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Search Results: 1 - 10 of 1422 matches for " Arjun Dey "
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The Early History of Powerful Radio Galaxies
Arjun Dey
Physics , 1998,
Abstract: I briefly review the current status of observations of AGN-powered UV/optical light, starlight, dust and outflow phenomena in high-redshift powerful radio galaxies. The existing data are consistent with the hypothesis that powerful radio galaxies undergo a major episode of star formation at high redshift z>4 during which they form most of their stars, and subsequently evolve `passively', with the UV continuum emission in the z~1 galaxies being dominated by AGN-related processes rather than starlight from the underlying, aging population.
The Radio Galaxy 3C265 Contains a Hidden Quasar Nucleus
Arjun Dey,Hyron Spinrad
Physics , 1995, DOI: 10.1086/176874
Abstract: We report the discovery of broad MgII emission from the high redshift radio galaxy 3C265 (z=0.81). We detect the broad line in the nuclear spectrum and in the spatially extended galaxian component, both near the nucleus and in the spectrum of an off-nuclear knot located 31 kpc south east of the nucleus of the galaxy. These data provide strong support for the simplest form of the unification hypothesis, that radio galaxies are quasars whose optical radiation is directed in the plane of the sky rather than into our line of sight. These data also strongly support the scattering model for the alignment of the UV continuum emission with the radio axis. In 3C265, if the axis of the anisotropically emitted UV continuum radiation is identified with the major axis of the radio source, then the observed rest frame UV continuum emission implies that the opening angle of the radiation cone is large (half angle approximately 45 degrees). We also derive a mass estimate of 8x10^{10} Msun for the central region of 3C265 from its rotation curve. The implied mass-to-light ratio is low (M/L is roughly 2), and suggests that a significant fraction of the rest frame UV continuum emission from this galaxy is dominated by reprocessed radiation from the buried AGN. Finally, we detect the CaII\lambda3933 K line in absorption in the integrated spectrum of 3C265. This provides direct spectroscopic evidence for the existence of stars in a high redshift radio galaxy.
The Pedigrees of DOGs (Dust-Obscured Galaxies)
Arjun Dey,the NDWFS/MIPS Collaboration
Physics , 2009,
Abstract: A simple mid-infrared-to-optical color criterion of R-[24]>14 Vega mag results in a robust selection of approximately half of the redshift 2 ultraluminous infrared galaxy (ULIRG) population. These `Dust-Obscured Galaxies', or DOGs, have many properties that suggest that they are good candidates for systems in a transition phase between gas-rich mergers and QSOs.
The Spectroscopic Redshift of an Extremely Red Object and the Nature of the Very Red Galaxy Population
James R. Graham,Arjun Dey
Physics , 1996, DOI: 10.1086/178000
Abstract: Infrared surveys have discovered a significant population of bright (K<19) extremely red (R-K>6) objects. Little is known about the properties of these objects on account of their optical faintness (R>24). Here, we report deep infrared imaging and spectroscopy of one of the extremely red objects (EROs) discovered by Hu \& Ridgway (1994) in the field of the z=3.79 quasar PC1643+4631A. The infrared images were obtained in 0.5" seeing, and show that the object (denoted HR10) is not a dynamically relaxed elliptical galaxy dominated by an old stellar population as was previously suspected, but instead has an asymmetric morphology suggestive of either a disk or an interacting system. The infrared spectrum of HR10 shows a single, possibly broad emission feature at 1.60um which we identify as Halpha+[NII] at z=1.44. The luminosity and width of this emission line indicates either intense star formation (about 20h^{-2} solar masses/yr) or the presence of an active nucleus. Based on the rest frame UV-optical spectral energy distribution, the luminosity of HR10 is estimated to be 3 to 8 L*. The colors of HR10 are unusually red for a galaxy (at z=1.44 the age of HR10 is at most 2 to 8 Gyr depending on cosmology), and indicate that HR10 is dusty. HR10 is detected weakly at radio wavelengths; this is consistent with either the starburst or AGN hypothesis. If HR10 is a typical representative of its class, EROs are numerous and represent a significant component of the luminous objects in the Universe at z approx 1.5.
Micro-Pop-In Issues in Nanoscale Contact Deformation Resistance of Tooth Enamel
Nilormi Biswas,Arjun Dey,Anoop Kumar Mukhopadhyay
ISRN Biomaterials , 2013, DOI: 10.5402/2013/545791
Abstract: Human tooth enamel is a natural nanocomposite with a hierarchical structural architecture that spans from macroscale to nanoscale. Thus it offers the unique opportunity to study the physics of deformation at the nanoscale in a controlled manner using the novel nanoindentation technique. In spite of the wealth of literature, however, the information about the effect of loading rate on the nanoindentation behavior of human tooth enamel is far from being significant. Therefore, the major objective of the present work was to study the loading rate effect on nanoindentation behavior of enamel with a view to improve our understanding that could be used for development of better bioinspired synthetic structures for functional as well as biomedical utilities. The nanoindentation experiments were conducted at loading rates in the range of to ? at peak load of ? at room temperature with a Berkovich tip on the longitudinal section from a freshly extracted premolar tooth enamel surface from a 65-year-old Indian male. To the best of our knowledge here we report for the first time the experimental observation of the increase in intrinsic resistance against contact-induced deformation at the nanoscale with the loading rate applied to the enamel surface. The results were explained by considering the microstructural details and the shear stress underneath the nanoindenter. 1. Introduction The nanocomposite structure of the human tooth enamel provides a unique opportunity to study the physics of deformation during the nanoscale contact events because it consists of hydroxyapatite (HAP) nanocrystals embedded in an organic-protein matrix with hierarchical structures from macrostructure to microstructure to sub-microstructure to nanostructure to sub-nanostructure [1–3]. The tooth enamel can survive normally up to even a billion contacts between themselves. The tooth enamel has nanohardness ( ) in the range of 3 to 5?GPa [4–10]. The indenter shape [5], teeth type [6], location [7, 8], degree of biomineralization [9, 10], and depth of indentation [11] can affect the data. The HAP single crystals have nanohardness higher than that of enamel [12]. The tooth makes various mastications during all oral cavity movements. All these contact-induced events that determine the life of the enamel are the sum total of a multitude of nanoscale contact events under a wide variety of loading rates which have not been studied in significant details [1–12]. Neither the role of micro-pop-in nor micro-pop-out issues during nanoscale contact events happening under different loading rates in
What is 3C 324?
Mark Dickinson,Arjun Dey,Hyron Spinrad
Physics , 1995, DOI: 10.1007/BFb0102374
Abstract: We report ground based and HST observations of the z=1.206 radio galaxy 3C 324, a prototypical example of the radio-optical ``alignment effect.'' While infrared images shows a simple, round object reminiscent of a giant elliptical galaxy, the HST images reveal a spectacular, linear chain of UV-bright subcomponents closely aligned with the radio axis. In light of the available data, we consider various scenarios to explain the properties of 3C 324, as well as evidence for the presence of dust which may obscure the central active nucleus and scatter its light to produce the polarized, aligned continuum seen in the rest-frame UV.
Discovery of a z=2.76 Dusty Radio Galaxy?
Arjun Dey,Hyron Spinrad,Mark Dickinson
Physics , 1994, DOI: 10.1086/175293
Abstract: We report the discovery of a z=2.76 radio galaxy, MG1019+0535, with an unusual spectrum. Ly$\alpha$, which usually dominates the spectra of high redshift radio galaxies, is very weak and the strongest lines in the spectrum are CIV$\lambda$1549 and HeII$\lambda$1640. We speculate that dust is responsible for attenuating the Ly$\alpha$ line. This object provides more evidence for dust formation at early epochs. The optical counterpart of the compact radio source has a double morphology which is orthogonal to the radio source axis and most likely due to the superposition of two distinct objects which may or may not be physically related. There is only one detected line from the brighter component (B) which is close in observed wavelength to the redshifted HeII$\lambda$1640 line from the z=2.76 galaxy (A). We discuss whether B is at the same redshift as A or whether it is a foreground galaxy at z=0.66. We also report the serendipitous discovery of a very red ($R-K>7$) field object.
Keck Observations of the Most Distant Galaxy: 8C1435+63 at z=4.25
Hyron Spinrad,Arjun Dey,James R. Graham
Physics , 1994, DOI: 10.1086/187713
Abstract: We report on Keck observations and confirm the redshift of the most distant galaxy known: 8C1435+63 at z=4.25. The spectrum shows a strong Ly$\alpha$ line, a Ly$\alpha$ forest continuum break and a continuum break at $\lambda_{rest}=912$\AA. The Ly$\alpha$ emission is spatially extended and roughly aligned with the radio source. The galaxy shows a double structure in the $I$-band ($\lambda_{rest}\approx$1500\AA) which is aligned with the radio axis; the two $I$-band components spatially coincide with the nuclear and southern radio components. Some fraction of the $I$ band emission could be due to a nonthermal process such as inverse compton scattering. In the $K$-band ($\lambda_{rest}\approx$4200\AA), which may be dominated by starlight, the galaxy has a very low surface brightness, diffuse morphology. The $K$ morphology shows little relationship to the radio source structure, although the major axis of the $K$ emission is elongated roughly in the direction of the radio source axis. The galaxian continuum is very red ($I-K>4$) and if the $K$ continuum is due to starlight, implies a formation redshift of $z_f > 5$. We speculate that this galaxy may be the progenitor of a present day cD galaxy.
Red Quasars and Quasar Evolution: the Case of BALQSO FIRST J155633.8+351758
Joan Najita,Arjun Dey,Michael Brotherton
Physics , 2000, DOI: 10.1086/316862
Abstract: We present the first near-IR spectroscopy of the z=1.5 radio-loud BALQSO FIRST J155633.8+351758. 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 BLR (E(B-V)~0.5 for SMC-type screen extinction at the QSO redshift). The implied gas column density along the line of sight is much less than is implied by the weak X-ray flux of the object, suggesting that either the BLR and BAL region 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. Comparing the maximal stellar mass with the black hole mass estimated from the bolometric luminosity of the QSO, we find that the ratio of the black hole to stellar mass may be 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 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. [ABRIDGED]
A Successful Broad-band Survey for Giant Lya Nebulae I: Survey Design and Candidate Selection
Moire K. M. Prescott,Arjun Dey,Buell T. Jannuzi
Physics , 2011, DOI: 10.1088/0004-637X/748/2/125
Abstract: Giant Lya nebulae (or Lya "blobs") are likely sites of ongoing massive galaxy formation, but the rarity of these powerful sources has made it difficult to form a coherent picture of their properties, ionization mechanisms, and space density. Systematic narrow-band Lya nebula surveys are ongoing, but the small redshift range covered and the observational expense limit the comoving volume that can be probed by even the largest of these surveys and pose a significant problem when searching for such rare sources. We have developed a systematic search technique designed to find large Lya nebulae at 2
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