359 research outputs found
The halo masses and galaxy environments of hyperluminous QSOs at z~2.7 in the Keck Baryonic Structure Survey
We present an analysis of the galaxy distribution surrounding 15 of the most
luminous (>10^{14} L_sun; M_1450 ~ -30) QSOs in the sky with z~2.7. Our data
are drawn from the Keck Baryonic Structure Survey (KBSS). In this work, we use
the positions and spectroscopic redshifts of 1558 galaxies that lie within ~3',
(4.2 h^{-1} comoving Mpc; cMpc) of the hyperluminous QSO (HLQSO) sightline in
one of 15 independent survey fields, together with new measurements of the
HLQSO systemic redshifts. We measure the galaxy-HLQSO cross-correlation
function, the galaxy-galaxy autocorrelation function, and the characteristic
scale of galaxy overdensities surrounding the sites of exceedingly rare,
extremely rapid, black hole accretion. On average, the HLQSOs lie within
significant galaxy overdensities, characterized by a velocity dispersion
sigma_v ~ 200 km s^{-1} and a transverse angular scale of ~25", (~200 physical
kpc). We argue that such scales are expected for small groups with
log(M_h/M_sun)~13. The galaxy-HLQSO cross-correlation function has a best-fit
correlation length r_0_GQ = (7.3 \pm 1.3) h^{-1} cMpc, while the galaxy
autocorrelation measured from the spectroscopic galaxy sample in the same
fields has r_0_GG = (6.0 \pm 0.5) h^{-1} cMpc. Based on a comparison with
simulations evaluated at z ~ 2.6, these values imply that a typical galaxy
lives in a host halo with log(M_h/M_sun) = 11.9\pm0.1, while HLQSOs inhabit
host halos of log(M_h/M_sun) = 12.3\pm0.5. In spite of the extremely large
black hole masses implied by their observed luminosities [log(M_BH/M_sun) >
9.7], it appears that HLQSOs do not require environments very different from
their much less luminous QSO counterparts. Evidently, the exceedingly low space
density of HLQSOs (< 10^{-9} cMpc^{-3}) results from a one-in-a-million event
on scales << 1 Mpc, and not from being hosted by rare dark matter halos.Comment: 15 pages, 6 figures. Accepted for publication in Ap
Wallace L. W. Sargent (1935–2012)
By any measure, Professor Wallace L. W. Sargent (known to colleagues and friends as “Wal”) was one of the most influential and consistently productive astronomers of the last 50 years; he authored or coauthored more than 320 refereed journal articles, which have received more than 25,000 citations. He never rested on past laurels—65 of his papers (with >5000 citations) have appeared since the year 2000. Wal remained active as a teacher and researcher until just weeks before the end of his life (he officially retired from the Caltech faculty on 2012 October 1). His impact on the field of astrophysics was remarkably broad as well as seminal; among many other awards, he was elected in 1981 as a Fellow of the Royal Society (U.K.) and as a member of the National Academy of Sciences in 2005, his first year of eligibility after becoming a naturalized U.S. citizen
Galaxies at high redshift: progress and prospects
There has been considerable progress made in the discovery, observation, and understanding of high redshift galaxies in the last few years; most of this progress is attributable to greatly improved spectroscopy throughput made possible by state-of-the-art instruments on the new generation of 8-10m telescopes. Here we review a few of the areas in which substantial progress has been made, and discuss the future of high redshift galaxy work in the context of the observational facilities that are either in operation or soon to come
Filamentary Large-scale Structure Traced by Six Lyα Blobs at z = 2.3
Extended nebulae of Lyα emission ("Lyα blobs") are known to be associated with overdense regions at high redshift. Here we present six large Lyα blobs in a previously known protocluster with galaxy overdensity δ ~ 7 at z = 2.3; this is the richest field of giant Lyα blobs detected to date. The blobs have linear sizes of ≳ 100 kpc and Lyα luminosities of ~10^(43) erg s^(–1). The positions of the blobs define two linear filaments with an extent of at least 12 comoving Mpc; these filaments intersect at the center of one of the blobs. Measurement of the position angles of the blobs indicates that five of the six are aligned with these filaments to within ~10°, suggesting a connection between the physical processes powering extended Lyα emission and those driving structure on larger scales
Substructure within the SSA22 protocluster at
We present the results of a densely sampled spectroscopic survey of the SSA22
protocluster at . Our sample with Keck/LRIS spectroscopy
includes 106 Ly Emitters (LAEs) and 40 Lyman Break Galaxies (LBGs) at
. These galaxies are contained within the region in
which the protocluster was discovered, which also hosts the maximum galaxy
overdensity in the SSA22 region. The redshift histogram of our spectroscopic
sample reveals two distinct peaks, at (blue, 43 galaxies) and
(red, 103 galaxies). Furthermore, objects in the blue and red peaks
are segregated on the sky, with galaxies in the blue peak concentrating towards
the western half of the field. These results suggest that the blue and red
redshift peaks represent two distinct structures in physical space. Although
the double-peaked redshift histogram is traced in the same manner by LBGs and
LAEs, and brighter and fainter galaxies, we find that nine out of 10 X-ray AGNs
in SSA22, and all seven spectroscopically-confirmed giant Ly "blobs,"
reside in the red peak. We combine our dataset with sparsely sampled
spectroscopy from the literature over a significantly wider area, finding
preliminary evidence that the double-peaked structure in redshift space extends
beyond the region of our dense spectroscopic sampling. In order to fully
characterize the three-dimensional structure, dynamics, and evolution of
large-scale structure in the SSA22 overdensity, we require the measurement of
large samples of LAE and LBG redshifts over a significantly wider area, as well
as detailed comparisons with cosmological simulations of massive cluster
formation.Comment: 6 pages, 4 figures, Accepted to ApJ Letter
Spectroscopic Measurements of the Far-Ultraviolet Dust Attenuation Curve at z~3
We present the first measurements of the shape of the far-ultraviolet
(far-UV; lambda=950-1500 A) dust attenuation curve at high redshift (z~3). Our
analysis employs rest-frame UV spectra of 933 galaxies at z~3, 121 of which
have very deep spectroscopic observations (>7 hrs) at lambda=850-1300 A, with
the Low Resolution Imaging Spectrograph on the Keck Telescope. By using an
iterative approach in which we calculate the ratios of composite spectra in
different bins of continuum color excess, E(B-V), we derive a dust curve that
implies a lower attenuation in the far-UV for a given E(B-V) than those
obtained with standard attenuation curves. We demonstrate that the UV composite
spectra of z~3 galaxies can be modeled well by assuming our new attenuation
curve, a high covering fraction of HI, and absorption from the Lyman-Werner
bands of H2 with a small (<20%) covering fraction. The low covering fraction of
H2 relative to that of the HI and dust suggests that most of the dust in the
ISM of typical galaxies at z~3 is unrelated to the catalysis of H2, and is
associated with other phases of the ISM (i.e., the ionized and neutral gas).
The far-UV dust curve implies a factor of ~2 lower dust attenuation of Lyman
continuum (ionizing) photons relative to those inferred from the most commonly
assumed attenuation curves for L* galaxies at z~3. Our results may be utilized
to assess the degree to which ionizing photons are attenuated in HII regions
or, more generally, in the ionized or low column density (N(HI)<10^17.2 cm^-2)
neutral ISM of high-redshift galaxies.Comment: 12 pages, 1 table, 8 figures, accepted to the Astrophysical Journa
The Spectroscopic Properties of Ly{\alpha}-Emitters at z 2.7: Escaping Gas and Photons from Faint Galaxies
We present a spectroscopic survey of 318 faint , ,
Ly{\alpha}-emission-selected galaxies (LAEs) at 2.5<z<3. A sample of 32 LAEs
with rest-frame optical spectra from Keck/MOSFIRE are used to interpret the LAE
spectra in the context of their systemic redshifts. We find that the Ly{\alpha}
emission of LAEs is typically less spectrally extended than among samples of
more luminous continuum-selected galaxies (LBGs) at similar redshifts. Using
the MOSFIRE subsample, we find that the peak of the Ly{\alpha} line is shifted
by +200 km/s with respect to systemic across a diverse set of galaxies
including both LAEs and LBGs. We also find a small number of objects with
significantly blueshifted Ly{\alpha} emission, a potential indicator of
accreting gas. The Ly{\alpha}-to-H{\alpha} line ratios suggest that the LAEs
have Ly{\alpha} escape fractions %,
significantly higher than typical LBG samples. Using redshifts calibrated by
our MOSFIRE sample, we construct composite LAE spectra, finding the first
evidence for metal-enriched outflows in such intrinsically-faint high-redshift
galaxies. These outflows have smaller continuum covering fractions and velocities km/s, km/s than those associated with typical LBGs, suggesting that
gas covering fraction is a likely driver of the high Ly{\alpha} and
Ly-continuum escape fractions of LAEs. Our results suggest a similar scaling of
outflow velocity with star formation rate as is observed at lower redshifts
and indicate that a substantial
fraction of gas is ejected with
X-Ray and Radio Emission from UV-Selected Star Forming Galaxies at Redshifts 1.5<Z<3.0 in the GOODS-North Field
We have examined the stacked radio and X-ray emission from UV-selected
galaxies spectroscopically confirmed to lie between redshifts 1.5 < z < 3.0 in
the GOODS-North field to determine their average extinction and star formation
rates (SFRs). The X-ray and radio data are obtained from the Chandra 2 Msec
survey and the Very Large Array, respectively. There is a good agreement
between the X-ray, radio, and de-reddened UV estimates of the average SFR for
our sample of z~2 galaxies of ~50 solar masses per year, indicating that the
locally-calibrated SFR relations appear to be statistically valid from
redshifts 1.5 < z < 3.0. We find that UV-estimated SFRs (uncorrected for
extinction) underestimate the bolometric SFRs as determined from the 2-10 keV
X-ray luminosity by a factor of ~4.5 to 5.0 for galaxies over a large range in
redshift from 1.0 < z < 3.5.Comment: 5 pages, 1 figure, Accepted by ApJ Letter
Calibrating Galaxy Redshifts Using Absorption by the Surrounding Intergalactic Medium
Rest-frame UV spectral lines of star-forming galaxies are systematically
offset from the galaxies' systemic redshifts, probably because of large-scale
outflows. We calibrate galaxy redshifts measured from rest-frame UV lines by
utilizing the fact that the mean HI Ly-alpha absorption profiles around the
galaxies, as seen in spectra of background objects, must be symmetric with
respect to the true galaxy redshifts if the galaxies are oriented randomly with
respect to the lines of sight to the background objects. We use 15 QSOs at
z~2.5-3 and more than 600 foreground galaxies with spectroscopic redshifts at
z~1.9-2.5. All galaxies are within 2 Mpc proper from the lines of sight to the
background QSOs. We find that LyA emission and ISM absorption redshifts require
systematic shifts of v_LyA=-295(+35)(-35) km/s and v_ISM=145(+70)(-35) km/s.
Assuming a Gaussian distribution, we put 1-sigma upper limits on possible
random redshift offsets of <220 km/s for LyA and <420 km/s for ISM redshifts.
For the small subset (<10%) of galaxies for which near-IR spectra have been
obtained, we can compare our results to direct measurements based on nebular
emission lines which we confirm to mark the systemic redshifts. While our v_ISM
agrees with the direct measurements, our v_LyA is significantly smaller.
However, when we apply our method to the near-IR subsample which is
characterized by slightly different selection effects, the best-fit velocity
offset comes into agreement with the direct measurement. This confirms the
validity of our approach, and implies that no single number appropriately
describes the whole population of galaxies, in line with the observation that
the line offset depends on galaxy spectral morphology. This method provides
accurate redshift calibrations and will enable studies of circumgalactic matter
around galaxies for which rest-frame optical observations are not available.Comment: 7 pages, 3 figures, accepted for publication in MNRA
Physical Properties of a Pilot Sample of Spectroscopic Close Pair Galaxies at z ~ 2
We use Hubble Space Telescope Wide-Field Camera 3 (HST/WFC3) rest-frame
optical imaging to select a pilot sample of star-forming galaxies in the
redshift range z = 2.00-2.65 whose multi-component morphologies are consistent
with expectations for major mergers. We follow up this sample of major merger
candidates with Keck/NIRSPEC longslit spectroscopy obtained in excellent seeing
conditions (FWHM ~ 0.5 arcsec) to obtain Halpha-based redshifts of each of the
morphological components in order to distinguish spectroscopic pairs from false
pairs created by projection along the line of sight. Of six pair candidates
observed, companions (estimated mass ratios 5:1 and 7:1) are detected for two
galaxies down to a 3sigma limiting emission-line flux of ~ 10^{-17} erg/s/cm2.
This detection rate is consistent with a ~ 50% false pair fraction at such
angular separations (1-2 arcsec), and with recent claims that the
star-formation rate (SFR) can differ by an order of magnitude between the
components in such mergers. The two spectroscopic pairs identified have total
SFR, SFR surface densities, and stellar masses consistent on average with the
overall z ~ 2 star forming galaxy population.Comment: 11 pages, 5 figures. Accepted for publication in Ap
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