175 research outputs found
Line Emitting Galaxies Beyond a Redshift of 7: An Improved Method for Estimating the Evolving Neutrality of the Intergalactic Medium
The redshift-dependent fraction of color-selected galaxies revealing Lyman
alpha emission has become the most valuable constraint on the evolving
neutrality of the early intergalactic medium. However, in addition to resonant
scattering by neutral gas, the visibility of Lyman alpha is also dependent on
the intrinsic properties of the host galaxy, including its stellar population,
dust content and the nature of outflowing gas. Taking advantage of significant
progress we have made in determining the line emitting properties of galaxies, we propose an improved method, based on using the measured
slopes of the rest-frame ultraviolet continua of galaxies, to interpret the
growing body of near-infrared spectra of galaxies in order to take into
account these host galaxy dependencies. In a first application of our new
method, we demonstrate its potential via a new spectroscopic survey of
galaxies undertaken with the Keck MOSFIRE spectrograph. Together with earlier
published data our data provides improved estimates of the evolving visibility
of Lyman alpha, particularly at redshift . As a byproduct, we also
present a new line emitting galaxy at a redshift which supersedes an
earlier redshift record. We discuss the improving constraints on the evolving
neutral fraction over and the implications for cosmic reionization.Comment: To be submitted to Ap
Planning for Decentralized Control of Multiple Robots Under Uncertainty
We describe a probabilistic framework for synthesizing control policies for
general multi-robot systems, given environment and sensor models and a cost
function. Decentralized, partially observable Markov decision processes
(Dec-POMDPs) are a general model of decision processes where a team of agents
must cooperate to optimize some objective (specified by a shared reward or cost
function) in the presence of uncertainty, but where communication limitations
mean that the agents cannot share their state, so execution must proceed in a
decentralized fashion. While Dec-POMDPs are typically intractable to solve for
real-world problems, recent research on the use of macro-actions in Dec-POMDPs
has significantly increased the size of problem that can be practically solved
as a Dec-POMDP. We describe this general model, and show how, in contrast to
most existing methods that are specialized to a particular problem class, it
can synthesize control policies that use whatever opportunities for
coordination are present in the problem, while balancing off uncertainty in
outcomes, sensor information, and information about other agents. We use three
variations on a warehouse task to show that a single planner of this type can
generate cooperative behavior using task allocation, direct communication, and
signaling, as appropriate
Contamination of Broadband Photometry by Nebular Emission in High-redshift Galaxies: Investigations with Keck's MOSFIRE Near-infrared Spectrograph
Earlier work has raised the potential importance of nebular emission in the derivation of the physical characteristics of high-redshift Lyman break galaxies. Within certain redshift ranges, and especially at z ≃ 6-7, such lines may be strong enough to reduce estimates of the stellar masses and ages of galaxies compared with those derived assuming the broadband photometry represents stellar light alone. To test this hypothesis at the highest redshifts where such lines can be probed with ground-based facilities, we examine the near-infrared spectra of a representative sample of 28 3.0 < z < 3.8 Lyman break galaxies using the newly commissioned MOSFIRE near-infrared spectrograph at the Keck I telescope. We use these data to derive the rest-frame equivalent widths (EWs) of [O III] emission and show that these are comparable with estimates derived using the spectral energy distribution (SED) fitting technique introduced for sources of known redshift by Stark et al. Although our current sample is modest, its [O III] EW distribution is consistent with that inferred for Hα based on SED fitting of Stark et al.'s larger sample of 3.8 < z < 5 galaxies. For a subset of survey galaxies, we use the combination of optical and near-infrared spectroscopy to quantify kinematics of outflows in z ≃ 3.5 star-forming galaxies and discuss the implications for reionization measurements. The trends we uncover underline the dangers of relying purely on broadband photometry to estimate the physical properties of high-redshift galaxies and emphasize the important role of diagnostic spectroscopy
A Strong-Lens Survey in AEGIS: the influence of large scale structure
We report on the results of a visual search for galaxy-scale strong
gravitational lenses over 650 arcmin^2 of HST/ACS imaging in the DEEP2-EGS
field. In addition to a previously-known Einstein Cross (the "Cross," HST
J141735+52264, with z_lens=0.8106 and a published z_source=3.40), we identify
two new strong galaxy-galaxy lenses with multiple extended arcs. The first, HST
J141820+52361 (the ``Dewdrop''; z_lens=0.5798, lenses two distinct extended
sources into two pairs of arcs z_source=0.while), 9818 the second, HST
J141833+52435 (the ``Anchor''; z_lens=0.4625), produces a single pair of arcs
(source redshift not yet known). All three definite lenses are fit well by
simple singular isothermal ellipsoid models including external shear. Using the
three-dimensional line-of-sight (LOS) information on galaxies from the DEEP2
data, we calculate the convergence and shear contributions, assuming singular
isothermal sphere halos truncated at 200 h^-1 kpc. These are also compared
against three-dimensional local-density estimates. We find that even strong
lenses in demonstrably underdense local environments may be considerably
affected by LOS contributions, which in turn, may be underestimates of the
effect of large scale structure.Comment: ApJ Letters, submitted. Part of the AEGIS ApJL Special Issue. 4
Figures, 1 Table. For a version with full-resolution figures, please see
http://www.slac.stanford.edu/~pjm/HAGGLeS/astroph/legs.pd
On the Origin of [OII] Emission in Red Sequence and Post-starburst Galaxies
We investigate the emission-line properties of galaxies with red rest-frame
colors using spectra from SDSS DR4. Emission lines are detected in more than
half of the red galaxies. We focus on the relationship between two emission
lines commonly used as star formation rate indicators: Ha 6563 and [OII] 3727.
There is a strong bimodality in [OII]/Ha ratio in the full SDSS sample which
closely corresponds to the bimodality in rest-frame color. Nearly all of the
line-emitting red galaxies have line ratios typical of various types of AGN --
most commonly LINERs, a small fraction of transition objects and, more rarely,
Seyferts. Only ~6% of red galaxies display star-forming line ratios. A straight
line in the [OII]-Ha equivalent width plane separates LINER-like galaxies from
other categories. Quiescent galaxies with no detectable emission lines and
LINER-like galaxies combine to form a single, tight red sequence in
color-magnitude-concentration space. [OII] EWs in LINER- and AGN-like galaxies
can be as large as in star-forming galaxies. Thus, unless objects with
AGN/LINER-like line ratios are excluded, [OII] emission cannot be used directly
as a proxy for star formation rate. Lack of [OII] emission is generally used to
indicate lack of star formation when post-starburst galaxies are selected at
high redshift. Our results imply, however, that these samples have been cut on
AGN properties as well as star formation, and therefore may provide seriously
incomplete sets of post-starburst galaxies. Furthermore, post-starburst
galaxies identifed in SDSS by requiring minimal Ha EW generally exhibit weak
but nonzero line emission with ratios typical of AGNs; few of them show
residual star formation. This suggests that most post-starbursts may harbor
AGNs/LINERs.Comment: 21 pages, 15 figures. v2: Added 4 new figures and updated many;
extended text. No conclusions change. v3: minor modifications and figure
updates to match version accepted by Ap
The DEEP2 Galaxy Redshift Survey: Mean Ages and Metallicities of Red Field Galaxies at z ~ 0.9 from Stacked Keck/DEIMOS Spectra
As part of the DEEP2 galaxy redshift survey, we analyze absorption line
strengths in stacked Keck/DEIMOS spectra of red field galaxies with weak to no
emission lines, at redshifts 0.7 <= z <= 1. Comparison with models of stellar
population synthesis shows that red galaxies at z ~ 0.9 have mean
luminosity-weighted ages of the order of only 1 Gyr and at least solar
metallicities. This result cannot be reconciled with a scenario where all stars
evolved passively after forming at very high z. Rather, a significant fraction
of stars can be no more than 1 Gyr old, which means that star formation
continued to at least z ~ 1.2. Furthermore, a comparison of these distant
galaxies with a local SDSS sample, using stellar populations synthesis models,
shows that the drop in the equivalent width of Hdelta from z ~ 0.9 to 0.1 is
less than predicted by passively evolving models. This admits of two
interpretations: either each individual galaxy experiences continuing low-level
star formation, or the red-sequence galaxy population from z ~ 0.9 to 0.1 is
continually being added to by new galaxies with younger stars.Comment: A few typos were corrected and numbers in Table 1 were revise
Strong Nebular Line Ratios in the Spectra of z~2-3 Star-forming Galaxies: First Results from KBSS-MOSFIRE
We present initial results of a deep near-IR spectroscopic survey covering
the 15 fields of the Keck Baryonic Structure Survey (KBSS) using MOSFIRE on the
Keck 1 telescope, focusing on a sample of 251 galaxies with redshifts 2.0< z <
2.6, star-formation rates 2 < SFR < 200 M_sun/yr, and stellar masses 8.6 <
log(M*/M_sun) < 11.4, with high-quality spectra in both H- and K-band
atmospheric windows. We show unambiguously that the locus of z~2.3 galaxies in
the "BPT" nebular diagnostic diagram exhibits a disjoint, yet similarly tight,
relationship between the ratios [NII]6585/Halpha and [OIII]/Hbeta as compared
to local galaxies. Using photoionization models, we argue that the offset of
the z~2.3 locus relative to z~ 0 is explained by a combination of harder
ionizing radiation field, higher ionization parameter, and higher N/O at a
given O/H than applies to most local galaxies, and that the position of a
galaxy along the z~2.3 star-forming BPT locus is surprisingly insensitive to
gas-phase oxygen abundance. The observed nebular emission line ratios are most
easily reproduced by models in which the net ionizing radiation field resembles
a blackbody with effective temperature T_eff = 50000-60000 K and N/O close to
the solar value at all O/H. We critically assess the applicability of
commonly-used strong line indices for estimating gas-phase metallicities, and
consider the implications of the small intrinsic scatter in the empirical
relationship between excitation-sensitive line indices and stellar mass (i.e.,
the "mass-metallicity" relation), at z~2.3.Comment: 41 pages, 25 figures, accepted for publication in the Astrophysical
Journal. Version with full-resolution figures available at
http://www.astro.caltech.edu/~ccs/mos_bpt_submit.pd
RLang: A Declarative Language for Describing Partial World Knowledge to Reinforcement Learning Agents
We introduce RLang, a domain-specific language (DSL) for communicating domain
knowledge to an RL agent. Unlike existing RL DSLs that ground to
\textit{single} elements of a decision-making formalism (e.g., the reward
function or policy), RLang can specify information about every element of a
Markov decision process. We define precise syntax and grounding semantics for
RLang, and provide a parser that grounds RLang programs to an
algorithm-agnostic \textit{partial} world model and policy that can be
exploited by an RL agent. We provide a series of example RLang programs
demonstrating how different RL methods can exploit the resulting knowledge,
encompassing model-free and model-based tabular algorithms, policy gradient and
value-based methods, hierarchical approaches, and deep methods
The DEEP2 Redshift Survey: Lyman Alpha Emitters in the Spectroscopic Database
We present the first results of a search for Lyman-alpha emitters (LAEs) in
the DEEP2 spectroscopic database that uses a search technique that is different
from but complementary to traditional narrowband imaging surveys. We have
visually inspected ~20% of the available DEEP2 spectroscopic data and have
found nine high-quality LAEs with clearly asymmetric line profiles and an
additional ten objects of lower quality, some of which may also be LAEs. Our
survey is most sensitive to LAEs at z=4.4-4.9 and that is indeed where all but
one of our high-quality objects are found. We find the number density of our
spectroscopically-discovered LAEs to be consistent with those found in
narrowband imaging searches. The combined, averaged spectrum of our nine
high-quality objects is well fit by a two-component model, with a second,
lower-amplitude component redshifted by ~420 km/s with respect to the primary
Lyman-alpha line, consistent with large-scale outflows from these objects. We
conclude by discussing the advantages and future prospects of blank-sky
spectroscopic surveys for high-z LAEs.Comment: Accepted for publication in Ap
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