293 research outputs found
Radio and Far-Infrared Emission as Tracers of Star Formation and AGN in Nearby Cluster Galaxies
We have studied the radio and far-infrared (FIR) emission from 114 galaxies
in the 7 nearest clusters (<100 Mpc) with prominent X-ray emission to
investigate the impact of the cluster environment on the star formation and AGN
activity in the member galaxies. The X-ray selection criterion is adopted to
focus on the most massive and dynamically relaxed clusters. A large majority of
cluster galaxies show an excess in radio emission over that predicted from the
radio-FIR correlation, the fraction of sources with radio excess increases
toward cluster cores, and the radial gradient in the FIR/radio flux ratio is a
result of radio enhancement. Of the radio-excess sources, 70% are early-type
galaxies and the same fraction host an AGN. The galaxy density drops by a
factor of 10 from the composite cluster center out to 1.5 Mpc, yet galaxies
show no change in FIR properties over this region, and show no indication of
mass segregation. We have examined in detail the physical mechanisms that might
impact the FIR and radio emission of cluster galaxies. While collisional
heating of dust may be important for galaxies in cluster centers, it appears to
have a negligible effect on the observed FIR emission for our sample galaxies.
The correlations between radio and FIR luminosity and radius could be explained
by magnetic compression from thermal ICM pressure. We also find that simple
delayed harassment cannot fully account for the observed radio, FIR, and mid-IR
properties of cluster galaxies.Comment: 21 pages, 15 figures, Accepted by Ap
Spectroscopic Measurements of the Far-Ultraviolet Dust Attenuation Curve at z~3
We present the first measurements of the shape of the far-ultraviolet
(far-UV; lambda=950-1500 A) dust attenuation curve at high redshift (z~3). Our
analysis employs rest-frame UV spectra of 933 galaxies at z~3, 121 of which
have very deep spectroscopic observations (>7 hrs) at lambda=850-1300 A, with
the Low Resolution Imaging Spectrograph on the Keck Telescope. By using an
iterative approach in which we calculate the ratios of composite spectra in
different bins of continuum color excess, E(B-V), we derive a dust curve that
implies a lower attenuation in the far-UV for a given E(B-V) than those
obtained with standard attenuation curves. We demonstrate that the UV composite
spectra of z~3 galaxies can be modeled well by assuming our new attenuation
curve, a high covering fraction of HI, and absorption from the Lyman-Werner
bands of H2 with a small (<20%) covering fraction. The low covering fraction of
H2 relative to that of the HI and dust suggests that most of the dust in the
ISM of typical galaxies at z~3 is unrelated to the catalysis of H2, and is
associated with other phases of the ISM (i.e., the ionized and neutral gas).
The far-UV dust curve implies a factor of ~2 lower dust attenuation of Lyman
continuum (ionizing) photons relative to those inferred from the most commonly
assumed attenuation curves for L* galaxies at z~3. Our results may be utilized
to assess the degree to which ionizing photons are attenuated in HII regions
or, more generally, in the ionized or low column density (N(HI)<10^17.2 cm^-2)
neutral ISM of high-redshift galaxies.Comment: 12 pages, 1 table, 8 figures, accepted to the Astrophysical Journa
A Glimpse of the First Galaxies
The recently refurbished Hubble Space Telescope reveals a galaxy from a time
when the Universe was just 500 million years old, providing insights into the
first throes of galaxy formation and the reionization of the Universe.Comment: Invited Nature "News and Views" Commentary on Bouwens et al. 2011,
Nature, 469, 504-507; 5 pages, 1 figur
The Connection Between Reddening, Gas Covering Fraction, and the Escape of Ionizing Radiation at High Redshift
We use a large sample of galaxies at z~3 to establish a relationship between
reddening, neutral gas covering fraction (fcov(HI)), and the escape of ionizing
photons at high redshift. Our sample includes 933 galaxies at z~3, 121 of which
have very deep spectroscopic observations (>7 hrs) in the rest-UV
(lambda=850-1300 A) with Keck/LRIS. Based on the high covering fraction of
outflowing optically-thick HI indicated by the composite spectra of these
galaxies, we conclude that photoelectric absorption, rather than dust
attenuation, dominates the depletion of ionizing photons. By modeling the
composite spectra as the combination of an unattenuated stellar spectrum
including nebular continuum emission with one that is absorbed by HI and
reddened by a line-of-sight extinction, we derive an empirical relationship
between E(B-V) and fcov(HI). Galaxies with redder UV continua have larger
covering fractions of HI characterized by higher line-of-sight extinctions. Our
results are consistent with the escape of Lya through gas-free lines-of-sight.
Covering fractions based on low-ionization interstellar absorption lines
systematically underpredict those deduced from the HI lines, suggesting that
much of the outflowing gas may be metal-poor. We develop a model which connects
the ionizing escape fraction with E(B-V), and which may be used to estimate the
escape fraction for an ensemble of high-redshift galaxies. Alternatively,
direct measurements of the escape fraction for our data allow us to constrain
the intrinsic 900-to-1500 A flux density ratio to be >0.20, a value that favors
stellar population models that include weaker stellar winds, a flatter initial
mass function, and/or binary evolution. Lastly, we demonstrate how the
framework discussed here may be used to assess the pathways by which ionizing
radiation escapes from high-redshift galaxies. [Abridged]Comment: 22 pages, 3 tables, 14 figures, accepted to the Astrophysical Journa
X-Ray and Radio Emission from UV-Selected Star Forming Galaxies at Redshifts 1.5<Z<3.0 in the GOODS-North Field
We have examined the stacked radio and X-ray emission from UV-selected
galaxies spectroscopically confirmed to lie between redshifts 1.5 < z < 3.0 in
the GOODS-North field to determine their average extinction and star formation
rates (SFRs). The X-ray and radio data are obtained from the Chandra 2 Msec
survey and the Very Large Array, respectively. There is a good agreement
between the X-ray, radio, and de-reddened UV estimates of the average SFR for
our sample of z~2 galaxies of ~50 solar masses per year, indicating that the
locally-calibrated SFR relations appear to be statistically valid from
redshifts 1.5 < z < 3.0. We find that UV-estimated SFRs (uncorrected for
extinction) underestimate the bolometric SFRs as determined from the 2-10 keV
X-ray luminosity by a factor of ~4.5 to 5.0 for galaxies over a large range in
redshift from 1.0 < z < 3.5.Comment: 5 pages, 1 figure, Accepted by ApJ Letter
A High-Resolution Hubble Space Telescope Study of Apparent Lyman Continuum Leakers at
We present follow-up observations of 16
candidate LyC emitters in the HS1549+1919 field. With these data, we
obtain high spatial-resolution photometric redshifts of all sub-arcsecond
components of the LyC candidates in order to eliminate foreground contamination
and identify robust candidates for leaking LyC emission. Of the 16 candidates,
we find one object with a robust LyC detection that is not due to foreground
contamination. This object (MD5) resolves into two components; we refer to the
LyC-emitting component as MD5b. MD5b has an observed 1500\AA\ to 900\AA\
flux-density ratio of , compatible with
predictions from stellar population synthesis models. Assuming minimal IGM
absorption, this ratio corresponds to a relative (absolute) escape fraction of
% (%). The stellar
population fit to MD5b indicates an age of Myr, which is in the
youngest 10% of the sample and the youngest third of typical
Lyman break galaxies, and may be a contributing factor to its LyC detection. We
obtain a revised, contamination-free estimate for the comoving specific
ionizing emissivity at , indicating (with large uncertainties) that
star-forming galaxies provide roughly the same contribution as QSOs to the
ionizing background at this redshift. Our results show that foreground
contamination prevents ground-based LyC studies from obtaining a full
understanding of LyC emission from star-forming galaxies. Future
progress in direct LyC searches is contingent upon the elimination of
foreground contaminants through high spatial-resolution observations, and upon
acquisition of sufficiently deep LyC imaging to probe ionizing radiation in
high-redshift galaxies.Comment: 31 pages, 5 tables, 19 figures. Accepted to ApJ. Version with
full-resolution figures is available at:
http://www.astro.ucla.edu/~aes/Mostardi_HST_LyC.pd
Investigating Hα, UV, and IR Star-formation Rate Diagnostics for a Large Sample of z ~ 2 Galaxies
We use a sample of 262 spectroscopically confirmed star-forming galaxies at redshifts 2.08 ≤ z ≤ 2.51 to compare Hα, ultraviolet (UV), and IR star formation rate (SFR) diagnostics and to investigate the dust properties of the galaxies. At these redshifts, the Hα line shifts to the K_s band. By comparing K_s-band photometry to underlying stellar population model fits to other UV, optical, and near-infrared data, we infer the Hα flux for each galaxy. We obtain the best agreement between Hα- and UV-based SFRs if we assume that the ionized gas and stellar continuum are reddened by the same value and that the Calzetti attenuation curve is applied to both. Aided with MIPS 24 μm data, we find that an attenuation curve steeper than the Calzetti curve is needed to reproduce the observed IR/UV ratios of galaxies younger than 100 Myr. Furthermore, using the bolometric SFR inferred from the UV and mid-IR data (SFR_(IR)+SFR_(UV), we calculated the conversion between the Hα luminosity and SFR to be (7.5 ± 1.3) x 10^(-42) for a Salpeter initial mass function, which is consistent with the Kennicutt conversion. The derived conversion factor is independent of any assumption of the dust correction and is robust to stellar population model uncertainties
Optical Selection of Galaxies at Redshifts 1<z<3
Few galaxies have been found between the redshift ranges z < ~1 probed by
magnitude-limited surveys and z > ~3 probed by Lyman-break surveys. Comparison
of galaxy samples at lower and higher redshift suggests that large numbers of
stars were born and the Hubble sequence began to take shape at the intermediate
redshifts 1<z<3, but observational challenges have prevented us from observing
the process in much detail. We present simple and efficient strategies that can
be used to find large numbers of galaxies throughout this important but
unexplored redshift range. All the strategies are based on selecting galaxies
for spectroscopy on the basis of their colors in ground-based images taken
through a small number of optical filters: GRi for redshifts 0.85<z<1.15, GRz
for 1<z<1.5, and UGR for 1.4<z<2.1 and 1.9<z<2.7. The performance of our
strategies is quantified empirically through spectroscopy of more than 2000
galaxies at 1<z<3.5. We estimate that more than half of the UV-luminosity
density at 1<z<3 is produced by galaxies that satisfy our color-selection
criteria. Our methodology is described in detail, allowing readers to devise
analogous selection criteria for other optical filter sets.Comment: 13 pages including 20 figures. Accepted for publication in the Ap
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