86 research outputs found
The Detection of Intergalactic Halpha Emission from the Slug Nebula at z~2.3
The Slug Nebula is one of the largest and most luminous Lyman-alpha (LyA)
nebulae discovered to date, extending over 450 kiloparsecs (kpc) around the
bright quasar UM287 at z=2.283. Characterized by high surface brightnesses over
intergalactic scales, its LyA emission may either trace high-density ionized
gas ("clumps") or large column densities of neutral material. To distinguish
between these two possibilities, information from a non-resonant line such as
Halpha is crucial. Therefore, we analyzed a deep MOSFIRE observation of one of
the brightest LyA emitting regions in the Slug Nebula with the goal of
detecting associated Halpha emission. We also obtained a deep, moderate
resolution LyA spectrum of the nearby brightest region of the Slug. We detected
an Halpha flux of F_(Halpha)= 2.62 +/- 0.47 x 10^-17 erg/cm^2/s
(SB_(Halpha)=2.70 +/- 0.48 x 10^-18 erg/cm^2/s/sq") at the expected spatial and
spectral location. Combining the Halpha detection with its corresponding LyA
flux (determined from the narrow-band imaging) we calculate a flux ratio of
F_(LyA_/F_(Halpha)= 5.5 +/- 1.1. The presence of a skyline at the location of
the Halpha emission decreases the signal to noise ratio of the detection and
our ability to put stringent constraints on the Halpha kinematics. Our
measurements argue for the origin of the LyA emission being recombination
radiation, suggesting the presence of high-density ionized gas. Finally, our
high-resolution spectroscopic study of the LyA emission does not show evidence
of a rotating disk pattern and suggest a more complex origin for at least some
parts of the Slug Nebula.Comment: 17 pages, 9 figures, final version including referee's comments after
acceptanc
A Low Lyman Continuum Escape Fraction of for Extreme [OIII] Emitters in an Overdensity at z
Recent work has suggested extreme [OIII] emitting star-forming galaxies are
important to reionization. Relatedly, [OIII]/[OII] has been put forward as an
indirect estimator of the Lyman Continuum (LyC) escape fraction () at
when the opaque IGM renders LyC photons unobservable. Using deep
archival U-band (VLT/VIMOS) imaging of a recently confirmed overdensity at
we calculate tight constraints on for a sample (N=73)
dominated by extreme [OIII] emitters. We find no Lyman Continuum signal
( at ) in a deep U-band stack of
our sample (31.98 mag at 1). This constraint is in agreement with
recent studies of star-forming galaxies spanning that have found
very low average . Despite the galaxies in our study having an
estimated average rest-frame EW([OIII]) and
[OIII]/[OII] from composite SED-fitting, we find no LyC detection,
which brings into question the potential of [OIII]/[OII] as an effective probe
of the LyC--a majority of LyC emitters have [OIII]/[OII], but we establish
here that [OIII]/[OII] does not guarantee significant LyC leakage for a
population. Since even extreme star-forming galaxies are unable to produce the
required by most theoretical calculations for star-forming
galaxies to drive reionization, there must either be a rapid evolution of
between and the Epoch of Reionization, or hitherto
observationally unstudied sources (e.g. ultra-faint low-mass galaxies with
) must make an outsized contribution to reionization.Comment: 10 pages, 8 figures (key result in Figures 7, 8). Accepted for
publication in MNRAS. Comments welcom
The Dependence of Star Formation Rates on Stellar Mass and Environment at z~0.8
We examine the star formation rates (SFRs) of galaxies in a redshift slice
encompassing the z=0.834 cluster RX J0152.7-1357. We used a low-dispersion
prism in the Inamori Magellan Areal Camera and Spectrograph (IMACS) to identify
galaxies with z<23.3 AB mag in diverse environments around the cluster out to
projected distances of ~8 Mpc from the cluster center. We utilize a
mass-limited sample (M>2x10^{10} M_sun) of 330 galaxies that were imaged by
Spitzer MIPS at 24 micron to derive SFRs and study the dependence of specific
SFR (SSFR) on stellar mass and environment. We find that the SFR and SSFR show
a strong decrease with increasing local density, similar to the relation at
z~0. Our result contrasts with other work at z~1 that find the SFR-density
trend to reverse for luminosity-limited samples. These other results appear to
be driven by star-formation in lower mass systems (M~10^{10} M_sun). Our
results imply that the processes that shut down star-formation are present in
groups and other dense regions in the field. Our data also suggest that the
lower SFRs of galaxies in higher density environments may reflect a change in
the ratio of star-forming to non-star-forming galaxies, rather than a change in
SFRs. As a consequence, the SFRs of star-forming galaxies, in environments
ranging from small groups to clusters, appear to be similar and largely
unaffected by the local processes that truncate star-formation at z~0.8.Comment: 5 pages, 3 figures, accepted for publication in ApJ
Three Super-Earths Orbiting HD 7924
We report the discovery of two super-Earth mass planets orbiting the nearby
K0.5 dwarf HD 7924 which was previously known to host one small planet. The new
companions have masses of 7.9 and 6.4 M, and orbital periods of 15.3
and 24.5 days. We perform a joint analysis of high-precision radial velocity
data from Keck/HIRES and the new Automated Planet Finder Telescope (APF) to
robustly detect three total planets in the system. We refine the ephemeris of
the previously known planet using five years of new Keck data and high-cadence
observations over the last 1.3 years with the APF. With this new ephemeris, we
show that a previous transit search for the inner-most planet would have
covered 70% of the predicted ingress or egress times. Photometric data
collected over the last eight years using the Automated Photometric Telescope
shows no evidence for transits of any of the planets, which would be detectable
if the planets transit and their compositions are hydrogen-dominated. We detect
a long-period signal that we interpret as the stellar magnetic activity cycle
since it is strongly correlated with the Ca II H and K activity index. We also
detect two additional short-period signals that we attribute to
rotationally-modulated starspots and a one month alias. The high-cadence APF
data help to distinguish between the true orbital periods and aliases caused by
the window function of the Keck data. The planets orbiting HD 7924 are a local
example of the compact, multi-planet systems that the Kepler Mission found in
great abundance.Comment: Accepted to ApJ on 4/7/201
A Wide-Field Study of the z~0.8 Cluster RX J0152.7-1357: the Role of Environment in the Formation of the Red-Sequence
[ABRIDGED] We present the first results from the largest spectroscopic survey
to date of an intermediate redshift galaxy cluster, the z=0.834 cluster RX
J0152.7-1357. We use the colors of galaxies, assembled from a D~12 Mpc region
centered on the cluster, to investigate the properties of the red-sequence as a
function of density and clustercentric radius. Our wide-field multi-slit survey
with a low-dispersion prism in the IMACS spectrograph at Magellan allowed us to
identify 475 new members of the cluster and its surrounding large scale
structure with a redshift accuracy of dz/(1+z)~1% and a contamination rate of
~2% for galaxies with i<23.75 mag. We combine these new members with the 279
previously known spectroscopic members to give a total of 754 galaxies from
which we obtain a mass-limited sample of 300 galaxies with stellar masses
M>4x10^{10} M_sun. We find that the red galaxy fraction is 93+/-3% in the two
merging cores of the cluster and declines to a level of 64+/-3% at projected
clustercentric radii R>~3 Mpc. At these large projected distances, the
correlation between clustercentric radius and local density is nonexistent.
This allows an assessment of the influence of the local environment on galaxy
evolution, as opposed to mechanisms that operate on cluster scales. Even beyond
R>3 Mpc we find an increasing fraction of red galaxies with increasing local
density. The red fraction at the highest local densities in two groups at R>3
Mpc matches the red fraction found in the two cores. Strikingly, galaxies at
intermediate densities at R>3 Mpc, that are not group members, also show signs
of an enhanced red fraction. Our results point to such intermediate density
regions and the groups in the outskirts of the cluster, as sites where the
local environment influences the transition of galaxies onto the red-sequence.Comment: 15 pages, 10 figures, 1 table, accepted for publication in ApJ,
expanded introduction and additional references adde
Recent structural evolution of early-type galaxies : size growth from z=1 to z=0
Strong size and internal density evolution of early-type galaxies between z similar to 2 and the present has been reported by several authors. Here we analyze samples of nearby and distant (z similar to 1) galaxies with dynamically measured masses in order to confirm the previous, model-dependent results and constrain the uncertainties that may play a role. Velocity dispersion (sigma) measurements are taken from the literature for 50 morphologically selected 0.8 < z < 1.2 field and cluster early-type galaxies with typical masses M-dyn = 2 x 10(11) M-circle dot. Sizes (R-eff) are determined with Advanced Camera for Surveys imaging. We compare the distant sample with a large sample of nearby (0.04 < z < 0.08) early-type galaxies extracted from the Sloan Digital Sky Survey for which we determine sizes, masses, and densities in a consistent manner, using simulations to quantify systematic differences between the size measurements of nearby and distant galaxies. We find a highly significant difference between the sigma-R-eff distributions of the nearby and distant samples, regardless of sample selection effects. The implied evolution in R-eff at fixed mass between z = 1 and the present is a factor of 1.97 +/- 0.15. This is in qualitative agreement with semianalytic models; however, the observed evolution is much faster than the predicted evolution. Our results reinforce and are quantitatively consistent with previous, photometric studies that found size evolution of up to a factor of 5 since z similar to 2. A combination of structural evolution of individual galaxies through the accretion of companions and the continuous formation of early-type galaxies through increasingly gas-poor mergers is one plausible explanation of the observations
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