ORCID Profile
0000-0003-4327-1460
Current Organisation
University of North Texas
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Publisher: American Astronomical Society
Date: 26-09-2018
Publisher: American Astronomical Society
Date: 20-06-2007
DOI: 10.1086/518082
Publisher: American Astronomical Society
Date: 03-02-2020
Publisher: Oxford University Press (OUP)
Date: 16-02-2022
Abstract: We present deeper Chandra observations for weak-line quasars (WLQs) in a representative s le that previously had limited X-ray constraints, and perform X-ray photometric analyses to reveal the full range of X-ray properties of WLQs. Only 5 of the 32 WLQs included in this representative s le remain X-ray undetected after these observations, and a stacking analysis shows that these 5 have an average X-ray weakness factor of & . One of the WLQs in the s le that was known to have extreme X-ray variability, SDSS J1539+3954, exhibited dramatic X-ray variability again: it changed from an X-ray normal state to an X-ray weak state within ≈3 months in the rest frame. This short time-scale for an X-ray flux variation by a factor of ≳9 further supports the thick disc and outflow (TDO) model proposed to explain the X-ray and multiwavelength properties of WLQs. The overall distribution of the X-ray-to-optical properties of WLQs suggests that the TDO has an average covering factor of the X-ray emitting region of ∼0.5, and the column density of the TDO can range from $N_{\\rm H}\\, \\sim 10^{23-24}~{\\rm cm}^{-2}$ to $N_{\\rm H}\\, \\gtrsim 10^{24}~{\\rm cm}^{-2}$, which leads to different levels of absorption and Compton reflection (and/or scattering) among WLQs.
Publisher: American Astronomical Society
Date: 04-2022
Abstract: We present ultraviolet spectroscopy covering the Ly α + N v complex of six candidate low-redshift (0.9 z 1.5) weak emission-line quasars (WLQs) based on observations with the Hubble Space Telescope. The original systematic searches for these puzzling Type 1 quasars with intrinsically weak broad emission lines revealed an N ≈ 100 WLQ population from optical spectroscopy of high-redshift ( z 3) quasars, defined by a Ly α + N v rest-frame equivalent width (EW) threshold .4 Å. Identification of lower-redshift ( z 3) WLQ candidates, however, has relied primarily on optical spectroscopy of weak broad emission lines at longer rest-frame wavelengths. With these new observations expanding existing optical coverage into the ultraviolet, we explore unifying the low- and high- z WLQ populations via EW[Ly α +N v ]. Two objects in the s le unify with high- z WLQs, three others appear consistent with the intermediate portion of the population connecting WLQs and normal quasars, and the final object is consistent with typical quasars. The expanded wavelength coverage improves the number of available line diagnostics for our in idual targets, allowing a better understanding of the shapes of their ionizing continua. The ratio of EW[Ly α +N v ] to EW[Mg ii ] in our s le is generally small but varied, favoring a soft ionizing continuum scenario for WLQs, and we find a lack of correlation between EW[Ly α +N v ] and the X-ray properties of our targets, consistent with a “slim-disk” shielding gas model. We also find indications that weak absorption may be a more significant contaminant in low- z WLQ populations than previously thought.
Publisher: American Astronomical Society
Date: 05-2022
Abstract: We present an overview of best practices for publishing data in astronomy and astrophysics journals. These recommendations are intended as a reference for authors to help prepare and publish data in a way that will better represent and support science results, enable better data sharing, improve reproducibility, and enhance the reusability of data. Observance of these guidelines will also help to streamline the extraction, preservation, integration and cross-linking of valuable data from astrophysics literature into major astronomical databases, and consequently facilitate new modes of science discovery that will better exploit the vast quantities of panchromatic and multidimensional data associated with the literature. We encourage authors, journal editors, referees, and publishers to implement the best practices reviewed here, as well as related recommendations from international astronomical organizations such as the International Astronomical Union for publication of nomenclature, data, and metadata. A convenient Checklist of Recommendations for Publishing Data in the Literature (Appendix A) is included for authors to consult before the submission of the final version of their journal articles and associated data files. We recommend that publishers of journals in astronomy and astrophysics incorporate a link to this document in their Instructions to Authors.
Publisher: American Astronomical Society
Date: 06-2023
Abstract: Weak emission-line quasars (WLQs) are a subset of type 1 quasars that exhibit extremely weak Ly α + N v λ 1240 and/or C iv λ 1549 emission lines. We investigate the relationship between emission-line properties and accretion rate for a s le of 230 “ordinary” type 1 quasars and 18 WLQs at z 0.5 and 1.5 z 3.5 that have rest-frame ultraviolet and optical spectral measurements. We apply a correction to the H β -based black hole mass ( M BH ) estimates of these quasars using the strength of the optical Fe ii emission. We confirm previous findings that WLQs’ M BH values are overestimated by up to an order of magnitude using the traditional broad-emission-line region size–luminosity relation. With this M BH correction, we find a significant correlation between H β -based Eddington luminosity ratios and a combination of the rest-frame C iv equivalent width and C iv blueshift with respect to the systemic redshift. This correlation holds for both ordinary quasars and WLQs, which suggests that the two-dimensional C iv parameter space can serve as an indicator of accretion rate in all type 1 quasars across a wide range of spectral properties.
Publisher: American Astronomical Society
Date: 06-2023
Abstract: Quasars at z ≳ 1 most often have redshifts measured from rest-frame ultraviolet emission lines. One of the most common such lines, C iv λ 1549, shows blueshifts up to ≈5000 km s −1 and in rare cases even higher. This blueshifting results in highly uncertain redshifts when compared to redshift determinations from rest-frame optical emission lines, e.g., from the narrow [O iii ] λ 5007 feature. We present spectroscopic measurements for 260 sources at 1.55 ≲ z ≲ 3.50 having −28.0 ≲ M i ≲ − 30.0 mag from the Gemini Near Infrared Spectrograph–Distant Quasar Survey (GNIRS-DQS) catalog, augmenting the previous iteration, which contained 226 of the 260 sources whose measurements are improved upon in this work. We obtain reliable systemic redshifts based on [O iii ] λ 5007 for a subset of 121 sources, which we use to calibrate prescriptions for correcting UV-based redshifts. These prescriptions are based on a regression analysis involving C iv full-width-at-half-maximum intensity and equivalent width, along with the UV continuum luminosity at a rest-frame wavelength of 1350 Å. Applying these corrections can improve the accuracy and the precision in the C iv -based redshift by up to ∼850 km s −1 and ∼150 km s −1 , respectively, which correspond to ∼8.5 and ∼1.5 Mpc in comoving distance at z = 2.5. Our prescriptions also improve the accuracy of the best available multifeature redshift determination algorithm by ∼100 km s −1 , indicating that the spectroscopic properties of the C iv emission line can provide robust redshift estimates for high-redshift quasars. We discuss the prospects of our prescriptions for cosmological and quasar studies utilizing upcoming large spectroscopic surveys.
Publisher: American Astronomical Society
Date: 06-2023
Abstract: The most reliable single-epoch supermassive black hole mass ( M BH ) estimates in quasars are obtained by using the velocity widths of low-ionization emission lines, typically the H β λ 4861 line. Unfortunately, this line is redshifted out of the optical band at z ≈ 1, leaving M BH estimates to rely on proxy rest-frame ultraviolet (UV) emission lines, such as C iv λ 1549 or Mg ii λ 2800, which contain intrinsic challenges when measuring, resulting in uncertain M BH estimates. In this work, we aim at correcting M BH estimates derived from the C iv and Mg ii emission lines based on estimates derived from the H β emission line. We find that employing the equivalent width of C iv in deriving M BH estimates based on Mg ii and C iv provides values that are closest to those obtained from H β . We also provide prescriptions to estimate M BH values when only C iv , only Mg ii , and both C iv and Mg ii are measurable. We find that utilizing both emission lines, where available, reduces the scatter of UV-based M BH estimates by ∼15% when compared to previous studies. Lastly, we discuss the potential of our prescriptions to provide more accurate and precise estimates of M BH given a much larger s le of quasars at 3.20 ≲ z ≲ 3.50, where both Mg ii and H β can be measured in the same near-infrared spectrum.
Publisher: Oxford University Press (OUP)
Date: 27-07-2018
Publisher: American Astronomical Society
Date: 06-2021
Publisher: Oxford University Press (OUP)
Date: 25-04-2018
Publisher: American Astronomical Society
Date: 19-01-2021
Publisher: American Astronomical Society
Date: 22-12-2021
Abstract: Vera C. Rubin Observatory is a ground-based astronomical facility under construction, a joint project of the National Science Foundation and the U.S. Department of Energy, designed to conduct a multipurpose 10 yr optical survey of the Southern Hemisphere sky: the Legacy Survey of Space and Time. Significant flexibility in survey strategy remains within the constraints imposed by the core science goals of probing dark energy and dark matter, cataloging the solar system, exploring the transient optical sky, and mapping the Milky Way. The survey’s massive data throughput will be transformational for many other astrophysics domains and Rubin’s data access policy sets the stage for a huge community of potential users. To ensure that the survey science potential is maximized while serving as broad a community as possible, Rubin Observatory has involved the scientific community at large in the process of setting and refining the details of the observing strategy. The motivation, history, and decision-making process of this strategy optimization are detailed in this paper, giving context to the science-driven proposals and recommendations for the survey strategy included in this Focus Issue.
Publisher: American Astronomical Society
Date: 02-12-2011
No related grants have been discovered for Ohad Shemmer.