109 research outputs found
Exploring the cosmic dark ages with the next generation of space and ground-based facilities
This paper reviews our current understanding of the process of re-ionization
of the Universe, focusing especially on those models where re-ionization is
caused by UV radiation from massive stars. After reviewing the expected
properties of stars at zero metallicity, I discuss the properties of primordial
HII regions and their observability.Comment: 14 pages, 10 figures. To appear in the proceedings of the SPIE
conference 4835 "Future research directions and visions for astronomy" held
in Waikaloa, HI on August 25-26, 2002, A. Dressler Ed. Needs spie.cls
(included
Physical Characterization of Early Galaxies in the Webb's First Deep Field SMACS J0723.3-7323
This paper highlights initial photometric analyses of JWST NIRCam imaging
data in the sightline of SMACS0723, aiming to identify galaxies at redshift
. By applying a conservative Lyman-break selection followed by photometric
redshift analysis and visual inspection, we identify four F090W-dropout and two
F150W-dropout sources, three of which were recently confirmed in an independent
spectroscopic analysis to , 7.665, and 8.499. We then supplement our
sample with a photometric-redshift selection, and identify five additional
candidates at . The NIRCam images clearly resolve all
sources and reveal their sub-galactic components that were not
resolved/detected in the previous imaging by Hubble Space Telescope. Our
spectral energy distribution analysis reveals that the selected galaxies are
characterized by young stellar populations (median age of Myr) of
sub-solar metallicity () and little dust attenuation
(). In several cases, we observe extreme Hb+[OIII] lines being
captured in the F444W band and seen as color excess, which is consistent with
their observed high star formation rate surface density. Eight of the 11
sources identified in this study appear in at least one of the recent studies
of the same fields (Adams et al., Atek et al., Donnan et al., Harikane et al.,
Yan et al.), implying the high fidelity of our selection. We cross-match all
high- galaxy candidates presented in the five studies with our catalog and
discuss the possible causes of discrepancy in the final lists.Comment: Resubmitted to ApJL after having addressed the reviewer's comments.
This version includes a few updates, 1. identification of z>10 galaxy
candidates, 2. estimates of number densities, and 3. comparison of the final
candidates to those in other studies, all of which were pointed out during
the review process. The analyses in this version are based on the latest
stable version of NIRCam zeropoint
The imprint of dark matter haloes on the size and velocity dispersion evolution of early-type galaxies
Early-type galaxies (ETGs) are observed to be more compact, on average, at than at , at fixed stellar mass. Recent observational
works suggest that such size evolution could reflect the similar evolution of
the host dark matter halo density as a function of the time of galaxy
quenching. We explore this hypothesis by studying the distribution of halo
central velocity dispersion () and half-mass radius () as
functions of halo mass and redshift , in a cosmological -CDM
-body simulation. In the range , we find
and , close to
the values expected for homologous virialized systems. At fixed in the
range we find
and . We show that
such evolution of the halo scaling laws is driven by individual haloes growing
in mass following the evolutionary tracks and , consistent with simple dissipationless merging models in
which the encounter orbital energy is accounted for. We compare the -body
data with ETGs observed at by populating the haloes with
a stellar component under simple but justified assumptions: the resulting
galaxies evolve consistently with the observed ETGs up to , but the
model has difficulty reproducing the fast evolution observed at .
We conclude that a substantial fraction of the size evolution of ETGs can be
ascribed to a systematic dependence on redshift of the dark matter haloes
structural properties.Comment: 15 pages, 14 figures, 1 table. Matches the Accepted version from
MNRA
The Dynamical Evolution of Substructure
The evolution of substructure embedded in non-dissipative dark halos is
studied through N-body simulations of isolated systems, both in and out of
initial equilibrium, complementing cosmological simulations of the growth of
structure. We determine by both analytic calculations and direct analysis of
the N-body simulations the relative importance of various dynamical processes
acting on the clumps, such as the removal of material by global tides,
clump-clump heating, clump-clump merging and dynamical friction. Our comparison
between merging and disruption processes implies that spiral galaxies cannot be
formed in a proto-system that contains a few large clumps, but can be formed
through the accretion of many small clumps; elliptical galaxies form in a more
clumpy environment than do spiral galaxies. Our results support the idea that
the central cusp in the density profiles of dark halos is the consequence of
self-limiting merging of small, dense halos. This implies that the collapse of
a system of clumps/substructure is not sufficient to form a cD galaxy, with an
extended envelope; plausibly subsequent accretion of large galaxies is
required. Persistent streams of material from disrupted clumps can be found in
the outer regions of the final system, and at an overdensity of around 0.75,
can cover 10% to 30% of the sky.Comment: Accepted for publication in MNRAS. 61 pages, 22 figures; figures 2-7
and 21-22 are separate gif files. Complete paper plus high resolution figures
available from http://www.stsci.edu/~mstiavel/Bing_et_al_02.htm
Does the Fine Structure Constant Really Vary in Time?
We discuss how laboratory experiments can be used to place constraints on
possible variations of the fine structure constant alpha in the observationally
relevant redshift interval z ~= 0 - 5, within a rather general theoretical
framework. We find a worst case upper limit for Delta alpha / alpha of 8 x
10^-6 for z <= 5 and Delta alpha / alpha of 0.9 x 10^-6 for z <= 1.6. The
derived limits are at variance with the recent findings by Webb et al. (1998),
who claim an observed variation of Delta alpha/alpha = -2.6 +- 0.4 x 10^-5 at
1<z<1.6.Comment: 11 pages, 2 figures, ApJL in pres
James Webb Space Telescope Studies of Dark Energy
The Hubble Space Telescope (HST) has contributed significantly to studies of dark energy. It was used to find the first evidence of deceleration at z=1.8 (Riess et al. 2001) through the serendipitous discovery of a type 1a supernova (SN1a) in the Hubble Deep Field. The discovery of deceleration at z greater than 1 was confirmation that the apparent acceleration at low redshift (Riess et al. 1998; Perlmutter et al. 1999) was due to dark energy rather than observational or astrophysical effects such as systematic errors, evolution in the SN1a population or intergalactic dust. The GOODS project and associated follow-up discovered 21 SN1a, expanding on this result (Riess et al. 2007). HST has also been used to constrain cosmological parameters and dark energy through weak lensing measurements in the COSMOS survey (Massey et al 2007; Schrabback et al 2009) and strong gravitational lensing with measured time delays (Suyu et al 2010). Constraints on dark energy are often parameterized as the equation of state, w = P/p. For the cosmological constant model, w = -1 at all times; other models predict a change with time, sometimes parameterized generally as w(a) or approximated as w(sub 0)+(1-a)w(sub a), where a = (1+z)(sup -1) is the scale factor of the universe relative to its current scale. Dark energy can be constrained through several measurements. Standard candles, such as SN1a, provide a direct measurement of the luminosity distance as a function of redshift, which can be converted to H(z), the change in the Hubble constant with redshift. An analysis of weak lensing in a galaxy field can be used to derive the angular-diameter distance from the weak-lensing equation and to measure the power spectrum of dark-matter halos, which constrains the growth of structure in the Universe. Baryonic acoustic oscillations (BAO), imprinted on the distribution of matter at recombination, provide a standard rod for measuring the cosmological geometry. Strong gravitational lensing of a time-variable source gives the angular diameter distance through measured time delays of multiple images. Finally, the growth of structure can also be constrained by measuring the mass of the largest galaxy clusters over cosmic time. HST has contributed to the study of dark energy through SN1a and gravitational lensing, as discussed above. HST has also helped to characterize galaxy clusters and the HST-measured constraints on the current Hubble constant H(sub 0) are relevant to the interpretation of dark energy measurements (Riess et al 2009a). HST has not been used to constrain BAO as the large number of galaxy redshifts required, of order 100 million, is poorly matched to HST's capabilities. As the successor to HST, the James Webb Space Telescope (JWST; Gardner et al 2006) will continue and extend HST's dark energy work in several ways
Characterization and modeling of contamination for Lyman break galaxy samples at high redshift
The selection of high redshift sources from broad-band photometry using the
Lyman-break galaxy (LBG) technique is a well established methodology, but the
characterization of its contamination for the faintest sources is still
incomplete. We use the optical and near-IR data from four (ultra)deep Hubble
Space Telescope legacy fields to investigate the contamination fraction of LBG
samples at z~5-8 selected using a colour-colour method. Our approach is based
on characterizing the number count distribution of interloper sources, that is
galaxies with colors similar to those of LBGs, but showing detection at
wavelengths shorter than the spectral break. Without sufficient sensitivity at
bluer wavelengths, a subset of interlopers may not be properly classified, and
contaminate the LBG selection. The surface density of interlopers in the sky
gets steeper with increasing redshift of LBG selections. Since the intrinsic
number of dropouts decreases significantly with increasing redshift, this
implies increasing contamination from misclassified interlopers with increasing
redshift, primarily by intermediate redshift sources with unremarkable
properties (intermediate ages, lack of ongoing star formation and low/moderate
dust content). Using Monte Carlo simulations, we estimate that the CANDELS deep
data have contamination induced by photometric scatter increasing from ~2% at
z~5 to ~6% at z~8 for a typical dropout color >1 mag, with contamination
naturally decreasing for a more stringent dropout selection. Contaminants are
expected to be located preferentially near the detection limit of surveys,
ranging from 0.1 to 0.4 contaminants per arcmin2 at J=30, depending on the
field considered. This analysis suggests that the impact of contamination in
future studies of z>10 galaxies needs to be carefully considered.Comment: 17 pages, 13 figures, ApJ in pres
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