arXiv subjects

D. Stern

Publications and source records attributed to D. Stern.

At least 325 records · Page 18Linked to original sources

arXiv2007

The spatial clustering of mid-IR selected star forming galaxies at z ~ 1 in the GOODS fields

We present the first spatial clustering measurements of z~1, 24um-selected, star forming galaxies in the Great Observatories Origins Deep Survey (GOODS). The sample under investigation includes 495 objects in GOODS-South and 811 objects in GOODS-North selected down to flux densities of f_24>20 uJy and z_AB<23.5 mag, for which spectroscopic redshifts are available. The median redshift, IR luminosity and star formation rate (SFR) of the samples are z~0.8, L_IR~4.4 x 10^10 L_sun, and SFR~7.6 M_sun/yr, respectively. We measure the projected correlation function w(r_p) on scales of r_p=0.06-10 h^-1 Mpc, from which we derive a best fit comoving correlation length of r_0 = 4.0 +- 0.4 h^-1 Mpc and slope of gamma=1.5 +- 0.1 for the whole f_24>20uJy sample after combining the two fields. We find indications of a larger correlation length for objects of higher luminosity, with Luminous Infrared Galaxies (LIRGs, L_IR>10^11 L_sun) reaching r_0~5.1 h^-1 Mpc. This would imply that galaxies with larger SFRs are hosted in progressively more massive halos, reaching minimum halo masses of ~3 x 10^12 M_sun for LIRGs. We compare our measurements with the predictions from semi-analytic models based on the Millennium simulation. The variance in the models is used to estimate the errors in our GOODS clustering measurements, which are dominated by cosmic variance. The measurements from the two GOODS fields are found to be consistent within the errors. On scales of the GOODS fields, the real sources appear more strongly clustered than objects in the Millennium-simulation based catalogs, if the selection function is applied consistently. This suggests that star formation at z~0.5-1 is being hosted in more massive halos and denser environments than currently predicted by galaxy formation models.[truncated]

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arXiv2007

Wide-field mid-infrared and millimetre imaging of the high-redshift radio galaxy, 4C41.17

We present deep 350- and 1200-micron imaging of the region around 4C41.17 -- one of the most distant (z = 3.792) and luminous known radio galaxies -- obtained with the Submillimeter High Angular Resolution Camera (SHARC-II) and the Max Planck Millimeter Bolometer Array (MAMBO). The radio galaxy is robustly detected at 350- and 1200-micron, as are two nearby 850-micron-selected galaxies; a third 850-micron source is detected at 350-micron and coincides with a ~ 2-sigma feature in the 1200-micron map. Further away from the radio galaxy an additional nine sources are detected at 1200-micron, bringing the total number of detected (sub)millimeter selected galaxies (SMGs) in this field to 14. Using radio images from the Very Large Array (VLA) and Spitzer mid-infrared (mid-IR) data, we find statistically robust radio and/or 24-micron counterparts to eight of the 14 SMGs in the field around 4C41.17. Follow-up spectroscopy with Keck/LRIS has yielded redshifts for three of the eight robustly identified SMGs, placing them in the redshift range 0.5 < z < 2.7, i.e. well below that of 4C41.17. We infer photometric redshifts for a further four sources using their 1.6-micron (rest-frame) stellar feature as probed by the IRAC bands; only one of them is likely to be at the same redshift as 4C41.17. Thus at least four, and as many as seven, of the SMGs within the 4C41.17 field are physically unrelated to the radio galaxy. With the redshift information at hand we are able to constrain the observed over-densities of SMGs within radial bins stretching to R=50 and 100" (~ 0.4 and ~ 0.8Mpc at z ~ 3.8) from the radio galaxy to ~ 5x and ~ 2x that of the field, dropping off to the background value at R=150". [Abridged]

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arXiv2007

The reversal of the star formation-density relation in the distant universe

We study the relationship between the local environment of galaxies and their star formation rate (SFR) in the Great Observatories Origins Deep Survey, GOODS, at z~1, from ultradeep imaging at 24 microns with the MIPS camera onboard Spitzer. We show that the star formation-density relation observed locally was reversed at z~1: the average SFR of an individual galaxy increased with local galaxy density when the universe was less than half its present age. Hierarchical galaxy formation models (simulated lightcones from the Millennium model) predicted such a reversal to occur only at only at earlier epochs (z>2) and at a lower level. We present a remarkable structure at z~1.016, containing X-ray traced galaxy concentrations, which will eventually merge into a Virgo-like cluster. This structure illustrates how the individual SFR of galaxies increases with density at the ~1-2 Mpc scale. The SFR of z~1 galaxies is found to correlate with stellar mass suggesting that mass plays a role in the observed star formation-density trend. However the specific SFR (=SFR/M*) decreases with stellar mass while it increases with galaxy density, which implies that the environment does directly affect the star formation activity of galaxies. Major mergers do not appear to be the unique or even major cause for this effect since nearly half (46%) of the luminous infrared galaxies (LIRGs) at z~1 present the HST-ACS morphology of spirals, while only a third present a clear signature of major mergers. The remaining galaxies are divided into compact (9%) and irregular (14%) galaxies. Moreover, the specific SFR of major mergers is only marginally stronger than that of spirals. Reproducing the SFR-density relation at z ~ 1 is a new challenge for models, requiring a correct balance between mergers and in-situ star formation at early epochs.

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arXiv2006

Mid-Infrared Selection of Brown Dwarfs and High-Redshift Quasars

We discuss color selection of rare objects in a wide-field, multiband survey spanning from the optical to the mid-infrared. Simple color criteria simultaneously identify and distinguish two of the most sought after astrophysical sources: the coolest brown dwarfs and the most distant quasars. We present spectroscopically-confirmed examples of each class identified in the IRAC Shallow Survey of the Bootes field of the NOAO Deep Wide-Field Survey. ISS J142950.9+333012 is a T4.5 brown dwarf at a distance of approximately 42 pc, and ISS J142738.5+331242 is a radio-loud quasar at redshift z=6.12. Our selection criteria identify a total of four candidates over 8 square degrees of the Bootes field. The other two candidates are both confirmed 5.5<z<6 quasars, previously reported by Cool et al. (2006). We discuss the implications of these discoveries and conclude that there are excellent prospects for extending such searches to cooler brown dwarfs and higher redshift quasars.

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arXiv2006

Comparing dynamical and photometric-stellar masses of early-type galaxies at z ~ 1

The purpose of this study is to explore the relationship between galaxy stellar masses, based on multiwavelength photometry spectral template fitting and dynamical masses based on published velocity dispersion measurements, for a sample of 48 early-type galaxies at z ~ 1 with HST/ACS morphological information. We determine photometric-stellar masses and perform a quantitative morphological analysis of cluster and field galaxies at redshift 0.6 < z < 1.2, using ground- and space-based multiwavelegth data available on the GOODS-S field and on the field around the X-ray luminous cluster RDCS1252.9-2927 at z = 1.24. We use multi-band photometry over 0.4-8um from HST/ACS, VLT/ISAAC and Spitzer/IRAC to estimate photometric-stellar masses using Composite Stellar Population (CSP) templates computed with PEGASE.2 models. We compare stellar masses with those obtained using CSPs built with Bruzual & Charlot and Maraston models. We then compare photometric-stellar mass and dynamical mass estimates as a function of morphological parameters obtained from HST/ACS imaging. Based on our sample, which spans the mass range log(Mphot)=[10, 11.5], we find that 1) PEGASE.2, BC03, M05 yield consistent photometric-stellar masses for early-type galaxies at z ~ 1 with a small scatter (0.15 dex rms); 2) adopting a Kroupa IMF, photometric-stellar masses match dynamical mass estimates for early-type galaxies with an average offset of 0.27 dex; 3) assuming a costant IMF, increasing dark matter fraction with the increasing galaxy mass can explain the observed trend.

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arXiv2006

Spitzer Observations of Centaurus A: Infrared Synchrotron Emission from the Northern Lobe

We present measurements obtained with the Spitzer Space Telescope in five bands from 3.6-24 microns of the northern inner radio lobe of Centaurus A, the nearest powerful radio galaxy. We show that this emission is synchrotron in origin. Comparison with ultraviolet observations from GALEX shows that diffuse ultraviolet emission exists in a smaller region than the infrared but also coincides with the radio jet. We discuss the possibility, that synchrotron emission is responsible for the ultraviolet emission and conclude that further data are required to confirm this.

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arXiv2006

Photometric Redshifts in the IRAC Shallow Survey

Accurate photometric redshifts are calculated for nearly 200,000 galaxies to a 4.5 micron flux limit of ~13 uJy in the 8.5 deg^2 Spitzer/IRAC Shallow survey. Using a hybrid photometric redshift algorithm incorporating both neural-net and template-fitting techniques, calibrated with over 15,000 spectroscopic redshifts, a redshift accuracy of σ= 0.06(1+z) is achieved for 95% of galaxies at 0 1) galaxy clusters. We present one such spectroscopically confirmed cluster at =1.24, ISCS J1434.5+3427. Finally, we present a measurement of the 4.5 micron-selected galaxy redshift distribution.

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arXiv2006

The XMM Cluster Survey: A Massive Galaxy Cluster at z=1.45

We report the discovery of XMMXCS J2215.9-1738, a massive galaxy cluster at z =1.45, which was found in the XMM Cluster Survey. The cluster candidate was initially identified as an extended X-ray source in archival XMM data. Optical spectroscopy shows that 6 galaxies within a 60 arcsec diameter region lie at z = 1.45 +/- 0.01. Model fits to the X-ray spectra of the extended emission yield kT = 7.4 (+2.7,-1.8) keV (90 % confidence); if there is an undetected central X-ray point source then kT = 6.5 (+2.6,-1.8) keV. The bolometric X-ray luminosity is Lx = 4.4 (+0.8,-0.6) x 10^44 ergs/s over a 2 Mpc radial region. The measured Tx, which is the highest known for a cluster at z > 1, suggests that this cluster is relatively massive for such a high redshift. The redshift of XMMXCS J2215.9-1738 is the highest currently known for a spectroscopically-confirmed cluster of galaxies.

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arXiv2006

The Great Observatories Origins Deep Survey VLT/FORS2 Spectroscopy in the GOODS-South Field: Part II

We present the second campaign of the ESO/GOODS program of spectroscopy of faint galaxies in the GOODS-South field. Objects were selected as candidates for VLT/FORS2 observations primarily based on the expectation that the detection and measurement of their spectral features would benefit from the high throughput and spectral resolution of FORS2. The reliability of the redshift estimates is assessed using the redshift-magnitude and color-redshift diagrams and comparing the results with public data. 807 spectra of 652 individua targets have been obtained in service mode with the FORS2 spectrograph at the ESO/VLT, providing 501 redshift determinations. The typical redshift uncertainty is estimated to be sigma_z ~ 0.001. Galaxies have been selected adopting three different color criteria and using the photometric redshifts.The resulting redshift distribution typically spans two redshift domains: from z=0.5 to 2 and z=3.5 to 6.2. In particular, 94 B435-,V606-,i775 "dropout" Lyman break galaxies have been observed, yielding redshifts for 65 objects in the interval 3.4<z<6.2. Three sources have been serendipitously discovered in the redshift interval 4.8<z<5.5. Together with the previous release, 930 sources have now been observed and 724 redshift determinations have been carried out. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/. Large scale structures are clearly detected at z=0.666, 0.734, 1.096, 1.221, 1.300, and 1.614. A sample of 34 sources with tilted [OII]3727 emission has been identified, 32 of them in the redshift range 0.9<z<1.5.

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arXiv2006

Spitzer Observations of High Redshift Radio Galaxies

We present the results of a comprehensive Spitzer survey of 70 radio galaxies across 1<z<5.2. Using IRAC, IRS and MIPS imaging we determine the rest-frame AGN contribution to the stellar emission peak at 1.6um. The stellar luminosities are found to be consistent with that of a giant elliptical with a stellar mass of 10^11-12Msun. The mean stellar mass remains constant at \~10^11.5Msun up to z=3 indicating that the upper end of the mass function is already in place by this redshift. The mid-IR luminosities imply bolometric IR luminosities that would classify all sources as ULIRGs. The mid-IR to radio luminosity generally correlate implying a common origin for these emissions. The ratio is higher than that found for lower redshift, ie z<1, radio galaxies.

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arXiv2005

VIMOS-VLT and Spitzer observations of a radio galaxy at z=2.5

We present: 1) a kinematic and morphological study of the giant Lya nebula associated with the radio galaxy MRC2104-242 (z = 2.49) based on integral field spectroscopic VIMOS data from VLT; 2) a photometric study of the host (proto?) galaxy based on Spitzer Space Telescope data. The galaxy appears to be embedded in a giant (>120 kpc) gas reservoir that surrounds it completely. The kinematic properties of the nebula suggest that it is a rotating structure, which would imply a lower limit to the dynamical mass of ~3 x 1e11 Msun. An alternate scenario is that the gas is infalling. Such a process would be able to initiate and sustain significant central starburst activity, although it is likely to contribute with less than 10% of the total stellar mass of the galaxy. The near- to mid-IR spectral energy distribution of the radio galaxy suggests the existence of a reddened, E(B-V)=0.4(+/-)0 .1$, evolved stellar population of age >~ 1.8 Gyr and mass 5(+/-)2 x 1e11 Msun. The implied formation redshift is z_f >~ 6. This stellar mass is similar to the stellar masses found for massive early-type galaxies at z~2 in deep, near-infrared surveys.

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arXiv2005

Discovery of an Optically-Faint Quasar at z=5.70 and Implications for the Faint End of the Quasar Luminosity Function

We present observations of an optically-faint quasar, RD J114816.2+525339, discovered from deep multi-color observations of the field around the z = 6.42 quasar SDSS J1148+5251. The two quasars have a projected separation of 109 arcsec and both are outliers in r-z versus z-J color-color space. Keck spectroscopy reveals RD J114816.2+525339 to be a broad-absorption line quasar at z = 5.70. With z_AB = 23.0, RD J114816.2+525339 is 3.3 mag fainter than SDSS J1148+5251, making it the faintest quasar known at z>5.5. This object was identified in a survey of ~2.5 square degrees. The implied surface density of quasars at these redshifts and luminosities is broadly consistent with previous extrapolations of the faint end of the quasar luminosity function and supports the idea that active galaxies provide only a minor component of the reionizing ultraviolet flux at these redshifts.

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arXiv2005

Evidence for a Massive Post-Starburst Galaxy at z ~ 6.5

We present results from a search for high-redshift J--band ``dropout'' galaxies in the portion of the GOODS southern field that is covered by extremely deep imaging from the Hubble Ultradeep Field (HUDF).Using observations at optical, near-infrared and mid-infrared wavelengths from the Hubble and Spitzer Space Telescopes and the ESO-VLT, we search for very massive galaxies at high redshifts and find one particularly remarkable candidate. Its spectral energy distribution is consistent with a galaxy at z ~ 6.5 and a stellar mass of 6x10e11 M(sun) (for a Salpeter IMF). We interpret a prominent photometric break between the near-infrared and Spitzer bandpasses as the 3646A Balmer discontinuity. The best-fitting models have low reddening and ages of several hundred Myr, placing the formation of the bulk of the stars at z > 9. Alternative models of dusty galaxies at z ~ 2.5 are possible but provide significantly poorer fits. The object is detected with Spitzer at 24 micron. This emission originats from an obscured active nucleus or star formation. We present optical and near-infrared spectroscopy which has, thus far, failed to detect any spectral features. This helps limit the solution in which the galaxy is a starburst or active galaxy at z ~ 2.5. If the high-redshift interpretation is correct, this object would be an example of a galaxy that formed by a process strongly resembling traditional models of monolithic collapse, in a way which a very large mass of stars formed within a remarkably short period of time, at very high redshift.

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arXiv2005

Direct evidence for an early reionization of the Universe?

We examine the possible reionization of the intergalactic medium (IGM) by the source UDF033238.7-274839.8 (hereafter HUDF-JD2), which was discovered in deep {\it HST}/VLT/{\it Spitzer} images obtained as part of the Great Observatory Origins Deep Survey and {\it Hubble} Ultra-Deep Field projects. Mobasher et al (2005) have identified HUDF-JD2 as a massive ($\sim6\times10^{11}M_\odot$) post-starburst galaxy at redshift z$\gtrsim6.5$. We find that HUDF-JD2 may be capable of reionizing its surrounding region of the Universe, starting the process at a redshift as high as z$\approx 15 \pm5$.

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arXiv2005

MIPS J142824.0+352619: A Hyperluminous Starburst Galaxy at z=1.325

Using the SHARC-II camera at the Caltech Submillimeter Observatory to obtain 350 micron images of sources detected with the MIPS instrument on Spitzer, we have discovered a remarkable object at z=1.325+/-0.002 with an apparent Far-Infrared luminosity of 3.2(+/-0.7) x 10^13 Lsun. Unlike other z>1 sources of comparable luminosity selected from mid-IR surveys, MIPS J142824.0+352619 lacks any trace of AGN activity, and is likely a luminous analog of galaxies selected locally by IRAS, or at high redshift in the submillimeter. This source appears to be lensed by a foreground elliptical galaxy at z=1.034, although the amplification is likely modest (~10). We argue that the contribution to the observed optical/Near-IR emission from the foreground galaxy is small, and hence are able to present the rest-frame UV through radio Spectral Energy Distribution of this galaxy. Due to its unusually high luminosity, MIPS J142824.0+352619 presents a unique chance to study a high redshift dusty starburst galaxy in great detail.

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arXiv2005

Near-Infrared Photometry of the High-Redshift Quasar RDJ030117+002025: Evidence for a Massive Starburst at z=5.5

With a redshift of z=5.5 and an optical blue magnitude M_B ~ -24.2 mag (~4.5 10^12 L_sun), RDJ030117+002025 is the most distant optically faint (M_B > -26 mag) quasar known. MAMBO continuum observations at lambda=1.2 mm (185 micrometer rest-frame) showed that this quasar has a far-IR luminosity comparable to its optical luminosity. We present near-infrared J- and K-band photometry obtained with NIRC on the Keck I telescope, tracing the slope of the rest frame UV spectrum of this quasar. The observed spectral index is close to the value of alpha_nu ~ -0.44 measured in composite spectra of optically-bright SDSS quasars. It thus appears that the quasar does not suffer from strong dust extinction, which further implies that its low rest-frame UV luminosity is due to an intrinsically-faint AGN. The FIR to optical luminosity ratio is then much larger than that observed for the more luminous quasars, supporting the suggestion that the FIR emission is not powered by the AGN but by a massive starburst.

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arXiv2005

Evidence for Primordial Clustering Around the QSO SDSS J1030+0524 at z=6.28

We present tentative evidence for primordial clustering, manifested as an excess of color-selected objects in the field of the QSO SDSS J1030+0524 at redshift z=6.28. We have selected objects red in i_{775}-z_{850} on the basis of Hubble Space Telescope Advanced Camera for Surveys imaging of a field centered on the QSO. Compared to data at comparable depth obtained by the GOODS survey, we find an excess of objects with (i_{775}-z_{850}) \geq 1.5 in the QSO field. The significance of the detection is estimated to be ~97% on the basis of the counts alone and increases to 99.4% if one takes into account the color distribution. If confirmed this would represent the highest redshift example of galaxy clustering and would have implications on models for the growth of structure. Bias-driven clustering of first luminous objects forming in the highest peaks of the primordial density field is expected in most models of early structure formation. The redshift of one of the candidates has been found to be z=5.970 by our spectroscopy with Keck I/LRIS, confirming the validity of our color selection.

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arXiv2004

Obscured AGN and the X-ray, Optical and Far-Infrared Number Counts of AGN in the GOODS Fields

The deep X-ray, optical, and far-infrared fields that constitute GOODS are sensitive to obscured AGN (N_H>10^{22} cm^{-2}) at the quasar epoch (z~2-3), as well as to unobscured AGN as distant as z~7. Luminous X-ray emission is a sign of accretion onto a supermassive black hole and thus reveals all but the most heavily obscured AGN. We combine X-ray luminosity functions with appropriate spectral energy distributions for AGN to model the X-ray, optical and far-infrared flux distributions of the X-ray sources in the GOODS fields. A simple model based on the unified paradigm for AGN, with ~3 times as many obscured AGN as unobscured, successfully reproduces the z-band flux distributions measured in the deep HST ACS observations on the GOODS North and South fields. This model is also consistent with the observed spectroscopic and photometric redshift distributions once selection effects are considered. The previously reported discrepancy between observed spectroscopic redshift distributions and the predictions of population synthesis models for the X-ray background can be explained by bias against the most heavily obscured AGN generated both by X-ray observations and the identification of sources via optical spectroscopy. We predict the AGN number counts for Spitzer MIPS 24 um and IRAC 3.6-8 um observations in the GOODS fields, which will verify whether most AGN in the early Universe are obscured in the optical. Such AGN should be very bright far-infrared sources and include some obscured AGN missed even by X-ray observations.

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