Stars were born in significantly denser regions in the early universe

Journal Article · Sun Jun 01 04:00:00 EDT 2014 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/787/2/120· OSTI ID:22356793

Shirazi, M.; Brinchmann, J.

The density of the warm ionized gas in high-redshift galaxies is known to be higher than what is typical in local galaxies on similar scales. At the same time, the mean global properties of the high- and low-redshift galaxies are quite different. Here, we present a detailed differential analysis of the ionization parameters of 14 star-forming galaxies at redshift 2.6-3.4, compiled from the literature. For each of those high-redshift galaxies, we construct a comparison sample of low-redshift galaxies closely matched in specific star formation rate (sSFR) and stellar mass, thus ensuring that their global physical conditions are similar to the high-redshift galaxy. We find that the median log [O III] 5007/[O II] 3727 line ratio of the high-redshift galaxies is 0.5 dex higher than their local counterparts. We construct a new calibration between the [O III] 5007/[O II] 3727 emission line ratio and ionization parameter to estimate the difference between the ionization parameters in the high- and low-redshift samples. Using this, we show that the typical density of the warm ionized gas in star-forming regions decreases by a median factor of 7.1{sub −5.4}{sup +10.2} from z ∼ 3.3 to z ∼ 0 at fixed mass and sSFR. We show that metallicity differences cannot explain the observed density differences. Because the high- and low-redshift samples are comparable in size, we infer that the relationship between star formation rate density and gas density must have been significantly less efficient at z ∼ 2-3 than what is observed in nearby galaxies with similar levels of star formation activity.

• OSTI ID: 22356793
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 787; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CALIBRATION
DENSITY
EMISSION
EVOLUTION
GALAXIES
MASS
METALLICITY
MILKY WAY
RED SHIFT
STARS
UNIVERSE