arXiv subjects

Joseph Shields

Publications and source records attributed to Joseph Shields.

11 recordsLinked to original sources

arXiv2026

Commissioning and on-sky performance verification of iLocater

iLocater is a high-resolution near-infrared extreme precision radial velocity (EPRV) spectrograph that was deployed to the Large Binocular Telescope (LBT) in June 2026. iLocater operates over $\lambda=966-1312$ nm with a median resolving power of $R=205,000$ as measured in the laboratory. We present the commissioning and initial on-sky verification program using solar and night-time observations at the LBT. First light was achieved on 27 June 2026, and nearly 150 on-sky spectra have now been recorded. Observations include single stars ranging in spectral type from B5 to M6. iLocater uses the LBT AO system, and we have demonstrated its ability to obtain spatially resolved spectra of close ($\theta < 1''$) binary stars.

astro-ph.IM↗

arXiv2013

Extreme-Velocity Quasar Outflows and the Role of X-ray Shielding

Quasar accretion disk winds observed via broad absorption lines (BALs) in the UV produce strong continuous absorption in X-rays. The X-ray absorber is believed to serve critically as a radiative shield to enable radiative driving. However, "mini-BAL" and narrow absorption line outflows have dramatically less X-ray absorption than BALs. Here we examine X-ray and rest-frame UV spectra of 8 mini-BAL quasars with outflow speeds in the range 0.1c to 0.2c to test whether extreme speeds require a strong shield. We find that the X-ray absorption is weak or moderate, with neutral-equivalent column densities N_H < few times 10^22 cm^-2, consistent with mini-BALs at lower speeds. We use photoionization models to show that this amount of shielding is too weak to control the outflow ionizations and, therefore, it is not important for the acceleration. Shielding in complex geometries also seems unlikely because the alleged shield would need to extinguish the ionizing far-UV flux while avoiding detection in X-rays and the near-UV. We argue that the outflow ionizations are kept moderate, instead, by high gas densities in small clouds. If the mini-BALs form at radial distances of order R ~ 2 pc from the central quasar (broadly consistent with theoretical models and with the mini-BAL variabilities observed here and in previous work), and the total column densities in the mini-BAL gas are N_H ~< 10^21 cm^-2, then the total radial extent of outflow clouds is only Delta-R_clouds ~< 3 x 10^13 cm and the radial filling factor is Delta-R_clouds/R ~< 5 x 10^-6 for weak/negligible shielding. Compared to the transverse sizes >~ 8 x 10^15 cm (based on measured line depths), the outflows have shapes like thin "pancakes" viewed face-on, or they occupy larger volumes like a spray of many dense clouds with a small volume filling factor. These results favor models with magnetic confinement in magnetic disk winds.

astro-ph.CO↗

arXiv2013

The Physics and Physical Properties of Quasar Outflows

We describe two studies designed to characterize the total column densities, kinetic energies, and acceleration physics of broad absorption line (BAL) outflows in quasars. The first study uses new Chandra X-ray and ground-based rest-frame UV observations of 7 quasars with mini-BALs at extreme high speeds, in the range 0.1c to 0.2c, to test the idea that strong radiative shielding is needed to moderate the mini-BAL ionizations and facilitate their acceleration to extreme speeds. We find that the X-ray absorption is weak or absent, with generally N_H < few x 10^22 mc^-2, and that radiative shielding is not important. We argue that the mini-BAL ionizations are controlled, instead, by high gas densities of order n_H ~ 4 x 10^8 cm^-3 in small outflow substructures. If we conservatively assume that the total column density in the mini-BAL gas is N_H < 10^22 cm^-2, covering >15% of the UV continuum source along our lines of sight (based on measured line depths), then the radial thickness of these outflows is only Delta_R < 3 x 10^13 cm and their transverse size is > 8 x 10^15 cm. Thus the outflow regions have the shape of very thin "pancakes" viewed face-on, or they occupy larger volumes like a spray of dense cloudlets with a very small volume filling factor. We speculate that this situation (with ineffective shielding and small dense outflow substructures) applies to most quasar outflows, including BALs. Our second study focuses from BALs of low-abundance ions, mainly PV 1118,1128 A, whose significant strengths imply large column densities, N_H > 10^22 cm^-2, that can further challenge models of the outflow acceleration. In spite of the difficulties of finding this line in the Ly-alpha forest, a search through the SDSS DR9 quasar catalog reveals >50 BAL sources at redshifts z>2.3 with strong PV BALs, which we are now using to characterize the general properties of high-column outflows.

astro-ph.CO↗

arXiv2012

Addressing Decadal Survey Science through Community Access to Highly Multiplexed Spectroscopy with BigBOSS on the KPNO Mayall Telescope

This document summarizes the results of a community-based discussion of the potential science impact of the Mayall+BigBOSS highly multiplexed multi-object spectroscopic capability. The KPNO Mayall 4m telescope equipped with the DOE- and internationally-funded BigBOSS spectrograph offers one of the most cost-efficient ways of accomplishing many of the pressing scientific goals identified for this decade by the "New Worlds, New Horizons" report. The BigBOSS Key Project will place unprecedented constraints on cosmological parameters related to the expansion history of the universe. With the addition of an open (publicly funded) community access component, the scientific impact of BigBOSS can be extended to many important astrophysical questions related to the origin and evolution of galaxies, stars, and the IGM. Massive spectroscopy is the critical missing ingredient in numerous ongoing and planned ground- and space-based surveys, and BigBOSS is unique in its ability to provide this to the US community. BigBOSS data from community-led projects will play a vital role in the education and training of students and in maintaining US leadership in these fields of astrophysics. We urge the NSF-AST division to support community science with the BigBOSS multi-object spectrograph through the period of the BigBOSS survey in order to ensure public access to the extraordinary spectroscopic capability.

astro-ph.CO↗

arXiv2012

Variability of Mini-BAL and BAL Outflows in Quasars

We report the results of several programs to study the variability of high-velocity (up to 0.2c) mini-"broad absorption lines" (mini-BALs) and BALs in quasar spectra, and thus to better characterize the structural and physical properties of these outflows. After the report of a highly variable mini-BAL outflow at a speed of ~0.17c in the quasar PG0935+417, we created the first systematic accounting of outflows in Sloan Digital Sky Survey (SDSS) quasar spectra that includes mini-BALs and extremely high velocity outflows (up to 0.2c) to measure their frequency. Following this study, we began a monitoring campaign to study the location, and dynamical and evolutionary effects of these outflows. This program covers a range of 0.9-3.3 years in the quasars' rest-frame by comparing new spectra (using facilities at the Kitt Peak National Observatory and MDM Observatory) with archival SDSS spectra. We find that ~57% of quasars with mini-BALs and BALs varied between just two observations. This variability tends to occur in complex ways; however, all the variable lines vary in intensity and not in velocity, not finding evidence for acceleration/deceleration in these outflows. Due to the variations in strength, mini-BALs can become BALs and vice versa, suggesting they share a similar nature. We include as an example the discovery of the transition of a mini-BAL into a BAL in the spectra of the SDSS quasar J115122+020426.

astro-ph.CO↗

arXiv2007

High Velocity Outflows in Quasars

High velocity (HV) outflows are an important but poorly understood aspect of quasar/SMBH evolution. Outflows during the luminous accretion phase might play a critical role in "unveiling" young dusty AGN and regulating star formation in the host galaxies. Most quasar studies have focussed on the broad absorption lines (BALs). We are involved in a program to study a nearly unexplored realm of quasar outflow parameter space: HV winds with v>10,000 km/s up to v~0.2c but small velocity dispersions (narrow absorption lines), such that (Delta v)/v << 1. Narrow-line HV flows merit specific attention because they complement the BAL work and pose unique challenges for models of the wind acceleration, mass loss rates, launch radii, geometry, etc. We have selected the brightest quasars at 1.8<z<3.5 with candidate narrow HV outflow lines (CIV 1548 A) in existing SDSS spectra and followed up with monitoring observations to i) characterize, for the first time, the variability in a sample of absorbers spanning a wide range of velocities and FWHMs, including some marginal BALs, ii) identify/confirm more of the true outflow systems, e.g., among the more ambiguous narrow lines, and iii) test the wind models and derive better constraints on basic outflow properties, such as the kinematics, locations, and physical conditions.

astro-ph↗

arXiv2005

The XBootes Chandra Survey Paper II: The X-ray Source Catalog

We present results from a Chandra survey of the nine square degree Bootes field of the NOAO Deep Wide-Field Survey (NDWFS). This XBootes survey consists of 126 separate contiguous ACIS-I observations each of approximately 5000 seconds in duration. These unique Chandra observations allow us to search for large scale structure and to calculate X-ray source statistics o ver a wide, contiguous field of view with arcsecond angular resolution and uniform coverage. Optical spectroscopic follow-up observations and the rich NDWFS data set will allow us to identify and classify these X-ray selected sources. Using wavelet decomposition, we detect 4642 point sources with n $\ge$ 2 counts. In order to keep our detections ~99% reliable, we limit our list to sources with n $\ge$ 4 counts. The full 0.5--7 keV band n $\ge$ 4 count list has 3293 point sources. In addition to the point sources, 43 extended sources have been detected consistent, with the depth of these observations and the number counts of clusters. We present here the X-ray catalog for the XBootes survey, including source positions, X-ray fluxes, hardness ratios and their uncertainties. We calculate and present the differential number of sources per flux densit y interval, $N(S)$, for the point sources.

astro-ph↗

arXiv2005

XBootes: An X-Ray Survey of the NDWFS Bootes Field - Paper I Overview and Initial Results

We obtained a 5 ksec deep Chandra X-ray Observatory ACIS-I map of the 9.3 square degree Bootes field of the NOAO Deep Wide-Field Survey. Here we describe the data acquisition and analysis strategies leading to a catalog of 4642 (3293) point sources with 2 or more (4 or more) counts, corresponding to a limiting flux of roughly 4(8)x10^{-15} erg cm^{-2}s^{-1} in the 0.5-7 keV band. These Chandra XBootes data are unique in that they consitute the widest contiguous X-ray field yet observed to such a faint flux limit. Because of the extraordinarily low background of the ACIS, we expect only 14% (0.7%) of the sources to be spurious. We also detected 43 extended sources in this survey. The distribution of the point sources among the 126 pointings (ACIS-I has a 16 x 16 arcminute field of view) is consistent with Poisson fluctuations about the mean of 36.8 sources per pointing. While a smoothed image of the point source distribution is clumpy, there is no statistically significant evidence of large scale filamentary structure. We do find however, that for theta>1 arcminute, the angular correlation function of these sources is consistent with previous measurements, following a power law in angle with slope -0.7. In a 1.4 deg^{2} sample of the survey, approximately 87% of the sources with 4 or more counts have an optical counterpart to R ~26 mag. As part of a larger program of optical spectroscopy of the NDWFS Bootes area, spectra have been obtained for \~900 of the X-ray sources, most of which are QSOs or AGN.

astro-ph↗

arXiv2002

Chandra Survey of Radio-quiet, High-redshift Quasars

We observed 17 optically-selected, radio-quiet high-redshift quasars with the Chandra Observatory ACIS, and detected 16 of them. The quasars have redshift between 3.70 and 6.28 and include the highest redshift quasars known. When compared to low-redshift quasars observed with ROSAT, these high redshift quasars are significantly more X-ray quiet. We also find that the X-ray spectral index of the high redshift objects is flatter than the average at lower redshift. These trends confirm the predictions of models where the accretion flow is described by a cold, optically-thick accretion disk surrounded by a hot, optically thin corona, provided the viscosity parameter alpha >= 0.02. The high redshift quasars have supermassive black holes with masses ~10^{10} M_{sun}, and are accreting material at ~0.1 the Eddington limit. We detect 10 X-ray photons from the z=6.28 quasar SDS 1030+0524, which may have a Gunn-Peterson trough and be near the redshift of reionization of the intergalactic medium. The X-ray data place an upper limit on the optical depth of the intergalactic medium tau(IGM) < 10^6, compared to the lower limit from the spectrum of Lyalpha and Lybeta, which implies tau(IGM) > 20.

astro-ph↗

arXiv2000

The Nature of Associated Absorption and the UV-X-ray Connection in 3C 288.1

We discuss new Hubble Space Telescope spectroscopy of the radio-loud quasar, 3C 288.1. The data cover ~590 A to ~1610 A in the quasar rest frame. They reveal a wealth of associated absorption lines (AALs) with no accompanying Lyman-limit absorption. The metallic AALs range in ionization from C III and N III to Ne VIII and Mg X. We use these data and photoionization models to derive the following properties of the AAL gas: 1) There are multiple ionization zones within the AAL region, spanning a factor of at least ~50 in ionization parameter. 2) The overall ionization is consistent with the ``warm'' X-ray continuum absorbers measured in Seyfert 1 nuclei and other QSOs. However, 3) the column densities implied by the AALs in 3C 288.1 are too low to produce significant bound-free absorption at any UV-X-ray wavelengths. Substantial X-ray absorption would require yet another zone, having a much higher ionization or a much lower velocity dispersion than the main AAL region. 4) The total hydrogen column density in the AAL gas is log N_H (cm-2)= 20.2. 5) The metallicity is roughly half solar. 6) The AALs have deconvolved widths of ~900 km/s and their centroids are consistent with no shift from the quasar systemic velocity (conservatively within +/-1000 km/s). 7) There are no direct indicators of the absorber's location in our data, but the high ionization and high metallicity both suggest a close physical relationship to the quasar/host galaxy environment. Finally, the UV continuum shape gives no indication of a ``blue bump'' at higher energies. There is a distinct break of unknown origin at ~1030 A, and the decline toward higher energies (with spectral index alpha = -1.73, for f_nu ~ nu^alpha) is even steeper than a single power-law interpolation from 1030 A to soft X-rays.

astro-ph↗

arXiv1997

Broad NeVIII 774 Emission From Quasars

NeVIII 774 is an important tracer of the high-ionization gas in QSOs. We examine the NeVIII emission-line properties using new HST-FOS spectra of four sources, mean spectra derived from two QSO samples in the HST archives, and new photoionization calculations. The results support our previous claim that broad NeVIIIlines are common in QSOs, with an average flux of ~42% of OVI 1034 and velocity widths that are ~2 to 5 times larger than OVI, CIV 1549 and other broad lines in the same spectra. The strongest and most reliably measured NeVIII 774 lines (in two sources) have FWHM ~ 14,500 km/s. Line profile fits in these cases show that the unusually large widths might be caused by blending with emission from NIV 765 and OIV~\lam 789. However, standard photoionization calculations indicate that NIV, OIV and all other lines near this wavelength should be too weak, leaving (very broad) NeVIII as the only viable identification for the ~774 A feature. (This conclusion might be avoided if there are large radial velocity dispersions [>~1000 km/s] in the emitting region and the resonant absorption of continuum photons enhances the flux in weaker lines.) The calculations also indicate that the NeVIII emitting regions have ionization parameters in the range 5 > U > 30, total hydrogen column densities of 10^22 < N_H < 3 x 10^23 cm-2, and an average covering factor of >30% (for solar abundances and a nominal QSO continuum shape). The NeVIII emitting region is therefore more extensive, more highly ionized, and has much higher velocities than the rest of the broad emission line region (BELR). This highly-ionized BELR component would be a strong X-ray ``warm'' absorber if it lies along our line-of-sight to the X-ray continuum source.

astro-ph↗