Dr. Timothy Heckman

1 December 2017 in The Astrophysical Journal Supplement Series

Journal Article

xCOLD GASS: The Complete IRAM 30 m Legacy Survey of Molecular Gas for Galaxy Evolution Studies

AS

Amélie Saintonge

BC

Barbara Catinella

LT

Linda J. Tacconi

GK

Guinevere Kauffmann

RG

Reinhard Genzel

GR

Guido W. Roberts-Borsani

+15

We introduce xCOLD GASS, a legacy survey providing a census of molecular gas in the local universe. Building on the original COLD GASS survey, we present here the full sample of 532 galaxies with CO (1–0) measurements from the IRAM 30 m telescope. The sample is mass-selected in the redshift interval from the Sloan Digital Sky Survey (SDSS) and therefore representative of the local galaxy population with . The CO (1–0) flux measurements are complemented by observations of the CO (2–1) line with both the IRAM 30 m and APEX telescopes, H i observations from Arecibo, and photometry from SDSS, WISE, and GALEX. Combining the IRAM and APEX data, we find that the ratio of CO (2–1) to CO (1–0) luminosity for integrated measurements is , with no systematic variations across the sample. The CO (1–0) luminosity function is constructed and best fit with a Schechter function with parameters , , and . With the sample now complete down to stellar masses of 10⁹ , we are able to extend our study of gas scaling relations and confirm that both molecular gas fractions ( ) and depletion timescale ( ) vary with specific star formation rate (or offset from the star formation main sequence) much more strongly than they depend on stellar mass. Comparing the xCOLD GASS results with outputs from hydrodynamic and semianalytic models, we highlight the constraining power of cold gas scaling relations on models of galaxy formation.

21 April 2022 in The Astrophysical Journal Supplement Series

Journal Article

The Low-redshift Lyman Continuum Survey. I. New, Diverse Local Lyman Continuum Emitters

SF

AJ

HF

GW

KM

XX

+41

The origins of Lyman continuum (LyC) photons responsible for the reionization of the universe are as of yet unknown and highly contested. Detecting LyC photons from the Epoch of Reionization is not possible due to absorption by the intergalactic medium, which has prompted the development of several indirect diagnostics to infer the rate at which galaxies contribute LyC photons to reionize the universe by studying lower-redshift analogs. We present the Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with the Hubble Space Telescope Cosmic Origins Spectrograph for a z = 0.2–0.4 sample of 66 galaxies. After careful processing of the far-UV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected with 97.725% confidence, nearly tripling the number of known local LCEs. We estimate escape fractions from the detected LyC flux and upper limits on the undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the 35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling the number of known local LCEs with cosmologically relevant LyC escape.

1 May 2022 in The Astrophysical Journal

Journal Article

The Low-redshift Lyman Continuum Survey. II. New Insights into LyC Diagnostics

SF

AJ

HF

GW

KM

AV

Anne Verhamme

+40

The Lyman continuum (LyC) cannot be observed at the epoch of reionization (z ≳ 6) owing to intergalactic H i absorption. To identify LyC emitters (LCEs) and infer the fraction of escaping LyC, astronomers have developed various indirect diagnostics of LyC escape. Using measurements of the LyC from the Low-redshift Lyman Continuum Survey (LzLCS), we present the first statistical test of these diagnostics. While optical depth indicators based on Lyα, such as peak velocity separation and equivalent width, perform well, we also find that other diagnostics, such as the [O iii]/[O ii] flux ratio and star formation rate surface density, predict whether a galaxy is an LCE. The relationship between these galaxy properties and the fraction of escaping LyC flux suggests that LyC escape depends strongly on H i column density, ionization parameter, and stellar feedback. We find that LCEs occupy a range of stellar masses, metallicities, star formation histories, and ionization parameters, which may indicate episodic and/or different physical causes of LyC escape.

30 June 2017 in The Astrophysical Journal

Journal Article

z ∼ 2: An Epoch of Disk Assembly

RS

Raymond C. Simons

SK

Susan A. Kassin

BW

Benjamin J. Weiner

SF

Sandra M. Faber

JT

Jonathan R. Trump

AV

Alexander de la Vega

+5

We explore the evolution of the internal gas kinematics of star-forming galaxies from the peak of cosmic star formation at

z \sim 2

to today. Measurements of galaxy rotation velocity V _rot, which quantify ordered motions, and gas velocity dispersion

σ_{g}

, which quantify disordered motions, are adopted from the DEEP2 and SIGMA surveys. This sample covers a continuous baseline in redshift over

0.1 < z < 2.5

, spanning 10 Gyr. At low redshift, nearly all sufficiently massive star-forming galaxies are rotationally supported (

V_{rot} > σ_{g}

). By z = 2, 50% and 70% of galaxies are rotationally supported at low (

10^{9} - 10^{10} M_{⊙}

) and high (

10^{10} - 10^{11} M_{⊙}

) stellar mass, respectively. For

V_{rot} > 3 σ_{g}

, the percentage drops below 35% for all masses. From z = 2 to now, galaxies exhibit remarkably smooth kinematic evolution on average. All galaxies tend toward rotational support with time, and higher-mass systems reach it earlier. This is largely due to a mass-independent decline in

σ_{g}

by a factor of 3 since z = 2. Over the same time period, V _rot increases by a factor of 1.5 in low-mass systems but does not evolve at high mass. These trends in V _rot and

σ_{g}

are at a fixed stellar mass and therefore should not be interpreted as evolutionary tracks for galaxy populations. When populations are linked in time via abundance matching,

σ_{g}

declines as before and V _rot strongly increases with time for all galaxy populations, enhancing the evolution in

V_{rot} / σ_{g}

. These results indicate that

z = 2

is a period of disk assembly, during which strong rotational support is only just beginning to emerge.

1 April 2005 in The Astronomical Journal

Journal Article

Active Galactic Nuclei in the Sloan Digital Sky Survey. II. Emission-Line Luminosity Function

LH

Lei Hao

MS

Michael A. Strauss

XF

Xiaohui Fan

CT

Christy A. Tremonti

DS

David J. Schlegel

GS

Gyula P. Szokoly

+14

The emission line luminosity function of active galactic nuclei (AGN) is measured from about 3000 AGN included in the main galaxy sample of the Sloan Digital Sky Survey within a redshift range of $0

27 July 2022 in The Astrophysical Journal Supplement Series

Journal Article

The COS Legacy Archive Spectroscopy Survey (CLASSY) Treasury Atlas*

DB

BJ

TK

MM

ZC

XX

+42

Far-ultraviolet (FUV; ∼1200–2000 Å) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of the James Webb Space Telescope will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before; however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of ample wavelength coverage, signal-to-noise, spectral-resolution, and sample diversity that has not yet been achieved by any FUV spectral database. We present the Cosmic Origins Spectrograph Legacy Spectroscopic Survey (CLASSY) treasury and its first high-level science product, the CLASSY atlas. CLASSY builds on the Hubble Space Telescope (HST) archive to construct the first high-quality (S/N_{1500 Å} ≳ 5/resel), high-resolution (R ∼ 15,000) FUV spectral database of 45 nearby (0.002 < z < 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY sample covers a broad range of properties including stellar mass (6.2 < log M _⋆(M _⊙) < 10.1), star formation rate (−2.0 < log SFR (M _⊙ yr⁻¹) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O₃₂ < 38.0), reddening (0.02 < E(B − V) < 0.67), and nebular density (10 < n _e (cm⁻³) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with the z ∼ 0 mass–metallicity relationship, but is offset to higher star formation rates by roughly 2 dex, similar to z ≳ 2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.

Dr. Timothy Heckman

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