124 research outputs found
Chemical Abundances in Star-Forming Galaxies at High Redshift
A galaxy's metallicity provides a record of star formation, gas accretion,
and gas outflow, and is therefore one of the most informative measurements that
can be made at high redshift. It is also one of the most difficult. I review
methods of determining chemical abundances in distant star-forming galaxies,
and summarize results for galaxies at 1<z<3. I then focus on the
mass-metallicity relation, its evolution with redshift, and its uses in
constraining inflows and outflows of gas, and conclude with a brief discussion
of future prospects for metallicity measurements at high redshift.Comment: Invited review to appear in "Chemical Abundances in the Universe:
Connecting First Stars to Planets", Proceedings of IAU Symposium 265, Rio de
Janeiro 2009, K. Cunha, M. Spite, B. Barbuy, ed
Filamentary Large-scale Structure Traced by Six Lyα Blobs at z = 2.3
Extended nebulae of Lyα emission ("Lyα blobs") are known to be associated with overdense regions at high redshift. Here we present six large Lyα blobs in a previously known protocluster with galaxy overdensity δ ~ 7 at z = 2.3; this is the richest field of giant Lyα blobs detected to date. The blobs have linear sizes of ≳ 100 kpc and Lyα luminosities of ~10^(43) erg s^(–1). The positions of the blobs define two linear filaments with an extent of at least 12 comoving Mpc; these filaments intersect at the center of one of the blobs. Measurement of the position angles of the blobs indicates that five of the six are aligned with these filaments to within ~10°, suggesting a connection between the physical processes powering extended Lyα emission and those driving structure on larger scales
A Model for Star Formation, Gas Flows and Chemical Evolution in Galaxies at High Redshifts
Motivated by the increasing use of the Kennicutt-Schmidt (K-S) star formation
law to interpret observations of high redshift galaxies, the importance of gas
accretion to galaxy formation, and the recent observations of chemical
abundances in galaxies at z~2-3, I use simple analytical models to assess the
consistency of these processes of galaxy evolution with observations and with
each other. I derive the time dependence of star formation implied by the K-S
law, and show that the sustained high star formation rates observed in galaxies
at z~2-3 require the accretion of additional gas. A model in which the gas
accretion rate is approximately equal to the combined star formation and
outflow rates broadly reproduces the observed trends of star formation rate
with galaxy age. Using an analytical description of chemical evolution, I also
show that this model, further constrained to have an outflow rate roughly equal
to the star formation rate, reproduces the observed mass-metallicity relation
at z~2.Comment: 7 pages, 3 figures. Accepted for publication in Ap
The Direct Detection of Lyman Continuum Emission from Star-forming Galaxies at z~3
We present the results of rest-frame UV spectroscopic observations of a sample of 14 z ~ 3 star-forming galaxies in the SSA 22a field. These spectra are characterized by unprecedented depth in the Lyman continuum region. For the first time, we have detected escaping ionizing radiation from individual galaxies at high redshift, with 2 of the 14 objects showing significant emission below the Lyman limit. We also measured the ratio of emergent flux density at 1500 Å to that in the Lyman continuum region, for the individual detections (C49 and D3) and the sample average. If a correction for the average IGM opacity is applied to the spectra of the objects C49 and D3, we find f_(1500)/f_(900,corr,C49) = 4.5 and f_(1500)/f_(900,corr,D3) = 2.9. The average emergent flux density ratio in our sample is = 22, implying an escape fraction ~4.5 times lower than inferred from the composite spectrum from Steidel and coworkers. If this new estimate is representative of LBGs, their contribution to the metagalactic ionizing radiation field is J_ν(900) ~ 2.6 × 10^(-22) ergs s^(-1) cm^(-2) Hz^(-1) sr^(-1), comparable to the contribution of optically selected quasars at the same redshift. The sum of the contributions from galaxies and quasars is consistent with recent estimates of the level of the ionizing background at z ~ 3, inferred from the H I Lyα forest optical depth. There is significant variance among the emergent far-UV spectra in our sample, yet the factors controlling the detection or nondetection of Lyman continuum emission from galaxies are not well determined. Because we do not yet understand the source of this variance, significantly larger samples will be required to obtain robust constraints on the galaxy contribution to the ionizing background at z ~ 3 and beyond
The Kinematics of Morphologically Selected z~2 Galaxies in the GOODS-N Field
We present near-IR spectra of H-alpha emission from 13 galaxies at z~2 in the
GOODS-N field. The galaxies were selected primarily because they appear to have
elongated morphologies, and slits were aligned with the major axes (as
determined from the rest-frame UV emission) of 11 of the 13. If the galaxies
are elongated because they are highly inclined, alignment of the slit and major
axis should maximize the observed velocity and reveal velocity shear, if
present. In spite of this alignment, we see spatially resolved velocity shear
in only two galaxies. We show that the seeing makes a large difference in the
observed velocity spread of a tilted emission line, and use this information to
place limits on the velocity spread of the ionized gas of the galaxies in the
sample: we find that all 13 have v_{0.5} < 110 km/s, where v_{0.5} is the
velocity shear (half of the velocity range of a tilted emission line) that
would be observed under our best seeing conditions of ~0.5". When combined with
previous work, our data also indicate that aligning the slit along the major
axis does not increase the probability of observing a tilted emission line. We
then focus on the one-dimensional velocity dispersion \sigma, which is much
less affected by the seeing, and see that the elongated subsample exhibits a
significantly lower velocity dispersion than galaxies selected at random from
our total H-alpha sample, not higher as one might have expected. We also see
some evidence that the elongated galaxies are less reddened than those randomly
selected using only UV colors. Both of these results are counter to what would
be expected if the elongated galaxies were highly inclined disks. It is at
least as likely that the galaxies' elongated morphologies are due to merging
subunits.Comment: 9 pages, 5 figures. Accepted for publication in Ap
A Window On The Earliest Star Formation: Extreme Photoionization Conditions of a High-Ionization, Low-Metallicity Lensed Galaxy at z~2
We report new observations of SL2SJ021737-051329, a lens system consisting of
a bright arc at z=1.84435, magnified ~17x by a massive galaxy at z=0.65.
SL2SJ0217 is a low-mass (M <10^9 M*), low-metallicity (Z~1/20 Z*) galaxy, with
extreme star-forming conditions that produce strong nebular UV emission lines
in the absence of any apparent outflows. Here we present several notable
features from rest-frame UV Keck/LRIS spectroscopy: (1) Very strong narrow
emission lines are measured for CIV 1548,1550, HeII 1640, OIII] 1661,1666,
SiIII] 1883,1892, and CIII] 1907,1909. (2) Double-peaked LyA emission is
observed with a dominant blue peak and centered near the systemic velocity. (3)
The low- and high-ionization absorption features indicate very little or no
outflowing gas along the sightline to the lensed galaxy. The relative emission
line strengths can be reproduced with a very high-ionization, low-metallicity
starburst with binaries, with the exception of He \ii, which indicates an
additional ionization source is needed. We rule out large contributions from
AGN and shocks to the photoionization budget, suggesting that the emission
features requiring the hardest radiation field likely result from extreme
stellar populations that are beyond the capabilities of current models.
Therefore, SL2S0217 serves as a template for the extreme conditions that are
important for reionization and thought to be more common in the early Universe.Comment: 28 pages, 16 figures, 8 tables, re-submitted to ApJ, comments welcom
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