958 research outputs found
Emission-Line Properties of z > 4 Quasars
We present results of a program of high signal-to-noise spectroscopy for 44
QSOs at redshifts > 4 using the MMT and Keck observatories. The quasar spectra
cover 1100 -- 1700 A in the rest frame for sources spanning a luminosity range
of approximately 2 orders of magnitude. Comparisons between these data and
spectra of lower redshift quasars reveal a high degree of similarity, although
differences are present in the profiles and the strengths of some emission
features. An examination of the luminosity dependence of the emission lines
reveals evidence for a weak or absent Baldwin effect among z > 4 QSOs. We
compare measurements for objects in our sample with results from other high
redshift surveys characterized by different selection techniques. Distributions
of equivalent widths for these different ensembles are consistent with a common
parent population, suggesting that our sample is not strongly biased, or in any
case, subject to selection effects that are not significantly different from
other surveys, including the Sloan Digital Sky Survey. Based on this
comparison, we tentatively conclude that the trends identified here are
representative of high z QSOs. In particular, the data bolster indications of
supersolar metallicities in these luminous, high-z sources, which support
scenarios that assume substantial star formation at epochs preceding or
concurrent with the QSO phenomena.Comment: 26 pages (incl. 9 figures), AASTeX v5.0, to appear in The
Astrophysical Journa
Galaxy Merger Candidates in High-Redshift Cluster Environments
We compile a sample of spectroscopically- and photometrically-selected
cluster galaxies from four high-redshift galaxy clusters ()
from the Spitzer Adaptation of the Red-Sequence Cluster Survey (SpARCS), and a
comparison field sample selected from the UKIDSS Deep Survey. Using
near-infrared imaging from the \textit{Hubble Space Telescope} we classify
potential mergers involving massive () cluster members by eye, based on morphological
properties such as tidal distortions, double nuclei, and projected near
neighbors within 20 kpc. With a catalogue of 23 spectroscopic and 32
photometric massive cluster members across the four clusters and 65
spectroscopic and 26 photometric comparable field galaxies, we find that after
taking into account contamination from interlopers, of
the cluster members are involved in potential mergers, compared to
of the field galaxies. We see no evidence of merger
enhancement in the central cluster environment with respect to the field,
suggesting that galaxy-galaxy merging is not a stronger source of galaxy
evolution in cluster environments compared to the field at these redshifts.Comment: Accepted by Ap
The FIRST Bright Quasar Survey. II. 60 Nights and 1200 Spectra Later
We have used the VLA FIRST survey and the APM catalog of the POSS-I plates as
the basis for constructing a new radio-selected sample of optically bright
quasars. This is the first radio-selected sample that is competitive in size
with current optically selected quasar surveys. Using only two basic criteria,
radio-optical positional coincidence and optical morphology, quasars and BL
Lacs can be identified with 60% selection efficiency; the efficiency increases
to 70% for objects fainter than magnitude 17. We show that a more sophisticated
selection scheme can predict with better than 85% reliability which candidates
will turn out to be quasars.
This paper presents the second installment of the FIRST Bright Quasar Survey
with a catalog of 636 quasars distributed over 2682 square degrees. The quasar
sample is characterized and all spectra are displayed. The FBQS detects both
radio-loud and radio-quiet quasars out to a redshift z>3. We find a large
population of objects of intermediate radio-loudness; there is no evidence in
our sample for a bimodal distribution of radio characteristics. The sample
includes ~29 broad absorption line quasars, both high and low ionization, and a
number of new objects with remarkable optical spectra.Comment: 41 pages plus 39 gifs which contain all quasar spectra. Accepted for
publication in the Astrophysical Journal Supplement Serie
Red and Blue Shifted Broad Lines in Luminous Quasars
We have observed a sample of 22 luminous quasars, in the range 2.0<z<2.5, at
1.6 microns with the near-infrared (NIR) spectrograph FSPEC on the Multiple
Mirror Telescope. Our sample contains 13 radio-loud and 9 radio-quiet objects.
We have measured the systemic redshifts z_(sys) directly from the strong [O
III]5007 line emitted from the narrow-line-region. From the same spectra, we
have found that the non-resonance broad H lines have a systematic mean
redward shift of 520+/-80 km/s with respect to systemic. Such a shift was not
found in our identical analysis of the low-redshift sample of Boroson & Green.
The amplitude of this redshift is comparable to half the expected gravitational
redshift and transverse Doppler effects, and is consistent with a correlation
between redshift differences and quasar luminosity. From data in the
literature, we confirm that the high-ionization rest-frame ultraviolet broad
lines are blueshifted ~550-1050 km/s from systemic, and that these velocity
shifts systematically increase with ionization potential. Our results allow us
to quantify the known bias in estimating the ionizing flux from the
inter-galactic-medium J_(IGM) via the Proximity Effect. Using redshift
measurements commonly determined from strong broad line species, like Ly\alpha
or CIV1549, results in an over-estimation of J_(IGM) by factors of ~1.9-2.3.
Similarly, corresponding lower limits on the density of baryon Omega_b will be
over-estimated by factors of ~1.4-1.5. However, the low-ionization MgII2798
broad line is within ~50 km/s of systemic, and thus would be the line of choice
for determining the true redshift of 1.0<z<2.2 quasars without NIR
spectroscopy, and z>3.1 objects using NIR spectroscopy.Comment: 12 pages, Latex, 2 figures, 2 tables, Accepted for publication in ApJ
Letter
ALMA Observations of Gas-Rich Galaxies in z~1.6 Galaxy Clusters: Evidence for Higher Gas Fractions in High-Density Environments
We present ALMA CO (2-1) detections in 11 gas-rich cluster galaxies at z~1.6,
constituting the largest sample of molecular gas measurements in z>1.5 clusters
to date. The observations span three galaxy clusters, derived from the Spitzer
Adaptation of the Red-sequence Cluster Survey. We augment the >5sigma
detections of the CO (2-1) fluxes with multi-band photometry, yielding stellar
masses and infrared-derived star formation rates, to place some of the first
constraints on molecular gas properties in z~1.6 cluster environments. We
measure sizable gas reservoirs of 0.5-2x10^11 solar masses in these objects,
with high gas fractions and long depletion timescales, averaging 62% and 1.4
Gyr, respectively. We compare our cluster galaxies to the scaling relations of
the coeval field, in the context of how gas fractions and depletion timescales
vary with respect to the star-forming main sequence. We find that our cluster
galaxies lie systematically off the field scaling relations at z=1.6 toward
enhanced gas fractions, at a level of ~4sigma, but have consistent depletion
timescales. Exploiting CO detections in lower-redshift clusters from the
literature, we investigate the evolution of the gas fraction in cluster
galaxies, finding it to mimic the strong rise with redshift in the field. We
emphasize the utility of detecting abundant gas-rich galaxies in high-redshift
clusters, deeming them as crucial laboratories for future statistical studies.Comment: 8 pages, 3 figures, published in ApJ Letters; updated to match
published versio
The young age of the extremely metal-deficient blue compact dwarf galaxy SBS 1415+437
We use Multiple Mirror Telescope (MMT) spectrophotometry and Hubble Space
Telescope (HST) Faint Object Spectrograph (FOS) spectra and Wide Field and
Planetary Camera 2 (WFPC2) V and I images to study the properties and
evolutionary status of the nearby (D = 11.4 Mpc) extremely metal-deficient blue
compact dwarf (BCD) galaxy SBS 1415+437=CG 389. The oxygen abundance in the
galaxy is 12+log(O/H)=7.60+/-0.01 or Zsun/21. The helium mass fraction in SBS
1415+437 is Y=0.246+/-0.004 which agrees with the primordial helium abundance
determined by Izotov & Thuan using a much larger sample of BCDs. The
alpha-elements-to-oxygen abundance ratios (Ne/O, S/O, Ar/O) are in very good
agreement with the mean values for other metal-deficient BCDs and are
consistent with the scenario that these elements are made in massive stars. The
Fe/O abundance ratio is ~2 times smaller than the solar ratio. The Si/O ratio
is close to the solar value, implying that silicon is not significantly
depleted into dust grains. The values of the N/O and C/O ratios imply that
intermediate-mass stars have not had time to evolve in SBS 1415+437 and release
their nucleosynthesis products and that both N and C in the BCD have been made
by massive stars only. This sets an upper limit of ~100 Myr on the age of SBS
1415+437. The (V-I) color of the low-surface-brightness component of the galaxy
is blue (<0.4 mag) indicative of a very young underlying stellar population.
The (V-I) - I color-magnitude diagrams of the resolved stellar populations in
different regions of SBS 1415+437 suggest propagating star formation from the
NE side of the galaxy to the SW. All regions in SBS 1415+437 possess very blue
spectral energy distributions (SED). We find that the ages of the stellar
populations in SBS 1415+437 to range from a few Myr to 100 Myr.Comment: 25 pages, 12 PS and 5 JPG figures, to appear in Ap
The Evolution of Environmental Quenching Timescales to
Using a sample of 4 galaxy clusters at and 10 galaxy
clusters at , we measure the environmental quenching
timescale, , corresponding to the time required after a galaxy is accreted
by a cluster for it to fully cease star formation. Cluster members are selected
by a photometric-redshift criterion, and categorized as star-forming,
quiescent, or intermediate according to their dust-corrected rest-frame colors
and magnitudes. We employ a "delayed-then-rapid" quenching model that relates a
simulated cluster mass accretion rate to the observed numbers of each type of
galaxy in the cluster to constrain . For galaxies of mass , we find a quenching timescale of 1.24 Gyr
in the cluster sample, and 1.50 Gyr at . Using values
drawn from the literature, we compare the redshift evolution of to
timescales predicted for different physical quenching mechanisms. We find
to depend on host halo mass such that quenching occurs over faster timescales
in clusters relative to groups, suggesting that properties of the host halo are
responsible for quenching high-mass galaxies. Between and , we
find that evolves faster than the molecular gas depletion timescale and
slower than an SFR-outflow timescale, but is consistent with the evolution of
the dynamical time. This suggests that environmental quenching in these
galaxies is driven by the motion of satellites relative to the cluster
environment, although due to uncertainties in the atomic gas budget at high
redshift, we cannot rule out quenching due to simple gas depletion
Helium abundance in the most metal-deficient blue compact galaxies: I Zw 18 and SBS 0335-052
We present high-quality spectroscopic observations of the two most-metal
deficient blue compact galaxies known, I Zw 18 and SBS 0335-052 to determine
the helium abundance. The underlying stellar absorption strongly influences the
observed intensities of He I emission lines in the brightest NW component of I
Zw 18, and hence this component should not be used for primordial He abundance
determination. The effect of underlying stellar absorption, though present, is
much smaller in the SE component. Assuming all systematic uncertainties are
negligible, the He mass fraction derived in this component is Y =
0.243+/-0.007. The high signal-to-noise ratio spectrum (> 100 in the continuum)
of SBS 0335-052 allows us to measure the helium mass fraction with a precision
better than 2% -- 5% in nine different regions along the slit. Assuming all
systematic uncertainties are negligible, the weighted mean He mass fraction in
SBS 0335-052 is Y = 0.2437+/-0.0014 when the three He I 4471, 5876 and 6678
emission lines are used, and is 0.2463+/-0.0015 when the He I 4471 emission
line is excluded. The weighted mean helium mass fraction in the two most
metal-deficient BCGs I Zw 18 and SBS 0335-052, Y=0.2462+/-0.0015, after
correction for the stellar He production results in a primordial He mass
fraction Yp = 0.2452+/-0.0015. The derived Yp leads to a baryon-to-photon ratio
of (4.7+/-1.0) 10^{-10}, consistent with the values derived from the primordial
D and 7Li abundances, and supporting the standard big bang nucleosynthesis
theory. For the most consistent set of primordial D, 4He, and 7Li abundances we
derive an equivalent number of light neutrino species 3.0+/-0.3 (95% C.L.).Comment: 28 pages, 10 figures. To appear in Ap
A spectroscopic study of component C and the extended emission around I Zw 18
Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are
used to investigate the stellar population and the evolutionary status of I Zw
18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18.
Hydrogen H and H emission lines are detected in the spectra of I
Zw 18C, implying that ionizing massive stars are present. High signal-to-noise
Keck II spectra of different regions in I Zw 18C reveal H, H
and higher order hydrogen lines in absorption. Several techniques are used to
constrain the age of the stellar population in I Zw 18C. Ages derived from two
different methods, one based on the equivalent widths of the H,
H emission lines and the other on H, H absorption lines
are consistent with a 15 Myr instantaneous burst model. We find that a small
extinction in the range = 0.20 -- 0.65 mag is needed to fit the observed
spectral energy distribution of I Zw 18C with that model. In the case of
constant star formation, all observed properties are consistent with stars
forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available
observational constraints for I Zw 18C, including those obtained from Hubble
Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18
should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized
gas emission around I Zw 18. H emission is detected as far as 30" from
it. To a B surface brightness limit of ~ 27 mag arcsec we find no
observational evidence for extended stellar emission in the outermost regions,
at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the
Astrophysical Journa
The HST Survey of BL Lacertae Objects. II. Host Galaxies
We have used the HST WFPC2 camera to survey 132 BL Lac objects comprising
seven complete radio-, X-ray-, and optically-selected samples. We obtained
useful images for 110 targets spanning the redshift range 0 < z < 1.3. In two
thirds of the BL Lac images, host galaxies are detected, including nearly all
for z < 0.5 (58 of 63). The highest redshift host galaxy detected is in a BL
Lac object at z=0.664. In 58 of the 72 resolved host galaxies, a de Vaucouleurs
profile is significantly preferred, at >99% confidence, over a pure exponential
disk; the two fits are comparable in the remaining 14 cases. These results
limit the number of disk systems to at most 8% of BL Lacs (at 99% confidence),
and are consistent with all BL~Lac host galaxies being ellipticals. The
detected host galaxies are luminous ellipticals with a median absolute
K-corrected magnitude of M_R= -23.7 +- 0.6 mag, at least one magnitude brighter
than M* and comparable to brightest cluster galaxies. The galaxy morphologies
are generally smooth and undisturbed, with small or negligible ellipticities
(<0.2). There is no correlation between host galaxy and observed nuclear
magnitude or estimated jet power corrected for beaming. If black hole mass is
correlated linearly with bulge mass in general, this implies a large range in
Eddington ratio. Present data strongly support the unification picture with FR
I galaxies constituting the bulk of the parent population of BL Lac objects.Comment: Accepted for publication on ApJ. 38 pages, 8 figure
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