2,920 research outputs found
SPITZER IRS spectra of Virgo early type galaxies: detection of stellar silicate emission
We present high signal to noise ratio Spitzer Infrared Spectrograph
observations of 17 Virgo early-type galaxies. The galaxies were selected from
those that define the colour-magnitude relation of the cluster, with the aim of
detecting the silicate emission of their dusty, mass-losing evolved stars. To
flux calibrate these extended sources we have devised a new procedure that
allows us to obtain the intrinsic spectral energy distribution and to
disentangle resolved and unresolved emission within the same object. We have
found that thirteen objects of the sample (76%) are passively evolving galaxies
with a pronounced broad silicate feature which is spatially extended and likely
of stellar origin, in agreement with model predictions. The other 4 objects
(24%) are characterized by different levels of activity. In NGC 4486 (M 87) the
line emission and the broad silicate emission are evidently unresolved and,
given also the typical shape of the continuum, they likely originate in the
nuclear torus. NGC 4636 shows emission lines superimposed on extended (i.e.
stellar) silicate emission, thus pushing the percentage of galaxies with
silicate emission to 82%. Finally, NGC 4550 and NGC 4435 are characterized by
polycyclic aromatic hydrocarbon (PAH) and line emission, arising from a central
unresolved region. A more detailed analysis of our sample, with updated models,
will be presented in a forthcoming paper.Comment: 6 pages; ApJ Letters, accepte
Modelling Intermediate Age and Old Stellar Populations in the Infrared
We have investigated the spectro-photometric properties of the Asymptotic
Giant Branch (AGB) stars and their contribution to the integrated infrared
emission in simple stellar populations (SSP). Adopting analytical relations
describing the evolution of these stars in the HR diagram and empirical
relations for the mass-loss rate and the wind terminal velocity, we were able
to model the effects of the dusty envelope around these stars, with a minimal
number of parameters. We computed isochrones at different age and initial metal
content. We compare our models with existing infrared colors of M giants and
Mira stars and with IRAS PSC data. Contrary to previous models, in the new
isochrones the mass-loss rate, which establishes the duration of the AGB phase,
also determines the spectral properties of the stars. The contribution of these
stars to the integrated light of the population is thus obtained in a
consistent way. We find that the emission in the mid infrared is about one
order of magnitude larger when dust is taken into account in an intermediate
age population, irrespective of the particular mixture adopted. The dependence
of the integrated colors on the metallicity and age is discussed, with
particular emphasis on the problem of age-metallicity degeneracy. We show that,
contrary to the case of optical or near infrared colors, the adoption of a
suitable pass-band in the mid infrared allows a fair separation of the two
effects. We suggest intermediate redshift elliptical galaxies as possible
targets of this method of solving the age-metallicity dilemma. The new SSP
models constitute a first step in a more extended study aimed at modelling the
spectral properties of the galaxies from the ultraviolet to the far infrared.Comment: 16 pages, 10 figures, to appear in A&
The role of the synchrotron component in the mid infrared spectrum of M 87
We study in detail the mid-infrared Spitzer-IRS spectrum of M 87 in the range
5 to 20 micron. Thanks to the high sensitivity of our Spitzer-IRS spectra we
can disentangle the stellar and nuclear components of this active galaxy. To
this end we have properly subtracted from the M 87 spectrum, the contribution
of the underlying stellar continuum, derived from passive Virgo galaxies in our
sample. The residual is a clear power-law, without any additional thermal
component, with a zero point consistent with that obtained by high spatial
resolution, ground based observations. The residual is independent of the
adopted passive template. This indicates that the 10 micron silicate emission
shown in spectra of M 87 can be entirely accounted for by the underlying old
stellar population, leaving little room for a possible torus contribution. The
MIR power-law has a slope alpha ~ 0.77-0.82 (S),
consistent with optically thin synchrotron emission.Comment: 5 pages, 4 figures, accepted for publication in ApJ main journa
Calcium Triplet Synthesis
We present theoretical equivalent widths for the sum of the two strongest
lines of the Calcium Triplet, CaT index, in the near-IR, using evolutionary
techniques and the most recent models and observational data for this feature
in individual stars. We compute the CaT index for Single Stellar Populations
(instantaneous burst, standard Salpeter-type IMF) at four metallicities,
Z=0.004, 0.008, 0.02 (solar) and 0.05, and ranging in age from very young
bursts of star formation (few Myr) to old stellar populations, up to 17 gyr,
representative of globular clusters, elliptical galaxies and bulges of spirals.
The interpretation of the observed equivalent widths of CaT in different
stellar systems is discussed. Composite-population models are also computed as
a tool to interpret the CaT detections in star-forming regions, in order to
disantangle between the component due to Red Supergiants stars, RSG, and the
underlying, older, population. CaT is found to be an excellent
metallicity-indicator for populations older than 1 Gyr, practically independent
of the age. We discuss its application to remove the age- metallicity
degeneracy, characteristic of all studies of galaxy evolution based on the
usual integrated indices (both broad band colors and narrow band indices). The
application of the models computed here to the analysis of a sample of
elliptical galaxies will be discussed in a forthcoming paper (Gorgas et al.
1998).Comment: 17 pages, 7 figures, to be published in A&
Panchromatic models of galaxies: GRASIL
We present here a model for simulating the panchromatic spectral energy
distribution of galaxies, which aims to be a complete tool to study the complex
multi-wavelength picture of the universe. The model take into account all
important components that concur to the SED of galaxies at wavelengths from
X-rays to the radio. We review the modeling of each component and provide
several applications, interpreting observations of galaxy of different types at
all the wavelengths.Comment: 10 pages, 4 figures, invited talk, to appear in the proceedings of:
"The Spectral Energy Distribution of Gas-Rich Galaxies: Confronting Models
with Data", Heidelberg, 4-8 Oct. 2004, eds. C.C. Popescu and R.J. Tuffs, AIP
Conf. Ser., in pres
Star Formation History and Extinction in the central kpc of M82-like Starbursts
We report on the star formation histories and extinction in the central kpc
region of a sample of starburst galaxies that have similar far infrared (FIR),
10 micron and K-band luminosities as those of the archetype starburst M82. Our
study is based on new optical spectra and previously published K-band
photometric data, both sampling the same area around the nucleus. Model
starburst spectra were synthesized as a combination of stellar populations of
distinct ages formed over the Hubble time, and were fitted to the observed
optical spectra and K-band flux. The model is able to reproduce simultaneously
the equivalent widths of emission and absorption lines, the continuum fluxes
between 3500-7000 Ang, the K-band and the FIR flux. We require a minimum of 3
populations -- (1) a young population of age < 8 Myr, with its corresponding
nebular emission, (2) an intermediate-age population (age < 500 Myr), and (3)
an old population that forms part of the underlying disk or/and bulge
population. The contribution of the old population to the K-band luminosity
depends on the birthrate parameter and remains above 60% in the majority of the
sample galaxies. Even in the blue band, the intermediate age and old
populations contribute more than 40% of the total flux in all the cases. A
relatively high contribution from the old stars to the K-band nuclear flux is
also apparent from the strength of the 4000 Ang break and the CaII K line. The
extinction of the old population is found to be around half of that of the
young population. The contribution to the continuum from the relatively old
stars has the effect of diluting the emission equivalent widths below the
values expected for young bursts. The mean dilution factors are found to be 5
and 3 for the Halpha and Hbeta lines respectively.Comment: 20 pages, uses emulateapj.cls. Scheduled to appear in ApJ Jan 1, 200
Ultra-luminous X-ray sources and remnants of massive metal-poor stars
Massive metal-poor stars might form massive stellar black holes (BHs), with
mass 25<=mBH/Msun<=80, via direct collapse. We derive the number of massive BHs
(NBH) that are expected to form per galaxy through this mechanism. Such massive
BHs might power most of the observed ultra-luminous X-ray sources (ULXs). We
select a sample of 64 galaxies with X-ray coverage, measurements of the star
formation rate (SFR) and of the metallicity. We find that NBH correlates with
the number of observed ULXs per galaxy (NULX) in this sample. We discuss the
dependence of our model on the SFR and on the metallicity. The SFR is found to
be crucial, consistently with previous studies. The metallicity plays a role in
our model, since a lower metallicity enhances the formation of massive BHs.
Consistently with our model, the data indicate that there might be an
anticorrelation between NULX, normalized to the SFR, and the metallicity. A
larger and more homogeneous sample of metallicity measurements is required, in
order to confirm our results.Comment: 21 pages, 8 figures, accepted for publication in MNRA
SBV Regularity for Genuinely Nonlinear, Strictly Hyperbolic Systems of Conservation Laws in one space dimension
We prove that if is the entropy
solution to a strictly hyperbolic system of conservation laws with
genuinely nonlinear characteristic fields then up to a
countable set of times the function is in
, i.e. its distributional derivative is a measure with no
Cantorian part.
The proof is based on the decomposition of into waves belonging to
the characteristic families and the balance
of the continuous/jump part of the measures in regions bounded by
characteristics. To this aim, a new interaction measure \mu_{i,\jump} is
introduced, controlling the creation of atoms in the measure .
The main argument of the proof is that for all where the Cantorian part
of is not 0, either the Glimm functional has a downward jump, or there is
a cancellation of waves or the measure is positive
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