1,174 research outputs found
On the relation between sSFR and metallicity
In this paper we present an exact general analytic expression
linking the gas metallicity Z to the specific
star formation rate (sSFR), that validates and extends the approximate relation
put forward by Lilly et al. (2013, L13), where is the yield per stellar
generation, is the instantaneous ratio between inflow and star
formation rate expressed as a function of the sSFR, and is the integral of
the past enrichment history, respectively. We then demonstrate that the
instantaneous metallicity of a self-regulating system, such that its sSFR
decreases with decreasing redshift, can be well approximated by the first term
on the right-hand side in the above formula, which provides an upper bound to
the metallicity. The metallicity is well approximated also by the L13 ideal
regulator case, which provides a lower bound to the actual metallicity. We
compare these approximate analytic formulae to numerical results and infer a
discrepancy <0.1 dex in a range of metallicities and almost three orders of
magnitude in the sSFR. We explore the consequences of the L13 model on the
mass-weighted metallicity in the stellar component of the galaxies. We find
that the stellar average metallicity lags 0.1-0.2 dex behind the gas-phase
metallicity relation, in agreement with the data. (abridged)Comment: 14 pages, 6 figures, MNRAS accepte
Model atmosphere analysis of the extreme DQ white dwarf GSC2U J131147.2+292348
A new model atmosphere analysis for the peculiar DQ white dwarf discovered by
Carollo et al. (2002) is presented. The effective temperature and carbon
abundance have been estimated by fitting both the photometric data
(UBJ,VRF,IN,JHK) and a low resolution spectrum (3500<lambda<7500 A) with a new
model grid for helium-rich white dwarfs with traces of carbon (DQ stars). We
estimate Teff ~ 5120 +/- 200 K and log[C/He] ~ -5.8 +/- 0.5, which make GSC2U
J131147.2+292348 the coolest DQ star ever observed. This result indicates that
the hypothetical transition from C2 to C2H molecules around Teff = 6000 K,
which was inferred to explain the absence of DQ stars at lower temperatures,
needs to be reconsidered.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and
Astrophysics Letter
The Zurich Environmental Study (ZENS) of galaxies in groups along the cosmic web. V. properties and frequency of merging satellites and centrals in different environments
We use the Zurich ENvironmental Study (ZENS) database to investigate the
environmental dependence of the merger fraction and merging galaxy
properties in a sample of ~1300 group galaxies with and
0.05<z<0.0585. In all galaxy mass bins investigated in our study, we find that
decreases by a factor of ~2-3 in groups with halo masses
relative to less massive systems, indicating a
suppression of merger activity in large potential wells. In the fiducial case
of relaxed groups only, we measure a variation dex, which is almost independent of galaxy mass
and merger stage. At galaxy masses , most mergers are dry
accretions of quenched satellites onto quenched centrals, leading to a strong
increase of with decreasing group-centric distance at these mass
scales.Both satellite and central galaxies in these high mass mergers do not
differ in color and structural properties from a control sample of nonmerging
galaxies of equal mass and rank. At galaxy masses , where
we mostly probe satellite-satellite pairs and mergers between star-forming
systems, close pairs (projected distance kpc) show instead
enhanced (specific) star formation rates and
larger sizes than similar mass, nonmerging satellites. The increase in both
size and SFR leads to similar surface star-formation densities in the merging
and control-sample satellite populations.Comment: Published in ApJ, 797, 12
Oxygen Gas Abundances at 0.4<z<1.5: Implications for the Chemical Evolution History of Galaxies
We report VLT-ISAAC and Keck-NIRSPEC near-infrared spectroscopy for a sample
of 30 0.47<z<0.92 CFRS galaxies and five [OII]-selected, M_B,AB<-21.5, z~1.4
galaxies. We have measured Halpha and [NII] line fluxes for the CFRS galaxies
which have [OII], Hbeta and [OIII] line fluxes available from optical
spectroscopy. For the z~1.4 objects we measured Hbeta and [OIII] emission line
fluxes from J-band spectra, and Halpha line fluxes plus upper limits for [NII]
fluxes from H-band spectra. We derive the extinction and oxygen abundances for
the sample using a method based on a set of ionisation parameter and oxygen
abundance diagnostics, simultaneously fitting the [OII], Hbeta, [OIII], Halpha
and [NII] line fluxes. Our most salient conclusions are: a) the source of gas
ionisation in the 30 CFRS and in all z~1.4 galaxies is not due to AGN activity;
b) about one third of the 0.47<z<0.92 CFRS galaxies in our sample have
substantially lower metallicities than local galaxies with similar luminosities
and star formation rates; c) comparison with a chemical evolution model
indicates that these low metallicity galaxies are unlikely to be the
progenitors of metal-poor dwarf galaxies at z~0, but more likely the
progenitors of massive spirals; d) the z~1.4 galaxies are characterized by the
high [OIII]/[OII] line ratios, low extinction and low metallicity that are
typical of lower luminosity CADIS galaxies at 0.4<z<0.7, and of more luminous
Lyman Break Galaxies at z~3.1, but not seen in CFRS galaxies at 0.4<z<1.0; e)
the properties of the z~1.4 galaxies suggest that the period of rapid chemical
evolution takes place progressively in lower mass systems as the universe ages,
and thus provides further support for a downsizing picture of galaxy formation,
at least from z~1.4 to today.Comment: Proceedings contribution for "The Fabulous Destiny of Galaxies;
Bridging Past and Present", Marseille, 200
A parsimonious model for generating arbitrage-free scenario trees
Simulation models of economic, financial and business risk factors are widely used to assess risks and support decision-making. Extensive literature on scenario generation methods aims at describing some underlying stochastic processes with the least number of scenarios to overcome the âcurse of dimensionalityâ. There is, however, an important requirement that is usually overlooked when one departs from the application domain of security pricing: the no-arbitrage condition. We formulate a moment matching model to generate multi-factor scenario trees for stochastic optimization satisfying no-arbitrage restrictions with a minimal number of scenarios and without any distributional assumptions. The resulting global optimization problem is quite general. However, it is non-convex and can grow significantly with the number of risk factors, and we develop convex lower bounding techniques for its solution exploiting the special structure of the problem. Applications to some standard problems from the literature show that this is a robust approach for tree generation. We use it to price a European basket option in complete and incomplete markets
WFPC2 Observations of NGC 454: an Interacting Pair of Galaxies
We present WFPC2 images in the F450W, F606W and F814W filters of the
interacting pair of galaxies NGC 454. Our data indicate that the system is in
the early stages of interaction. A population of young star-clusters has formed
around the late component, and substantial amounts of gas have sunk into the
center of the earlier component, where it has not yet produced significant
visible star formation or nuclear activity. We have photometric evidence that
the star-clusters have strong line emission, which indicate the presence of a
substantial component of hot, massive stars which formed less than 5-10 Myrs
ago.Comment: 14 pages, 4 figures, Latex (AAS macros), ApJL in pres
Spin-1/2 geometric phase driven by decohering quantum fields
We calculate the geometric phase of a spin-1/2 system driven by a one and two
mode quantum field subject to decoherence. Using the quantum jump approach, we
show that the corrections to the phase in the no-jump trajectory are different
when considering an adiabatic and non-adiabatic evolution. We discuss the
implications of our results from both the fundamental as well as quantum
computational perspective.Comment: 4 page
On the relation between specific star formation rate and metallicity
In this paper, we present an exact general analytic expression linking the gas metallicity Z to the specific star formation rate (sSFR), which validates and extends the approximate relation put forward by Lilly etal. (L13), where yz is the yield per stellar generation, Î(sSFR) is the instantaneous ratio between inflow and star formation rate expressed as a function of the sSFR and I is the integral of the past enrichment history, respectively. We then demonstrate that the instantaneous metallicity of a self-regulating system, such that its sSFR decreases with decreasing redshift, can be well approximated by the first term on the right-hand side in the above formula, which provides an upper bound to the metallicity. The metallicity is well approximated also by (L13 ideal regulator case), which provides a lower bound to the actual metallicity. We compare these approximate analytic formulae to numerical results and infer a discrepancy <0.1 dex in a range of metallicities (, for yz ⥠Zâ=0.02) and almost three orders of magnitude in the sSFR. We explore the consequences of the L13 model on the mass-weighted metallicity in the stellar component of the galaxies. We find that the stellar average metallicity lags âŒ0.1-0.2 dex behind the gas-phase-metallicity relation, in agreement with the dat
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