Arjun Dey

LBDS 53W091: An Old, Red Galaxy at z = 1.552

The weak radio source LBDS 53W091 is associated with a very faint (R ? 24.5) red (R - K ? 5.8) galaxy. Long spectroscopic integrations with the W. M. Keck telescope have provided an absorption-line redshift, z = 1.552 ? 0.002. The galaxy has a rest frame ultraviolet spectrum very similar to that of an F6 V star, and a single-burst old stellar population that matches the IR colors, the optical energy distribution and the spectral discontinuities has a minimum age of 3.5 Gyr. We present detailed population synthesis analyses of the observed spectrum in order to estimate the time since the last major epoch of star formation. We discuss the discrepancies in these estimates resulting from using different models, subjecting the UV spectrum of M32 to the same tests as a measure of robustness of these techniques. The models most consistent with the data tend to yield ages at z = 1.55 of 3.5 Gyr, similar to that inferred for the intermediate-age population in M32. Depending upon the assumed Hubble constant and the value of ?0, only certain cosmological expansion times are consistent with the age of LBDS 53W091; in particular, for ?0 = 1, only models with H0 45 km s-1 Mpc-1 are permitted. For H0 = 50 km s-1 Mpc-1 and ?0 = 0.2, we derive a formation redshift, zf ? 5.

Observations of a z = 1.44 Dusty, Ultraluminous Galaxy and Implications for Deep Submillimeter Surveys* ** ***

We present new near-infrared and optical spectroscopic observations that confirm the redshift of the z = 1.44 extremely red object ERO J164502+4626.4 (object 10 of Hu & Ridgway, formerly known as HR 10 or [HR94] 10) and a Hubble Space Telescope image that reveals a reflected S-shaped morphology at (rest-frame) near-ultraviolet wavelengths. The contrast between the rest-frame far-red (λλ8200-9800 A) and near-UV (λλ2900-3900 A) morphologies suggests that the central regions of the galaxy are heavily obscured by dust and that the galaxy is most likely an interacting or disturbed system. We also present new photometry of this object at 450, 850, and 1350 μm obtained using the Submillimeter Common-User Bolometer Array (SCUBA) submillimeter camera on the James Clerk Maxwell Telescope. Our submillimeter data are extremely sensitive to emission from cold dust at high redshift. The rest-frame spectral energy distribution of ERO J164502+4626.4 is best understood in terms of a highly reddened stellar population with ongoing star formation, as originally suggested by Graham & Dey. The new submillimeter data presented here indicate that the remarkable similarity to ultraluminous infrared galaxies (ULIRGs) such as Arp 220 and Mrk 231 extends into the rest-frame far-infrared, which bears the signature of thermal emission from dust, presumably heated by young stars. ERO J164502+4626.4 is extremely luminous (L ≈ 7 × 1012 h-250 L☉) and dusty [Mdust ≈ 7 × 108(Tdust/40 K)-5 h-250 M☉]. If its luminosity is powered by young hot stars, then ERO J164502+4626.4 is forming stars at the prodigious rate of = 1000-2000 h-250 M☉ yr-1. We conclude that ERO J164502+4626.4 is a distant analog of the nearby ULIRG population, the more distant or less luminous counterparts of which may be missed by even the deepest existing optical surveys. The submillimeter emitters recently discovered by deep SCUBA surveys may be galaxies similar to ERO J164502+4626.4 (but perhaps more distant). This population of extremely dusty galaxies may also contribute significantly to the cosmic submillimeter background emission.

Chandra detection of a Type II quasar at z = 3.288

We report on observations of a type II quasar at redshift z ¼ 3:288, identified as a hard X-ray source in a 185 ks observation with the Chandra X-Ray Observatory and as a high-redshift photometric candidate from deep, multiband optical imaging. CXO J084837.9+445352 (hereafter CXO 52) shows an unusually hard Xray spectrum from which we infer an absorbing column density NH ¼ð 4:8 � 2:1 Þ� 10 23 cm � 2 (90% confidence) and an implied unabsorbed 2 10 keV rest-frame luminosity of L2 10 ¼ 3:3 � 10 44 ergs s � 1 , well within the quasar regime. Hubble Space Telescope imaging shows CXO 52 to be elongated with slight morphological differences between the WFPC2 F814W and NICMOS F160W bands. Optical and near-infrared spectroscopy of CXO 52 shows high-ionization emission lines with velocity widths � 1000 km s � 1 and flux ratios similar to a Seyfert 2 galaxy or radio galaxy. The latter are the only class of high-redshift type II luminous active galactic nuclei that have been extensively studied to date. Unlike radio galaxies, however, CXO 52 is radio quiet, remaining undetected at radio wavelengths to fairly deep limits, f4:8 GHz < 40 lJy. High-redshift type II quasars, expected from unification models of active galaxies and long thought necessary to explain the X-ray background, are poorly constrained observationally, with few such systems known. We discuss recent observations of similar type II quasars and detail search techniques for such systems, namely, (1) X-ray selection, (2) radio selection, (3) multicolor imaging selection, and (4) narrowband imaging selection. Such studies are likely to begin identifying luminous, high-redshift type II systems in large numbers. We discuss the prospects for these studies and their implications for our understanding of the X-ray background. Subject headings: cosmology: observations — galaxies: active — quasars: individual (CXO J084837.9+445352) — X-rays: galaxies

A Galaxy at z = 5.34*

We report the discovery of Lyα emission from a galaxy at z=5.34, the first object at z>5 with a spectroscopically confirmed redshift. The faint continuum emission [mAB(8000 A) ≈ 27], the relatively small rest-frame equivalent width of the emission line (WrestLyα ≈ 95 A), and the limits on the N V/Lyα ratio suggest that this is a star-forming galaxy and not an active galactic nucleus. The star formation rates implied by the UV continuum emission and the Lyα emission are (in the absence of dust extinction) fairly modest (~6 h−250 M☉ yr-1 for q0=0.5). The continuum luminosity is similar to that of sub-L*1500 star-forming galaxies at z~3, and the width of the Lyα line yields an upper limit to the mass of less than 2.6×1010 M☉. The strong emission line detected in this low-luminosity galaxy provides hope for the discovery of higher luminosity primeval galaxies at redshifts z>5.

A z = 5.34 galaxy pair in the Hubble Deep Field

?????The system as a whole is slightly brighter than L relative to the z ~ 3 Lyman break population, and the total star formation rate inferred from the UV continuum is ?22 h M? yr-1 (q0 = 0.5) assuming the absence of dust extinction. The two individual galaxies are quite small (size scales 1 h kpc). Thus these galaxies superficially resemble the building blocks of Pascarelle and coworkers; if they comprise a gravitationally bound system, the pair will likely merge in a timescale ~100 Myr.

Massive Elliptical Galaxies at High Redshift: NICMOS* Imaging of z ≈ 1 Radio Galaxies

We present deep, ≈1.6 μm, continuum images of 11 high-redshift (0.811 < z < 1.875) 3CR radio galaxies observed with NICMOS on board the Hubble Space Telescope. Our NICMOS images probe the rest-frame optical light where stars are expected to dominate the galaxy luminosity. The rest-frame ultraviolet light of eight of these galaxies demonstrates the well-known alignment effect, with extended and often complex morphologies elongated along an axis close to that of the Fanaroff-Riley type II (FRII) radio source. As has been previously noted from ground-based near-infrared imaging, most of the radio galaxies have rounder, more symmetric morphologies at rest-frame optical wavelengths. Here we show the most direct evidence that in most cases the stellar hosts are normal elliptical galaxies with r1/4-law light profiles. For a few galaxies, very faint traces (less than 4% of the total H-band light) of the UV-bright aligned component are also visible in the infrared images. We derive both the effective radius and surface brightness for nine of 11 sample galaxies by fitting one- and two-dimensional surface-brightness models to them. We compare the high-redshift radio galaxies to lower redshift counterparts. We find that their sizes are similar to those of local FRII radio source hosts and are in general larger than other local galaxies. The derived host galaxy luminosities are very high and lie at the bright end of luminosity functions constructed at similar redshifts. This indicates that the high-redshift radio galaxies are likely rare, massive sources. The galaxies in our sample are also brighter than the rest-frame size-surface-brightness locus defined by the low-redshift sources. Passive evolution roughly aligns the z ≈ 1 galaxies with the low-redshift samples with a slope equal to 4.7. This value is intermediate between the canonical Kormendy relation (≈3.5) and a constant luminosity line (=5). The optical host is sometimes centered on a local minimum in the rest-frame UV emission, suggesting the presence of substantial dust obscuration. We also see good evidence of nuclear point sources (no brighter than 5% of the total H-band light) in three galaxies. Overall, our results are consistent with the hypothesis that these galaxies have already formed the bulk of their stars at redshifts greater than z ≈ 2 and that the active galactic nucleus phenomenon takes place within otherwise normal, perhaps passively evolving, galaxies.

Spectropolarimetry of FIRST Broad Absorption Line Quasars

We present Keck spectropolarimetry of two rare low-ionization broad absorption line (BAL) quasars (QSOs), FIRST J084044.5+363328 and FIRST J155633.8+351758, that also exhibit narrow absorption lines from metastable excited levels of Fe II (iron Lo-BALs). These QSOs were discovered in optical follow-ups to a deep radio survey; FIRST J155633.8+351758 is radio-loud, the first BAL QSO so identified. FIRST J084044.5+363328 is highly polarized and exhibits many features found in other BAL QSOs. The continuum is ? 4% polarized near the 2000 ? rest frame, falling to ? 2% at longer wavelengths, at a position angle of ? 50?. The emission lines are unpolarized. The polarization rises to ? 8% in the low-ionization troughs of Mg II ?2800 and Al III ?1860. The polarization and its position angle vary in a complicated manner across the metastable Fe II absorption lines, which suggests that more than one mechanism is at work or that the system geometry is complex. FIRST J155633.8+351758 may be the most highly polarized BAL QSO known, and it exhibits other unusual polarization properties compared with those of other highly polarized BAL QSOs. The continuum is ? 13% polarized near the 2000 ? rest frame, falling to ? 7% at longer wavelengths, at a position angle of 153?. The emission lines are polarized like the continuum, but in the absorption troughs the polarization drops to zero. Currently available data cannot yet discriminate among the possible lines of sight to BAL QSOs (edge-on, pole-on, or random).We present Keck spectropolarimetry of two rare low-ionization broad absorption line (BAL) QSOs, FIRST J084044.5+363328 and FIRST J155633.8+351758, that also exhibit narrow absorption lines from metastable excited levels of Fe II (“Iron Lo-BALs”). These QSOs were discovered in optical follow-ups to a deep radio survey; FIRST J155633.8+351758 is radio-loud, the first BAL QSO so identified. FIRST J084044.5+363328 is highly polarized and exhibits many features found in other BAL QSOs. The continuum is ≈4% polarized near 2000 Å rest-frame, falling to ≈2% at longer wavelengths, at a position angle of ≈50. The emission lines are unpolarized. The polarization rises to ≈8% in the low-ionization troughs of Mg II λ2800 and Al III λ1860. The polarization and its position angle vary in a complicated manner across the metastable Fe II absorption lines, suggesting that more than one mechanism is at work, or that the system geometry is complex. FIRST J155633.8+351758 may be the most highly polarized BAL QSO known, and exhibits other unusual polarization properties compared to other highly polarized BAL QSOs. The continuum is ≈13% polarized near 2000 Å rest-frame, falling to ≈7% at longer wavelengths, at a position angle of 153. The emission lines are polarized like the continuum, but in the absorption troughs the polarization drops to zero. Currently available data cannot yet discriminate among the possible lines of sight to BAL QSOs (edge-on, pole-on, or random). Subject headings: quasars: absorption lines, quasars: emission lines, quasars:

New High-Redshift Radio Galaxies from the MIT-Green Bank Catalog*

We present optical identifications and redshifts for 17 new high-redshift radio sources. Fifteen of these sources are radio galaxies; the remaining two are high-redshift, steep-spectrum, radio-loud quasars. These objects were discovered as part of an ongoing study of compact (θ 0.75,Sν ∝ ν-α) sources from the MIT–Green Bank (MG) radio catalog (S5 GHz 50 mJy). Spectra for the optical counterparts were obtained at the W. M. Keck Telescopes and are among the optically faintest radio galaxies thus far identified. Redshifts range between 0.3 and 3.6, with 13 of the 17 at redshifts greater than 1.5. Combining these new radio galaxies with two published MG radio galaxy spectra, we synthesize a composite MG radio galaxy spectrum and discuss the properties of these galaxies in comparison with other, more powerful, radio galaxies at similar redshifts. We suggest a radio power–ionization state relation.

A Polarimetric Search for Hidden Quasars in Three Radio-selected Ultraluminous Infrared Galaxies

We have carried out a spectropolarimetric search for hidden broad-line quasars in three ultraluminous infrared galaxies (ULIRGs) discovered in the positional correlations between sources detected in deep radio surveys and the IRAS Faint Source Catalog. Only the high-ionization Seyfert 2 galaxy TF J1736+1122 is highly polarized, displaying a broad-line spectrum visible in polarized light. The other two objects, TF J1020+6436 and FF J1614+3234, display spectra dominated by a population of young (A type) stars similar to those of E+A galaxies. They are unpolarized, showing no sign of hidden broad-line regions. The presence of young starburst components in all three galaxies indicates that the ULIRG phenomenon encompasses both active galactic nuclei (AGNs) and starburst activity, but the most energetic ULIRGs do not necessarily harbor buried quasars. We find that a luminous infrared galaxy is most likely to host an obscured quasar if it exhibits a high-ionization ([O III] λ5007/Hβ 5) spectrum typical of a classic Seyfert 2 galaxy with little or no Balmer absorption lines, is ultraluminous (LIR1012 L☉), and has a warm IR color (f25/f600.25). The detection of hidden quasars in this group but not in the low-ionization, starburst-dominated ULIRGs (classified as LINERs or H II galaxies) may indicate an evolutionary connection, with the latter being found in younger systems.

Red Quasars and Quasar Evolution: The Case of BAL QSO FIRST J155633.8+351758*

We present the first near-infrared spectroscopic observations of the radio-loud broad absorption line quasi-stellar object (QSO), FIRST J155633.8+351758. The spectrum is similar to that of a reddened QSO and shows strong emission lines of H? and H?, as well as strong Fe II emission blends near H?. The redshift of the object, measured from the H? and H? lines, is zBLR = 1.5008 ? 0.0007, slightly larger than the redshift of zmetal = 1.48, estimated from the broad metal absorption features. Thus, the broad metal absorption features are blue shifted with respect to the systemic velocity. The width of the H? emission line (FWHM ? 4100 km s -1) is typical of that observed in QSO broad-line regions, but the Balmer decrement (H?/H? ? 5.8) is larger than that of most optically selected QSOs. Both the Balmer decrement and the slope of the rest-frame UV?optical continuum independently suggest a modest amount of extinction along the line of sight to the broad-line region (EB-V ? 0.5 for SMC-type screen extinction at the redshift of the QSO). The implied gas column density along the line of sight is much less than that implied by the weak X-ray flux of the object, suggesting that either the broad emission and absorption line regions have a low dust-to-gas ratio or that the rest-frame optical light encounters significantly lower mean column density lines of sight than the X-ray emission. From the rest-frame UV-optical spectrum, we are able to constrain the stellar mass content of the system (<3 ? 1011 M?). Comparing this mass limit with the black hole mass estimated from the bolometric luminosity of the QSO, we find it possible that the ratio of the black hole to stellar mass is comparable to the Magorrian value, which would imply that the Magorrian relation is already in place at z = 1.5. However, multiple factors favor a much larger black hole?to?stellar mass ratio. This would imply that if the Magorrian relation characterizes the late history of QSOs and if the situation observed for F1556+3517 is typical of the early evolutionary history of QSOs, central black hole masses develop more rapidly than bulge masses.