116 research outputs found
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
Feedback in the local LBG Analog Haro 11 as probed by far-UV and X-ray observations
We have re-analyzed FUSE data and obtained new Chandra observations of Haro
11, a local (D_L=88 Mpc) UV luminous galaxy. Haro 11 has a similar far-UV
luminosity (10^10.3 L_\odot), UV surface brightness (10^9.4 L_\odot kpc^-2),
SFR, and metallicity to that observed in Lyman Break Galaxies (LBGs). We show
that Haro 11 has extended, soft thermal (kT~0.68 keV) X-ray emission with a
luminosity and size which scales with the physical properties (e.g. SFR,
stellar mass) of the host galaxy. An enhanced alpha/Fe, ratio of ~4 relative to
solar abundance suggests significant supernovae enrichment. These results are
consistent with the X-ray emission being produced in a shock between a
supernovae driven outflow and the ambient material. The FUV spectra show strong
absorption lines similar to those observed in LBG spectra. A blueshifted
absorption component is identified as a wind outflowing at ~200-280 km/s.
OVI\lambda\lambda1032,1038 emission, the dominant cooling mechanism for coronal
gas at T~10^5.5 K is also observed. If associated with the outflow, the
luminosity of the OVI emission suggests that <20% of the total mechanical
energy from the supernovae and solar winds is being radiated away. This implies
that radiative cooling through OVI is not significantly inhibiting the growth
of the outflowing gas. In contradiction to the findings of Bergvall et al 2006,
we find no convincing evidence of Lyman continuum leakage in Haro 11. We
conclude that the wind has not created a `tunnel' allowing the escape of a
significant fraction of Lyman continuum photons and place a limit on the escape
fraction of f_{esc}<2%. Overall, both Haro 11 and a previously observed LBG
analogue VV 114, provide an invaluable insight into the X-ray and FUV
properties of high redshift LBGs.Comment: Accepted for publication in ApJ, 40 pages, 17 figure
Mid Infrared Properties of Low Metallicity Blue Compact Dwarf Galaxies From Spitzer/IRS
We present a {\em Spitzer}-based mid-infrared study of a large sample of Blue
Compact Dwarf galaxies (BCD) using the Infrared Spectrograph (IRS), including
the first mid-IR spectrum of IZw18, the archetype for the BCD class and among
the most metal poor galaxies known. We show the spectra of Polycyclic Aromatic
Hydrocarbon (PAH) emission in low-metallicity environment. We find that the
equivalent widths (EW) of PAHs at 6.2, 7.7, 8.6 and 11.2 m are generally
weaker in BCDs than in typical starburst galaxies and that the fine structure
line ratio, [NeIII]/[NeII], has a weak anti-correlation with the PAH EW. A much
stronger anti-correlation is shown between the PAH EW and the product of the
[NeIII]/[NeII] ratio and the UV luminosity density divided by the metallicity.
We conclude that PAH EW in metal-poor high-excitation environments is
determined by a combination of PAH formation and destruction effects.Comment: 41 pages, 14 figure
The dark nature of GRB 051022 and its host galaxy
We present multiwavelength (X-ray/optical/near-infrared/millimetre)
observations of GRB 051022 between 2.5 hours and ~1.15 yr after the event. It
is the most intense gamma-ray burst (~ 10^-4 erg cm^-2) detected by HETE-2,
with the exception of the nearby GRB 030329. Optical and near infrared
observations did not detect the afterglow despite a strong afterglow at X-ray
wavelengths. Millimetre observations at Plateau de Bure (PdB) detected a source
and a flare, confirming the association of this event with a moderately bright
(R = 21.5) galaxy. Spectroscopic observations of this galaxy show strong [O
II], Hbeta and [O III] emission lines at a redshift of 0.809. The spectral
energy distribution of the galaxy implies Av (rest frame) = 1.0 and a starburst
occuring ~ 25 Myr ago, during which the star-forming-rate reached >= 25
Msun/yr. In conjunction with the spatial extent (~ 1'') it suggests a very
luminous (Mv = - 21.8) blue compact galaxy, for which we also find with Z Zsun.
The X-ray spectrum shows evidence of considerable absorption by neutral gas
with NH, X-ray = 3.47(+0.48/-0.47) x 10^22 cm^-2 (rest frame). Absorption by
dust in the host galaxy at z = 0.809 certainly cannot account for the
non-detection of the optical afterglow, unless the dust-to-gas ratio is quite
different than that seen in our Galaxy (i.e. large dust grains). It is likely
that the afterglow of the dark GRB 051022 was extinguished along the line of
sight by an obscured, dense star forming region in a molecular cloud within the
parent host galaxy. This galaxy is different from most GRB hosts being brighter
than L* by a factor of 3. We have also derived a SFR ~ 50 Msun/yr and predict
that this host galaxy will be detected at sub-mm wavelengths.Comment: 7 Pages, 7 figures. Accepted in Astronomy and Astrophysic
Star forming dwarf galaxies
Star forming dwarf galaxies (SFDGs) have a high gas content and low
metallicities, reminiscent of the basic entities in hierarchical galaxy
formation scenarios. In the young universe they probably also played a major
role in the cosmic reionization. Their abundant presence in the local volume
and their youthful character make them ideal objects for detailed studies of
the initial stellar mass function (IMF), fundamental star formation processes
and its feedback to the interstellar medium. Occasionally we witness SFDGs
involved in extreme starbursts, giving rise to strongly elevated production of
super star clusters and global superwinds, mechanisms yet to be explored in
more detail. SFDGs is the initial state of all dwarf galaxies and the relation
to the environment provides us with a key to how different types of dwarf
galaxies are emerging. In this review we will put the emphasis on the exotic
starburst phase, as it seems less important for present day galaxy evolution
but perhaps fundamental in the initial phase of galaxy formation.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy
Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon,
September 2010, Springer Verlag, in pres
HS 2134+0400 - new very metal-poor galaxy, a representative of void population?
We present the SAO 6m telescope spectroscopy of a blue compact galaxy (BCG)
HS 2134+0400 discovered in frame of the dedicated Hamburg/SAO survey for Low
Metallicity BCGs (HSS-LM). Its very low abundance of oxygen (12+log(O/H) =
7.44), as well as other heavy elements (S, N, Ne, Ar), assigns this dwarf
galaxy to the group of BCGs with the lowest metal content. There are only eight
that low metallicity among several thousand known BCGs in the nearby Universe.
The abundance ratios for the heavy elements (S/O, Ne/O, N/O, and Ar/O) are well
consistent with the typical values of other very metal-poor BCGs. The global
environment of HS 2134+0400 is atypical of the majority of BCGs. The object
falls within the Pegasus void, the large volume with the very low density of
galaxies with the normal (M_B* = -19.6) or high luminosity. Since we found in
voids a dozen more the very metal-poor galaxies, we discuss the hypothesis that
such objects can be representative of a substantial fraction of the void dwarf
galaxy population.Comment: 6 pages, including 2 tables and 2 postscript figures. Submitted to
Astronomy Letter
The Extraordinary `Superthin' Spiral Galaxy UGC7321. I. Disk Color Gradients and Global Properties from Multiwavelength Observations
We present B- and R-band imaging and photometry, H-alpha narrow-band imaging,
NIR H-band imaging, and HI 21-cm spectroscopy of the nearby Sd spiral galaxy
UGC7321. UGC7321 exhibits a remarkably thin stellar disk with no bulge
component. The galaxy has a very diffuse, low surface brightness disk, which
appears to suffer little internal extinction in spite of its edge-on geometry.
The UGC7321 disk shows significant B-R color gradients in both the radial and
vertical directions. These color gradients cannot be explained solely by dust
and are indicative of changes in the mix of stellar ages and/or metallicity as
a function of both radius and height above the galaxy plane. The outer regions
of the UGC7321 disk are too blue to be explained by low metallicity alone
(B-R<0.6), and must be relatively young. However, the galaxy also contains
stellar populations with B-R>1.1, indicating it is not a young or
recently-formed galaxy. The disk of UGC7321 is not a simple exponential, but
exhibits a light excess at small radii, as well as distinct surface brightness
zones. Together the properties of UGC7321 imply that it is an under-evolved
galaxy in both a dynamical and in a star-formation sense. (Abridged)Comment: Accepted to the Astronomical Journal; 28 pages, 1 table and 21
figures (GIF and postscript
The Hamburg/SAO survey for low metallicity blue compact/HII-galaxies (HSS-LM). I. The first list of 46 strong-lined galaxies
We present the description and the first results of a new project devoted to
the search for extremely metal-deficient blue compact/HII-galaxies (BCGs) and
to the creation of a well selected large BCG sample with strong emission lines.
Such galaxies should be suitable for reliable determination of their oxygen
abundance through the measurement of the faint [OIII]4363A line. The goals of
the project are two-fold: a) to discover a significant number of new extremely
metal-poor galaxies (Z <= 1/20 Zo), and b) to study the metallicity
distribution of local BCGs. Selection of candidates for follow-up slit
spectroscopy is performed on the database of objective prism spectra of the
Hamburg Quasar Survey. The sky region is limited by delta >= 0 deg. and b^ii <=
-30 deg. In this paper we present the results of the follow-up spectroscopy
conducted with the Russian 6m telescope. The list of observed candidates
contained 52 objects, of which 46 were confirmed as strong-lined BCGs
(EW([OIII]5007) >= 100 A). The remaining five lower excitation ELGs include
three BCGs, and two galaxies classified as SBN (Starburst Nucleus) and DANS
(Dwarf Amorphous Nucleus Starburst). One object is identified as a quasar with
a strong Ly_alpha emission line near 5000 A (z~3). We provide a list with
coordinates, measured radial velocities, B-magnitudes, equivalent widths
EW([OIII]5007) and EW(H_beta) and for the 46 strong-lined BCGs the derived
oxygen abundances 12+log(O/H). The abundances range between 7.42 and 8.4
(corresponding to metallicities between 1/30 and 1/3 Zo). The sample contains
four galaxies with Z < 1/20 Zo, of which three are new discoveries.Comment: 19 pages, 13 figures, accepted for publication in A&A, corrected
typos, reference
The GalMer database: Galaxy Mergers in the Virtual Observatory
We present the GalMer database, a library of galaxy merger simulations, made
available to users through tools compatible with the Virtual Observatory (VO)
standards adapted specially for this theoretical database. To investigate the
physics of galaxy formation through hierarchical merging, it is necessary to
simulate galaxy interactions varying a large number of parameters:
morphological types, mass ratios, orbital configurations, etc. On one side,
these simulations have to be run in a cosmological context, able to provide a
large number of galaxy pairs, with boundary conditions given by the large-scale
simulations, on the other side the resolution has to be high enough at galaxy
scales, to provide realistic physics. The GalMer database is a library of
thousands simulations of galaxy mergers at moderate spatial resolution and it
is a compromise between the diversity of initial conditions and the details of
underlying physics. We provide all coordinates and data of simulated particles
in FITS binary tables. The main advantages of the database are VO access
interfaces and value-added services which allow users to compare the results of
the simulations directly to observations: stellar population modelling, dust
extinction, spectra, images, visualisation using dedicated VO tools. The GalMer
value-added services can be used as virtual telescope producing broadband
images, 1D spectra, 3D spectral datacubes, thus making our database oriented
towards the usage by observers. We present several examples of the GalMer
database scientific usage obtained from the analysis of simulations and
modelling their stellar population properties, including: (1) studies of the
star formation efficiency in interactions; (2) creation of old counter-rotating
components; (3) reshaping metallicity profiles in elliptical galaxies; (4)
orbital to internal angular momentum transfer; (5) reproducing observed colour
bimodality of galaxies.Comment: 15 pages, 11 figures, 10 tables accepted to A&A. Visualisation of
GalMer simulations, access to snapshot files and value-added tools described
in the paper are available at http://galmer.obspm.fr
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