377 research outputs found
New Wolf-Rayet Galaxies with Detection of WC Stars
We report the discovery of two new Wolf-Rayet (WR) galaxies: Mrk~1039, and
F082082816. Two broad WR bumps at 5808\AA~ and 4650\AA~ indicate the
presence of WCE and WNL star population in all two sources. We also confirm the
presenceof WR features in Mrk~35, previously detected in a different position.
The observed equivalent width of the WR bump at 4650\AA~ and the derived number
ratios of WR/(WRO) imply that star formation in these sources takes place
inshort burst duration. Comparisons with the recent models of WR populations in
young starbursts with the observed EW(\HeII)/EW(\CIV)/EW(WRbump) and their
relative intensitie provide an indication that the stellar initial mass
function in some WR galaxies might not be Salpeter-like. It is interesting to
find that the luminous IRAS source, F082082816, has little dust reddening,
probably because of the existence of a powerful superwind. By comparisons with
other starbursts observed with the Hopkins Ultraviolet Telescope, F082082816
as a merging system renders a chance to study the contribution from young
starbursts to the UV background radiation in universe.Comment: 22 pages, 6 figures, 4 tables, accepted by The Astrophysical Journa
The Frontier Fields Lens Modeling Comparison Project
Gravitational lensing by clusters of galaxies offers a powerful probe of
their structure and mass distribution. Deriving a lens magnification map for a
galaxy cluster is a classic inversion problem and many methods have been
developed over the past two decades to solve it. Several research groups have
developed techniques independently to map the predominantly dark matter
distribution in cluster lenses. While these methods have all provided
remarkably high precision mass maps, particularly with exquisite imaging data
from the Hubble Space Telescope (HST), the reconstructions themselves have
never been directly compared. In this paper, we report the results of comparing
various independent lens modeling techniques employed by individual research
groups in the community. Here we present for the first time a detailed and
robust comparison of methodologies for fidelity, accuracy and precision. For
this collaborative exercise, the lens modeling community was provided simulated
cluster images -- of two clusters Ares and Hera -- that mimic the depth and
resolution of the ongoing HST Frontier Fields. The results of the submitted
reconstructions with the un-blinded true mass profile of these two clusters are
presented here. Parametric, free-form and hybrid techniques have been deployed
by the participating groups and we detail the strengths and trade-offs in
accuracy and systematics that arise for each methodology. We note in conclusion
that lensing reconstruction methods produce reliable mass distributions that
enable the use of clusters as extremely valuable astrophysical laboratories and
cosmological probes.Comment: 38 pages, 25 figures, submitted to MNRAS, version with full
resolution images can be found at
http://pico.bo.astro.it/~massimo/papers/FFsims.pd
Silicates in D-type symbiotic stars: an ISO overview
We investigate the IR spectral features of a sample of D-type symbiotic
stars. Analyzing unexploited ISO-SWS data, deriving the basic observational
parameters of dust bands and comparing them with respect to those observed in
other astronomical sources, we try to highlight the effect of environment on
grain chemistry and physic. We find strong amorphous silicate emission bands at
10 micron and 18 micron in a large fraction of the sample. The analysis of the
10 micron band, along with a direct comparison with several astronomical
sources, reveals that silicate dust in symbiotic stars shows features between
the characteristic circumstellar environments and the interstellar medium. This
indicates an increasing reprocessing of grains in relation to specific
symbiotic behavior of the objects. A correlation between the central wavelength
of the 10 and 18 micron dust bands is found. By the modeling of IR spectral
lines we investigate also dust grains conditions within the shocked nebulae.
Both the unusual depletion values and the high sputtering efficiency might be
explained by the formation of SiO moleculae, which are known to be a very
reliable shock tracer. We conclude that the signature of dust chemical
disturbance due to symbiotic activity should be looked for in the outer,
circumbinary, expanding shells where the environmental conditions for grain
processing might be achieved. Symbiotic stars are thus attractive targets for
new mid-infrared and mm observations.Comment: 24 pages, 6 figures, 5 tables - to be published in A
zCOSMOS 20k: Satellite galaxies are the main drivers of environmental effects in the galaxy population at least to z~0.7
We explore the role of environment in the evolution of galaxies over
0.1<z<0.7 using the final zCOSMOS-bright data set. Using the red fraction of
galaxies as a proxy for the quenched population, we find that the fraction of
red galaxies increases with the environmental overdensity and with the stellar
mass, consistent with previous works. As at lower redshift, the red fraction
appears to be separable in mass and environment, suggesting the action of two
processes: mass and environmental quenching. The parameters describing these
appear to be essentially the same at z~0.7 as locally. We explore the relation
between red fraction, mass and environment also for the central and satellite
galaxies separately, paying close attention to the effects of impurities in the
central-satellite classification and using carefully constructed samples
matched in stellar mass. There is little evidence for a dependence of the red
fraction of centrals on overdensity. Satellites are consistently redder at all
overdensities, and the satellite quenching efficiency increases with
overdensity at 0.1<z<0.4. This is less marked at higher redshift, but both are
nevertheless consistent with the equivalent local measurements. At a given
stellar mass, the fraction of galaxies that are satellites also increases with
the overdensity. At a given overdensity and mass, the obtained relation between
the environmental quenching and the satellite fraction agrees well with the
satellite quenching efficiency, demonstrating that the environmental quenching
in the overall population is consistent with being entirely produced through
the satellite quenching process at least up to z=0.7. However, despite the
unprecedented size of our high redshift samples, the associated statistical
uncertainties are still significant and our statements should be understood as
approximations to physical reality, rather than physically exact formulae.Comment: 22 pages, 19 figures, submitted to MNRA
Extreme emission-line galaxies out to z1 in zCOSMOS. I. Sample and characterization of global properties
We present a thorough characterization of a large sample of 183 extreme
emission-line galaxies (EELGs) at redshift 0.11 < z < 0.93 selected from the
20k zCOSMOS Bright Survey because of their unusually large emission line
equivalent widths. We use multiwavelength COSMOS photometry, HST-ACS I-band
imaging and optical zCOSMOS spectroscopy to derive the main global properties
of EELGs, such as sizes, masses, SFRs, reliable metallicities from both
"direct" and "strong-line" methods. The EELGs are compact (R_50 ~ 1.3 kpc),
low-mass (log(M*/Msol)~7-10) galaxies forming stars at unusually high specific
SFR (log(sSFR/yr) up to ~ -7) compared to main sequence SFGs of the same
stellar mass and redshift. At UV wavelengths, the EELGs are luminous and show
high surface brightness and include strong Ly emitters, as revealed by
GALEX spectroscopy. We show that zCOSMOS EELGs are high-ionization,
low-metallicity systems, with median 12+log(O/H)=8.16, including a handful of
extremely metal-deficient galaxies (<10% solar). While ~80% of the EELGs show
non-axisymmetric morphologies, including clumpy and tadpole galaxies, we find
that ~29% of them show additional low surface-brightness features, which
strongly suggest recent or ongoing interactions. As star-forming dwarfs in the
local Universe, EELGs are most often found in relative isolation. While only
very few EELGs belong to compact groups, almost one third of them are found in
spectroscopically confirmed loose pairs or triplets. We conclude that EELGs are
galaxies caught in a transient and probably early period of their evolution,
where they are efficiently building-up a significant fraction of their
present-day stellar mass in an ongoing galaxy-wide starburst. Therefore, the
EELGs constitute an ideal benchmark for comparison studies between low- and
high-redshift low-mass star-forming galaxies.Comment: Accepted in A&A. Final replacement to match the version in press. It
includes a minor change in the title and a new figur
Interpreting high [O III]/H β ratios with maturing starbursts
Star-forming galaxies at high redshift show ubiquitously high-ionization parameters, as measured by the ratio of optical emission lines. We demonstrate that local (z < 0.2) sources selected as Lyman break analogues also manifest high line ratios with a typical [O III]/Hβ=3.36+0.14−0.04 – comparable to all but the highest ratios seen in star-forming galaxies at z ∼ 2–4. We argue that the stellar population synthesis code BPASS can explain the high-ionization parameters required through the ageing of rapidly formed star populations, without invoking any AGN contribution. Binary stellar evolution pathways prolong the age interval over which a starburst is likely to show elevated line ratios, relative to those predicted by single stellar evolution codes. As a result, model galaxies at near-solar metallicities and with ages of up to ∼100 Myr after a starburst typically have a line ratio [O III]/Hβ ∼ 3, consistent with those seen in Lyman break galaxies and local sources with similar star formation densities. This emphasises the importance of including binary evolution pathways when simulating the nebular line emission of young or bursty stellar populations
The zCOSMOS 20k Group Catalog
We present an optical group catalog between 0.1 < z < 1 based on 16,500
high-quality spectroscopic redshifts in the completed zCOSMOS-bright survey.
The catalog published herein contains 1498 groups in total and 192 groups with
more than five observed members. The catalog includes both group properties and
the identification of the member galaxies. Based on mock catalogs, the
completeness and purity of groups with three and more members should be both
about 83% with respect to all groups that should have been detectable within
the survey, and more than 75% of the groups should exhibit a one-to-one
correspondence to the "real" groups. Particularly at high redshift, there are
apparently more galaxies in groups in the COSMOS field than expected from mock
catalogs. We detect clear evidence for the growth of cosmic structure over the
last seven billion years in the sense that the fraction of galaxies that are
found in groups (in volume-limited samples) increases significantly with cosmic
time. In the second part of the paper, we develop a method for associating
galaxies that only have photo-z to our spectroscopically identified groups. We
show that this leads to improved definition of group centers, improved
identification of the most massive galaxies in the groups, and improved
identification of central and satellite galaxies, where we define the former to
be galaxies at the minimum of the gravitational potential wells. Subsamples of
centrals and satellites in the groups can be defined with purities up to 80%,
while a straight binary classification of all group and non-group galaxies into
centrals and satellites achieves purities of 85% and 75%, respectively, for the
spectroscopic sample.Comment: 26 pages, 21 figures, published in ApJ (along with machine-readable
tables
An Imaging Survey of Early-Type Barred Galaxies
This paper presents the results of a high-resolution imaging survey, using
both ground-based and Hubble Space Telescope images, of a complete sample of
nearby barred S0--Sa galaxies in the field, with a particular emphasis on
identifying and measuring central structures within the bars: secondary bars,
inner disks, nuclear rings and spirals, and off-plane dust. A discussion of the
frequency and statistical properties of the various types of inner structures
has already been published. Here, we present the data for the individual
galaxies and measurements of their bars and inner structures. We set out the
methods we use to find and measure these structures, and how we discriminate
between them. In particular, we discuss some of the deficiencies of ellipse
fitting of the isophotes, which by itself cannot always distinguish between
bars, rings, spirals, and dust, and which can produce erroneous measurements of
bar sizes and orientations.Comment: LaTeX, 66 pages (including 42 figures, 36 in color). To appear in The
Astrophysical Journal Supplement. Full-resolution and text-only versions
available at http://www.iac.es/galeria/erwin/research
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