5,999 research outputs found
Do galaxies that leak ionizing photons have extreme outflows?
To reionize the early universe, high-energy photons must escape the galaxies
that produce them. It has been suggested that stellar feedback drives galactic
outflows out of star-forming regions, creating low density channels through
which ionizing photons escape into the inter-galactic medium. We compare the
galactic outflow properties of confirmed Lyman continuum (LyC) leaking galaxies
to a control sample of nearby star-forming galaxies to explore whether the
outflows from leakers are extreme as compared to the control sample. We use
data from the Cosmic Origins Spectrograph on the Hubble Space Telescope to
measure the equivalent widths and velocities of Si II and Si III absorption
lines, tracing neutral and ionized galactic outflows. We find that the Si II
and Si III equivalent widths of the LyC leakers reside on the low-end of the
trend established by the control sample. The leakers' velocities are not
statistically different than the control sample, but their absorption line
profiles have a different asymmetry: their central velocities are closer to
their maximum velocities. The outflow kinematics and equivalent widths are
consistent with the scaling relations between outflow properties and host
galaxy properties -- most notably metallicity -- defined by the control sample.
Additionally, we use the Ly\alpha\ profiles to show that the Si II equivalent
width scales with the Ly\alpha\ peak velocity separation. We determine that the
low equivalent widths of the leakers are likely driven by low metallicities and
low H I column densities, consistent with a density-bounded ionization region,
although we cannot rule out significant variations in covering fraction. While
we do not find that the LyC leakers have extreme outflow velocities, the low
maximum-to-central velocity ratios demonstrate the importance of the
acceleration and density profiles for LyC and Ly\alpha\ escape. [abridged]Comment: 17 pages, 8 Figures. Accepted for publication in Astronomy &
Astrophysic
The Lyman Alpha Reference Sample. VIII. Characterizing Lyman-Alpha Scattering in Nearby Galaxies
We examine the dust geometry and Ly{\alpha} scattering in the galaxies of the
Lyman Alpha Reference Sample (LARS), a set of 14 nearby (0.02 < < 0.2)
Ly{\alpha} emitting and starbursting systems with Hubble Space Telescope
Ly{\alpha}, H{\alpha}, and H{\beta} imaging. We find that the global dust
properties determined by line ratios are consistent with other studies, with
some of the LARS galaxies exhibiting clumpy dust media while others of them
show significantly lower Ly{\alpha} emission compared to their Balmer
decrement. With the LARS imaging, we present Ly{\alpha}/H{\alpha} and
H{\alpha}/H{\beta} maps with spatial resolutions as low as 40 pc, and
use these data to show that in most galaxies, the dust geometry is best modeled
by three distinct regions: a central core where dust acts as a screen, an
annulus where dust is distributed in clumps, and an outer envelope where
Ly{\alpha} photons only scatter. We show that the dust that affects the escape
of Ly{\alpha} is more restricted to the galaxies' central regions, while the
larger Ly{\alpha} halos are generated by scattering at large radii. We present
an empirical modeling technique to quantify how much Ly{\alpha} scatters in the
halo, and find that this "characteristic" scattering distance correlates with
the measured size of the Ly{\alpha} halo. We note that there exists a slight
anti-correlation between the scattering distance of Ly{\alpha} and global dust
properties.Comment: 32 pages, 51 figures, accepted to Ap
Constraining the metallicities, ages, star formation histories, and ionizing continua of extragalactic massive star populations
We infer the properties of massive star populations using the far-ultraviolet
stellar continua of 61 star-forming galaxies: 42 at low-z observed with HST and
19 at z~2 from the Megasaura sample. We fit each stellar continuum with a
linear combination of up to 50 single age and single metallicity Starburst99
models. From these fits, we derive light-weighted ages and metallicities, which
agree with stellar wind and photospheric spectral features, and infer the
spectral shapes and strengths of the ionizing continua. Inferred light-weighted
stellar metallicities span 0.05-1.5 Z and are similar to the measured
nebular metallicities. We quantify the ionizing continua using the ratio of the
ionizing flux at 900\AA\ to the non-ionizing flux at 1500\AA\ and demonstrate
the evolution of this ratio with stellar age and metallicity using theoretical
single burst models. These single burst models only match the inferred ionizing
continua of half of the sample, while the other half are described by a mixture
of stellar ages. Mixed age populations produce stronger and harder ionizing
spectra than continuous star formation histories, but, contrary to previous
studies that assume constant star formation, have similar stellar and nebular
metallicities. Stellar population age and metallicity affect the far-UV
continua in different and distinguishable ways; assuming a constant star
formation history diminishes the diagnostic power. Finally, we provide simple
prescriptions to determine the ionizing photon production efficiency
() from the stellar population properties. has a range
of log( Hz erg that depends on stellar age,
metallicity, star formation history, and contributions from binary star
evolution. These stellar population properties must be observationally
determined to determine the number of ionizing photons generated by massive
stars.Comment: 31 pages, 23 figures, resubmitted to ApJ after incorporating the
referee's comments. Comments encourage
Keck Spectroscopy of Faint 3<z<7 Lyman Break Galaxies: - I. New constraints on cosmic reionisation from the luminosity and redshift-dependent fraction of Lyman-alpha emission
We present results from a new Keck spectroscopic survey of UV-faint LBGs in
the redshift range 3<z<7. Combined with earlier Keck and published ESO VLT
data, our sample contains more than 600 dropouts, offering new insight into the
nature of sub-L* sources typical of those likely to dominate the cosmic
reionisation process. Here we use this sample to characterise the fraction of
strong Lya emitters within the continuum-selected dropouts. By quantifying how
the "Lya fraction" varies with redshift, we seek to constrain changes in Lya
transmission associated with reionisation. In order to distinguish the effects
of reionisation from other factors which affect the Lya fraction (e.g. dust,
ISM kinematics), we study the luminosity and redshift-dependence of the Lya
fraction over 3<z<6, when the IGM is known to be ionised. These results reveal
that low luminosity galaxies show strong Lya emission much more frequently than
luminous systems, and that at fixed luminosity, the prevalence of strong Lya
emission increases moderately with redshift over 3 < z < 6. Based on the
correlation between blue UV slopes and strong Lya emitting galaxies in our
dataset, we argue that the Lya fraction trends are governed by redshift and
luminosity-dependent variations in the dust obscuration, with likely additional
contributions from trends in the kinematics and covering fraction of neutral
hydrogen. We find a tentative decrease in the Lya fraction at z~7 based on the
limited IR spectra for candidate z~7 lensed LBGs, a result which, if confirmed
with future surveys, would suggest an increase in the neutral fraction by this
epoch. Given the supply of z and Y-drops now available from Hubble WFC3/IR
surveys, we show it will soon be possible to significantly improve estimates of
the Lya fraction using optical and near-IR spectrographs, thereby extending the
study conducted in this paper to 7<z<8.Comment: 23 pages, 15 figures, submitted to MNRA
An HST Search for Lyman Continuum Emission From Galaxies at z=1.1--1.4
If enough of their Lyman limit continuum escapes, star-forming galaxies could
be significant contributors to the cosmic background of ionizing photons. To
investigate this possibility, we obtained the first deep imaging in the far
ultraviolet of eleven bright blue galaxies at intermediate redshift
(z=1.1--1.4). NO Lyman continuum emission was detected. Sensitive,
model-independent, upper limits of typically 2 x 10**-19 erg/sec/cm2/Ang were
obtained for the ionizing flux escaping from these normal galaxies. This
corresponds to lower limits on the observed ratio of 1500 to 700Ang flux of 150
up to 1000. Based on a wide range of stellar synthesis models, this suggests
that less than 6%, down to less than 1%, of the available ionizing flux emitted
by hot stars is escaping these galaxies. The magnitude of this spectral break
at the Lyman l imit confirms that the basic premise of `Lyman break' searches
for galaxies at high redshift can also be applied at intermediate redshifts.
This implies that the integrated contribution of galaxies to the UV cosmic
background at z around 1.2 is less than 15%, and may be less than 2%.Comment: 20 manuscript pages, which includes two tables and two figures. To be
published in 1 December 2003 issue of The Astrophysical Journa
On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback From Momentum-Driven Winds
We investigate large-scale galactic winds driven by momentum deposition.
Momentum injection is provided by (1) radiation pressure produced by the
continuum absorption and scattering of UV photons on dust grains and (2)
supernovae. UV radiation can be produced by a starburst or AGN activity. We
argue that momentum-driven winds are an efficient mechanism for feedback during
the formation of galaxies. We show that above a limiting luminosity, momentum
deposition from star formation can expel a significant fraction of the gas in a
galaxy. The limiting, Eddington-like luminosity is , where is the galaxy velocity dispersion and is the
gas fraction. A starburst that attains moderates its star formation
rate and its luminosity does not increase significantly further. We argue that
ellipticals attain this limit during their growth at and that
this is the origin of the Faber-Jackson relation. We show that Lyman break
galaxies and ultra-luminous infrared galaxies have luminosities near . Star formation is unlikely to efficiently remove gas from very small
scales in galactic nuclei, i.e., scales much smaller than that of a nuclear
starburst. This gas is available to fuel a central black hole (BH). We argue
that a BH clears gas out of its galactic nucleus when the luminosity of the BH
itself reaches . This shuts off the fuel supply to the BH
and may also terminate star formation in the surrounding galaxy. As a result,
the BH mass is fixed to be , where is the electron scattering opacity. This
limit is in accord with the observed relation. (Abridged)Comment: 21 pages, emulateapj, accepted to ApJ, minor changes to discussio
Mission: Impossible (Escape from the Lyman Limit)
We investigate the intrinsic opacity of high-redshift galaxies to outgoing
ionising photons using high-quality photometry of a sample of 27
spectroscopically-identified galaxies of redshift 1.9<z<3.5 in the Hubble Deep
Field. Our measurement is based on maximum-likelihood fitting of model galaxy
spectral energy distributions-including the effects of intrinsic Lyman-limit
absorption and random realizations of intervening Lyman-series and Lyman-limit
absorption-to photometry of galaxies from space- and ground-based broad-band
images. Our method provides several important advantages over the methods used
by previous groups, including most importantly that two-dimensional sky
subtraction of faint-galaxy images is more robust than one-dimensional sky
subtraction of faint-galaxy spectra. We find at the 3sigma statistical
confidence level that on average no more than 4% of the ionising photons escape
galaxies of redshift 1.9<z<3.5. This result is consistent with observations of
low- and moderate-redshift galaxies but is in direct contradiction to a recent
result based on medium-resolution spectroscopy of high-redshift (z~3) galaxies.
Dividing our sample in subsamples according to luminosity, intrinsic
ultraviolet colour, and redshift, we find no evidence for selection effects
that could explain such discrepancy. Even when all systematic effects are
included, the data could not realistically accomodate any escape fraction value
larger than ~15%.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 8 pages, 4 b/w figures, MNRAS styl
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