2,385 research outputs found
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
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
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
Infinite reflections of shock fronts in driven diffusive systems with two species
Interaction of a domain wall with boundaries of a system is studied for a
class of stochastic driven particle models. Reflection maps are introduced for
the description of this process. We show that, generically, a domain wall
reflects infinitely many times from the boundaries before a stationary state
can be reached. This is in an evident contrast with one-species models where
the stationary density is attained after just one reflection.Comment: 11 pages, 8 eps figs, to appearin JPhysA 01.200
From First Galaxies to QSOs: feeding the baby monsters
We present a physical model for the coevolution of massive spheroidal
galaxies and active nuclei at their centers. Supernova heating is increasingly
effective in slowing down the star formation and in driving gas outflows in
smaller and smaller dark matter halos. Thus the more massive protogalaxies
virializing at early times are the sites of faster star formation. The
correspondingly higher radiation drag causes a faster angular momentum loss by
the gas and induces a larger accretion rate onto the central black hole. In
turn, the kinetic energy of the outflows powered by the active nuclei can
unbind the residual gas in a time shorter for larger halos. The model accounts
for a broad variety of dynamical, photometric and metallicity properties of
early-type galaxies, for the M_BH -- \sigma relation and for the local
supermassive black-hole mass function.Comment: 6 pages, contributed paper to Proceedings of the Conference on
"Growing Black Holes" held in Garching, Germany, on June 21-25, 2004, edited
by A. Merloni, S. Nayakshin and R. Sunyaev, Springer-Verlag series of "ESO
Astrophysics Symposia
Low Star Formation Rates for z=1 Early-Type Galaxies in the Very Deep GOODS-MIPS Imaging: Implications for their Optical/Near-Infrared Spectral Energy Distributions
We measure the obscured star formation in z~1 early-type galaxies. This
constrains the influence of star formation on their optical/near-IR colors,
which, we found, are redder than predicted by the model by Bruzual & Charlot
(2003). From deep ACS imaging we construct a sample of 95 morphologically
selected early-type galaxies in the HDF-N and CDF-S with spectroscopic
redshifts in the range 0.85<z<1.15. We measure their 24 micron fluxes from the
deep GOODS-MIPS imaging and derive the IR luminosities and star formation
rates. The fraction of galaxies with >2 sigma detections (~25 muJy} is
17(-4,+9)%. Of the 15 galaxies with significant detections at least six have an
AGN. Stacking the MIPS images of the galaxies without significant detections
and adding the detected galaxies without AGN we find an upper limit on the mean
star formation rate (SFR) of 5.2+/-3.0 Msol yr^-1, and on the mean specific SFR
of 4.6+/-2.2 * 10^-11 yr^-1. Under the assumption that the average SFR will
decline at the same rate as the cosmic average, the in situ growth in stellar
mass of the early-type galaxy population is less than 14+/-7% between z=1 and
the present. We show that the typically low IR luminosity and SFR imply that
the effect of obscured star formation (or AGN) on their rest-frame
optical/near-IR SEDs is negligible for ~90% of the galaxies in our sample.
Hence, their optical/near-IR colors are most likely dominated by evolved
stellar populations. This implies that the colors predicted by the Bruzual &
Charlot (2003) model for stellar populations with ages similar to those of z~1
early-type galaxies (~1-3 Gyr) are most likely too blue, and that stellar
masses of evolved, high-redshift galaxies can be overestimated by up to a
factor of ~2.Comment: Accepted for publication in ApJ, 8 pages, 4 figures, 1 tabl
On the Demand for Meat in Brazil
We used alternative specifications of the Almost Ideal Demand System to estimate the aggregate demand for beef, pork, chicken, and other consumption goods and their elasticities in Brazil. We detected the need for using time trend variables in models’ equations so that an upward trend for each meat demand and a downward trend for other consumption goods were found. The dummy variable for the prices stabilization macroeconomic Real Plan indicated it has not affected demands. According to Marshallian own-price elasticities, demands for meats are inelastic and the demand for other consumption goods is elastic. Cross-price Marshallian and Hicksian elasticities confirm beef, pork and chicken are substitutes one to each other. Expenditure elasticities showed all goods are normal, except for pork which is an inferior good. As personal consumption expenditure increases over time, ceteris paribus, meat consumption will lose importance to other consumption goods, beef consumption will lose importance to chicken, and pork consumption will lose importance to the other types of meat.Meat demand; Almost Ideal Demand System; elasticities; Brazil
CAVITATION EROSION WEAR OF METALLIC SPECIMENS USING THE NEW COMPACT ROTATING DISK DEVICE
A more compact version of the rotating disk device (where a disk with cavitation inducers and specimens fixed on it rotates in water to provide cavitating flow) is used here to study the failure by cavitation in metallic specimens. The damage in the specimens is measured by mass loss versus exposition time to cavitating flow diagrams. Images of the test specimens obtained by electronic microscope are also shown and discussed. After 25 hours working in cavitating conditions, aluminum, brass and cast iron are weared by the cavitation phenomenon, resulting in pitting formation and mass loss. After each 5 hours operating in cavitating conditions, the specimens are cleaned by ultrasound, dried and weighted in a digital balance to obtain the mass loss in the process by comparing to its initial weight. All specimens, aluminum, brass and cast iron, were eroded by cavitation although in the aluminum the mass loss was lesser than the expected. The brass specimen was eroded as expected, mas no mass loss could be attained for the cast iron due to specimen oxidation
Cepheid Mass-loss and the Pulsation -- Evolutionary Mass Discrepancy
I investigate the discrepancy between the evolution and pulsation masses for
Cepheid variables. A number of recent works have proposed that non-canonical
mass-loss can account for the mass discrepancy. This mass-loss would be such
that a 5Mo star loses approximately 20% of its mass by arriving at the Cepheid
instability strip; a 14Mo star, none. Such findings would pose a serious
challenge to our understanding of mass-loss. I revisit these results in light
of the Padova stellar evolutionary models and find evolutionary masses are
()% greater than pulsation masses for Cepheids between 5<M/Mo<14. I
find that mild internal mixing in the main-sequence progenitor of the Cepheid
are able to account for this mass discrepancy.Comment: 15 pages, 3 figures, ApJ accepte
- …