75 research outputs found
The LyAlpha Line Profiles of Ultraluminous Infrared Galaxies: Fast Winds and Lyman Continuum Leakage
We present new Hubble Space Telescope Cosmic Origins Spectrograph
far-ultraviolet (far-UV) spectroscopy and Keck Echellete optical spectroscopy
of 11 ultraluminous infrared galaxies (ULIRGs), a rare population of local
galaxies experiencing massive gas inflows, extreme starbursts, and prominent
outflows. We detect H Lyman alpha emission from 8 ULIRGs and the companion to
IRAS09583+4714. In contrast to the P Cygni profiles often seen in galaxy
spectra, the H Lyman alpha profiles exhibit prominent, blueshifted emission out
to Doppler shifts exceeding -1000 km/s in three HII-dominated and two
AGN-dominated ULIRGs. To better understand the role of resonance scattering in
shaping the H Lyman alpha line profiles, we directly compare them to
non-resonant emission lines in optical spectra. We find that the line wings are
already present in the intrinsic nebular spectra, and scattering merely
enhances the wings relative to the line core. The H Lyman alpha attenuation (as
measured in the COS aperture) ranges from that of the far-UV continuum to over
100 times more. A simple radiative transfer model suggests the H Lyman alpha
photons escape through cavities which have low column densities of neutral
hydrogen and become optically thin to the Lyman continuum in the most advanced
mergers. We show that the properties of the highly blueshifted line wings on
the H Lyman alpha and optical emission-line profiles are consistent with
emission from clumps of gas condensing out of a fast, hot wind. The luminosity
of the H Lyman alpha emission increases non-linearly with the ULIRG bolometric
luminosity and represents about 0.1 to 1% of the radiative cooling from the hot
winds in the HII-dominated ULIRGs.Comment: Submitted to Ap
The metallicity evolution of low mass galaxies: New constraints at intermediate redshift
We present abundance measurements from 26 emission-line selected galaxies at
z~0.6-0.7. By reaching stellar masses as low as 10^8 M_{\sun}, these
observations provide the first measurement of the intermediate redshift
mass-metallicity (MZ) relation below 10^9 M_{\sun} For the portion of our
sample above M > 10^9 M_{\sun} (8/26 galaxies), we find good agreement with
previous measurements of the intermediate redshift MZ relation. Compared to the
local relation, we measure an evolution that corresponds to a 0.12 dex decrease
in oxygen abundances at intermediate redshifts. This result confirms the trend
that metallicity evolution becomes more significant towards lower stellar
masses, in keeping with a downsizing scenario where low mass galaxies evolve
onto the local MZ relation at later cosmic times. We show that these galaxies
follow the local fundamental metallicity relation, where objects with higher
specific (mass-normalized) star formation rates (SFRs) have lower
metallicities. Furthermore, we show that the galaxies in our sample lie on an
extrapolation of the SFR-M_{*} relation (the star-forming main sequence).
Leveraging the MZ relation and star-forming main sequence (and combining our
data with higher mass measurements from the literature), we test models that
assume an equilibrium between mass inflow, outflow and star formation. We find
that outflows are required to describe the data. By comparing different outflow
prescriptions, we show that momentum driven winds can describe the MZ relation;
however, this model under-predicts the amount of star formation in low mass
galaxies. This disagreement may indicate that preventive feedback from
gas-heating has been overestimated, or it may signify a more fundamental
deviation from the equilibrium assumption.Comment: Accepted for publication in Ap
A search for Lyman Break Galaxies at z>8 in the NICMOS Parallel Imaging Survey
We have selected 14 J-dropout Lyman Break Galaxy (LBG) candidates with J110 -
H160 > 2.5 from the NICMOS Parallel Imaging Survey. This survey consists of 135
square arcminutes of imaging in 228 independent sight lines, reaching average 5
sigma sensitivities of J110 = 25.8 and H160 = 25.6 (AB). Distinguishing these
candidates from dust reddened star forming galaxies at z ~ 2-3 is difficult,
and will require longer wavelength observations. We consider the likelihood
that any J-dropout LBGs exist in this survey, and find that if L*(z=9.5) is
significantly brighter than L*(z=6) (a factor of four), then a few J-dropout
LBGs are likely. A similar increase in luminosity has been suggested by Eyles
et al. and Yan et al., but the magnitude of this increase is uncertain.Comment: Accepted to ApJ Letter
Predicting Future Space Near-IR Grism Surveys using the WFC3 Infrared Spectroscopic Parallels Survey
We present near-infrared emission line counts and luminosity functions from
the HST WFC3 Infrared Spectroscopic Parallels (WISP) program for 29 fields
(0.037 deg^2) observed using both the G102 and G141 grisms. Altogether we
identify 1048 emission line galaxies with observed equivalent widths greater
than 40 Angstroms, 467 of which have multiple detected emission lines. The WISP
survey is sensitive to fainter flux levels (3-5x10^-17 ergs/s/cm^2) than the
future space near-infrared grism missions aimed at baryonic acoustic
oscillation cosmology (1-4x10^-16 ergs/s/cm^2), allowing us to probe the
fainter emission line galaxies that the shallower future surveys may miss.
Cumulative number counts of 0.7<z<1.5 galaxies reach 10,000 deg^-2 above an
H-alpha flux of 2x10^-16 ergs/s/cm^2. H-alpha-emitting galaxies with comparable
[OIII] flux are roughly 5 times less common than galaxies with just H-alpha
emission at those flux levels. Galaxies with low H-alpha/[OIII] ratios are very
rare at the brighter fluxes that future near-infrared grism surveys will probe;
our survey finds no galaxies with H-alpha/[OIII] < 0.95 that have H-alpha flux
greater than 3x10^-16 ergs/s/cm^2. Our H-alpha luminosity function contains a
comparable number density of faint line emitters to that found by the NICMOS
near-infrared grism surveys, but significantly fewer (factors of 3-4 less) high
luminosity emitters. We also find that our high redshift (z=0.9-1.5) counts are
in agreement with the high redshift (z=1.47) narrow band H-alpha survey of
HiZELS (Sobral et al. 2013), while our lower redshift luminosity function
(z=0.3-0.9) falls slightly below their z=0.84 result. The evolution in both the
H-alpha luminosity function from z=0.3--1.5 and the [OIII] luminosity function
from z=0.7-2.3 is almost entirely in the L* parameter, which steadily increases
with redshift over those ranges.Comment: 18 pages, 14 figures, Accepted by Ap
The Grism Lens-Amplified Survey from Space (GLASS) X. Sub-kpc resolution gas-phase metallicity maps at cosmic noon behind the Hubble Frontier Fields cluster MACS1149.6+2223
(Abridged) We combine deep HST grism spectroscopy with a new Bayesian method
to derive maps of gas-phase metallicity, nebular dust extinction, and
star-formation rate for 10 star-forming galaxies at high redshift
(). Exploiting lensing magnification by the foreground cluster
MACS1149.6+2223, we reach sub-kpc spatial resolution and push the stellar mass
limit associated with such high-z spatially resolved measurements below
for the first time. Our maps exhibit diverse morphologies,
indicative of various effects such as efficient radial mixing from tidal
torques, rapid accretion of low-metallicity gas, etc., which can affect the gas
and metallicity distributions in individual galaxies. Based upon an exhaustive
sample of all existing sub-kpc metallicity gradients at high-z, we find that
predictions given by analytical chemical evolution models assuming a relatively
extended star-formation profile in the early disk formation phase can explain
the majority of observed gradients, without involving galactic feedback or
radial outflows. We observe a tentative correlation between stellar mass and
metallicity gradient, consistent with the downsizing galaxy formation picture
that more massive galaxies are more evolved into a later phase of disk growth,
where they experience more coherent mass assembly at all radii and thus show
shallower metallicity gradients. In addition, we compile a sample of
homogeneously cross-calibrated integrated metallicity measurements spanning
three orders of magnitude in stellar mass at . We use this sample to
study the mass-metallicity relation (MZR) and test the fundamental metallicity
relation (FMR). The slope of the observed MZR can rule out the momentum-driven
wind model at 3- confidence level. We find no significant offset with
respect to the FMR, taking into account the intrinsic scatter and measurement
uncertainties.Comment: 26 pages, 14 figures, and 6 table
Physical Properties of Emission-Line Galaxies at z ~ 2 from Near-Infrared Spectroscopy with Magellan FIRE
We present results from near-infrared spectroscopy of 26 emission-line
galaxies at z ~ 2 obtained with the FIRE spectrometer on the Magellan Baade
telescope. The sample was selected from the WISP survey, which uses the
near-infrared grism of the Hubble Space Telescope Wide Field Camera 3 to detect
emission-line galaxies over 0.3 < z < 2.3. Our FIRE follow-up spectroscopy
(R~5000) over 1.0-2.5 micron permits detailed measurements of physical
properties of the z~2 emission-line galaxies. Dust-corrected star formation
rates for the sample range from ~5-100 M_sun yr-1. We derive a median
metallicity for the sample of ~0.45 Z_sun, and the estimated stellar masses
range from ~10^8.5 - 10^9.5 M_sun. The average ionization parameters measured
for the sample are typically much higher than what is found for local
star-forming galaxies. We derive composite spectra from the FIRE sample, from
which we infer typical nebular electron densities of ~100-400 cm^-3. Based on
the location of the galaxies and composite spectra on BPT diagrams, we do not
find evidence for significant AGN activity in the sample. Most of the galaxies
as well as the composites are offset in the BPT diagram toward higher [O
III]/H-beta at a given [N II]/H-alpha, in agreement with other observations of
z > 1 star-forming galaxies, but composite spectra derived from the sample do
not show an appreciable offset from the local star-forming sequence on the [O
III]/H-beta versus [S II]/H-alpha diagram. We infer a high nitrogen-to-oxygen
abundance ratio from the composite spectrum, which may contribute to the offset
of the high-redshift galaxies from the local star-forming sequence in the [O
III]/H-beta versus [N II]/H-alpha diagram. We speculate that the elevated
nitrogen abundance could result from substantial numbers of Wolf-Rayet stars in
starbursting galaxies at z~2. (Abridged)Comment: Accepted for publication in Ap
First Census of Gas-phase Metallicity Gradients of Star-forming Galaxies in Overdense Environments at Cosmic Noon
We report the first spatially resolved measurements of gas-phase metallicity
radial gradients in star-forming galaxies in overdense environments at
. The spectroscopic data are acquired by the \mg\ survey, a Hubble
Space Telescope (HST) cycle-28 medium program. This program is obtaining 45
orbits of WFC3/IR grism spectroscopy in the density peak regions of three
massive galaxy protoclusters (BOSS 1244, BOSS 1542 and BOSS 1441) at .
Our sample in the BOSS 1244 field consists of 20 galaxies with stellar-mass
ranging from to \Msun\ , star formation rate (SFR) from
10 to 240 \Msun\,yr, and global gas-phase metallicity (\oh) from 8.2 to
8.6. At confidence level, 2/20 galaxies in our sample show positive
(inverted) gradients -- the relative abundance of oxygen increasing with
galactocentric radius, opposite the usual trend. Furthermore, 1/20 shows
negative gradients and 17/20 are consistent with flat gradients. This high
fraction of flat/inverted gradients is uncommon in simulations and previous
observations conducted in blank fields at similar redshifts. To understand
this, we investigate the correlations among various observed properties of our
sample galaxies. We find an anticorrelation between metallicity gradient and
global metallicity of our galaxies residing in extreme overdensities, and a
marked deficiency of metallicity in our massive galaxies as compared to their
coeval field counterparts. We conclude that the cold-mode gas accretion plays
an active role in shaping the chemical evolution of galaxies in the
protocluster environments, diluting their central chemical abundance, and
flattening/inverting their metallicity gradients.Comment: 13 pages, 6 figures, 1 table. Accepted for publication in ApJ
Expanding the search for galaxies at z ~7-10 with new NICMOS Parallel Fields
We have carried out a search for galaxies at z ~ 7-10 in ~14.4 sq. arcmin of
new NICMOS parallel imaging taken in the Great Observatories Origins Deep
Survey (GOODS, 5.9 sq. arcmin), the Cosmic Origins Survey (COSMOS, 7.2 sq.
arcmin), and SSA22 (1.3 sq. arcmin). These images reach 5 sigma sensitivities
of J110 = 26.0-27.5 (AB), and combined they increase the amount of deep
near-infrared data by more than 60% in fields where the investment in deep
optical data has already been made. We find no z>7 candidates in our survey
area, consistent with the Bouwens et al. (2008) measurements at z~7 and 9 (over
23 sq. arcmin), which predict 0.7 galaxies at z~7 and <0.03 galaxies at z~9. We
estimate that 10-20% of z>7 galaxies are missed by this survey, due to
incompleteness from foreground contamination by faint sources. For the case of
luminosity evolution, assuming a Schecter parameterization with a typical phi*
= 10^-3 Mpc^-3, we find M* > -20.0 for z~7 and M* > -20.7 for z~9 (68%
confidence). This suggests that the downward luminosity evolution of LBGs
continues to z~7, although our result is marginally consistent with the z~6 LF
of Bouwens et al.(2006, 2007). In addition we present newly-acquired deep
MMT/Megacam imaging of the z~9 candidate JD2325+1433, first presented in Henry
et al. (2008). The resulting weak but significant detection at i' indicates
that this galaxy is most likely an interloper at z~2.7.Comment: Accepted to ApJ. Replacement includes updated discussion of
incompleteness from foreground contaminatio
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