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 ($1.59 < z < 1.71$) 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 ($M_* \geq 3\times 10^{10}\mathrm{M}_\odot$) 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, $11.0 ^{+7.0}_{-5.6}\%$ of the cluster members are involved in potential mergers, compared to $24.7^{+5.3}_{-4.6}\%$ 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$\beta$ 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 z∼1.6z\sim1.6

Using a sample of 4 galaxy clusters at $1.35 < z < 1.65$ and 10 galaxy clusters at $0.85 < z < 1.35$, we measure the environmental quenching timescale, $t_Q$, 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 $t_Q$. For galaxies of mass $M_* \gtrsim 10^{10.5}~ \mathrm{M}_\odot$, we find a quenching timescale of $t_Q=$ 1.24 Gyr in the $z\sim1.5$ cluster sample, and $t_Q=$ 1.50 Gyr at $z\sim1$. Using values drawn from the literature, we compare the redshift evolution of $t_Q$ to timescales predicted for different physical quenching mechanisms. We find $t_Q$ 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 $z=0$ and $z=1.5$, we find that $t_Q$ 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$\alpha$ and H$\beta$ 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$\gamma$, H$\delta$ 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$\alpha$, H$\beta$ emission lines and the other on H$\gamma$, H$\delta$ absorption lines are consistent with a 15 Myr instantaneous burst model. We find that a small extinction in the range $A_V$ = 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$\alpha$ emission is detected as far as 30" from it. To a B surface brightness limit of ~ 27 mag arcsec$^{-2}$ 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