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X. Fan

Publications and source records attributed to X. Fan.

At least 109 records · Page 6Linked to original sources
arXiv2003

High-excitation CO in a quasar host galaxy at z=6.42

We report the detection of high excitation CO emission from the most distant quasar currently known, SDSS J114816.64+525150.3 (hereafter J1148+5251), at a redshift z=6.419. The CO (J=6-5) and (J=7-6) lines were detected using the IRAM Plateau de Bure interferometer, showing a width of ~280 km/s. An upper flux limit for the CO (J=1-0) line was obtained from observations with the Effelsberg 100-meter telescope. Assuming no gravitational magnification, we estimate a molecular gas mass of ~2x10^10 M_sun. Using the CO (3-2) observations by Walter et al. (2003), a comparison of the line flux ratios with predictions from a large velocity gradient model suggests that the gas is likely of high excitation, at densities ~10^5 cm^-3 and a temperature ~100 K. Since in this case the CO lines appear to have moderate optical depths, the gas must be extended over a few kpc. The gas mass detected in J1148+5251 can fuel star formation at the rate implied by the far-infrared luminosity for less than 10 million years, a time comparable to the dynamical time of the region. The gas must therefore be replenished quickly, and metal and dust enrichment must occur fast. The strong dust emission and massive, dense gas reservoir at z~6.4 provide further evidence that vigorous star formation is co-eval with the rapid growth of massive black holes at these early epochs of the Universe.

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arXiv2003

VLT+UVES Spectroscopy of the CaII LoBAL Quasar SDSS J030000.56+004828.0

We study high-resolution spectra of the `overlapping-trough' low-ionization broad absorption line (LoBAL) quasar SDSS J030000.56+004828.0. The CaII, MgII and MgI column densities in this object are the largest reported to date for any BAL outflow. The broad CaII absorption is mildly blended, but the blending can be disentangled to measure the CaII column density, which is large enough that the outflow must include a strong hydrogen ionization front. The outflow begins at a blueshift of ~1650 km/s from the systemic redshift. The lowest velocity BAL region produces strong CaII absorption but does not produce significant excited FeII absorption, while the higher velocity excited FeII absorption region produces very little CaII absorption. We have found that only a disk wind outflow can explain this segregation. Whether the outflow is smooth or clumpy, we conclude that the CaII BAL region has a density high enough to populate excited levels of FeII, but a temperature low enough to prevent them from being significantly populated. This requirement means the CaII BAL region has T<1000 K, and perhaps even T<550 K. This quasar also has an associated absorption line system (AAL) that exhibits partial covering, and therefore is likely located near the central engine. Its association with the BAL outflow is unclear. Blending of the AAL with the BAL trough shows that the spatial region covered by the BAL outflow can vary over velocity differences of ~1700 km/s.

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arXiv2003

An Initial Survey of White Dwarfs in the Sloan Digital Sky Survey

An initial assessment is made of white dwarf and hot subdwarf stars observed in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees), observed much like the full survey will be, 269 white dwarfs and 56 hot subdwarfs are identified spectroscopically where only 44 white dwarfs and 5 hot subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and DB (helium) types. In addition, in the full survey to date, a number of WDs have been found with uncommon spectral types. Among these are blue DQ stars displaying lines of atomic carbon; red DQ stars showing molecular bands of C_2 with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na, and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry alone allows identification of stars hotter than 12000 K, and the density of these stars for 15<g<20 is found to be ~2.2 deg^{-2} at Galactic latitudes 29-62 deg. Spectra are obtained for roughly half of these hot stars. The spectra show that, for 15<g<17, 40% of hot stars are WDs and the fraction of WDs rises to ~90% at g=20. The remainder are hot sdB and sdO stars.

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arXiv2003

Dust emission from the most distant quasars

We report observations of three SDSS z>6 QSOs at 250 GHz (1.2mm) using the 117-channel Max-Planck Millimeter Bolometer (MAMBO-2) array at the IRAM 30-meter telescope. J1148+5251 (z=6.41) and J1048+4637 (z=6.23) were detected with 250 GHz flux densities of 5.0 +- 0.6 mJy and 3.0 +- 0.4 mJy, respectively. J1630+4012 (z=6.05) was not detected with a 3 sigma upper limit of 1.8 mJy. Upper flux density limits from VLA observations at 43 GHz for J1148+5251 and J1048+4637 imply steeply rising spectra, indicative of thermal infrared emission from warm dust. The far-infrared luminosities are estimated to be \~10^13 L_sun, and the dust masses ~10^8 M_sun, assuming Galactic dust properties. The presence of large amounts of dust in the highest redshift QSOs indicates that dust formation must be rapid during the early evolution of QSO host galaxies. Dust absorption may hinder the escape of ionizing photons which reionize the intergalactic medium at this early epoch.

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arXiv2003

Chandra and XMM-Newton observations of the first quasars: X-rays from the age of cosmic enlightenment

We report on Chandra and XMM-Newton observations of a sample of 13 quasars at z~4.7-5.4 mostly taken from the Sloan Digital Sky Survey (SDSS). The present sample complements previous X-ray studies of z>4 quasars, in which the majority of the objects are optically more luminous and at lower redshifts. All but two of our quasars have been detected in the X-ray band, thus doubling the number of z>4.8 X-ray detected quasars. The two non-detections are likely to be due to a short exposure time and to the presence of intrinsic absorption. We confirm and extend to the highest redshifts the presence of a correlation between AB1450(1+z) magnitude and soft X-ray flux for z>4 quasars, and the presence of a steeper optical-to-X-ray spectral energy distribution (parameterized by aox) for high-luminosity, high-redshift quasars than for lower-luminosity, lower-redshift quasars. The second effect is likely due to the known anti-correlation between aox and rest-frame 2500 Angstrom luminosity, whose significance is confirmed via partial correlation analysis. The joint ~2.5-36 keV rest-frame spectrum of the z>4.8 SDSS quasars observed thus far by Chandra is well parameterized by a power-law with photon index Gamma=1.84+/-0.30; this photon index is consistent with those of z~0-3 quasars and that obtained from joint spectral fitting of z~4.1-4.5 optically luminous Palomar Digital Sky Survey quasars. No evidence for widespread intrinsic X-ray absorption has been found (Nh<4.0x10^{22} cm^{-2} on average at 90% confidence). We also obtained Hobby-Eberly Telescope (HET) photometric observations for eight of our target quasars. None of these shows significant (>30%) optical variability over the time interval of a few years (in the observed frame) between the SDSS and HET observations.

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arXiv2003

X-rays from the First Massive Black Holes

X-ray studies of high-redshift (z > 4) active galaxies have advanced substantially over the past few years, largely due to results from the new generation of X-ray observatories. As of this writing X-ray emission has been detected from nearly 60 high-redshift active galaxies. This paper reviews the observational results and their implications for models of the first massive black holes, and it discusses future prospects for the field.

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arXiv2002

Characterization of M,L and T dwarfs in the Sloan Digital Sky Survey

An extensive sample of M, L and T dwarfs identified in the Sloan Digital Sky Survey (SDSS) has been compiled. The sample of 718 dwarfs includes 677 new objects (629 M dwarfs, 48 L dwarfs) together with 41 that have been previously published. All new objects and some of the previously published ones have new optical spectra obtained either with the SDSS spectrographs or with the Apache Point Observatory 3.5m ARC telescope. Spectral types and SDSS colors are available for all objects; approximately 35% also have near-infrared magnitudes measured by 2MASS or on the Mauna Kea system. We use this sample to characterize the color--spectral type and color--color relations of late type dwarfs in the SDSS filters, and to derive spectroscopic and photometric parallax relations for use in future studies of the luminosity and mass functions based on SDSS data. We find that the (i*-z*) and (i*-J) colors provide good spectral type and absolute magnitude (M_i*) estimates for M and L dwarfs. Our distance estimates for the current sample indicate that SDSS is finding early M dwarfs out to about 1.5 kpc, L dwarfs to approximately 100 pc and T dwarfs to near 20 pc. The T dwarf photometric data show large scatter and are therefore less reliable for spectral type and distance estimation.

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arXiv2002

Unusual Broad Absorption Line Quasars from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has confirmed the existence of populations of broad absorption line (BAL) quasars with various unusual properties. We present and discuss twenty-three such objects and consider the implications of their wide range of properties for models of BAL outflows and quasars in general. We have discovered one BAL quasar with a record number of absorption lines. Two other similarly complex objects with many narrow troughs show broad MgII absorption extending longward of their systemic host galaxy redshifts. This can be explained as absorption of an extended continuum source by the rotation-dominated base of a disk wind. Five other objects have absorption which removes an unprecedented ~90% of all flux shortward of MgII. The absorption in one of them has varied across the ultraviolet with an amplitude and rate of change as great as ever seen. This same object may also show broad H-beta absorption. Numerous reddened BAL quasars have been found, including at least one reddened mini-BAL quasar with very strong FeII emission. The five reddest objects have continuum reddenings of E(B-V)~0.5, and in two of them we find strong evidence that the reddening curve is even steeper than that of the SMC. We have found at least one object with absorption from FeIII but not FeII. This may be due to a high column density of moderately high-ionization gas, but the FeIII level populations must also be affected by some sort of resonance. Finally, we have found two luminous, probably reddened high-redshift objects which may be BAL quasars whose troughs partially cover different regions of the continuum source as a function of velocity.

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arXiv2002

Exploratory Chandra Observations of the Three Highest Redshift Quasars Known

We report on exploratory Chandra observations of the three highest redshift quasars known (z = 5.82, 5.99, and 6.28), all found in the Sloan Digital Sky Survey. These data, combined with a previous XMM-Newton observation of a z = 5.74 quasar, form a complete set of color-selected, z > 5.7 quasars. X-ray emission is detected from all of the quasars at levels that indicate that the X-ray to optical flux ratios of z ~ 6 optically selected quasars are similar to those of lower redshift quasars. The observations demonstrate that it will be feasible to obtain quality X-ray spectra of z ~ 6 quasars with current and future X-ray missions.

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arXiv2002

VLT observations of the z=6.28 quasar SDSS 1030+0524

We present new VLT spectroscopic observations of the most distant quasar known, SDSS J1030+0524 at z=6.28 which was recently discovered by the Sloan Digital Sky Survey. We confirm the presence of a complete Gunn-Peterson trough caused by neutral hydrogen in the intergalactic medium. There is no detectable flux over the wavelength range from 8450 to 8710 A. We set an improved limit on the drop of the flux level blueward of the Ly alpha line: a factor of > 200. Below 8450 A the spectrum shows a rise in flux, with a large fraction (> 60 %) of the total emission produced by few narrow features of transmitted flux. We discuss the obvious proximity effect around this quasar, with the presence of transmitted flux with many absorption features in a region of about 23h^{-1} comoving Mpc. If assuming the surrounding medium were completely neutral, the size of this region would imply a quasar lifetime of \~1.3x10^7 years. We also present near-IR spectroscopy of both SDSS J1030+0524 and of SDSS J1306+05, the second most distant quasar known at redshift 6.0. We combine measurements of the CIV line and limits on the HeII emission with the NV line measurements from the optical spectra to derive line ratios, and by implication the abundances of these early quasar environments. The results are indistinguishable from those of lower redshift quasars and indicate little or no evolution in the abundances from z ~ 6 to z ~ 2. The line ratios suggest supersolar metallicities, implying that the first stars around the quasars must have formed at least a few hundreds of Myrs prior to the observation, i.e. at redshift higher than 8.

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arXiv2001

Towards Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis

We present 0.6-2.5um, R~400 spectra of twenty-seven cool, low luminosity stars and substellar objects. Based on these and previously published spectra we develop a preliminary spectral classification system for L and T dwarfs. For late L and T types the classification system is based entirely on four spectral indices in the 1-2.5um interval. Two of these indices are derived from water absorption bands at 1.15um and 1.4um, the latter of which shows a smooth increase in depth through the L and T sequences and can be used to classify both spectral types. The other two indices make use of methane absorption features in the H and K bands, with the K band index also applicable to mid to late L dwarfs. Continuum indices shortward of 1um used by previous authors to classify L dwarfs are found to be useful only through mid L subclasses. We employ the 1.5um water index and the 2.2um methane index to complete the L classification through L9.5 and to link the new system with a modified version of the 2MASS ``Color-d'' index. By correlating the depths of the methane and water absorption features, we establish a T spectral sequence from types T0 to T8, based on all four indices, which is a smooth continuation of the L sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one SDSS object as L9. In the proposed system methane absorption appears in the K band approximately at L8, two subclasses earlier than its appearance in the H band. The L and T spectral classes are distinguished by the absence and presence, respectively, of H band methane absorption.

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arXiv2001

The Optical, Infrared and Radio Properties of Extragalactic Sources Observed by SDSS, 2MASS and FIRST Surveys

We positionally match sources observed by the Sloan Digital Sky Survey (SDSS), the Two Micron All Sky Survey (2MASS), and the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey. Practically all 2MASS sources are matched to an SDSS source within 2 arcsec; ~11% of them are optically resolved galaxies and the rest are dominated by stars. About 1/3 of FIRST sources are matched to an SDSS source within 2 arcsec; ~80% of these are galaxies and the rest are dominated by quasars. Based on these results, we project that by the completion of these surveys the matched samples will include about 10^7 stars and 10^6 galaxies observed by both SDSS and 2MASS, and about 250,000 galaxies and 50,000 quasars observed by both SDSS and FIRST. Here we present a preliminary analysis of the optical, infrared and radio properties for the extragalactic sources from the matched samples. In particular, we find that the fraction of quasars with stellar colors missed by the SDSS spectroscopic survey is probably not larger than ~10%, and that the optical colors of radio-loud quasars are ~0.05 mag. redder (with 4-sigma significance) than the colors of radio-quiet quasars.

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arXiv2001

J-Band Spectroscopy of the z = 5.74 BAL QSO SDSSp J104433.04-012502.2

We use the NIRSPEC near-IR spectrometer on Keck II to obtain moderate resolution (R = 1540) spectroscopy that shows conclusively that the C IV emission line in the z = 5.74 quasar SDSSp J104433.04-012502.2 is accompanied by broad, blueshifted C IV absorption. The line has a "balnicity index" of 900 km/s and a rest-frame equivalent width of 13.1 +/- 1.3 relative to the continuum. This confirms its membership in the class of objects called "broad-absorption line" (BAL) QSOs. SDSSP J1044-0125 was previously suggested to be a BAL QSO based on its larger UV-to-X-ray flux ratio than most non-BAL QSOs. The C IV emission is of normal strength, implying a metallicity similar to that found in other, lower-redshift QSOs.

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arXiv2001

Infrared Classification of L and T Dwarfs

We use new and published medium resolution 0.6--2.5 $μ$m spectra of L and T dwarfs to develop a unified classification system for both of these new spectral classes. Two indices of the system at 1.2 $μ$m and 1.5 $μ$m are based on nearby absorption bands of water vapor and two are associated with methane bands near 1.6 $μ$m and 2.2 $μ$m. The 1.5 $μ$m index is monotonic through the L and T sequences, and forms the backbone of the system; the indices for the other bands provide extensive, but only partial, coverage. We correlate the 1.5 $μ$m index with continuum indices shortward of 1 $μ$m devised by others for classifying L dwarfs, in order to obtain a tight link between optical and infrared classifications. Our proposed system defines ten spectral subclasses for L (L0--L9) and nine for T (T0--T8). The boundary between L and T is defined to be the onset of absorption by methane in the H band. Methane absorption in the K band near 2.2 $μ$m is found to begin approximately at L8.

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arXiv2001

Infrared Colors of L and T Dwarfs

We discuss the behaviour of the JHKL'M' colors of L and T dwarfs, based on photometric and spectroscopic data obtained at the United Kingdom Infrared Telescope in Hawaii. We have measured the first accurate M' photometry for L and T dwarfs. The K-M' colors of T dwarfs are much bluer than predicted by published models, suggesting that CO may be more abundant than expected, as has been found spectroscopically for the T6 dwarf Gl 229B. We also find that K-L' increases monotonically through most of the M, L, and T subclasses, but it is approximately constant between types L6 and T5, due to the onset of methane absorption at the blue edge of the L' bandpass. The JHK colors of L dwarfs show significant scatter, suggesting variations in the amount and properties of photospheric dust, and indicating that it may not be possible to associate a unique effective temperature with a given L spectral type. The H-K colors of the later T dwarfs also show some scatter which we suggest is due to variations in pressure-induced molecular hydrogen opacity, which is sensitive to gravity (or age for a brown dwarf) and metallicity.

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arXiv2001

Infrared Photometry of Late-M, L, and T Dwarfs

We present ZJHKL'M' photometry of a sample of 58 late-M, L, and T dwarfs, most of which are identified from the Sloan Digital Sky Survey and the Two Micron All-Sky Survey. Near-infrared spectra and spectral classifications for most of this sample are presented in a companion paper by Geballe et al. We derive the luminosities of 18 dwarfs in the sample and the results imply that the effective temperature range for the L dwarfs in our sample is approximately 2200-1300 K and for the T dwarfs 1300-800 K. We obtained new photometric data at the United Kingdom Infrared Telescope for: 42 dwarfs at Z, 34 dwarfs at JHK, 21 dwarfs at L', as well as M' data for two L dwarfs and two T dwarfs. The M' data provide the first accurate photometry for L and T dwarfs in this bandpass - for a T2 and a T5 dwarf, we find K-M'=1.2 and 1.6, respectively. These colors are much bluer than predicted by models suggesting that CO may be more abundant in these objects than expected, as has been found for the T6 dwarf Gl 229B. We also find that K-L' increases monotonically through most of the M, L, and T subclasses, but it is almost constant between types L6 and T5. The degeneracy is probably due to the onset of methane absorption at the blue edge of the L' bandpass. The JHK colors of L dwarfs show significant scatter, suggesting that the fluxes in these bandpasses are sensitive to variations in photospheric dust properties. The H-K colors of the later T dwarfs also show some scatter which we suggest is due to variations in pressure-induced molecular hydrogen opacity, which is sensitive to gravity and metallicity.

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arXiv2001

Exploratory Chandra Observations of the Highest-Redshift Quasars: X-rays from the Dawn of the Modern Universe

We report exploratory Chandra observations of 14 high-redshift (z=4.06-5.27), optically selected quasars. Ten of these quasars are detected, increasing the number of z>4 X-ray detected quasars by 71%. Our detections include four of the five highest-redshift X-ray detected quasars to date, among them SDSSp J021043.17-001818.4, the highest-redshift (z=4.77) radio-loud quasar detected in the X-ray band. The four undetected objects are the Broad Absorption Line quasars SDSSp J112956.10-014212.4 and SDSSp 160501.21-011220.0, the weak emission-line quasar SDSSp J153259.96-003944.1, and the quasar PSS 1435+3057. A comparison of the quasars' spectral energy distributions (by means of the optical-to-X-ray spectral index, alpha_ox) with those of lower-redshift samples indicates that the Chandra quasars are X-ray fainter by a factor of approx 2. X-ray faintness could be associated with the presence of large amounts of gas in the primeval galaxies harboring these high-redshift quasars, as suggested by recent studies conducted on z>4 quasars in other bands. Using the current Chandra data, predictions for the next generation of X-ray observatories, Constellation-X and Xeus, are also provided.

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