2,625 research outputs found
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
Fine-Structure FeII* Emission and Resonant MgII Emission in z = 1 Star-Forming Galaxies
We present a study of the prevalence, strength, and kinematics of ultraviolet
FeII and MgII emission lines in 212 star-forming galaxies at z = 1 selected
from the DEEP2 survey. We find FeII* emission in composite spectra assembled on
the basis of different galaxy properties, indicating that FeII* emission is
prevalent at z = 1. In these composites, FeII* emission is observed at roughly
the systemic velocity. At z = 1, we find that the strength of FeII* emission is
most strongly modulated by dust attenuation, and is additionally correlated
with redshift, star-formation rate, and [OII] equivalent width, such that
systems at higher redshifts with lower dust levels, lower star-formation rates,
and larger [OII] equivalent widths show stronger FeII* emission. We detect MgII
emission in at least 15% of the individual spectra and we find that objects
showing stronger MgII emission have higher specific star-formation rates,
smaller [OII] linewidths, larger [OII] equivalent widths, lower dust
attenuations, and lower stellar masses than the sample as a whole. MgII
emission strength exhibits the strongest correlation with specific
star-formation rate, although we find evidence that dust attenuation and
stellar mass also play roles in the regulation of MgII emission. Future
integral field unit observations of the spatial extent of FeII* and MgII
emission in galaxies with high specific star-formation rates, low dust
attenuations, and low stellar masses will be important for probing the
morphology of circumgalactic gas.Comment: 29 pages, 22 figures, 2 tables; accepted to Ap
Star-forming Galaxies in the 'Redshift Desert'
We describe results of optical and near-IR observations of a large
spectroscopic sample of star-forming galaxies photometrically-selected to lie
in the redshift range 1.4 < z < 2.5, often called the ``redshift desert''
because of historical difficulty in obtaining spectroscopic redshifts in this
range. We show that the former ``redshift desert'' is now very much open to
observation.Comment: 10 pages, 6 figures, to appear in the Proceedings of the ESO/USM/MPE
Workshop on "Multiwavelength Mapping of Galaxy Formation and Evolution", eds.
R. Bender and A. Renzin
Probing the Ionization State of the Universe at z>6
We present high signal-to-noise ratio Keck ESI spectra of the two quasars
known to have Gunn-Peterson absorption troughs, SDSS J1030+0524 (z=6.28) and
SDSS J1148+5251 (z=6.37). The Ly alpha and Ly beta troughs for SDSS J1030+0524
are very black and show no evidence for any emission over a redshift interval
of ~0.2 starting at z=6. On the other hand, SDSS J1148+5251 shows a number of
emission peaks in the Ly beta Gunn-Peterson trough along with a single weak
peak in the Ly alpha trough. The Ly alpha emission has corresponding Ly beta
emission, suggesting that it is indeed a region of lower optical depth in the
intergalactic medium at z=6.08.
The stronger Ly beta peaks in the spectrum of SDSS J1148+5251 could
conceivably also be the result of "leaks" in the IGM, but we suggest that they
are instead Ly alpha emission from an intervening galaxy at z=4.9. This
hypothesis gains credence from a strong complex of C IV absorption at the same
redshift and from the detection of continuum emission in the Ly alpha trough at
the expected brightness. If this proposal is correct, the quasar light has
probably been magnified through gravitational lensing by the intervening
galaxy. The Stromgren sphere observed in the absorption spectrum of SDSS
J1148+5251 is significantly smaller than expected based on its brightness,
which is consistent with the hypothesis that the quasar is lensed.
If our argument for lensing is correct, the optical depths derived from the
troughs of SDSS J1148+5251 are only lower limits (albeit still quite strong,
with tau(LyA)>16 inferred from the Ly beta trough.) The Ly beta absorption
trough of SDSS J1030+0524 gives the single best measurement of the IGM
transmission at z>6, with an inferred optical depth tau(LyA)>22.Comment: To appear in July 2003 AJ, 34 pages, 11 figures; minor changes/typos
fixe
Matching structure and bargaining outcomes in buyer–seller networks
We examine the relationship between the matching structure of a bipartite (buyer-seller) network and the (expected) shares of the unit surplus that each connected pair in this network can create. We show that in different bargaining environments, these shares are closely related to the Gallai-Edmonds Structure Theorem. This theorem characterizes the structure of maximum matchings in an undirected graph. We show that the relationship between the (expected) shares and the tructure Theorem is not an artefact of a particular bargaining mechanism or trade centralization. However, this relationship does not necessarily generalize to non-bipartite networks or to networks with heterogeneous link values
The Keck Lyman Continuum Spectroscopic Survey (KLCS): The Emergent Ionizing Spectrum of Galaxies at
We present results of a deep spectroscopic survey designed to quantify the
statistics of the escape of ionizing photons from star-forming galaxies at z~3.
We measure the ratio of ionizing to non-ionizing UV flux density
_obs, where f900 is the mean flux density evaluated over the range
[880,910] A. We quantify the emergent ratio of ionizing to non-ionizing UV flux
density by analyzing high-S/N composite spectra formed from sub-samples with
common observed properties and numbers sufficient to reduce the statistical
uncertainty in the modeled IGM+CGM correction to obtain precise values of
_out, including a full-sample average
_out=. We further show that _out
increases monotonically with Ly rest equivalent width, inducing an
inverse correlation with UV luminosity as a by-product. We fit the composite
spectra using stellar spectral synthesis together with models of the ISM in
which a fraction f_c of the stellar continuum is covered by gas with column
density N(HI). We show that the composite spectra simultaneously constrain the
intrinsic properties of the stars (L900/L1500)_int along with f_c, N(HI),
E(B-V), and , the absolute escape fraction of ionizing photons. We
find a sample-averaged , and that subsamples fall
along a linear relation . We use the FUV luminosity function, the distribution function
, and the relationship between and
_out to estimate the total ionizing emissivity of
star-forming galaxies with Muv < -19.5:
ergs/s/Hz/Mpc, exceeding the contribution of QSOs by a factor of ,
and accounting for % of the total at
estimated using indirect methods.Comment: 45 pages, 31 figures, ApJ, in pres
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
The Kinematics of Multiple-Peaked Lyα Emission in Star-Forming Galaxies at z ~ 2-3
We present new results on the Lyα emission-line kinematics of 18 z ~ 2-3 star-forming galaxies with multiple-peaked Lyα profiles. With our large spectroscopic database of UV-selected star-forming galaxies at these redshifts, we have determined that ~30% of such objects with detectable Lyα emission display multiple-peaked emission profiles. These profiles provide additional constraints on the escape of Lyα photons due to the rich velocity structure in the emergent line. Despite recent advances in modeling the escape of Lyα from star-forming galaxies at high redshifts, comparisons between models and data are often missing crucial observational information. Using Keck II NIRSPEC spectra of Hα (z ~ 2) and [O III]λ5007 (z ~ 3), we have measured accurate systemic redshifts, rest-frame optical nebular velocity dispersions, and emission-line fluxes for the objects in the sample. In addition, rest-frame UV luminosities and colors provide estimates of star formation rates and the degree of dust extinction. In concert with the profile sub-structure, these measurements provide critical constraints on the geometry and kinematics of interstellar gas in high-redshift galaxies. Accurate systemic redshifts allow us to translate the multiple-peaked Lyα profiles into velocity space, revealing that the majority (11/18) display double-peaked emission straddling the velocity-field zero point with stronger red-side emission. Interstellar absorption-line kinematics suggest the presence of large-scale outflows for the majority of objects in our sample, with an average measured interstellar absorption velocity offset of (Δv_(abs))=–230 km s^(–1). A comparison of the interstellar absorption kinematics for objects with multiple- and single-peaked Lyα profiles indicate that the multiple-peaked objects are characterized by significantly narrower absorption line widths. We compare our data with the predictions of simple models for outflowing and infalling gas distributions around high-redshift galaxies. While popular "shell" models provide a qualitative match with many of the observations of Lyα emission, we find that in detail there are important discrepancies between the models and data, as well as problems with applying the framework of an expanding thin shell of gas to explain high-redshift galaxy spectra. Our data highlight these inconsistencies, as well as illuminating critical elements for success in future models of outflow and infall in high-redshift galaxies
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