124 research outputs found
Galactic winds and stellar populations in Lyman emitting galaxies at z ~ 3.1
We present a sample of 33 spectroscopically confirmed z ~ 3.1
Ly-emitting galaxies (LAEs) in the Cosmological Evolution Survey
(COSMOS) field. This paper details the narrow-band survey we conducted to
detect the LAE sample, the optical spectroscopy we performed to confirm the
nature of these LAEs, and a new near-infrared spectroscopic detection of the [O
III] 5007 \AA\ line in one of these LAEs. This detection is in addition to two
[O III] detections in two z ~ 3.1 LAEs we have reported on previously (McLinden
et al 2011). The bulk of the paper then presents detailed constraints on the
physical characteristics of the entire LAE sample from spectral energy
distribution (SED) fitting. These characteristics include mass, age,
star-formation history, dust content, and metallicity. We also detail an
approach to account for nebular emission lines in the SED fitting process -
wherein our models predict the strength of the [O III] line in an LAE spectrum.
We are able to study the success of this prediction because we can compare the
model predictions to our actual near-infrared observations both in galaxies
that have [O III] detections and those that yielded non-detections. We find a
median stellar mass of 6.9 10 M and a median star
formation rate weighted stellar population age of 4.5 10 yr. In
addition to SED fitting, we quantify the velocity offset between the [O III]
and Ly lines in the galaxy with the new [O III] detection, finding that
the Ly line is shifted 52 km s redwards of the [O III] line,
which defines the systemic velocity of the galaxy.Comment: 38 pages, 27 figures, 4 tables, Accepted for publication in MNRA
The Dynamical Masses, Densities, and Star Formation Scaling Relations of Lyman Alpha Galaxies
We present the first dynamical mass measurements for Lyman alpha galaxies at
high redshift, based on velocity dispersion measurements from rest-frame
optical emission lines and size measurements from HST imaging, for a sample of
nine galaxies drawn from four surveys. These measurements enable us to study
the nature of Lyman alpha galaxies in the context of galaxy scaling relations.
The resulting dynamical masses range from 1e9 to 1e10 solar masses. We also fit
stellar population models to our sample, and use them to plot the Lyman alpha
sample on a stellar mass vs. line width relation. Overall, the Lyman alpha
galaxies follow well the scaling relation established by observing star forming
galaxies at lower redshift (and without regard for Lyman alpha emission),
though in 1/3 of the Lyman alpha galaxies, lower-mass fits are also acceptable.
In all cases, the dynamical masses agree with established stellarmass-linewidth
relation. Using the dynamical masses as an upper limit on gas mass, we show
that Lyman alpha galaxies resemble starbursts (rather than "normal" galaxies)
in the relation between gas mass surface density and star formation activity,
in spite of relatively modest star formation rates. Finally, we examine the
mass densities of these galaxies, and show that their future evolution likely
requires dissipational ("wet") merging. In short, we find that Lyman alpha
galaxies are low mass cousins of larger starbursts.Comment: Submitted to The Astrophysical Journal. 23 pp including three figures
and four table
The Role of Bulge Formation in the Homogenization of Stellar Populations at as revealed by Internal Color Dispersion in CANDELS
We use data from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy
Survey to study how the spatial variation in the stellar populations of
galaxies relate to the formation of galaxies at . We use the
Internal Color Dispersion (ICD), measured between the rest-frame UV and optical
bands, which is sensitive to age (and dust attenuation) variations in stellar
populations. The ICD shows a relation with the stellar masses and morphologies
of the galaxies. Galaxies with the largest variation in their stellar
populations as evidenced by high ICD have disk-dominated morphologies (with
S\'{e}rsic indexes ) and stellar masses between . There is a marked decrease in the ICD as the stellar mass and/or
the S\'ersic index increases. By studying the relations between the ICD and
other galaxy properties including sizes, total colors, star-formation rate, and
dust attenuation, we conclude that the largest variations in stellar
populations occur in galaxies where the light from newly, high star-forming
clumps contrasts older stellar disk populations. This phase reaches a peak for
galaxies only with a specific stellar mass range, , and prior to the formation of a substantial bulge/spheroid. In contrast,
galaxies at higher or lower stellar masses, and/or higher S\'{e}rsic index () show reduced ICD values, implying a greater homogeneity of their stellar
populations. This indicates that if a galaxy is to have both a quiescent bulge
along with a star forming disk, typical of Hubble Sequence galaxies, this is
most common for stellar masses and when the
bulge component remains relatively small ().Comment: 15 pages, 14 figure
Predicting the Merger Fraction of Lyman alpha Emitters from Redshift z~3 to z~7
Rapid mass assembly, likely from mergers or smooth accretion, has been
predicted to play a vital role in star-formation in high-redshift Lyman-alpha
(Lya) emitters. Here we predict the major merger, minor merger, and smooth
accreting Lya emitter fraction from z~3 to z~7 using a large dark matter
simulation, and a simple physical model that is successful in reproducing many
observations over this large redshift range. The central tenet of this model,
different from many of the earlier models, is that the star-formation in Lya
emitters is proportional to the mass accretion rate rather than the total halo
mass. We find that at z~3, nearly 35% of the Lya emitters accrete their mass
through major (3:1) mergers, and this fraction increases to about 50% at z~7.
This imply that the star-formation in a large fraction of high-redshift Lya
emitters is driven by mergers. While there is discrepancy between the model
predictions and observed merger fractions, some of this difference (~15%) can
be attributed to the mass-ratio used to define a merger in the simulation. We
predict that future, deeper observations which use a 3:1 definition of major
mergers will find >30% major merger fraction of Lya emitters at redshifts >3.Comment: Accepted in MNRA
First results from Faint Infrared Grism Survey (FIGS): first simultaneous detection of Lyman-alpha emission and Lyman break from a galaxy at z=7.51
Galaxies at high redshifts provide a valuable tool to study cosmic dawn, and
therefore it is crucial to reliably identify these galaxies. Here, we present
an unambiguous and first simultaneous detection of both the Lyman-alpha
emission and the Lyman break from a z = 7.512+/- 0.004 galaxy, observed in the
Faint Infrared Grism Survey (FIGS). These spectra, taken with G102 grism on
Hubble Space Telescope (HST), show a significant emission line detection (6
sigma) in multiple observational position angles (PA), with total integrated
Ly{\alpha} line flux of 1.06+/- 0.12 e10-17erg s-1cm-2. The line flux is nearly
a factor of four higher than the previous MOSFIRE spectroscopic observations of
faint Ly{\alpha} emission at {\lambda} = 1.0347{\mu}m, yielding z = 7.5078+/-
0.0004. This is consistent with other recent observations implying that
ground-based near-infrared spectroscopy underestimates total emission line
fluxes, and if confirmed, can have strong implications for reionization studies
that are based on ground-based Lyman-{\alpha} measurements. A 4-{\sigma}
detection of the NV line in one PA also suggests a weak Active Galactic Nucleus
(AGN), potentially making this source the highest-redshift AGN yet found. Thus,
this observation from the Hubble Space Telescope clearly demonstrates the
sensitivity of the FIGS survey, and the capability of grism spectroscopy to
study the epoch of reionization.Comment: Published in ApJL; matches published versio
The distribution of satellites around massive galaxies at 1<z<3 in ZFOURGE/CANDELS: dependence on star formation activity
We study the statistical distribution of satellites around star-forming and
quiescent central galaxies at 1<z<3 using imaging from the FourStar Galaxy
Evolution Survey (ZFOURGE) and the Cosmic Assembly Near-IR Deep Extragalactic
Legacy Survey (CANDELS). The deep near-IR data select satellites down to
at z<3. The radial satellite distribution around centrals
is consistent with a projected NFW profile. Massive quiescent centrals,
, have 2 times the number of satellites compared
to star-forming centrals with a significance of 2.7 even after
accounting for differences in the centrals' stellar-mass distributions. We find
no statistical difference in the satellite distributions of intermediate-mass
quiescent and star-forming centrals, . Comparing
to the Guo2011 semi-analytic model, the excess number of satellites indicates
that quiescent centrals have halo masses 0.3 dex larger than star-forming
centrals, even when the stellar-mass distributions are fixed. We use a simple
toy model that relates halo mass and quenching, which roughly reproduces the
observed quenched fractions and the differences in halo mass between
star-forming and quenched galaxies only if galaxies have a quenching
probability that increases with halo mass from 0 for
11 to 1 for 13.5. A single
halo-mass quenching threshold is unable to reproduce the quiescent fraction and
satellite distribution of centrals. Therefore, while halo quenching may be an
important mechanism, it is unlikely to be the only factor driving quenching. It
remains unclear why a high fraction of centrals remain star-forming even in
relatively massive halos.Comment: 19 pages, 17 figures, accepted by ApJ. Information on ZFOURGE can be
found at http://zfourge.tamu.ed
Onset of Cosmic Reionization: Evidence of An Ionized Bubble Merely 680 Myrs after the Big Bang
While most of the inter-galactic medium (IGM) today is permeated by ionized
hydrogen, it was largely filled with neutral hydrogen for the first 700 million
years after the Big Bang. The process that ionized the IGM (cosmic
reionization) is expected to be spatially inhomogeneous, with fainter galaxies
playing a significant role. However, we still have only a few direct
constraints on the reionization process. Here we report the first spectroscopic
confirmation of two galaxies and very likely a third galaxy in a group
(hereafter EGS77) at redshift z = 7.7, merely 680 Myrs after the Big Bang. The
physical separation among the three members is < 0.7 Mpc. We estimate the
radius of ionized bubble of the brightest galaxy to be about 1.02 Mpc, and show
that the individual ionized bubbles formed by all three galaxies likely overlap
significantly, forming a large yet localized ionized region, which leads to the
spatial inhomogeneity in the reionization process. It is striking that two of
three galaxies in EGS77 are quite faint in the continuum, thanks to our
selection of reionizing sources using their Lyman-alpha line emission. Indeed,
one is the faintest spectroscopically confirmed galaxy yet discovered at such
high redshifts. Our observations provide direct constraints in the process of
cosmic reionization, and allow us to investigate the properties of sources
responsible for reionizing the universe.Comment: Submitted after addressing referee's comment
Spectroscopic confirmation of z~7 LBGs: probing the earliest galaxies and the epoch of reionization
We present the final results from our ultra-deep spectroscopic campaign with
FORS2 at the ESO/VLT for the confirmation of z~7 "z--band dropout" candidates
selected from our VLT/Hawk-I imaging survey over three independent fields. In
particular we report on two newly discovered galaxies at redshift ~6.7 in the
NTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWs
of the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also present
the results of ultra-deep observations of a sample of i-dropout galaxies, from
which we set a solid upper limit on the fraction of interlopers. Out of the 20
z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is
systematically below the expectations drawn on the basis of lower redshift
observations: in particular there is a significant lack of objects with
intermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend for
the fraction of Ly-alpha emission in LBGs that is constantly increasing from
z~3 to z~6 is most probably reversed from z~6 to z~7.
Explaining the observed rapid change in the LAE fraction among the drop-out
population with reionization requires a fast evolution of the neutral fraction
of hydrogen in the Universe. Assuming that the Universe is completely ionized
at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we
find that our data require a change of the neutral hydrogen fraction of the
order Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escape
fraction does not increase dramatically over the same redshift interval.Comment: Submitted to Ap
Search for z~6.96 Ly-alpha emitters with Magellan/IMACS in the COSMOS field
We report a search for z~6.96 Ly-alpha emitters (LAEs) using a Narrow-Band
filter, centered at 9680 Angstroms, with the IMACS instrument on the Magellan
telescope at Las Campanas Observatory. We obtain a sample of 6 Ly-alpha emitter
candidates of luminosity ~10^42 erg/s in a total area of 465 square arcmin
corresponding to a comoving volume of ~ 72000 Mpc^3. From this result, we
derive a Ly-alpha luminosity function (LF) at z~6.96 and compare our sample
with the only z~6.96 Ly-alpha emitter spectroscopically confirmed to date (Iye
et al. 2006). We find no evolution between the z=5.7 and z~7 Ly-alpha
luminosity functions, if a majority of our candidates are confirmed.
Spectroscopic confirmation for this sample will enable more robust conclusions.Comment: Accepted to Ap
CANDELSz7: A large spectroscopic survey of CANDELS galaxies in the reionization epoch
We present the results of CANDELSz7, an ESO large program aimed at confirming
spectroscopically a homogeneous sample of z~6 and z~7 star forming galaxies.
The candidates were selected in the GOODS-South, UDS and COSMOS fields using
the official CANDELS catalogs based on H160-band detections. Standard color
criteria, which were tailored depending on the ancillary multi-wavelength data
available for each field, were applied to select more than 160 candidate
galaxies at z~6 and z~7. Deep medium resolution FORS2 spectroscopic
observations were then conducted with integration times ranging from 12 to 20
hours, to reach a Lyalpha flux limit of approximately 1-3x 10-18 erg/s/cm^2 at
3sigma. For about 40% of the galaxies we could determine a spectroscopic
redshift, mainly through the detection of a single emission line that we
interpret as Lyalpha emission, or for some of the brightest objects (H160<
25.5) from the presence of faint continuum and sharp drop that we interpret as
a Lyman break. In this paper we present the redshifts and main properties of 65
newly confirmed high redshift galaxies. Adding previous proprietary and
archival data we assemble a sample of ~260 galaxies that we use to explore the
evolution of the Lyalpha fraction in Lyman break galaxies and the change in the
shape of the emission line between z~6 and z~7. We also discuss the accuracy of
the CANDELS photometric redshifts in this redshift range.STFC
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