283 research outputs found
The role of tidal interactions in driving galaxy evolution
We carry out a statistical analysis of galaxy pairs selected from chemical
hydrodynamical simulations with the aim at assessing the capability of
hierarchical scenarios to reproduce recent observational results for galaxies
in pairs. Particularly, we analyse the effects of mergers and interactions on
the star formation (SF) activity, the global mean chemical properties and the
colour distribution of interacting galaxies. We also assess the effects of
spurious pairs.Comment: to appear in "Groups of galaxies in the nearby Universe" ESO
Workshop, (Dec 2005) Santiago, Chil
Measuring the Cosmic Equation of State with Counts of Galaxies II: Error Budget for the DEEP2 Redshift Survey
In a previous paper, we described a new variant on the classical dN/dz test
which could be performed using upcoming redshift surveys. By observing the
velocity function of galaxies rather than their luminosity function, it is
possible to avoid many of the uncertainties of galaxy evolution while employing
a directly measurable quantity. We previously assumed that counting statistics
would dominate the errors in this method. Here, we present the results of
including cosmic variance and determine the impact of systematic effects on
application of this test to the upcoming DEEP2 Redshift Survey. For DEEP2,
cosmic variance should yield errors roughly twice those predicted from Poisson
statistics. Through Monte Carlo simulations we find that if the functional
form, but not the strength, of any of the major systematic effects (baryonic
infall, velocity errors, and incompleteness) is known, the systematic may
successfully be corrected for using the observed velocity function, leaving a
much smaller residual error. The total uncertainty from systematics is
comparable to that from cosmic variance, but correlated amongst redshift bins.
We find that DEEP2 can provide a much more precise measurement of the cosmic
equation of state parameter w than any available today, albeit weaker than some
other proposed tests.Comment: 10 pages plus 11 figures and 2 tables; approved for publication in
the Astrophysical Journa
Confinement of supernova explosions in a collapsing cloud
We analyze the confining effect of cloud collapse on an expanding supernova
shockfront. We solve the differential equation for the forces on the shockfront
due to ram pressure, supernova energy, and gravity. We find that the expansion
of the shockfront is slowed and in fact reversed by the collapsing cloud.
Including radiative losses and a potential time lag between supernova explosion
and cloud collapse shows that the expansion is reversed at smaller distances as
compared to the non-radiative case. We also consider the case of multiple
supernova explosions at the center of a collapsing cloud. For instance, if we
scale our self-similar solution to a single supernova of energy 10^51 ergs
occurring when a cloud of initial density 10^2 H/cm^3 has collapsed by 50%, we
find that the shockfront is confined to ~15 pc in ~1 Myrs. Our calculations are
pertinent to the observed unusually compact non-thermal radio emission in blue
compact dwarf galaxies (BCDs). More generally, we demonstrate the potential of
a collapsing cloud to confine supernovae, thereby explaining how dwarf galaxies
would exist beyond their first generation of star formation.Comment: 3 pages, 4 figure
First orbital solution for the non-thermal emitter Cyg OB2 #9
After the first detection of its binary nature, the spectroscopic monitoring
of the non-thermal radio emitter Cyg OB2 #9 (P=2.4yrs) has continued, doubling
the number of available spectra of the star. Since the discovery paper of 2008,
a second periastron passage has occurred in February 2009. Using a variety of
techniques, the radial velocities could be estimated and a first, preliminary
orbital solution was derived from the HeI5876 line. The mass ratio appears
close to unity and the eccentricity is large, 0.7--0.75. X-ray data from 2004
and 2007 are also analyzed in quest of peculiarities linked to binarity. The
observations reveal no large overluminosity nor strong hardness, but it must be
noted that the high-energy data were taken after the periastron passage, at a
time where colliding wind emission may be low. Some unusual X-ray variability
is however detected, with a 10% flux decrease between 2004 and 2007. To clarify
their origin and find a more obvious signature of the wind-wind collision,
additional data, taken at periastron and close to it, are needed.Comment: 15 pages, 4 figures, accepted by Ap
X-ray emission from massive stars in Cyg OB2
We report on the analysis of the Chandra-ACIS data of O, B and WR stars in
the young association Cyg OB2. X-ray spectra of 49 O-stars, 54 B-stars and 3
WR-stars are analyzed and for the brighter sources, the epoch dependence of the
X-ray fluxes is investigated. The O-stars in Cyg\,OB2 follow a well-defined
scaling relation between their X-ray and bolometric luminosities: log(Lx/Lbol)
= -7.2 +/- 0.2. This relation is in excellent agreement with the one previously
derived for the Carina OB1 association. Except for the brightest O-star
binaries, there is no general X-ray overluminosity due to colliding winds in
O-star binaries. Roughly half of the known B-stars in the surveyed field are
detected, but they fail to display a clear relationship between Lx and Lbol.
Out of the three WR stars in Cyg OB2, probably only WR144 is itself responsible
for the observed level of X-ray emission, at a very low log(Lx/Lbol) = -8.8 +/-
0.2. The X-ray emission of the other two WR-stars (WR145 and 146) is most
probably due to their O-type companion along with a moderate contribution from
a wind-wind interaction zone.Comment: Accepted for an ApJS Special Issue devoted to the Chandra Cygnus OB2
Legacy Surve
The Metallicities of Low Stellar Mass Galaxies and the Scatter in the Mass-Metallicity Relation
In this investigation we quantify the metallicities of low mass galaxies by
constructing the most comprehensive census to date. We use galaxies from the
SDSS and DEEP2 survey and estimate metallicities from their optical emission
lines. We also use two smaller samples from the literature which have
metallicities determined by the direct method using the temperature sensitive
[OIII]4363 line. We examine the scatter in the local mass-metallicity (MZ)
relation determined from ~20,000 star-forming galaxies in the SDSS and show
that it is larger at lower stellar masses, consistent with the theoretical
scatter in the MZ relation determined from hydrodynamical simulations. We
determine a lower limit for the scatter in metallicities of galaxies down to
stellar masses of ~10^7 M_solar that is only slightly smaller than the expected
scatter inferred from the SDSS MZ relation and significantly larger than what
is previously established in the literature. The average metallicity of
star-forming galaxies increases with stellar mass. By examining the scatter in
the SDSS MZ relation, we show that this is mostly due to the lowest metallicity
galaxies. The population of low mass, metal-rich galaxies have properties which
are consistent with previously identified galaxies that may be transitional
objects between gas-rich dwarf irregulars and gas-poor dwarf spheroidals and
ellipticals.Comment: Accepted to ApJ. 17 pages, 17 figure
Distance Dependence in the Solar Neighborhood Age-Metallicity Relation
The age-metallicity relation for F and G dwarf stars in the solar
neighborhood, based on the stellar metallicity data of Edvardsson et al.
(1993), shows an apparent scatter that is larger than expected considering the
uncertainties in metallicities and ages. A number of theoretical models have
been put forward to explain the large scatter. However, we present evidence,
based on Edvardsson et al. (1993) data, along with Hipparcos parallaxes and new
age estimates, that the scatter in the age-metallicity relation depends on the
distance to the stars in the sample, such that stars within 30 pc of the Sun
show significantly less scatter in [Fe/H]. Stars of intermediate age from the
Edvardsson et al. sample at distances 30-80 pc from the Sun are systematically
more metal-poor than those more nearby. We also find that the slope of the
apparent age-metallicity relation is different for stars within 30 pc than for
those stars more distant. These results are most likely an artifact of
selection biases in the Edvardsson et al. star sample. We conclude that the
intrinsic dispersion in metallicity at fixed age is < 0.15 dex, consistent with
the < 0.1 dex scatter for Galactic open star clusters and the interstellar
medium.Comment: 15 pages, 5 figures, uses AASTex aaspp4 style; accepted for
publication in the Astrophysical Journa
Mode identification in the high-amplitude {\delta} Scuti star V2367 Cyg
We report on a multi-site photometric campaign on the high-amplitude
Scuti star V2367 Cyg in order to determine the pulsation modes. We also used
high-dispersion spectroscopy to estimate the stellar parameters and projected
rotational velocity. Time series multicolour photometry was obtained during a
98-d interval from five different sites. These data were used together with
model atmospheres and non-adiabatic pulsation models to identify the spherical
harmonic degree of the three independent frequencies of highest amplitude as
well as the first two harmonics of the dominant mode. This was accomplished by
matching the observed relative light amplitudes and phases in different
wavebands with those computed by the models. In general, our results support
the assumed mode identifications in a previous analysis of Kepler data.Comment: 9 pages, 5 figures, 4 tables. Accepted for publication in MNRA
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