The Star-Formation Histories of z~2 DOGs and SMGs

The Spitzer Space Telescope has identified a population of ultra-luminous infrared galaxies (ULIRGs) at z ~ 2 that may play an important role in the evolution of massive galaxies. We measure the stellar masses of two populations of Spitzer-selected ULIRGs, both of which have extremely red R-[24] colors (dust-obscured galaxies, or DOGs) and compare our results with sub-millimeter selected galaxies (SMGs). One set of 39 DOGs has a local maximum in their mid-IR spectral energy distribution (SED) at rest-frame 1.6um associated with stellar emission ("bump DOGs"), while the other set of 51 DOGs has a power-law dominated mid-IR SED with spectral features typical of obscured AGN ("power-law DOGs"). We use stellar population synthesis models applied self-consistently to broad-band photometry in the rest-frame ultra-violet, optical, and near-infrared of each of these populations and test a variety of stellar population synthesis codes, star-formation histories (SFHs), and initial mass functions (IMFs). Assuming a simple stellar population SFH and a Chabrier IMF, we find that the median and inner quartile stellar masses of SMGs, bump DOGs and power-law DOGs are given by log(M_*/M_sun) = 10.42_-0.36^+0.42, 10.62_-0.32^+0.36, and 10.71_-0.34^+0.40, respectively. Implementing more complicated SFHs with multiple age components increases these mass estimates by up to 0.5 dex. Our stellar mass estimates are consistent with physical mechanisms for the origin of z~2 ULIRGs that result in high star-formation rates for a given stellar mass. Such mechanisms are usually driven by a major merger of two gas-rich systems, rather than smooth accretion of gas and small satellites.Comment: 17 pages, 7 figures, 3 tables. Plus figures showing SEDs and best-fit synthesized stellar population model. Accepted to the Ap

A Star Catalog for the Open Cluster NGC188

We present new BVRI broad-band photometry for the old open cluster NGC188 based upon analysis of 299 CCD images either obtained by us, donated by colleagues, or retrieved from public archives. We compare our results on a star-by-star basis with data from eleven previous photometric investigations of the cluster. We homogenize and merge the data from all the photometric studies, and also merge membership probabilities from four previous proper-motion studies of the cluster field. Fiducial cluster sequences in the BV (Johnson) RI (Cousins) photometric system of Landolt (1992, AJ, 104, 340) represent the principal result of this paper. These have been compared to reference samples defined by (a) Landolt's standard stars, (b) the old open clusters M67 and NGC6791, and (c) stars within 25 pc having modern photometry and precise Hipparcos parallaxes. In a companion paper we show that our derived cluster results agree well with the predictions of modern stellar-interior and -evolution theory, given reasonable estimates of the cluster chemical abundances and foreground reddening. The individual and combined datasets for NGC188 have been made available through our web site.Comment: Accepted for PAS

Interpreting high [O III]/H β ratios with maturing starbursts

Star-forming galaxies at high redshift show ubiquitously high-ionization parameters, as measured by the ratio of optical emission lines. We demonstrate that local (z < 0.2) sources selected as Lyman break analogues also manifest high line ratios with a typical [O III]/Hβ=3.36+0.14−0.04 – comparable to all but the highest ratios seen in star-forming galaxies at z ∼ 2–4. We argue that the stellar population synthesis code BPASS can explain the high-ionization parameters required through the ageing of rapidly formed star populations, without invoking any AGN contribution. Binary stellar evolution pathways prolong the age interval over which a starburst is likely to show elevated line ratios, relative to those predicted by single stellar evolution codes. As a result, model galaxies at near-solar metallicities and with ages of up to ∼100 Myr after a starburst typically have a line ratio [O III]/Hβ ∼ 3, consistent with those seen in Lyman break galaxies and local sources with similar star formation densities. This emphasises the importance of including binary evolution pathways when simulating the nebular line emission of young or bursty stellar populations

A Direct Measurement of the Linear Bias of Mid-infrared-selected Quasars at z ap 1 Using Cosmic Microwave Background Lensing

We measure the cross-power spectrum of the projected mass density as traced by the convergence of the cosmic microwave background lensing field from the South Pole Telescope (SPT) and a sample of Type 1 and 2 (unobscured and obscured) quasars at langzrang ~ 1 selected with the Wide-field Infrared Survey Explorer, over 2500 deg2. The cross-power spectrum is detected at ≈7σ, and we measure a linear bias b = 1.61 ± 0.22, consistent with clustering analyses. Using an independent lensing map, derived from Planck observations, to measure the cross-spectrum, we find excellent agreement with the SPT analysis. The bias of the combined sample of Type 1 and 2 quasars determined in this work is similar to that previously determined for Type 1 quasars alone; we conclude that obscured and unobscured quasars trace the matter field in a similar way. This result has implications for our understanding of quasar unification and evolution schemes.Peer reviewe

Time Delay and Accretion Disk Size Measurements in the Lensed Quasar SBS 0909+532 from Multiwavelength Microlensing Analysis

We present three complete seasons and two half-seasons of Sloan Digital Sky Survey (SDSS) r-band photometry of the gravitationally lensed quasar SBS 0909+532 from the U.S. Naval Observatory, as well as two seasons each of SDSS g-band and r-band monitoring from the Liverpool Robotic Telescope. Using Monte Carlo simulations to simultaneously measure the system’s time delay and model the r-band microlensing variability, we confirm and significantly refine the precision of the system’s time delay to ΔtAB = 50+2 −4 days, where the stated uncertainties represent the bounds of the formal 1σ confidence interval. There may be a conflict between the time delay measurement and a lens consisting of a single galaxy. While models based on the Hubble Space Telescope astrometry and a relatively compact stellar distribution can reproduce the observed delay, the models have somewhat less dark matter than we would typically expect. We also carry out a joint analysis of the microlensing variability in the r and g bands to constrain the size of the quasar’s continuum source at these wavelengths, obtaining log{(rs,r/cm)[cos i/0.5]1/2} = 15.3 ± 0.3 and log{(rs,g/cm)[cos i/0.5]1/2} = 14.8 ± 0.9, respectively. Our current results do not formally constrain the temperature profile of the accretion disk but are consistent with the expectations of standard thin disk theory

Submillimeter Follow-up of WISE-Selected Hyperluminous Galaxies

We have used the Caltech Submillimeter Observatory (CSO) to follow-up a sample of WISE-selected, hyperluminous galaxies, so called W1W2-dropout galaxies. This is a rare (~ 1000 all-sky) population of galaxies at high redshift (peaks at z=2-3), that are faint or undetected by WISE at 3.4 and 4.6 um, yet are clearly detected at 12 and 22 um. The optical spectra of most of these galaxies show significant AGN activity. We observed 14 high-redshift (z > 1.7) W1W2-dropout galaxies with SHARC-II at 350 to 850 um, with 9 detections; and observed 18 with Bolocam at 1.1 mm, with five detections. Warm Spitzer follow-up of 25 targets at 3.6 and 4.5 um, as well as optical spectra of 12 targets are also presented in the paper. Combining WISE data with observations from warm Spitzer and CSO, we constructed their mid-IR to millimeter spectral energy distributions (SEDs). These SEDs have a consistent shape, showing significantly higher mid-IR to submm ratios than other galaxy templates, suggesting a hotter dust temperature. We estimate their dust temperatures to be 60-120 K using a single-temperature model. Their infrared luminosities are well over 10^{13} Lsun. These SEDs are not well fitted with existing galaxy templates, suggesting they are a new population with very high luminosity and hot dust. They are likely among the most luminous galaxies in the Universe. We argue that they are extreme cases of luminous, hot dust-obscured galaxies (DOGs), possibly representing a short evolutionary phase during galaxy merging and evolution. A better understanding of their long-wavelength properties needs ALMA as well as Herschel data.Comment: Will be Published on Sep 1, 2012 by Ap

General Approach for Combining Diverse Rare Variant Association Tests Provides Improved Robustness Across a Wider Range of Genetic Architectures

The widespread availability of genome sequencing data made possible by way of next-generation technologies has yielded a flood of different gene-based rare variant association tests. Most of these tests have been published because they have superior power for particular genetic architectures. However, for applied researchers it is challenging to know which test to choose in practice when little is known a priori about genetic architecture. Recently, tests have been proposed which combine two particular individual tests (one burden and one variance components) to minimize power loss while improving robustness to a wider range of genetic architectures. In our analysis we propose an expansion of these approaches, yielding a general method that works for combining any number of individual tests. We demonstrate that running multiple different tests on the same dataset and using a Bonferroni correction for multiple testing is never better than combining tests using our general method. We also find that using a test statistic that is highly robust to the inclusion of non-causal variants (Joint-infinity) together with a previously published combined test (SKAT-O) provides improved robustness to a wide range of genetic architectures and should be considered for use in practice. Software for this approach is supplied. We support the increased use of combined tests in practice-- as well as further exploration of novel combined testing approaches using the general framework provided here--to maximize robustness of rare-variant testing strategies against a wide range of genetic architectures

A Potential Galaxy Threshing System in the Cosmos Field

We report on the discovery of a new potential galaxy threshing system in the COSMOS 2 square degree field using the prime-focus camera, Suprime-Cam, on the 8.2 m Subaru Telescope. This system consists of a giant elliptical galaxy with $M_V \approx -21.6$ and a tidally disrupted satellite galaxy with $M_V \approx -17.7$ at a photometric redshift of $z \approx 0.08$. This redshift is consistent with the spectroscopic redshift of 0.079 for the giant elliptical galaxy obtained from the Sloan Digital Sky Survey (SDSS) archive. The luminosity masses of the two galaxies are $3.7 \times 10^{12} \cal{M}_{\odot}$ and $3.1 \times 10^{9} \cal{M}_{\odot}$, respectively. The distance between the two galaxies is greater than 100 kpc. The two tidal tails emanating from the satellite galaxy extend over 150 kpc. This system would be the second well-defined galaxy threshing system found so far.Comment: 17 pages, 7 figures, accepted for the COSMOS special issue of ApJ

Dust extinction from Balmer decrements of star-forming galaxies at 0.75<z<1.5 with HST/WFC3 spectroscopy from the WISP survey

Spectroscopic observations of Halpha and Hbeta emission lines of 128 star-forming galaxies in the redshift range 0.75<z<1.5 are presented. These data were taken with slitless spectroscopy using the G102 and G141 grisms of the Wide-Field-Camera 3 (WFC3) on board the Hubble Space Telescope as part of the WFC3 Infrared Spectroscopic Parallel (WISP) survey. Interstellar dust extinction is measured from stacked spectra that cover the Balmer decrement (Halpha/Hbeta). We present dust extinction as a function of Halpha luminosity (down to 3 x 10^{41} erg/s), galaxy stellar mass (reaching 4 x 10^{8} Msun), and rest-frame Halpha equivalent width. The faintest galaxies are two times fainter in Halpha luminosity than galaxies previously studied at z~1.5. An evolution is observed where galaxies of the same Halpha luminosity have lower extinction at higher redshifts, whereas no evolution is found within our error bars with stellar mass. The lower Halpha luminosity galaxies in our sample are found to be consistent with no dust extinction. We find an anti-correlation of the [OIII]5007/Halpha flux ratio as a function of luminosity where galaxies with L_{Halpha}<5 x 10^{41} erg/s are brighter in [OIII]5007 than Halpha. This trend is evident even after extinction correction, suggesting that the increased [OIII]5007/Halpha ratio in low luminosity galaxies is likely due to lower metallicity and/or higher ionization parameters.Comment: 11 pages, 9 figures, 2 tables; version addressing the referee comment

A population of z> 2 far-infrared Herschel-spire-selected starbursts

We present spectroscopic observations for a sample of 36 Herschel-SPIRE 250-500um selected galaxies (HSGs) at 2<z<5 from the Herschel Multi-tiered Extragalactic Survey (HerMES). Redshifts are confirmed as part of a large redshift survey of Herschel-SPIRE-selected sources covering ~0.93deg^2 in six extragalactic legacy fields. Observations were taken with the Keck I Low Resolution Imaging Spectrometer (LRIS) and the Keck II DEep Imaging Multi-Object Spectrograph (DEIMOS). Precise astrometry, needed for spectroscopic follow-up, is determined by identification of counterparts at 24um or 1.4GHz using a cross-identification likelihood matching method. Individual source luminosities range from log(L_IR/Lsun)=12.5-13.6 (corresponding to star formation rates 500-9000Msun/yr, assuming a Salpeter IMF), constituting some of the most intrinsically luminous, distant infrared galaxies yet discovered. We present both individual and composite rest-frame ultraviolet spectra and infrared spectral energy distributions (SEDs). The selection of these HSGs is reproducible and well characterized across large areas of sky in contrast to most z>2 HyLIRGs in the literature which are detected serendipitously or via tailored surveys searching only for high-z HyLIRGs; therefore, we can place lower limits on the contribution of HSGs to the cosmic star formation rate density at (7+-2)x10^(-3)Msun/yr h^3Mpc^(-3) at z~2.5, which is >10% of the estimated total star formation rate density (SFRD) of the Universe from optical surveys. The contribution at z~4 has a lower limit of 3x10^(-3)Msun/yr h^3 Mpc^(-3), ~>20% of the estimated total SFRD. This highlights the importance of extremely infrared-luminous galaxies with high star formation rates to the build-up of stellar mass, even at the earliest epochs.Comment: 25 pages, 10 figures; ApJ accepte