62 research outputs found
The Starburst Nature of Lyman-Break Galaxies: Testing UV Extinction with X-rays
We derive the bolometric to X-ray correlation for a local sample of normal
and starburst galaxies and use it, in combination with several UV reddening
schemes, to predict the 2--8 keV X-ray luminosity for a sample of 24
Lyman-break galaxies in the HDF/CDF-N. We find that the mean X-ray luminosity,
as predicted from the Meurer UV reddening relation for starburst galaxies,
agrees extremely well with the Brandt stacking analysis. This provides
additional evidence that Lyman-break galaxies can be considered as scaled-up
local starbursts and that the locally derived starburst UV reddening relation
may be a reasonable tool for estimating the UV extinction at high redshift. Our
analysis shows that the Lyman-break sample can not have far-IR to far-UV flux
ratios similar to nearby ULIGs, as this would predict a mean X-ray luminosity
100 times larger than observed, as well as far-IR luminosities large enough to
be detected in the sub-mm. We calculate the UV reddening expected from the
Calzetti effective starburst attenuation curve and the radiative transfer
models of Witt & Gordon for low metallicity dust in a shell geometry with
homogeneous or clumpy dust distributions and find that all are consistent with
the observed X-ray emission. Finally, we show that the mean X-ray luminosity of
the sample would be under predicted by a factor of 6 if the the far-UV is
unattenuated by dust.Comment: 7 pages, 3 figures. Accepted for publication in A
ACCESS - V. Dissecting ram-pressure stripping through integral-field spectroscopy and multi-band imaging
We study the case of a bright (L>L*) barred spiral galaxy from the rich
cluster A3558 in the Shapley supercluster core (z=0.05) undergoing ram-pressure
stripping. Integral-field spectroscopy, complemented by multi-band imaging,
allows us to reveal the impact of ram pressure on the interstellar medium. We
study in detail the kinematics and the physical conditions of the ionized gas
and the properties of the stellar populations. We observe one-sided extraplanar
ionized gas along the full extent of the galaxy disc. Narrow-band Halpha
imaging resolves this outflow into a complex of knots and filaments. The gas
velocity field is complex with the extraplanar gas showing signature of
rotation. In all parts of the galaxy, we find a significant contribution from
shock excitation, as well as emission powered by star formation. Shock-ionized
gas is associated with the turbulent gas outflow and highly attenuated by dust.
All these findings cover the whole phenomenology of early-stage ram-pressure
stripping. Intense, highly obscured star formation is taking place in the
nucleus, probably related to the bar, and in a region 12 kpc South-West from
the centre. In the SW region we identify a starburst characterized by a 5x
increase in the star-formation rate over the last ~100 Myr, possibly related to
the compression of the interstellar gas by the ram pressure. The scenario
suggested by the observations is supported and refined by ad hoc
N-body/hydrodynamical simulations which identify a rather narrow temporal range
for the onset of ram-pressure stripping around t~60 Myr ago, and an angle
between the galaxy rotation axis and the intra-cluster medium wind of ~45 deg.
Taking into account that the galaxy is found ~1 Mpc from the cluster centre in
a relatively low-density region, this study shows that ram-pressure stripping
still acts efficiently on massive galaxies well outside the cluster cores.Comment: 46 pages, 21 figures, accepted for publication; MNRAS 201
Old and Young X-ray Point Source Populations in Nearby Galaxies
We analyzed 1441 Chandra X-ray point sources in 32 nearby galaxies. The total
point-source X-ray luminosity L_XP is well correlated with B, K, and FIR+UV
luminosities of spiral host galaxies, and with the B and K luminosities for
ellipticals. This suggests an intimate connection between L_XP and both the old
and young stellar populations, for which K and FIR+UV luminosities are proxies
for the galaxy mass M and star-formation rate SFR. We derive proportionality
constants 1.3E29 erg/s/Msol and 0.7E39 erg/s/(Msol/yr), which can be used to
estimate the old and young components from M and SFR, respectively. The
cumulative X-ray luminosity functions for the point sources have quite
different slopes for the spirals (gamma ~= 0.5-0.8) and ellipticals (gamma ~=
1.4), implying *the most luminous point sources dominate L_XP* for the spirals.
Most of the point sources have X-ray colors that are consistent with either
LMXBs or Ultraluminous X-ray sources (ULXs a.k.a. IXOs) and we rule out
classical HMXBs (e.g. neutron-star X-ray pulsars) as contributing much to L_XP.
Thus, for spirals, the ULXs dominate L_XP. We estimate that >~20% of all ULXs
found in spirals originate from the older (pop II) stellar populations,
indicating that many of the ULXs that have been found in spiral galaxies are in
fact pop II ULXs, like those in elliptical galaxies. The linear dependence of
L_XP on the SFR argues for either a steepening in the X-ray luminosity function
of the young (pop I) X-ray source population at L_X >~10^(38.5-39) erg/s, or a
decreasing efficiency for producing all types of young X-ray point sources as
the galaxy SFR increases.Comment: 33 pages AASTEX, ApJ accepted. Please download full version with
figures from http://www.pha.jhu.edu/~colbert/chps_accepted.p
Far-Ultraviolet Radiation from Elliptical Galaxies
Far-ultraviolet radiation is a ubiquitous, if unanticipated, phenomenon in
elliptical galaxies and early-type spiral bulges. It is the most variable
photometric feature associated with old stellar populations. Recent
observational and theoretical evidence shows that it is produced mainly by
low-mass, small-envelope, helium-burning stars in extreme horizontal branch and
subsequent phases of evolution. These are probably descendents of the dominant,
metal rich population of the galaxies. Their lifetime UV outputs are remarkably
sensitive to their physical properties and hence to the age and the helium and
metal abundances of their parents. UV spectra are therefore exceptionally
promising diagnostics of old stellar populations, although their calibration
requires a much improved understanding of giant branch mass loss, helium
enrichment, and atmospheric diffusion.Comment: 46 pages; includes LaTeX text file, 9 PS figures, 1 JPG figure, 2
style files. Full resolution figures and PS version available at
http://www.astro.virginia.edu/~rwo/araa99/. Article to appear in Annual
Reviews of Astronomy & Astrophysics, 199
Multi-wavelength Observations of Dusty Star Formation at Low and High Redshift
This paper examines what can be learned about high-redshift star formation
from the small fraction of high-redshift galaxies' luminosities that is emitted
at accessible wavelengths. We review and quantify empirical correlations
between bolometric luminosities produced by star formation and the UV, mid-IR,
sub-mm, and radio luminosities of galaxies in the local universe. These
correlations suggest that observations of high-redshift galaxies at any of
these wavelengths should constrain their star-formation rates to within
0.2--0.3 dex. We assemble the limited evidence that high-redshift galaxies obey
these locally calibrated correlations. The characteristic luminosities and dust
obscurations of galaxies at z ~ 0, z ~ 1, and z ~ 3 are reviewed. After
discussing the relationship between the high-redshift populations selected in
surveys at different wavelengths, we calculate the contribution to the 850um
background from each. The available data show that a correlation between
star-formation rate and dust obscuration L_dust/L_UV exists at low and high
redshift. This correlation plays a central role in the major conclusion of this
paper: most star formation at high redshift occurred in galaxies with 1 <
L_dust/L_UV < 100 similar to those that host the majority of star formation in
the local universe and to those that are detected in UV-selected surveys.
(abridged)Comment: Scheduled for publication in ApJ v544 Dec 2000. Significant changes
to section 4. Characteristic UV and dust luminosities of star-forming
galaxies at redshifts z~0, z~1, and z~3 presented. Existence of extremely
obscured galaxies more clearly acknowledged. Original conclusions reinforced
by the observed correlation between bolometric luminosity and dust
obscuration at 0<z<
Ultraviolet Properties of Primeval Galaxies: Theoretical Models from Stellar Population Synthesis
The ultraviolet luminosity evolution of star-forming galaxies is explored
from the theoretical point of view, especially focusing on the theory of UV
energetics in simple and composite stellar populations and its relationship to
the star formation rate and other main evolutionary parameters.
Galaxy emission below 3000 Angstroms directly correlates with actual star
formation, not depending on the total mass of the system. A straightforward
calibration is obtained, in this sense, from the theoretical models at 1600,
2000 and 2800 Angstroms, and a full comparison is carried out with IUE data and
other balloon-borne observations for local galaxies.
The claimed role of late-type systems as prevailing contributors to the
cosmic UV background is reinforced by our results; at 2000 Angstroms Im
irregulars are found in fact nearly four orders of magnitude brighter than
ellipticals, per unit luminous mass.
The role of dust absorption in the observation of high-redshift galaxies is
assessed, comparing model output and observed spectral energy distribution of
local galaxy samples. Similar to what we observe in our own galaxy, a quick
evolution in the dust environment might be envisaged in primeval galaxies, with
an increasing fraction of luminous matter that would escape the regions of
harder and "clumpy" dust absorption on a timescale of some 10^7 yr, comparable
with the lifetime of stars of 5-10 solar masses.Comment: 20 pages and 14 PS figures - To appear in the March issue of the
Astronomical Journal. See http://www.merate.mi.astro.it/~eps/home.html for
more info and model retrieva
Galaxy properties from Voids to Clusters in the SDSS-DR4
We investigate the environmental dependence of galaxy population properties
in a complete volume-limited sample of 91566 galaxies in the redshift range
0.05 <= z <= 0.095 and with M_r <= -20.0, selected from the Sloan Digital Sky
Survey (SDSS) Data Release 4. Our aim is to search for systematic variations in
the properties of galaxies with the local galaxy density. In particular, we
analize how the (u - r) color index and the morphological type of galaxies (the
latter evaluated through the SDSS Eclass and FracDev parameters) are related to
the environment and to the luminosity of galaxies, in order to find hints that
can be related to the presence of a ''void'' galaxy population. Void galaxies
are identified through a highly selective criterion, which takes also into
account redshift and allows us to exclude from the sample all the galaxies that
are not really close to the center of underdense regions. We study the (u - r)
color distribution for galaxies in different luminosity bins, and we look for
correlations of color with the environment, the luminosity, and the
morphological type of the galaxies. We find that galaxies in underdense regions
(voids) have lower luminosity (M_r > -21) and are bluer than cluster galaxies.
The transition from overdense to underdense environments is smooth, the
fraction of late-type galaxies decreases while the fraction of early-type
galaxies increases smoothly from underdense to dense environments. We do not
find any sudden transition in the galaxy properties with density, which,
according to a suggestion by Peebles (2001), should mark the transition to a
population of "void" galaxies in LCDM models. On the contrary, our results
suggest a continuity of galaxy properties, from voids to clusters.Comment: A&A, accepted for publication. 17 pages, 9 figures. Constructive
comments welcom
Cepheid calibration of type Ia Supernovae and the Hubble constant
We investigate how a different calibration of the Cepheid Period-Luminosity
(PL) relation taking into account the metallicity corrections, affects the
absolute magnitude calibration of Supernovae (SNe) Ia and, in turn, the
determination of the Hubble constant H_0. We exploit SN Ia light curves from
literature and previously unpublished data, to build the M_B -- Delta m_{15}(B)
relation and we calibrate the zero point by means of 9 type Ia SNe with Cepheid
measured distances. This relation was then used to build the Hubble diagram and
in turn to derive H_0. In the attempt to correct for the host galaxy
extinction, we found that the data seems to suggest a value for the total to
selective absorption ratio, R_B=3.5, which is smaller than the standard value
for our own Galaxy R_B=4.315. Depending on different metallicity corrections
for the Cepheids P-L relation, values of R_B and SN sample selection criteria,
we found that the values of the Hubble constant H_0 is in the range 68--74 km
s^{-1} Mpc^{-1}, with associated uncertainties of the order of 10%. Unpublished
photometry is also presented for 18 SNe of our sample (1991S, 1991T, 1992A,
1992K, 1993H, 1993L, 1994D, 1994M, 1994ae, 1995D, 1995ac, 1995bd, 1996bo,
1997bp, 1997br, 1999aa, 1999dk, 2000cx), which are the results of a long
standing effort for supernova monitoring at ESO - La Silla and Asiago
Observatories.Comment: 19 pages, 9 figures, accepted for publication in MNRA
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