Updated: 14 September 2011

Advisor: J. Xavier Prochaska
UCO/Lick Observatory; UCSC

Jump to dissertation abstract and document.

The Last Eight-Billion Years of Intergalactic C IV and Si IV Evolution

Contributed talks to MKI Postdoc Symposium 2011, MIT, MA (4/14/2011; presentation PDF);
Gas in Galaxies: from Cosmic Web to Molecular Clouds, Kloster Seeon, Germany (6/14/2011; presentation PDF);
The Cosmic Odyssey of Baryons, Marseille, France (6/22/2011; presentation PDF); and
Santa Cruz Galaxy Workshop 2011, Santa Cruz, CA (8/10/2011; presentation PDF).

The low-redshift (z < 1) IGM probes the last eight-billion years of metal enrichment from galactic feedback processes. Outflows and mergers return to the IGM enriched material for future generations of galaxies and stars. The signature of this process is etched in the recycled gas: metallicity, abundances, density, distribution, etc. Observations of intergalactic absorbers provide "end-product" constraints on the physics driving chemical evolution and, therefore, the physics adopted in cosmological hydrodynamic simulations. We conducted the largest survey for C IV and/or Si IV systems at z < 1, and we compare our results to those from high-redshift (1.5 < z < 6) studies. For example, we present the frequency distribution ƒ(N) and mass density of C IV and Si IV absorbers in the HST archival spectra of 49 quasars. The changes in the properties of the low-redshift systems, compared to the high-redshift ones, indicate that C IV and Si IV more commonly trace circum-galactic gas at z < 1. Indeed, this agrees with simulated observations from several OverWhelmingly Large Simulations.

The Last Eight-Billion Years of Intergalactic C IV and Si IV Evolution

Colloquium at Boston University Astronomy Department (11/1/2010);
Astrophysics Journal Club at Brown University Department of Physics (11/10/2010);
AURA Colloquium at CTIO (11/19/2010)
(presentation PDF and notes)

The low-redshift (z < 1) IGM probes the last eight-billion years of metal enrichment from galactic feedback processes. Outflows and mergers return to the IGM enriched material for future generations of galaxies and stars. The signature of this process is etched in the recycled gas: metallicity, abundances, density, distribution, etc. We conducted the largest survey for C IV and/or Si IV systems at z < 1, and we compare our results to those from high-redshift (1.5 < z < 6) studies. For example, we present the frequency distribution ƒ(N) and mass density of C IV and Si IV absorbers in the HST archival spectra of 49 quasars. The changes in the properties of the low-redshift systems, compared to the high-redshift ones, indicate that C IV and Si IV more commonly trace circum-galactic gas at z < 1.

The Last Eight-Billion Years of Intergalactic Si IV Evolution

Cooksey, Prochaska, Thom, and Chen, 2010, ApJ, 729, 87 (PDF).
Online data can be found here.

We identified 24 Si IV absorption systems with z ≤ 1 from a blind survey of 49 low-redshift quasars with archival Hubble Space Telescope ultraviolet spectra. We relied solely on the characteristic wavelength separation of the doublet to automatically detect candidates. After visual inspection, we defined a sample of 20 definite (group G = 1) and 4 "highly likely" (G = 2) doublets with rest equivalent widths W_r for both lines detected at ≥ 3σ_W. The absorber line density of the G = 1 doublets was dN_Si IV/dX = 1.4+0.4/−0.3 for log N(Si⁺³) > 12.9. The best-fit power law to the G = 1 frequency distribution of column densities ƒ(N(Si⁺³)) had normalization k = (1.2+0.5/−0.4) × 10⁻¹⁴ cm² and slope α_N = −1.6+0.3/−0.3. Using the power-law model of ƒ(N(Si⁺³)), we measured the Si⁺³ mass density relative to the critical density: Ω_Si+3 = (3.71+2.82/−1.68) × 10⁻⁸ for 13 ≤ log N(Si⁺³) ≤ 15. From Monte Carlo sampling of the distributions, we estimated our value to be a factor of 4.8+3.0/−1.9 higher than the 2 ≤ z ≤ 4.5 ⟨Ω_Si+3⟩. From a simple linear fit to Ω_Si+3 over the age of the universe, we estimated a slow and steady increase from z = 5.5 → 0 with dΩ_Si+3/dt_age = (0.61±0.13) × 10⁻⁸ Gyr⁻¹. We compared our ionic ratios N(Si⁺³)/N(C⁺³) to a 2 < z < 4.5 sample and concluded, from survival analysis, that the two populations are similar, with median ⟨N(Si⁺³)/N(C⁺³)⟩ = 0.16.

Cooksey (September 2009; dissertation PDF; presentation PDF and notes)

Metals are produced in the stars in the galaxies, and a variety of feedback processes move metals from the sites of production into the intergalactic medium (IGM), enriching the material for future generations of stars. The signature of this process is etched in the recycled gas: its metallicity, elemental abundances, density, distribution, etc. The study of the low-redshift, z < 1, IGM is the study of the last eight-billion years of cosmic chemical evolution and all prior enrichment.

In this thesis, I characterize the cosmic enrichment cycle with the use of observations and simulations. The gas is observed through quasar absorption-line spectroscopy. As the light of a distant quasar travels to us, intervening clouds of gas absorb the light at wavelengths characteristic, albeit redshifted, of the elements in the clouds. By identifying and modeling the elements associated with the absorption systems, I learn the ionic composition and density of the cosmic web (voids, filaments, and/or groups) along the line of sight.

From a detailed study of a single sightline, I observe a multi-phase IGM, with kinematically-distinct, hot and warm components (T ≈ 10^5.5 K and ≈ 10⁴ K, respectively). By correlating the absorption systems with a complementary galaxy survey of the field around the background quasar, I find that the IGM systems arise in a variety of galactic environments. The metal-lines systems all have L > 0.1L_∗ galaxies within a few hundred kiloparsecs, which suggests this is the distance to which galactic feedback processes typically disperse metals.

I conduct a large, blind survey for triply-ionized carbon (C IV) absorption at z < 1 in the spectra of 49 low-redshift quasars and compare their properties with those detected at z > 1. The mass density in C IV doublets with 13 ≤ log N(C⁺³) ≤ 15 at z < 1 has increased by a factor of 2.8 ± 0.7 over the error-weighted mean of the 1.5 < z < 5 measurements, where the mass density has not evolved significantly. The line density dN_C IV/dX has not evolved as much, indicating that the average column density per doublet increases with decreasing redshift.

In addition, I compare the observed properties of C IV absorbers with those predicted by cosmological hydrodynamic simulations with a variety of physical models (e.g., feedback, cosmology). I also use the results from the simulations that reproduce well the observations to understand better the physical conditions giving rise to the C IV absorbing gas. The observations and simulations indicate that the log N(C⁺³) > 13 C IV absorption systems predominately come from circum-galactic (or halo) gas.

The Last Eight-Billion Years of Intergalactic C IV Evolution

Cooksey, Thom, Prochaska, and Chen, 2010, ApJ, 708, 868 (PDF [published version but our PDF is formatted better].

We surveyed the Hubble Space Telescope UV spectra of 49 low-redshift quasars for z < 1 C IV candidates, relying solely on the characteristic wavelength separation of the doublet. After consideration of the defining traits of C IV doublets (e.g., consistent line profiles, other associated transitions, etc.), we defined a sample of 38 definite (group G = 1) and five likely (G = 2) doublets with rest equivalent widthsW_r for both lines detected at ≥3σ. We conducted Monte Carlo completeness tests to measure the unblocked redshift (Δz) and co-moving pathlength (ΔX) over which we were sensitive to C IV doublets of a range of equivalent widths and column densities. The absorber line density of (G = 1+2) doublets is dN_C IV/dX = 4.1+0.7/−0.6 for log N(C⁺³) ≥ 13.2, and dN_C IV/dX has not evolved significantly since z = 5. The best-fit power law to the frequency distribution of column densities f(N(C⁺³)) = k(N(C⁺³)/N₀)^α has coefficient k = (0.67+0.18/−0.16) × 10⁻¹⁴ cm² and exponent α = −1.50+0.17/−0.19, where N₀ = 10¹⁴ cm⁻². Using the power-law fit to f(N(C⁺³)), we measured the C⁺³ mass density relative to the critical density: Ω(C⁺³) = (6.20+1.82/−1.52) × 10⁻⁸ for 13 ≤ log N(C⁺³) ≤ 15. This value is a 2.8±0.7 increase in Ω(C⁺³) compared to the error-weighted mean from several 1 < z < 5 surveys for C IV absorbers. The linear fit to Ω(C⁺³) over the age of the Universe reveals that Ω(C⁺³) has slowly but steadily increased from z = 5 → 0, with dΩ(C⁺³)/dt_age = (0.42±0.2) × 10⁻⁸ Gyr⁻¹.

The Last Eight-Billion Years of Intergalactic C IV Evolution

Conference presentation (PDF) from The Chemical Enrichment of the Intergalactic Medium, Leiden, The Netherlands, May 25−19, 2009

Metal enrichment from galactic feedback processes. Outflows and mergers return to the IGM enriched material for future generations of galaxies and stars. The signature of this process is etched in the recycled gas: metallicity, abundances, density, distribution, etc. We conducted the largest survey for C IV at z < 1, and we compare our results to those from high-redshift (1.5 < z < 6) studies. For example, we present the frequency distribution ƒ(N) and mass density of C IV absorbers in the HST archival spectra of 49 quasars. We also compare the observed properties of the C IV doublets with synthetic C IV absorbers from cosmological hydrodynamic simulations with different feedback prescriptions.

Metals in the Low-Redshift Universe:
From Galaxies to the Intergalactic Medium

Conference presentation (PDF) from 213th AAS Meeting, Long Beach, California, January 4−8, 2009

The low-redshift IGM probes the last ten billion years of metal enrichment from galactic feedback processes. Outflows and mergers return to the IGM enriched material for future generations of galaxies and stars. The signature of this process is etched in the recycled gas: metallicity, abundances, density, distribution, etc. We have been characterizing two different stages of the cosmic enrichment cycle−the low-density IGM and the extended gaseous halos of galaxies. We present the frequency distribution f(N) of C IV absorbers at z < 1.5, from a survey of HST archival spectra of ≈50 quasars. We compare the observed f(N) with that predicted by cosmological hydrodynamic simulations with different feedback prescriptions. We also discuss early results from a survey of galaxies with halos probed by background quasars. We look for correlations between the galaxies and the halo absorbers that distinguish the feedback process(es) responsible for enriching the extended halos. This work is funded by the HST archival grant 10679 and the NSF CAREER grant AST 05_48180.

Talk Notes

(AKA: Properties of Metal-line Absorption Systems and Their Neighboring Galaxies)

Conference presentation (PDF) from The Cosmic Odyssey of the Elements, Aegina, Greece, June 2008 (research done in part with Genevieve Graves)

Quasar absorption-line spectroscopy provides a unique probe of the extended gaseous halos of galaxies to low column densities. The detection of metal-line absorption systems constrain mechanisms for metal transport from the stars and/or from the intergalactic medium to the extended halos. The metallicity of the halo medium characterizes the metal production and mixing from the ISM and IGM. Here we present observations of strong log N_H I > 16 metal-line systems with galaxies within 200 kpc. Several of the closest galaxies are early-type (presumably gas poor) galaxies. We will show the similarities and differences between metal-line systems closest to early- versus late-type galaxies.

Talk Notes

[Actually, this project was the inspiration for the NSF AAPF project "Seeking the Lost Interstellar Medium of Red-Sequence Galaxies" described here.]

Metal-line System Survey: Characterizing the Low-redshift IGM

Cooksey and Prochaska 2007 (conference proceeding (PDF), submitted to Astronomy, Astrophysics & Space Science) from the first Space Astronomy: The UV Window to the Universe, El Escorial, Spain, May 2007

The low-redshift IGM probes the last ten billion years of metal enrichment from galactic feedback processes. We present preliminary results from a survey of intergalactic metal-line absorption systems in archival HST\STIS, GHRS, and FUSE spectra of ~50 z ≤ 2 UV-bright objects. We summarize the detailed analysis of one sightline (PKS1302−102, z_QSO = 0.2784) with which we set the methodology for the larger survey. We use simple CLOUDY models to constrain the ionizing mechanism(s) and metallicities for the metal-line systems. For about 15 sightlines, including PKS1302−102, we have a complementary galaxy survey, and we look for correlations between galaxies and absorption systems in order to understand the large-scale distribution of the metal-enriched IGM.

Metal-line System Survey: Characterizing the Low-z IGM

Conference presentation (PDF) from the first Space Astronomy: The UV Window to the Universe, El Escorial, Spain, May 2007

The low-redshift, z < 1.5, IGM probes the last ten billion years of metal enrichment from galactic feedback processes. We present preliminary results from a survey of intergalactic metal-line absorption systems in the UV spectra of 50 z < 1.5 UV-bright targets. We search archival HST/STIS and FUSE spectra for C IV, Si IV, and O VI doublets and any associated Lyα absorption. From CLOUDY models, we approximate the metallicity of the IGM and constrain the ionizing mechanism(s) for the various systems. For about 15 sight lines, we have a complementary galaxy survey, and we look for correlations between galaxies and absorption systems in order to understand the large-scale distribution of the metal-enriched IGM. We also compare the observations with synthetic spectra from cosmological hydrodynamic simulations where we know the input feedback physics.

Talk Notes