492 research outputs found
The Rest Frame UV to Optical Colors and SEDs of z~4-7 Galaxies
We use the ultra-deep HUDF09 and the deep ERS data from the HST WFC3/IR
camera, along with the wide area Spitzer/IRAC data from GOODS-S to derive SEDs
of star-forming galaxies from the rest-frame UV to the optical over a wide
luminosity range (M_1500 ~ -21 to M_1500 ~ -18) from z ~ 7 to z ~ 4. The sample
contains ~ 400 z ~ 4, ~ 120 z ~ 5, ~ 60 z ~ 6, and 36 prior z ~ 7 galaxies.
Median stacking enables the first comprehensive study of very faint high-z
galaxies at multiple redshifts (e.g., [3.6] = 27.4 +/- 0.1 AB mag for the
M_1500 ~ -18 sources at z ~ 4). At z ~ 4 our faint median-stacked SEDs reach to
~ 0.06 L*(z=4) and are combined with recently published results at high
luminosity L > L* that extend to M_1500 ~ -23. We use the observed SEDs and
template fits to derive rest frame UV-to-optical colors (U - V) at all
redshifts and luminosities. We find that this color does not vary significantly
with redshift at a fixed luminosity. The UV-to-optical color does show a weak
trend with luminosity, becoming redder at higher luminosities. This is most
likely due to dust. At z >~ 5 we find blue colors [3.6]-[4.5] ~ -0.3 mag that
are most likely due to rest-frame optical emission lines contributing to the
flux in the IRAC filter bandpasses. The scatter across our derived SEDs remains
substantial, but the results are most consistent with a lack of any evolution
in the SEDs with redshift at a given luminosity. The similarity of the SEDs
suggests a self-similar mode of evolution over a timespan from 0.7 Gyr to 1.5
Gyr that encompasses very substantial growth in the stellar mass density in the
universe (from ~ 4x10^6 to ~ 2x10^7 Msun Mpc^-3).Comment: 15 pages, 11 figures, 3 tables, submitted to Ap
The molecular gas content of z > 6.5 Lyman-alpha emitters
We present results from a sensitive search for CO J=1-0 line emission in two
z> 6.5 Lyman-alpha emitters (LAEs) with the Green Bank Telescope. CO J=1-0
emission was not detected from either object. For HCM 6A, at z ~ 6.56, the
lensing magnification factor of ~4.5 implies that the CO non-detection yields
stringent constraints on the CO J=1-0 line luminosity and molecular gas mass of
the LAE, L'(CO) < 6.1x10^9 x (dV/300)^(1/2) K km/s pc^2 and M(H_2) < 4.9x10^9 x
(dV/300)^(1/2) x (X(CO)/0.8) Msun. These are the strongest limits obtained so
far for a z >~ 6 galaxy. For IOK-1, the constraints are somewhat less
sensitive, L'(CO) < 2.3x10^10 x (dV/300)^(1/2) K km/s pc^2 and M(H_2) <
1.9x10^10 x (dV/300)^(1/2) x (X(CO)/0.8) Msun. The non-detection of CO J=1-0
emission in HCM~6A, whose high estimated star formation rate, dust extinction,
and lensing magnification make it one of the best high-z LAEs for such a
search, implies that typical z >~ 6 LAEs are likely to have significantly lower
CO line luminosities than massive sub-mm galaxies and hyperluminous infrared
quasars at similar redshifts, due to either a significantly lower molecular gas
content or a higher CO-to-H_2 conversion factor.Comment: Accepted for publication in ApJ Letter
Galaxy formation in the Planck cosmology - III. The high-redshift universe
We present high-redshift predictions of the star formation rate distribution function (SFRDF), UV luminosity function (UVLF), galactic stellar mass function (GSMF), and specific star formation rates (sSFRs) of galaxies from the latest version of the Munich semi-analytic model L-GALAXIES. We find a good fit to both the shape and normalization of the SFRDF at z = 4â7, apart from a slight underprediction at the low-SFR end at z = 4. Likewise, we find a good fit to the faint number counts for the observed UVLF at brighter magnitudes our predictions lie below the observations, increasingly so at higher redshifts. At all redshifts and magnitudes, the raw (unattenuated) number counts for the UVLF lie above the observations. Because of the good agreement with the SFR we interpret our underprediction as an overestimate of the amount of dust in the model for the brightest galaxies, especially at high redshift. While the shape of our GSMF matches that of the observations, we lie between (conflicting) observations at z = 4â5, and underpredict at z = 6â7. The sSFRs of our model galaxies show the observed trend of increasing normalization with redshift, but do not reproduce the observed mass dependence. Overall, we conclude that the latest version of L-GALAXIES, which is tuned to match observations at z †3, does a fair job of reproducing the observed properties of galaxies at z â„ 4. More work needs to be done on understanding observational bias at high redshift, and upon the dust model, before strong conclusions can be drawn on how to interpret remaining discrepancies between the model and observations
Discovery of a possibly old galaxy at , multiply imaged by the massive cluster Abell 383
We report the discovery of a unique galaxy, multiply imaged by the
cluster Abell 383 and detected in new Hubble Space Telescope ACS and WFC3
imaging, as well as in Warm Spitzer observations. This galaxy was selected as a
pair of i-dropouts; its suspected high redshift was confirmed by the
measurement of a strong Lyman-alpha line in both images using Keck/DEIMOS.
Combining Hubble and Spitzer photometry after correcting for contamination by
line emission (estimated to be a small effect), we identify a strong Balmer
break of 1.5 magnitudes. Taking into account the magnification factor of
11.4+/-1.9 (2.65+/-0.17 mag) for the brightest image, the unlensed AB magnitude
for the source is 27.2+/-0.05 in the H band, corresponding to a 0.4 L* galaxy,
and 25.7+/-0.08 at 3.6 um. The UV slope is consistent with beta~2.0, and from
the rest-frame UV continuum we measure a current star formation rate of
2.4+/-1.1 Msol/yr. The unlensed half-light radius is measured to be 300 pc,
from which we deduce a star-forming surface density of ~10 Msol/yr/kpc2. The
Lyman-alpha emission is found to be extended over ~3" along the slit,
corresponding to ~5 kpc in the source plane. This can be explained by the
presence of a much larger envelope of neutral hydrogen around the star-forming
region. Finally, fitting the spectral energy distribution using 7 photometric
data points with simple SED models, we derive the following properties: very
little reddening, an inferred stellar mass of M*=6e9 Msol, and an inferred age
of ~800 Myrs (corresponding to a redshift of formation of ~18). The
star-formation rate of this object was likely much stronger in the past than at
the time of observation, suggesting that we may be missing a fraction of
galaxies at z~6 which have already faded in rest-frame UV wavelengths.Comment: 6 pages, 3 figures, MNRAS in press, replaced with accepted version
including minor comment
A robust sample of galaxies at redshifts 6.0<z<8.7: stellar populations, star-formation rates and stellar masses
We present the results of a photometric redshift analysis designed to
identify z>6 galaxies from the near-IR HST imaging in three deep fields (HUDF,
HUDF09-2 & ERS). By adopting a rigorous set of criteria for rejecting low-z
interlopers, and by employing a deconfusion technique to allow the available
IRAC imaging to be included in the candidate selection process, we have derived
a robust sample of 70 Lyman-break galaxies (LBGs) spanning the redshift range
6.0<z<8.7. Based on our final sample we investigate the distribution of UV
spectral slopes (beta), finding a variance-weighted mean value of =-2.05
+/- 0.09 which, contrary to some previous results, is not significantly bluer
than displayed by lower-redshift starburst galaxies. We confirm the correlation
between UV luminosity and stellar mass reported elsewhere, but based on fitting
galaxy templates featuring a range of star-formation histories, metallicities
and reddening we find that, at z>=6, the range in mass-to-light ratio (M*/L_UV)
at a given UV luminosity could span a factor of ~50. Focusing on a sub-sample
of twenty-one candidates with IRAC detections at 3.6-microns we find that L*
LBGs at z~6.5 have a median stellar mass of M* = (2.1 +/- 1.1) x 10^9 Msun and
a median specific star-formation rate of 1.9 +/- 0.8 Gyr^-1. Using the same
sub-sample we have investigated the influence of nebular continuum and line
emission, finding that for the majority of candidates (16 out of 21) the
best-fitting stellar-mass estimates are reduced by less than a factor of 2.5.
Finally, a detailed comparison of our final sample with the results of previous
studies suggests that, at faint magnitudes, several high-redshift galaxy
samples in the literature are significantly contaminated by low-redshift
interlopers (abridged).Comment: 35 pages, 22 figures, replaced to match version accepted by MNRAS,
minor changes onl
In search of the QCD-Gravity correspondence
Quantum Chromodynamics (QCD) is the fundamental theory of strong
interactions. It describes the behavior of quarks and gluons which are the
smallest known constituents of nuclear matter. The difficulties in solving the
theory at low energies in the strongly interacting, non-perturbative regime
have left unanswered many important questions in QCD, such as the nature of
confinement or the mechanism of hadronization. In these lectures oriented
towards the students we introduce two classes of dualities that attempt to
reproduce many of the features of QCD, while making the treatment at strong
coupling more tractable: (1) the AdS/CFT correspondence between a specific
class of string theories and a conformal field theory and (2) an effective
low-energy theory of QCD dual to classical QCD on a curved conformal
gravitational background. The hope is that by applying these dualities to the
evaluation of various properties of the strongly-interacting matter produced in
heavy ion collisions one can understand how QCD behaves at strong coupling. We
give an outline of the applications, with emphasis on two transport
coefficients of QCD matter -- shear and bulk viscosities.Comment: 31 pages, 7 figures; Lectures delivered by D. Kharzeev at the
International QGP Winter School, Jaipur, India, February 1-3, 200
The Emission Line Properties of Gravitationally-lensed 1.5 < z < 5 Galaxies
We present and analyse near-infrared spectroscopy for a sample of 28
gravitationally- lensed star-forming galaxies in the redshift range 1.5 < z <
5, observed mostly with the Keck II telescope. With typical magnifications of
~1.5-4 magnitudes, our survey provides a valuable census of star formation
rates, gas-phase metallicities and dynamical masses for a representative sample
of low luminosity galaxies seen at a formative period in cosmic history. We
find less evolution in the mass-metallicity relation compared to earlier work
that focused on more luminous systems with z - 2-3, especially in the low mass
(- 10^9 Msol) where our sample is - 0.25 dex more metal-rich. We interpret this
offset as a result of the lower star formation rates (typically a factor of -10
lower) for a given stellar mass in our sub-luminous systems. Taking this effect
into account, we conclude our objects are consistent with a fundamental
metallicity relation recently proposed from unlensed observations.Comment: 22 pages, 12 figures, MNRAS, version including proof correction
Wormhole cosmic strings
We construct regular multi-wormhole solutions to a gravitating model
in three space-time dimensions, and extend these solutions to cylindrical
traversable wormholes in four and five dimensions. We then discuss the
possibility of identifying wormhole mouths in pairs to give rise to Wheeler
wormholes. Such an identification is consistent with the original field
equations only in the absence of the -model source, but with possible
naked cosmic string sources. The resulting Wheeler wormhole space-times are
flat outside the sources and may be asymptotically Minkowskian.Comment: 17 pages, LaTeX, 4 figures (hard copy available on request
Keck Spectroscopy of 3<z<7 Faint Lyman Break Galaxies: The Importance of Nebular Emission in Understanding the Specific Star Formation Rate and Stellar Mass Density
The physical properties inferred from the SEDs of z>3 galaxies have been
influential in shaping our understanding of early galaxy formation and the role
galaxies may play in cosmic reionization. Of particular importance is the
stellar mass density at early times which represents the integral of earlier
star formation. An important puzzle arising from the measurements so far
reported is that the specific star formation rates (sSFR) evolve far less
rapidly than expected in most theoretical models. Yet the observations
underpinning these results remain very uncertain, owing in part to the possible
contamination of rest-optical broadband light from strong nebular emission
lines. To quantify the contribution of nebular emission to broad-band fluxes,
we investigate the SEDs of 92 spectroscopically-confirmed galaxies in the
redshift range 3.8<z<5.0 chosen because the H-alpha line lies within the
Spitzer/IRAC 3.6 um filter. We demonstrate that the 3.6 um flux is
systematically in excess of that expected from stellar continuum, which we
derive by fitting the SED with population synthesis models. No such excess is
seen in a control sample at 3.1<z<3.6 in which there is no nebular
contamination in the IRAC filters. From the distribution of our 3.6 um flux
excesses, we derive an H-alpha equivalent width (EW) distribution. The mean
rest-frame H-alpha EW we infer at 3.8<z<5.0 (270 A) indicates that nebular
emission contributes at least 30% of the 3.6 um flux. Via our
empirically-derived EW distribution we correct the available stellar mass
densities and show that the sSFR evolves more rapidly at z>4 than previously
thought, supporting up to a 5x increase between z~2 and 7. Such a trend is much
closer to theoretical expectations. Given our findings, we discuss the
prospects for verifying quantitatively the nebular emission line strengths
prior to the launch of the James Webb Space Telescope.Comment: 16 pages, 9 figures, submitted to Ap
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