260 research outputs found
Massive (?) starburst hosts of blue compact galaxies (BCGs)
We present optical spectroscopy and deep optical/near-IR photometry of 4
luminous metal-poor blue compact galaxies (BCGs) and two of their companions.
With the aid of spectral evolutionary models (SEMs) and structural parameters
derived from the surface photometry we discuss the properties of the central
starbursts and the halo populations of the galaxies. Special attention is paid
to the effects of dust, chemical inhomogeneities and contamination of nebular
emission to the halo light. The optical/near-IR colour index profiles show a
sharp distinction between the starburst and the host. The hosts have luminosity
profiles characteristic of massive ellipticals and remarkably red colours,
typical of a relatively {\it metal-rich} stellar population of {\it old age}.
These properties are in conflict with the relatively low luminosities. The
situation can best be explained if the hosts have an unusually large amount of
dark matter that can hinder the outflow of metals from the system. The
indicated difference in metallicity between the halo and the young starburst
disproves the recurrent burst scenario and supports different origins of the
two populations. We conclude that these BCGs are undergoing mergers between
early type galaxies/thick disks and gas-rich galaxies or intergalactic HI
clouds, in many respects reminiscent of a retarded formation of massive
ellipticals.Comment: 30 pages, 21 figures, accepted for publication in Astronomy and
Astrophysic
The dark matter halos of the bluest low surface brightness galaxies
We present BVI photometry and long-slit Halpha rotation curve data obtained
with ESO VLT/FORS2 for six low surface brightness galaxies with extremely blue
colours and very faint central regions. We find no evidence for a steep central
density cusp of the type predicted by many N-body simulations of cold dark
matter (CDM) halos. Our observations are instead consistent with dark matter
halos characterized by cores of roughly constant density, in agreement with
previous investigations. While unremarkable in terms of the central density
slope, these galaxies appear very challenging for existing CDM halo models in
terms of average central halo density, as measured by the Delta_(V/2)
parameter. Since most of our target galaxies are bulgeless disks, our
observations also disfavour a recently suggested mechanism for lowering the
central mass concentration of the halo by means of a fast collapse phase, as
this scenario predicts that the original CDM profile should still be detectable
in bulgeless galaxies. Other potential ways of reconciling the CDM predictions
with these observations are discussed.Comment: 14 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Dynamics of blue compact galaxies, as revealed by their H-alpha velocity fields I. The data, velocity fields and rotation curves
Observations of six luminous blue compact galaxies (BCGs) and two star
forming companion galaxies were carried out with the CIGALE scanning
Fabry-Perot interferometer attached to the ESO 3.6m telescope on La Silla. The
observations were made in the H-alpha emission line which is prominent in BCGs.
A velocity sampling of 5 km/s and a pixel size of 0.9 arcseconds were used. In
this paper we present the observations and the data together with the velocity
fields and the derived rotation curves. In addition we provide rough estimates
of the total dynamical mass and of the ionised gas mass for each galaxy. All
galaxies display rotation, but while the companion galaxies have regular
velocity fields, those of the BCGs are complex and appear perturbed. This is
the most extensive study to date of the optical velocity fields of BCGs. The
interpretation of these results will be presented in a forthcoming paper (Paper
II).Comment: 26 pages, 14 figures. Accepted for publication in A&AS. The paper
(with figures in slightly higher resolution) and an electronic table is also
available at ftp://ftp.iap.fr/pub/from_users/ostlin/Articles/ . Replaced
version, figure captions fixe
Non-Makovian decoherence of a two-level system weakly coupled to a bosonic bath
Bloch-Redfield equation is a common tool for studying evolution of qubit
systems weakly coupled to environment. We investigate the accuracy of the Born
approximation underlying this equation. We find that the high order terms in
the perturbative expansion contain accumulating divergences that make
straightforward Born approximation inappropriate. We develop diagrammatic
technique to formulate, and solve the improved self-consistent Born
approximation. This more accurate treatment reveals an exponential time
dependent prefactor in the non-Markovian contribution dominating the qubit
long-time relaxation found in Phys. Rev. B 71, 035318 (2005). At the same time,
the associated dephasing is not affected and is described by the Born-Markov
approximation.Comment: To appear in EuroPhys. Let
Constraining Dark Matter with the Long-Term Variability of Quasars
By comparing the results of numerical microlensing simulations to the
observed long-term variability of quasars, strong upper limits on the
cosmological density of compact objects in the mass range 0.01 to 0.0001 solar
masses may be imposed. Using recently developed methods to better approximate
the amplification of large sources, we investigate in what way the constraints
are affected by assumptions concerning the size of the optical
continuum-emitting region of quasars in the currently favored (Omega_M=0.3,
Omega_Lambda=0.7) cosmology.Comment: 3 pages, 2 figures, to appear in the proceedings from the conference
"Lighthouses of the Universe: The Most Luminous Celestial Objects and Their
Use for Cosmology", Garching, August 2001 (Springer-Verlag
The impact of nebular emission on the broadband fluxes of high-redshift galaxies
A substantial fraction of the light emitted from young or star-forming
galaxies at ultraviolet to near-infrared wavelengths comes from the ionized
interstellar medium in the form of emission lines and a nebular continuum. At
high redshifts, star formation rates are on average higher and stellar
populations younger than in the local Universe. Both of these effects act to
boost the impact of nebular emission on the overall spectrum of galaxies. Even
so, the broadband fluxes and colours of high-redshift galaxies are routinely
analyzed under the assumption that the light observed originates directly from
stars. Here, we assess the impact of nebular emission on broadband fluxes in
Johnson/Cousins BVRIJHK, Sloan Digital Sky Survey griz and Spitzer IRAC/MIPS
filters as a function of observed redshift (up to z=15) for galaxies with
different star formation histories. We find that nebular emission may account
for a non-negligible fraction of the light received from high-redshift
galaxies. The ages and masses inferred for such objects through the use of
spectral evolutionary models that omit the nebular contribution are therefore
likely to contain systematic errors. We argue that a careful treatment of the
nebular component will be essential for the interpretation of the rest-frame
ultraviolet-to-infrared properties of the first galaxies formed, like the ones
expected to be detected with the James Webb Space Telescope.Comment: 5 pages, 3 figures, accepted for publication in ApJ
The temporal and spatial evolution of the starburst in ESO 338-IG04 as probed by its star clusters
In this paper we use ultra-violet (UV) and optical HST photometry in five
bands, and an extensive set of spectral evolutionary synthesis scenarios to
investigate the age and masses of 124 star clusters in the luminous blue
compact galaxy ESO338-IG04 (Tololo 1924-416). The very small internal reddening
makes ESO 338-IG04 an excellent laboratory for studying the formation of
massive star clusters. We have used the star clusters to trace the temporal and
spatial evolution of the starburst, and to put constraints on the star
formation activity over a cosmological time-scale. The present starburst has
been active for about 40 Myr. A standard Salpeter initial mass function (IMF)
extending up to 120 solar masses provides the best fit to the data, although a
flatter IMF cannot be excluded. The compact star clusters provide 30-40 percent
of the UV luminosity and star formation activity. We find no evidence for dust
obscuration even among the youngest (< 1 Myr) clusters. The fraction of stellar
mass contained in compact star clusters is found to be several percent, which
is an unusually high value. The intermediate age clusters show a flattened
space distribution which agrees with the isophotal shape of the galaxy, whereas
the oldest clusters seem to have a spherical distribution.(abridged)Comment: Accepted for publication in A&
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