32 research outputs found
The primordial Helium-4 abundance determination: systematic effects
By extrapolating to O/H = N/H = 0 the empirical correlations Y-O/H and Y-N/H
defined by a relatively large sample of ~ 45 Blue Compact Dwarfs (BCDs), we
have obtained a primordial 4Helium mass fraction Yp= 0.2443+/-0.0015 with dY/dZ
= 2.4+/-1.0. This result is in excellent agreement with the average Yp=
0.2452+/-0.0015 determined in the two most metal-deficient BCDs known, I Zw 18
(Zsun/50) and SBS 0335-052 (Zsun/41), where the correction for He production is
smallest. The quoted error (1sigma) of < 1% is statistical and does not include
systematic effects. We examine various systematic effects including collisional
excitation of Hydrogen lines, ionization structure and temperature fluctuation
effects, and underlying stellar HeI absorption, and conclude that combining all
systematic effects, our Yp may be underestimated by ~ 2-4%. Taken at face
value, our Yp implies a baryon-to-photon number ratio eta = 4.7x10^-10 and a
baryon mass fraction Omega_b h^2_{100} = 0.017+/-0.005 (2sigma), consistent
with the values obtained from deuterium and Cosmic Microwave Background
measurements. Correcting Yp upward by 2-4% would make the agreement even
better.Comment: 12 pages, 5 PS figures, to appear in "Matter in the Universe", ed P.
Jetzer, K. Pretzl and R. von Steiger, Kluwer, Dordrecht (2002
Star-forming galaxies at very high redshifts
Analysis of the deepest available images of the sky, obtained by the Hubble
Space Telescope, reveals a large number of candidate high-redshift galaxies. A
catalogue of 1,683 objects is presented, with estimated redshifts ranging from
to . The high-redshift objects are interpreted as regions of star
formation associated with the progenitors of present-day normal galaxies at
epochs reaching to 95\% of the time to the Big Bang.Comment: 10 pages, LaTeX type, aaspp4.sty macro provided. Supplementary
information, including the full catalog, plots of spectra and redshift
likelihood functions for all the objects, and composite spectra, are
available at ftp://ftp.ess.sunysb.edu/pub/hd
The Neutral ISM in Nearby Luminous Compact Blue Galaxies
We observed 20 nearby Luminous Compact Blue Galaxies (LCBGs) in HI and
CO(J=2-1) with the GBT and JCMT. These ~L^star galaxies are blue, high surface
brightness, starbursting, high metallicity galaxies with an underlying older
stellar population. They are common at z~1, but rare in the local Universe. It
has been proposed that intermediate redshift LCBGs may be the progenitors of
local dwarf ellipticals or low luminosity spirals, or that they may be more
massive disks forming from the center outward to become L^star galaxies. To
discriminate among various possible evolutionary scenarios, we have measured
the dynamical masses and gas depletion time scales of this sample of nearby
LCBGs.
We find that local LCBGs span a wide range of dynamical masses, from 4 x 10^9
to 1 x 10^11 M_solar (measured within R_25). Molecular gas in local LCBGs is
depleted quite quickly, in 30 to 200 million years. The molecular plus atomic
gas is depleted in 30 million to 10 billion years; however, ~80% of the local
LCBGs deplete their gas in less than 5 billion years. As LCBGs are
heterogeneous in both dynamical mass and gas depletion time scales, they are
not likely to evolve into one homogeneous galaxy class.Comment: 4 pages, 2 figures, to be published in 4th
Cologne-Bonn-Zermatt-Symposium, Eds. S. Pfalzner, C. Kramer, C. Straubmeier,
and A. Heithause
Properties of Galaxies in and around Voids
Two surveys for intrinsically faint galaxies towards nearby voids have been
conducted at the MPI f\"ur Astronomie, Heidelberg. One selected targets from a
new diameter limited () catalog with morphological criteria while
the other used digitized objective prism Schmidt plates to select mainly HII
dwarf galaxies. For some 450 galaxies, redshifts and other optical data were
obtained. We studied the spatial distribution of the sample objects, their
luminosity function, and their intrinsic properties. Most of the galaxies
belong to already well known sheets and filaments. But we found about a dozen
highly isolated galaxies in each sample (nearest neighborhood distance ). These tend to populate additional structures and are not
distributed homogeneously throughout the voids. As our results on 'void
galaxies' still suffer from small sample statistics, I also tried to combine
similar existing surveys of nearby voids to get further hints on the larger
structure and on the luminosity function of the isolated galaxies. No
differences in the luminosity function of sheet and void galaxies could be
found. The optical and infrared properties of both samples are in the normal
range for samples dominated by late-type dwarfs. Follow-up HI studies show that
the isolated dwarfs in both samples have unusual high amount of neutral gas for
a given luminosity.Comment: 10 pages, 4 figures, latex, to appear in the proceedings of the
'Ringberg workshop on Large Scale Structure', hold Sep. 23-28, 199
The Cosmological Baryon Density from the Deuterium Abundance at a redshift z = 3.57
We present a measurement of the deuterium to hydrogen ratio in a quasar
absorption system at redshift z = 3.57 towards QSO 1937-1009. We use a two
component fit, with redshifts determined from unsaturated metal lines, to fit
the hydrogen and deuterium features simultaneously. We find a low value of D/H
= 2.3 \pm 0.6 \times 10^{-5}, which does not agree with other measurements of
high D/H (Songaila et al. 1994, Carswell et al. 1994). The absorption system is
very metal poor, with metallicities less than 1/100 solar. Standard models of
chemical evolution show the astration of deuterium is limited to a few percent
from primordial for systems this metal-poor, so we believe our value represents
the primordial one. Using predictions of standard big-bang nucleosynthesis and
measurements of the cosmic microwave background, our measurement gives the
density of baryons in units of the critical density, , where H_0 = 100 h km s^{-1] Mpc^{-1}.Comment: 10 pages, 2 Figures, also available at http://nately.ucsd.edu/ ;
submitted to Natur
Detection of intergalactic red-giant-branch stars in the Virgo cluster
It has been suspected for nearly 50 years that clusters of galaxies contain a
population of intergalactic stars, ripped from galaxies during cluster
formation or when the galaxies' orbits take them through the cluster center.
Support for the existence of such a population of free-floating stars comes
from measurements of the diffuse light in clusters, and from recent detections
of planetary nebulae with positions and/or velocities far removed from any
observed cluster galaxy. But estimates for the mass of the diffuse population
and its distribution relative to the galaxies are still highly uncertain. Here
we report the direct detection of intergalactic stars in deep images of a blank
field in the Virgo Cluster. The data suggest that approximately 10% of the
stellar mass of the cluster is in intergalactic stars. We observe a relatively
homogeneous distribution of stars, with evidence of a slight gradient toward
M87.Comment: Accepted for publication in Nature. 10 pages, 2 postscript figures
included. Uses nature.sty and astrobib.sty. (Astrobib is available from
http://www.stsci.edu/software/TeX.html.
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
Gravitational Waves from Gravitational Collapse
Gravitational wave emission from the gravitational collapse of massive stars
has been studied for more than three decades. Current state of the art
numerical investigations of collapse include those that use progenitors with
realistic angular momentum profiles, properly treat microphysics issues,
account for general relativity, and examine non--axisymmetric effects in three
dimensions. Such simulations predict that gravitational waves from various
phenomena associated with gravitational collapse could be detectable with
advanced ground--based and future space--based interferometric observatories.Comment: 68 pages including 13 figures; revised version accepted for
publication in Living Reviews in Relativity (http://www.livingreviews.org
Observations of Ly Emitters at High Redshift
In this series of lectures, I review our observational understanding of
high- Ly emitters (LAEs) and relevant scientific topics. Since the
discovery of LAEs in the late 1990s, more than ten (one) thousand(s) of LAEs
have been identified photometrically (spectroscopically) at to . These large samples of LAEs are useful to address two major astrophysical
issues, galaxy formation and cosmic reionization. Statistical studies have
revealed the general picture of LAEs' physical properties: young stellar
populations, remarkable luminosity function evolutions, compact morphologies,
highly ionized inter-stellar media (ISM) with low metal/dust contents, low
masses of dark-matter halos. Typical LAEs represent low-mass high- galaxies,
high- analogs of dwarf galaxies, some of which are thought to be candidates
of population III galaxies. These observational studies have also pinpointed
rare bright Ly sources extended over kpc, dubbed
Ly blobs, whose physical origins are under debate. LAEs are used as
probes of cosmic reionization history through the Ly damping wing
absorption given by the neutral hydrogen of the inter-galactic medium (IGM),
which complement the cosmic microwave background radiation and 21cm
observations. The low-mass and highly-ionized population of LAEs can be major
sources of cosmic reionization. The budget of ionizing photons for cosmic
reionization has been constrained, although there remain large observational
uncertainties in the parameters. Beyond galaxy formation and cosmic
reionization, several new usages of LAEs for science frontiers have been
suggested such as the distribution of {\sc Hi} gas in the circum-galactic
medium and filaments of large-scale structures. On-going programs and future
telescope projects, such as JWST, ELTs, and SKA, will push the horizons of the
science frontiers.Comment: Lecture notes for `Lyman-alpha as an Astrophysical and Cosmological
Tool', Saas-Fee Advanced Course 46. Verhamme, A., North, P., Cantalupo, S., &
Atek, H. (eds.) --- 147 pages, 103 figures. Abstract abridged. Link to the
lecture program including the video recording and ppt files :
https://obswww.unige.ch/Courses/saas-fee-2016/program.cg