Updated: 11 January 2021

MIT Kavli Institute

Precious Metals in SDSS Quasar Spectra


Collaborators: Robert Simcoe, Melodie Kao (Caltech),
John O'Meara, Eduardo Seyffert (MIT), and J. Xavier Prochaska

| Papers | Presentations |


I. Tracking the Evolution of Strong, 1.5 < z < 4.5 C IV Absorbers with Thousands of Systems

Reference: Cooksey et al. (2013, ApJ, 763, 37: online, PDF)

Notice: 14 March 2016—Updated catalog available (see igmabsorbers.info).


We have vastly increased the C IV statistics at intermediate redshift by surveying the thousands of QSOs in the Sloan Digital Sky Survey Data-Release 7. We visually verified over 16,000 C IV systems with 1.46 < z < 4.55—a sample size that renders Poisson error negligible. Detailed Monte Carlo simulations show we are approximately 50% complete down to rest equivalent widths Wr ≈ 0.6 Å. We analyzed the sample as a whole and in ten small redshift bins with approximately 1500 doublets each. The equivalent width frequency distributions ƒ(Wr) were well modeled by an exponential, with little evolution in shape. In contrast with previous studies that modeled the frequency distribution as a single power law, the fitted exponential gives a finite mass density for C IV ions. The co-moving line density dNC IV/dX evolved smoothly with redshift, increasing by about a factor of 2.37 ± 0.09 from z = 4.55 to 1.96, then plateauing at dNC IV/dX ≈ 0.34 for z = 1.96 to 1.46. Comparing our SDSS sample with z < 1 (ultraviolet) and z > 5 (infrared) surveys, we see an approximately ten-fold increase in dNC IV/dX over z ≈ 6 → 0, for Wr ≥ 0.6 Å. This suggests a monotonic and significant increase in the enrichment of gas outside galaxies over the 12 Gyr lifetime of the universe.

II. Tracking the Evolution of Strong, 0.4 < z < 2.3 Mg II Absorbers with Thousands of Systems

Reference: Seyffert et al. (2013, ApJ, 779, 161: online, PDF)


We have performed an analysis of over 34,000 Mg II doublets at 0.36 < z < 2.29 in Sloan Digital Sky Survey (SDSS) Data-Release 7 quasar spectra. The catalog was divided into 14 small redshift bins with roughly 2,500 doublets in each, and from Monte-Carlo simulations, we estimate 50% completeness at rest equivalent width Wr ≈ 0.8 Å. The equivalent-width frequency distribution is described well by an exponential model for all redshifts, and the distribution becomes flatter with increasing redshift, i.e., there are more strong systems relative to weak ones. Direct comparison with previous SDSS Mg II surveys reveal that we recover at least 70% of the doublets in these other catalogs, in addition to detecting thousands of new systems. We discuss how these surveys come by their different results, which qualitatively agree but, due to the extremely small uncertainties, differ by a statistically significant amount. The estimated physical cross-section of Mg II-absorbing galaxy halos increased three-fold, approximately, from z = 0.4 → 2.3, while the Wr ≥ 1 Å absorber line density grew, dNMg II/dX, by roughly 45%. Finally, we explore the different evolution of various absorber populations—damped Lyman-$\alpha$ absorbers, Lyman-limit systems, strong C IV absorbers, and strong and weaker Mg II systems—across cosmic time (0 < z < 6).


"Precious Metals in SDSS Quasar Spectra: Toward a Multi-Ion Classification Scheme"

Conference: 235th Meeting of the American Astronomical Society, Honolulu, HI, 4−8 January 2020 (poster #205.08 PDF)


We aim to robustly "classify" absorption-line systems with our SDSS catalogs of SiIV, CIV, and MgII. Galaxies have a classification scheme—spirals versus ellipticals, emission-line versus absorption-line dominated—based on their fundamental properties, that also encapsulate similar formation and evolution processes. We are conducting unsupervised clustering analysis (a la Churchill et al. 2000) on the rest equivalent widths of SiIV (Wr,1393), CIV (Wr,1548), and MgII (Wr,2796), for systems where all can be detected. They span the decline in the cosmic star-formation-rate density (redshifts 1.7 to 2.3 or 3.9 to 2.9 Gyr after the Big Bang). The commonalities in the systems will be collected into "clusters," which are the absorber classes.

"Precious Metals (or Lack Thereof) in SDSS Quasar Spectra"

Conference: From Wall to Web, Berlin, Germany, 24−29 July 2016 (presentation slides and notes PDFs).

Colloquium: Institute for Astronomy, Mānoa, 8 April 2015 (presentation PDF)


Spectroscopic surveys of quasars yield a random sample of intervening absorbing gas clouds that can be used to constrain the on-going and summative enrichment processes in the universe. The CIV and MgII doublets have proven to be important tracers of the circumgalactic medium and its evolution from z = 6 to 0. As part of our on-going effort to compile fairly comparable samples of SDSS metal lines, we surveyed and analyzed thousands of z = 1.5−4.5 CIV and z = 0.4−2.3 MgII systems in DR7 quasar sightlines. We have also characterized the metallicity (spoiler: low!) of high-density Lyman-limit systems to trace inflowing gas (the fuel for future star formation) at z ≈ 3.5. I will summarize the metal-line absorbers (or lack thereof), relate them to galaxies and galaxy evolution, and discuss what's next (e.g., statistical analysis of multi-ion systems).

"Profile Fitting Absorption-Line Spectra of Circumgalactic Gaseous Structure"

by Chantelle Kiessner, UH Hilo astronomy & physics major and Hawai`i/NASA Space Grant Consortium (HSGC) Trainee

Poster: Fall 2017 HSGC Symposia