is a key
element in making the construction of reliable libraries of stellar spectra in
the infrared feasible.Comment: 17 pages, 19 figures, accepted for publication in A&
JIRAM, the Jovian Infrared Auroral Mapper
JIRAM is an imager/spectrometer on board the Juno spacecraft bound for a polar orbit around Jupiter. JIRAM is composed of IR imager and spectrometer channels. Its scientific goals are to explore the Jovian aurorae and the planet's atmospheric structure, dynamics and composition. This paper explains the characteristics and functionalities of the instrument and reports on the results of ground calibrations. It discusses the main subsystems to the extent needed to understand how the instrument is sequenced and used, the purpose of the calibrations necessary to determine instrument performance, the process for generating the commanding sequences, the main elements of the observational strategy, and the format of the scientific data that JIRAM will produce
The nuclear region of low luminosity flat radio spectrum sources. II. Emission-line spectra
International audienceWe report on the spectroscopic study of 19 low luminosity Flat Radio Spectrum (LL FRS) sources selected from MarchĂŁ's et al. (\cite{March96}) 200 mJy sample. In the optical, these objects are mainly dominated by the host galaxy starlight. After correcting the data for this effect, we obtain a new set of spectra clearly displaying weak emission lines; such features carry valuable information concerning the excitation mechanisms at work in the nuclear regions of LL FRS sources. We have used a special routine to model the spectra and assess the intensities and velocities of the emission lines; we have analyzed the results in terms of diagnostic diagrams. Our analysis shows that 79% of the studied objects harbour a Low Ionization Nuclear Emission-line Region (or LINER) whose contribution was swamped by the host galaxy starlight. The remaining objects display a higher ionization spectrum, more typical of Seyferts; due to the poor quality of the spectra, it was not possible to identify any possible large Balmer components. The fact that we observe a LINER-type spectrum in LL FRS sources supports the idea that some of these objects could be undergoing an ADAF phase; in addition, such a low ionization emission-line spectrum is in agreement with the black hole mass values and sub-Eddington accretion rates published for some FRS sources
The nuclear region of low luminosity flat radio spectrum sources. II. Emission-line spectra
International audienceWe report on the spectroscopic study of 19 low luminosity Flat Radio Spectrum (LL FRS) sources selected from MarchĂŁ's et al. (\cite{March96}) 200 mJy sample. In the optical, these objects are mainly dominated by the host galaxy starlight. After correcting the data for this effect, we obtain a new set of spectra clearly displaying weak emission lines; such features carry valuable information concerning the excitation mechanisms at work in the nuclear regions of LL FRS sources. We have used a special routine to model the spectra and assess the intensities and velocities of the emission lines; we have analyzed the results in terms of diagnostic diagrams. Our analysis shows that 79% of the studied objects harbour a Low Ionization Nuclear Emission-line Region (or LINER) whose contribution was swamped by the host galaxy starlight. The remaining objects display a higher ionization spectrum, more typical of Seyferts; due to the poor quality of the spectra, it was not possible to identify any possible large Balmer components. The fact that we observe a LINER-type spectrum in LL FRS sources supports the idea that some of these objects could be undergoing an ADAF phase; in addition, such a low ionization emission-line spectrum is in agreement with the black hole mass values and sub-Eddington accretion rates published for some FRS sources
Spectroscopic study of low luminosity BL Lac type objects
We have examined the spectroscopic properties of a sample of 19 optically
bright, low luminosity core-dominated radio sources; this sample was
expected to contain a high fraction of objects observed at small angles
to the line-of-sight (blazar-like objects). Our study focuses on the
properties - stellar populations and optical emission-line
spectra - of such nuclei.
Stellar population synthesis shows that their nuclear populations
are composed, in a general way, of old stars of solar metallicity or
lower; the dust content is weak.
After subtraction of the stellar contribution, we are
left with a set of nuclear emission-line spectra; their analysis shows
that most of the objects harbour a Low Ionisation Nuclear Emission
Region (or LINER), whose contribution was highly diluted by the
host galaxy starlight. Such a low ionisation spectrum is in agreement
with the black hole mass values and sub-Eddington accretion rates
published for some BL Lacs; also, the stellar populations derived
in this study are in agreement with those usually found in LINERs.
Our findings give strength to a unifying picture involving low
luminosity blazar-like objects and FRÂ I radio-galaxies (which usually
display LINER-type spectra)
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