arXiv subjects

A. Dey

Publications and source records attributed to A. Dey.

126 records · Page 7Linked to original sources

arXiv2004

Dissipation of angular momentum in light heavy ion collision

The inclusive energy distributions of fragments (4$\leq$Z$\leq$7) emitted in the reactions $^{16}$O (116 MeV) + $^{27}$Al, $^{28}$Si, $^{20}$Ne (145 MeV) + $^{27}$Al, $^{59}$Co have been measured in the angular range $θ_{lab} $= 10$^\circ$ - 65$^\circ$. Fusion-fission and deep inelastic components of the fragment emission have been extracted from the experimental data. The angular mometum dissipations in fully damped deep inelastic collisions have been estimated assming exit channel configuration similar to those for fusion-fission process. It has been found that, the angular momentum dissipations are more than those predicted by the empirical sticking limit in all cases. The deviation is found to increase with increasing charge transfer (lighter fragments). Qualitatively, this may be due to stronger friction in the exit channel. Moreover, for the heavier system $^{20}$Ne + $^{59}$Co, the overall magnitude of deviation is less as compared to those for the lighter systems, {\it i.e.}, $^{16}$O + $^{27}$Al, $^{28}$Si, $^{20}$Ne + $^{27}$Al. This may be due to lesser overlap in time scales of fusion and deep inelastic time scales for heavier systems.

nucl-ex↗

arXiv2003

172 ks Chandra Exposure of the LALA Boötes Field: X-ray Source Catalog

We present an analysis of a deep, 172 ks Chandra observation of the Large Area Lyman Alpha Survey (LALA) Boötes field, obtained with the Advanced CCD Imaging Spectrometer (ACIS-I) on the Chandra X-ray Observatory. This is one of the deepest Chandra images of the extragalactic sky; only the 2 Ms CDF-N and 1 Ms CDF-S are substantially deeper. A total of 168 X-ray sources were detected. The X-ray source counts were derived and compared with those from other Chandra deep surveys; the hard X-ray source density of the LALA Boötes field is 33% higher than that of CDF-S at the flux level of 2.0E-15 ergs/cm^2/s, confirming the field-to-field variances of the hard band source counts reported by previous studies. The deep exposure resolves > 72% of the 2-10 keV X-ray background. Our primary optical data are R-band imaging from NOAO Deep Wide-Field Survey (NDWFS), with limiting magnitude of R = 25.7 (Vega, 3sigma, 4" diameter aperture). We have found optical counterparts for 152 of the 168 Chandra sources (90%). Among the R-band non-detected sources, not more than 11 of them can possibly be at z > 5, based on the hardness ratios of their X-ray emission and nondetections in bluer bands. The majority (~76%) of the X-ray sources are found to have log(f_X/f_R) within 0.0+-1, which are believed to be AGNs.Most of the X-ray faint/optically bright sources (log(f_X/f_R) < -1.0) are optically extended, which are low-z normal galaxies or low luminosity AGNs. There is also a population of sources which are X-ray overluminous for their optical magnitudes (log(f_X/f_R) > 1.0), which are harder in X-ray and are probably obscured AGNs. (abridged)

astro-ph↗

arXiv2003

Color-selected galaxies at z~6 in the Great Observatories Origins Deep Survey

We report early results on galaxies at z~6, selected from Hubble Space Telescope imaging for the Great Observatories Origins Deep Survey. Spectroscopy of one object with the Advanced Camera for Surveys grism and from the Keck and VLT observatories a shows a strong continuum break and asymmetric line emission, identified as Lya at z=5.83. We detect only five spatially extended, z~6 candidates with signal-to-noise ratios > 10, two of which have spectroscopic confirmation. This is many fewer than would be expected if galaxies at z=6 had the same luminosity function as those at z=3. There are many fainter candidates, but we expect substantial contamination from foreground interlopers and spurious detections. Our best estimates favor a z=6 galaxy population with fainter luminosities, higher space density, and similar co-moving ultraviolet emissivity to that at z=3, but this depends critically on counts at fluxes fainter than those reliably probed by the current data.

astro-ph↗

arXiv2002

Massive Ellipticals at High Redshift: NICMOS Imaging of Z~1 Radio Galaxies

We present deep, continuum images of eleven high-redshift (0.811 < z < 1.875) 3CR radio galaxies observed with NICMOS. Our images probe the rest-frame optical light where stars are expected to dominate the galaxy luminosity. The rest-frame UV light of eight of these galaxies demonstrates the well-known ``alignment effect''. 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 de Vaucouleurs law light profiles. For a few galaxies very faint traces 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 eleven sample galaxies by fitting surface-brightness models to them. We find 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. 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. 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 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 AGN phenomenon takes place within otherwise normal, perhaps passively evolving, galaxies. (abridged)

astro-ph↗

arXiv2002

BRIGHT lights, BIG city: Massive galaxies, giant Ly-a nebulae, and proto--clusters

High redshift radio galaxies are great cosmological tools for pinpointing the most massive objects in the early Universe: massive forming galaxies, active super--massive black holes and proto--clusters. We report on deep narrow--band imaging and spectroscopic observations of several z > 2 radio galaxy fields to investigate the nature of giant Ly-alpha nebulae centered on the galaxies and to search for over--dense regions around them. We discuss the possible implications for our understanding of the formation and evolution of massive galaxies and galaxy clusters.

astro-ph↗

arXiv2002

Old elliptical galaxies at z=1.5 and the Kormendy relation

Deep spectroscopy of the two mJy radio galaxies LBDS 53W069 and 53W091 has previously shown them to have old (>3 Gyr) stellar populations at z=1.5. Here we present the results of HST observations of these galaxies. 53W069 has an r^1/4 profile in both the F814W & F110W data with an effective radius of 0.30 arcsec (2.7 kpc). The restframe U-B colour gradient requires a stellar population of super-solar (3Z_sun) metallicity that formed on a very short timescale at high redshift (z>5). 53W091 has a regular r^1/4 profile in F110W with an effective radius of 0.32 arcsec (2.9 kpc). The F814W profile is more extended and is consistent with a blue exponential disk that contributes 20% of the flux within r_e. The restframe U-B colour gradient is significantly larger than that observed in field ellipticals at z<1, implying a stellar population of mixed metallicity (1-3Z_sun) that formed in a high-redshift rapid burst. We have compared these two LBDS radio galaxies with the Kormendy relation of ten 3CR radio galaxies at z=0.8. The LBDS galaxies follow the same relation as the more radio-luminous 3CR galaxies, assuming passive evolution of their stellar populations, although they are smaller than the 3CR galaxies whose mean effective radius is 12 kpc. Their sizes and radio luminosities are consistent with scaling relations applied to the 3CR galaxies, in which both radio power and effective radius scale with galaxy mass. Compared with a sample of z=0.4 cluster ellipticals, 53W069 & 53W091 lie well within the scatter of the Kormendy relation. We conclude that the hosts of these mJy radio sources at z=1.5 are passively-evolving elliptical galaxies that will evolve into ordinary L* ellipticals by the present day. (Abridged)

astro-ph↗

arXiv2000

The Chandra Multi-wavelength Project (ChaMP): a serendipitous survey with Chandra archival data

The launch of the Chandra X-ray Observatory in July 1999 opened a new era in X-ray astronomy. Its unprecedented, <0.5" spatial resolution and low background are providing views of the X-ray sky 10-100 times fainter than previously possible. We have initiated a serendipitous survey (ChaMP) using Chandra archival data to flux limits covering the range between those reached by current satellites and those of the small area Chandra deep surveys. We estimate the survey will cover ~5 sq.deg./year to X-ray fluxes (2-10 keV) in the range 1E(-13)-6E(-16) erg/cm^2/s discovering ~2000 new X-ray sources, ~80% of which are expected to be AGN. The ChaMP has two parts, the extragalactic survey (ChaMP) and the galactic plane survey (ChaMPlane). ChaMP promises profoundly new science return on a number of key questions at the current frontier of many areas of astronomy including (1) locating and studying high redshift clusters and so constraining cosmological parameters (2) defining the true population of AGN, including those that are absorbed, and so constraining the accretion history of the universe, (3) filling in the gap in the luminosity/redshift plane between Chandra deep and previous surveys in studying the CXRB, (4) studying coronal emission from late-type stars and (5) search for CVs and quiescent Low-Mass X-ray Binaries (qLXMBs) to measure their luminosity functions. In this paper we summarize the status, predictions and initial results from the X-ray analysis and optical imaging.

astro-ph↗

arXiv2000

Three high-redshift millimeter sources and their radio and near-infrared identifications

We present millimeter wavelength detections of three faint sources that are most likely high-redshift starburst galaxies. For one of the sources, which was previously discovered with SCUBA at 850 mu m, we present a detection with the IRAM interferometer at 240 GHz (1.25 mm) that shows the object unresolved at an angular resolution of 2.''5, and coincident within 1'' with a radio source and a galaxy detected in the near-infrared. The two other sources were discovered in a deep 250 GHz (1.2 mm) survey with the Max-Planck Millimeter Bolometer (MAMBO) array at the IRAM 30m telescope. Both have fluxes of ~4 mJy and radio counterparts with a 1.4 GHz flux density of ~75 muJy. Their radio-to-mm flux ratios suggest redshifts larger than 2. Both sources are faint in the optical and near-infrared, one showing a 20.5 mag K-band counterpart. From our data and that available in the literature, we estimate the redshift distribution of twenty-two faint mm and sub-mm sources and conclude that the majority of them are likely to be at z>2.

astro-ph↗

arXiv2000

The Fraction of High Redshift Galaxies in Deep Infrared Selected Field Galaxy Samples

Recent results on the incidence of red galaxies in a > 100 square arcminute field galaxy survey to K=20 and a K=22 survey of the Hubble Deep Field are presented. We argue that a simple photometric redshift indicator, based on J-K color and supported by spectroscopic results obtained with Keck, gives a reliable lower limit of ~25% for the fraction of z>1 galaxies in the 100 square arcminute survey. This fraction is substantially higher than found in previous smaller samples, and is at least as consistent with predictions for pure luminosity evolution as with those for hierarchical models. The same technique yields a very low fraction for the HDF, which appears to be unusually underabundant in red galaxies.

astro-ph↗

arXiv1999

NICMOS Imaging of High-Redshift Radio Galaxies

We have obtained near-infrared (1.6 micron) images of 11 powerful 3CR radio galaxies at redshifts 0.8 < z < 1.8 using NICMOS on board HST. The high angular resolution permits a detailed study of galaxy morphology in these systems at rest-frame optical wavelengths, where starlight dominates over the extended, aligned UV continuum. The NICMOS morphologies are mostly symmetric and are consistent with dynamically relaxed, elliptical host galaxies dominated by a red, mature stellar population. The aligned structures are sometimes faintly visible, and nuclear point sources may be present in a few cases which manifest the ``unveiled'' AGN that is obscured from view at optical wavelengths. Our observations are consistent with the hypothesis that the host galaxies of z ~ 1-2 radio galaxies are similar to modern-day gE galaxies. Their sizes are typical of gE galaxies but smaller than present-day cD and brightest cluster galaxies, and their surface brightnesses are higher, as expected given simple luminosity evolution.

astro-ph↗

arXiv1999

Studying High Redshift Star Forming Galaxies at Centimeter and Millimeter Wavelengths

We discuss various aspects of centimeter and millimeter wavelength continuum and line observations of high redshift star forming galaxies. Perhaps the most important lesson is that sensitive observations at submm through cm wavelengths reveal a population of active star forming galaxies at high redshift which are unseen in deep optical surveys due to dust obscuration. Current models suggest that this population represents the formation of the spheroidal components of galaxies at z between 2 and 5, constituting about half of the total amount of cosmic star formation from the big bang to the present. High resolution imaging at cm wavelengths provides sub-arcsecond astrometry, and can be used to search for gravitational lensing and/or for the presence of an AGN. Radio continuum observations provide unique information on the magnetic fields in early galaxies, and give a gross indication of the star formation rate, while the radio-to-submm spectral index provides a rough indication of source redshift. Low J transitions of CO are redshifted into the cm bands for z > 2, allowing for sensitive searches for CO emission over large volumes at high redshift. We present recent results from the Very Large Array (VLA), and from the new 230 GHz MPIfR bolometer array at the IRAM 30m telescope. A wide field survey with the bolometer array indicates a cut-off in the source distribution function at FIR luminosities > 3e12 L_sun. Lastly, we summarize the scientific promise of the New VLA.

astro-ph↗

arXiv1999

The unusual afterglow of GRB 980326: evidence for the gamma-ray burst/supernova connection

Cosmic gamma-ray bursts (GRBs) have been firmly established as one of the most powerful phenomena in the Universe, releasing electromagnetic energy approaching the rest-mass energy of a neutron star in a few seconds. The two currently popular models for GRB progenitors are the coalescence of two compact objects (such as neutron stars or black holes) or collapse of a massive star. An unavoidable consequence of the latter model is that a bright supernovae should accompany the GRB. The emission from this supernova competes with the much brighter afterglow produced by the relativistic shock that gives rise to the GRB itself. Here we present evidence for an unusual light curve for GRB 980326 based on new optical observations. The transient brightened ~3 weeks after the burst to a flux sixty times larger than that extrapolated from the rapid decay seen at early time. Furthermore, the spectrum changed dramatically and became extremely red. We argue that the new source is the underlying supernova. If our hypothesis is true then this would be the first evidence for a supernova connection with GRBs at cosmological distances. We suggest that GRBs with long durations are associated with death of massive stars.

astro-ph↗

arXiv1999

Observations of a z=1.44 Dusty, Ultraluminous Galaxy and Implications for Deep Sub-mm Surveys

We present new near-IR and optical spectroscopic observations which confirm the redshift of the z=1.44 extremely red object ERO J164502+4626.4 (object 10 of Hu & Ridgway 1994) and a HST image which reveals a reflected-S-shaped morphology at (rest-frame) near-UV wavelengths. The contrast between the rest-frame far-red (8200-9800A) and near-UV (2900-3900A) 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 microns obtained using SCUBA on the JCMT. The rest-frame SED of this ERO is best understood in terms of a highly reddened stellar population with ongoing star formation, as originally suggested by Graham & Dey (1996). The new sub-mm data presented here indicate that the remarkable similarity to ultraluminous infrared galaxies (ULIRGs) such as Arp220 extends into the rest-frame far-IR which bears the signature of thermal emission from dust. ERO J164502+4626.4 is extremely luminous (7E12 Lsun) and dusty (M[dust] = 7E8 Msun). If its luminosity is powered by young hot stars, then ERO J164502+4626.4 is forming stars at the prodigious rate of 1000-2000 Msun/yr. We conclude that it is a distant analogue 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 sub-mm 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 sub-mm background emission.

astro-ph↗

arXiv1998

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 AGN 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] 5007/Hb >~ 5) spectrum typical of a ``classic'' Seyfert 2 galaxy with little or no Balmer absorption lines, is ``ultraluminous'' (L_IR >~ 10^12 L_sun), and has a ``warm'' IR color (f_25/f_60 >~ 0.25). 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.

astro-ph↗

arXiv1998

Induced star formation and morphological evolution in very high redshift radio galaxies

Near-infrared, sub-arcsecond seeing images obtained with the W. M. Keck I Telescope of show strong evolution at rest--frame optical} wavelengths in the morphologies of high redshift radio galaxies (HzRGs) with 1.9 3, to much more compact and symmetrical shapes at z<3. The linear sizes (~10 kpc) and luminosities (M_B~-20 to -22) of the individual components in the z>3 HzRGs are similar to the total sizes and luminosities of normal, radio--quiet, star forming galaxies seen at z=3--4. `R'-band, 0.1 arcsec resolution images with the Hubble Space Telescope of one of these HzRGs, 4C41.17 at z=3.800, show that at rest--frame UV wavelengths the galaxy morphology breaks up in even smaller, ~1 kpc--sized components embedded in a large halo of low surface brightness emission. The brightest UV emission is from a radio--aligned, edge-brightened feature (4C41.17-North) downstream from a bright radio knot. A narrow--band Lyman-alpha image, also obtained with HST, shows an arc--shaped Lyman-alpha feature at this same location, suggestive of a strong jet/cloud collision. Deep spectropolarimetric observations with the W. M. Keck II Telescope of 4C41.17 show that the radio--aligned UV continuum is unpolarized. (abridged)

astro-ph↗

arXiv1998

The Performance and Scientific Rationale for an IR Imaging Fourier Transform Spectrograph on a Large Space Telescope

We describe a concept for an imaging spectrograph for a large orbiting observatory such as NASA's proposed Next Generation Space Telescope (NGST) based on an imaging Fourier transform spectrograph (IFTS). An IFTS has several important advantages which make it an ideal instrument to pursue the scientific objectives of NGST. We review the operation of an IFTS and make a quantitative evaluation of the signal-to-noise performance of such an instrument in the context of NGST. We consider the relationship between pixel size, spectral resolution, and diameter of the beamsplitter for imaging and non-imaging Fourier transform spectrographs and give the condition required to maintain spectral modulation efficiency over the entire field of view. We give examples of scientific programs that could be performed with this facility.

astro-ph↗

arXiv1998

Old high-redshift galaxies and primordial density fluctuation spectra

We have discovered a population of extremely red galaxies at $z\simeq 1.5$ which have apparent stellar ages of $\gs 3$ Gyr, based on detailed spectroscopy in the rest-frame ultraviolet. In order for galaxies to have existed at the high collapse redshifts indicated by these ages, there must be a minimum level of power in the density fluctuation spectrum on galaxy scales. This paper compares the required power with that inferred from other high-redshift populations. If the collapse redshifts for the old red galaxies are in the range $z_c\simeq 6$ -- 8, there is general agreement between the various tracers on the required inhomogeneity on 1-Mpc scales. This level of small-scale power requires the Lyman-limit galaxies to be approximately $ν\simeq 3.0$ fluctuations, implying a very large bias parameter $b\simeq 6$. The high collapse redshifts of the red galaxies as deduced from gravitational collapse provides independent support for the ages estimated from their stellar populations. Such early-forming galaxies are rare, and their contribution to the cosmological stellar density is consistent with an extrapolation to higher redshifts of the star-formation rate measured at $z<5$; there is no evidence for a general era of spheroid formation at extreme redshifts.

astro-ph↗

arXiv1997

Optical Light Curves of the Type Ia Supernovae 1990N and 1991T

We present UBVRI light curves for the bright Type Ia supernovae SN 1990N in NGC 4639 and SN 1991T in NGC 4527 based on photometry gathered in the course of the Calan/Tololo supernova program. Both objects have well-sampled light curves starting several days before maximum light and spanning well through the exponential tail. These data supercede the preliminary photometry published by Leibundgut et al (1991) and Phillips et al (1992). The host galaxies for these supernovae have (or will have) accurate distances based on the Cepheid period-luminosity relationship. The photometric data in this paper provide template curves for the study of general population of Type Ia supernova and accurate photometric indices needed for the Cepheid-supernova distance scale.

astro-ph↗