arXiv subjects

Arjun Dey

Publications and source records attributed to Arjun Dey.

At least 289 records · Page 16Linked to original sources

arXiv2006

Evidence for merging or disruption of red galaxies from the evolution of their clustering

The formation and evolution of massive red galaxies form a crucial test of theories of galaxy formation based on hierarchical assembly. In this letter we use observations of the clustering of luminous red galaxies from the Bootes field and N-body simulations to argue that about 1/3 of the most luminous satellite galaxies appear to undergo merging or disruption within massive halos between z~0.9 and z~0.5.

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arXiv2006

Clustering of the IR Background Light with Spitzer: Contribution from Resolved Sources

We describe the angular power spectrum of resolved sources at 3.6 microns (L-band) in Spitzer imaging data of the GOODS HDF-N, the GOODS CDF-S, and the NDWFS Bootes field in several source magnitude bins. We also measure angular power spectra of resolved sources in the Bootes field at K_S and J-bands using ground-based IR imaging data. In the three bands, J, K_S, and L, we detect the clustering of galaxies on top of the shot-noise power spectrum at multipoles between ell ~ 10^2 and 10^5. The angular power spectra range from the large, linear scales to small, non-linear scales of galaxy clustering, and in some magnitude ranges, show departure from a power-law clustering spectrum. We consider a halo model to describe clustering measurements and to establish the halo occup ation number parameters of IR bright galaxies at redshifts around one. We also extend our clustering results and completeness-corrected faint source number counts in GOODS fields to understand the underlying nature of unresolved sources responsible for IR background (IRB) anisotropies that were detected in deep Spitzer images. While these unresolved fluctuations were measured at sub-arcminute angular scales, if a high-redshift diffuse component associated with first galaxies exists in the IRB, then it's clustering properties are best studied with shallow, wide-field images that allow a measurement of the clustering spectrum from a few degrees to arcminute angular scales.

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arXiv2006

The 1<z<5 Infrared Luminosity Function of Type I Quasars

We determine the rest-frame 8 micron luminosity function of type I quasars over the redshift range 1<z<5. Our sample consists of 292 24 micron sources brighter than 1 mJy selected from 7.17 square degrees of the Spitzer Space Telescope MIPS survey of the NOAO Deep Wide-Field Survey Bootes field. The AGN and Galaxy Evolution Survey (AGES) has measured redshifts for 270 of the R<21.7 sources and we estimate that the contamination of the remaining 22 sources by stars and galaxies is low. We are able to select quasars missed by ultra-violet excess quasar surveys, including reddened type I quasars and 2.2<z<3.0 quasars with optical colors similar to main sequence stars. We find reddened type I quasars comprise 20% of the type I quasar population. Nonetheless, the shape, normalization, and evolution of the rest-frame 8 micron luminosity function is comparable to that of quasars selected from optical surveys. The 8 micron luminosity function of type I quasars is well approximated by a power-law with index -2.75(+/-0.14). We directly measure the peak of the quasar space density to be at z=2.6(+/-0.3).

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arXiv2006

The Evolving Luminosity Function of Red Galaxies

We trace the assembly history of red galaxies since z=1, by measuring their evolving space density with the B-band luminosity function. Our sample of 39599 red galaxies, selected from 6.96 square degrees of imaging from the NOAO Deep Wide-Field and Spitzer IRAC Shallow surveys, is an order of magnitude larger, in size and volume, than comparable samples in the literature. We measure a higher space density of z=0.9 red galaxies than some of the recent literature, in part because we account for the faint yet significant galaxy flux which falls outside of our photometric aperture. The B-band luminosity density of red galaxies, which effectively measures the evolution of ~L* galaxies, increases by only 36 percent from z=0 to z=1. If red galaxy stellar populations have faded by 1.24 B-band magnitudes since z=1, the stellar mass contained within the red galaxy population has roughly doubled over the past 8 Gyr. This is consistent with star-forming galaxies being transformed into ~L* red galaxies after a decline in their star formation rates. In contrast, the evolution of 4L* red galaxies differs only slightly from a model with negligible star formation and no galaxy mergers since z=1. If this model approximates the luminosity evolution of red galaxy stellar populations, then 80 percent of the stellar mass contained within today's 4L* red galaxies was already in place at z=0.7. While red galaxy mergers have been observed, such mergers do not produce rapid growth of 4L* red galaxy stellar masses between z=1 and the present day.

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arXiv2006

A New Einstein Cross: A Highly Magnified, Intrinsically Faint Lyman-Alpha Emitter at z=2.7

We report the discovery of a new Einstein cross at redshift z_S = 2.701 based on Lyman-alpha emission in a cruciform configuration around an SDSS luminous red galaxy (z_L = 0.331). The system was targeted as a possible lens based on an anomalous emission line in the SDSS spectrum. Imaging and spectroscopy from the W. M. Keck Observatory confirm the lensing nature of this system. This is one of the widest-separation galaxy-scale lenses known, with an Einstein radius of ~1.84 arcsec. We present simple gravitational lens models for the system and compute the intrinsic properties of the lensed galaxy. The total mass of the lensing galaxy within the 8.8 +/- 0.1 kpc enclosed by the lensed images is (5.2 +/- 0.1) x 10^11 M_sun. The lensed galaxy is a low mass galaxy (0.2 L*) with a high equivalent-width Lyman-alpha line (EW_Lya_rest = 46 +/- 5 Angstroms). Follow-up studies of this lens system can probe the mass structure of the lensing galaxy, and can provide a unique view of an intrinsically faint, high-redshift, star-forming galaxy at high signal-to-noise ratio.

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arXiv2006

Black Hole Masses and Eddington Ratios at 0.3<z<4

We study the distribution of Eddington luminosity ratios, L_bol/L_edd, of active galactic nuclei (AGNs) discovered in the AGN and Galaxy Evolution Survey (AGES). We combine H-beta, MgII, and CIV line widths with continuum luminosities to estimate black hole (BH) masses in 407 AGNs, covering the redshift range z~0.3-4 and the bolometric luminosity range L_bol~10^45-10^47 erg/s. The sample consists of X-ray or mid-infrared (24 micron) point sources with optical magnitude R<=21.5 mag and optical emission line spectra characteristic of AGNs. For the range of luminosity and redshift probed by AGES, the distribution of estimated Eddington ratios is well described as log-normal with a peak at L_bol/L_edd ~ 1/4 and a dispersion of 0.3 dex. Since additional sources of scatter are minimal, this dispersion must account for contributions from the scatter between estimated and true BH mass and the scatter between estimated and true bolometric luminosity. Therefore, we conclude that: (1) neither of these sources of error can contribute more than \~0.3 dex rms; and (2) the true Eddington ratios of optically luminous AGNs are even more sharply peaked. Because the mass estimation errors must be smaller than ~0.3 dex, we can also investigate the distribution of Eddington ratios at fixed BH mass. We show for the first time that the distribution of Eddington ratios at fixed BH mass is peaked, and that the dearth of AGNs at a factor ~10 below Eddington is real and not an artifact of sample selection. These results provide strong evidence that supermassive BHs gain most of their mass while radiating close to the Eddington limit, and they suggest that the fueling rates in luminous AGNs are ultimately determined by BH self-regulation of the accretion flow rather than galactic scale dynamical disturbances.

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arXiv2006

The Discovery of Three New z>5 Quasars in the AGN and Galaxy Evolution Survey

We present the discovery of three z>5 quasars in the AGN and Galaxy Evolution Survey (AGES) spectroscopic observations of the NOAO Deep Wide-Field Survey (NDWFS) Bootes Field. These quasars were selected as part of a larger Spitzer mid-infrared quasar sample with no selection based on optical colors. The highest redshift object, NDWFS J142516.3+325409, z=5.85, is the lowest-luminosity z>5.8 quasar currently known. We compare mid-infrared techniques for identifying z>5 quasars to more traditional optical techniques and show that mid-infrared colors allow for selection of high-redshift quasars even at redshifts where quasars lie near the optical stellar locus and at z>7 where optical selection is impossible. Using the superb multi-wavelength coverage available in the NDWFS Bootes field, we construct the spectral energy distributions (SEDs) of high-redshift quasars from observed Bw-band to 24 microns (rest-frame 600 Angstroms - 3.7 microns). We show that the three high-redshift quasars have quite similar SEDs, and the rest-frame composite SED of low-redshift quasars from the literature shows little evolution compared to our high-redshift objects. We compare the number of z>5 quasars we have discovered to the expected number from published quasar luminosity functions. While analyses of the quasar luminosity function are tenuous based on only three objects, we find that a relatively steep luminosity function with Psi L^(-3.2) provides the best agreement with the number of high-redshift quasars discovered in our survey.

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arXiv2005

The Chandra XBootes Survey - III: Optical and Near-IR Counterparts

The XBootes Survey is a 5-ks Chandra survey of the Bootes Field of the NOAO Deep Wide-Field Survey (NDWFS). This survey is unique in that it is the largest (9.3 deg^2), contiguous region imaged in X-ray with complementary deep optical and near-IR observations. We present a catalog of the optical counterparts to the 3,213 X-ray point sources detected in the XBootes survey. Using a Bayesian identification scheme, we successfully identified optical counterparts for 98% of the X-ray point sources. The optical colors suggest that the optically detected galaxies are a combination of z<1 massive early-type galaxies and bluer star-forming galaxies whose optical AGN emission is faint or obscured, whereas the majority of the optically detected point sources are likely quasars over a large redshift range. Our large area, X-ray bright, optically deep survey enables us to select a large sub-sample of sources (773) with high X-ray to optical flux ratios (f_x/f_o>10). These objects are likely high redshift and/or dust obscured AGN. These sources have generally harder X-ray spectra than sources with 0.1<f_x/f_o<10. Of the 73 X-ray sources with no optical counterpart in the NDWFS catalog, 47 are truly optically blank down to R~25.5 (the average 50% completeness limit of the NDWFS R-band catalogs). These sources are also likely to be high redshift and/or dust obscured AGN.

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arXiv2005

The FLAMINGOS Extragalactic Survey

Using the Florida Multi-object Imaging Near-IR grism Observational Spectrometer (FLAMINGOS), we have conducted the FLAMINGOS Extragalactic Survey (FLAMEX), a deep imaging survey covering 7.1 square degrees within the 18.6 sq. deg NOAO Deep Wide-Field Survey (NDWFS) regions. FLAMEX is the first deep, wide-area near-infrared survey to image in both the J and Ks filters, and is larger than any previous NIR surveys of comparable depth. The intent of FLAMEX is to facilitate the study of galaxy and galaxy cluster evolution at 1 1 galaxy clusters detected using the joint FLAMEX, NDWFS, and Spitzer IRAC Shallow Survey data sets.

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arXiv2005

An IR-Selected Galaxy Cluster at z = 1.41

We report the discovery of a galaxy cluster at z = 1.41. ISCS J143809+341419 was found in the Spitzer/IRAC Shallow Survey of the Bootes field in the NOAO Deep Wide-Field Survey carried out by IRAC. The cluster candidate was initially identified as a high density region of objects with photometric redshifts in the range 1.3 < z < 1.5. Optical spectroscopy of a limited number of objects in the region shows that 5 galaxies within a ~120 arcsec diameter region lie at z = 1.41 +/- 0.01. Most of these member galaxies have broad--band colors consistent with the expected spectral energy distribution of a passively--evolving elliptical galaxy formed at high redshift. The redshift of ISCS J143809+341419 is the highest currently known for a spectroscopically-confirmed cluster of galaxies.

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arXiv2005

Dark Energy and Cosmic Sound: w(z) Surveys with the Gemini/Subaru Wide-Field Multi-Object Spectrograph

This white paper gives an overview of the proposed Gemini/Subaru Wide-Field Multi-Object Spectrograph (WFMOS) and the proposed redshift surveys of 2.6 million galaxies with 0.5<z<3.3 over 2000 deg^2 of sky. These surveys will probe the baryonic acoustic oscillations in the galaxy power spectrum with unprecedented precision and over a range of redshifts and deliver dark energy w(z) constraints an order of magnitude better than current limits. We discuss the requirements on precursor observations and on calibrations, the systematics in the method and the quantitative precision obtainaible in distance-redshift and expansion-rate-redshift measurements which feed in to the w(z) precision. We also outline the technological and scientific strengths and risks which might be associated with the project and the relationship of WFMOS to other baryon oscillation experiments.

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arXiv2005

The XBootes Chandra Survey Paper II: The X-ray Source Catalog

We present results from a Chandra survey of the nine square degree Bootes field of the NOAO Deep Wide-Field Survey (NDWFS). This XBootes survey consists of 126 separate contiguous ACIS-I observations each of approximately 5000 seconds in duration. These unique Chandra observations allow us to search for large scale structure and to calculate X-ray source statistics o ver a wide, contiguous field of view with arcsecond angular resolution and uniform coverage. Optical spectroscopic follow-up observations and the rich NDWFS data set will allow us to identify and classify these X-ray selected sources. Using wavelet decomposition, we detect 4642 point sources with n $\ge$ 2 counts. In order to keep our detections ~99% reliable, we limit our list to sources with n $\ge$ 4 counts. The full 0.5--7 keV band n $\ge$ 4 count list has 3293 point sources. In addition to the point sources, 43 extended sources have been detected consistent, with the depth of these observations and the number counts of clusters. We present here the X-ray catalog for the XBootes survey, including source positions, X-ray fluxes, hardness ratios and their uncertainties. We calculate and present the differential number of sources per flux densit y interval, $N(S)$, for the point sources.

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arXiv2005

XBootes: An X-Ray Survey of the NDWFS Bootes Field - Paper I Overview and Initial Results

We obtained a 5 ksec deep Chandra X-ray Observatory ACIS-I map of the 9.3 square degree Bootes field of the NOAO Deep Wide-Field Survey. Here we describe the data acquisition and analysis strategies leading to a catalog of 4642 (3293) point sources with 2 or more (4 or more) counts, corresponding to a limiting flux of roughly 4(8)x10^{-15} erg cm^{-2}s^{-1} in the 0.5-7 keV band. These Chandra XBootes data are unique in that they consitute the widest contiguous X-ray field yet observed to such a faint flux limit. Because of the extraordinarily low background of the ACIS, we expect only 14% (0.7%) of the sources to be spurious. We also detected 43 extended sources in this survey. The distribution of the point sources among the 126 pointings (ACIS-I has a 16 x 16 arcminute field of view) is consistent with Poisson fluctuations about the mean of 36.8 sources per pointing. While a smoothed image of the point source distribution is clumpy, there is no statistically significant evidence of large scale filamentary structure. We do find however, that for theta>1 arcminute, the angular correlation function of these sources is consistent with previous measurements, following a power law in angle with slope -0.7. In a 1.4 deg^{2} sample of the survey, approximately 87% of the sources with 4 or more counts have an optical counterpart to R ~26 mag. As part of a larger program of optical spectroscopy of the NDWFS Bootes area, spectra have been obtained for \~900 of the X-ray sources, most of which are QSOs or AGN.

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arXiv2005

Tracing the Nuclear Accretion History of the Red Galaxy Population

We investigate the evolution of the hard X-ray luminosity of the red galaxy population using a large sample of 3316 red galaxies selected over a wide range in redshift (0.3<z<0.9) from a 1.4 deg^2 region in the Bootes field of the NOAO Deep Wide-Field Survey (NDWFS). The red galaxies are early-type, bulge-dominated galaxies and are selected to have the same evolution corrected, absolute R-band magnitude distribution as a function of redshift to ensure we are tracing the evolution in the X-ray properties of a comparable optical population. Using a stacking analysis of 5-ks Chandra/ACIS observations within this field to study the X-ray emission from these red galaxies in three redshift bins, we find that the mean X-ray luminosity increases as a function of redshift. The large mean X-ray luminosity and the hardness of the mean X-ray spectrum suggests that the X-ray emission is largely dominated by AGN rather than stellar sources. The hardness ratio can be reproduced by either an absorbed (N_H ~2 x 10^22 cm^-2) Gamma=1.7 power-law source, consistent with that of a population of moderately obscured Seyfert-like AGN, or an unabsorbed Gamma=0.7 source suggesting a radiatively inefficient accretion flow (e.g., an advection-dominated accretion flow). We also find that the emission from this sample of red galaxies constitutes at least 5% of the hard X-ray background. These results suggest a global decline in the mean AGN activity of normal early-type galaxies from z~1 to the present, which indicates that we are witnessing the tailing off of the accretion activity onto SMBHs in early-type galaxies since the quasar epoch.

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arXiv2004

Spectroscopic Properties of the z=4.5 Lyman-alpha Emitters

We present Keck/LRIS optical spectra of 17 Lya-emitting galaxies and one Lyman break galaxy at z=4.5 discovered in the Large Area Lyman Alpha (LALA) survey. The survey has identified a sample of ~350 candidate Lya-emitting galaxies at z=4.5 in a search volume of 1.5 x 10^6 comoving Mpc^3. We targeted 25 candidates for spectroscopy; hence, the 18 confirmations presented herein suggest a selection reliability of 72%. The large equivalent widths (median W(rest)~80 A) but narrow physical widths (v < 500 km/s) of the Lya emission lines, along with the lack of accompanying high-ionization state emission lines, suggest that these galaxies are young systems powered by star formation rather than by AGN activity. Theoretical models of galaxy formation in the primordial Universe suggest that a small fraction of Lya-emitting galaxies at z=4.5 may still be nascent, metal-free objects. Indeed, we find with 90% confidence that 3 to 5 of the confirmed sources show W(rest) > 240 A, exceeding the maximum Lya equivalent width predicted for normal stellar populations. Nonetheless, we find no evidence for HeII 1640 emission in either individual or composite spectra, indicating that though these galaxies are young, they are not truly primitive, Population III objects.

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arXiv2004

The Clustering of Extragalactic Extremely Red Objects

We have measured the angular and spatial clustering of 671 K<18.40, R-K>5 Extremely Red Objects (EROs) from a 0.98 square degree sub-region of the NOAO Deep Wide-Field Survey (NDWFS). Our study covers nearly 5 times the area and has twice the sample size of any previous ERO clustering study. The wide field of view and BwRIK passbands of the NDWFS allow us to place improved constraints on the clustering of z=1 EROs. We find the angular clustering of EROs is slightly weaker than in previous measurements, and w(1')=0.25+/-0.05 for K<18.40 EROs. We find no significant correlation of ERO spatial clustering with redshift, apparent color or absolute magnitude, although given the uncertainties, such correlations remain plausible. We find the spatial clustering of K<18.40, R-K>5 EROs is well approximated by a power-law, with r_0=9.7+/-1.1 Mpc/h in comoving coordinates. This is comparable to the clustering of 4L* early-type galaxies at z<1, and is consistent with the brightest EROs being the progenitors of the most massive ellipticals. There is evidence of the angular clustering of EROs decreasing with increasing apparent magnitude, when NDWFS measurements of ERO clustering are combined with those from the literature. Unless the redshift distribution of K>20 EROs is very broad, the spatial clustering of EROs decreases from r_0=9.7+/-1.1 Mpc/h for K<18.40 to r_0=7.5 Mpc/h for K>20 EROs.

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arXiv2004

Mid-Infrared Selection of Active Galaxies

Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ultraviolet to mid-infrared continuum of normal galaxies is dominated by the composite stellar black body curve and peaks at approximately 1.6 microns, the ultraviolet to mid-infrared continuum of active galaxies is dominated by a power law. Consequently, with sufficient wavelength baseline, one can easily distinguish AGN from stellar populations. Mirroring the tendency of AGN to be bluer than galaxies in the ultraviolet, where galaxies (and stars) sample the blue, rising portion of stellar spectra, AGN tend to be redder than galaxies in the mid-infrared, where galaxies sample the red, falling portion of the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors, derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically identified sources from the AGN and Galaxy Evolution Survey. Based on this spectroscopic sample, we find that simple mid-infrared color criteria provide remarkably robust separation of active galaxies from normal galaxies and Galactic stars, with over 80% completeness and less than 20% contamination. Considering only broad-lined AGN, these mid-infrared color criteria identify over 90% of spectroscopically identified quasars and Seyfert 1s. Applying these color criteria to the full imaging data set, we discuss the implied surface density of AGN and find evidence for a large population of optically obscured active galaxies.

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arXiv2004

A Luminous Lyman-alpha Emitting Galaxy at Redshift z=6.535: Discovery and Spectroscopic Confirmation

We present a redshift z=6.535 galaxy discovered by its Lyman alpha emission in a 9180A narrowband image from the Large Area Lyman Alpha (LALA) survey. The Ly-a line luminosity (1.1e43 erg/s) is among the largest known for star forming galaxies at z=6.5. The line shows the distinct asymmetry that is characteristic of high-redshift Ly-a. The 2 sigma lower bound on the observer-frame equivalent width is > 530A. This is hard to reconcile with a neutral intergalactic medium unless the Ly-a line is intrinsically strong and is emitted from its host galaxy with an intrinsic Doppler shift of several hundred km/s. If the IGM is ionized, it corresponds to a rest frame equivalent width > 40A after correcting for Ly-a forest absorption. We also present complete spectroscopic followup of the remaining candidates with line flux > 2e-17 erg/cm2/s in our 1200 square arcminute narrowband image. These include another galaxy with a strong emission line at 9136A and no detected continuum flux, which however is most likely an [OIII] source at z=0.824 based on a weak detection of the [OIII] 4959A line.

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