Astrophys. J., 882, 168-168 (2019/September-2)
Low star formation efficiency in typical galaxies at z = 5-6.
PAVESI R., RIECHERS D.A., FAISST A.L., STACEY G.J. and CAPAK P.L.
Abstract (from CDS):
Using the Very Large Array and ALMA, we have obtained CO(2-1), [C II], and [N II] line emission and multiple dust continuum measurements in a sample of "normal" galaxies at z = 5-6. We report the highest-redshift detection of low-J CO emission from a Lyman break galaxy, at z ∼ 5.7. The CO line luminosity implies a massive molecular gas reservoir of (1.3 ± 0.3)(αCO/4.5 M☉ (K km s–1 pc2)–1) x 1011 M☉, suggesting low star formation efficiency with a gas depletion timescale of order ∼1 Gyr. This efficiency is much lower than traditionally observed in z >= 5 starbursts, indicating that star-forming conditions in main-sequence galaxies at z ∼ 6 may be comparable to those of normal galaxies probed up to z ∼ 3 to date but with rising gas fractions across the entire redshift range. We also obtain a deep CO upper limit for a main-sequence galaxy at z ∼ 5.3 with an approximately three times lower star formation rate, perhaps implying a high αCO conversion factor, as typically found in low-metallicity galaxies. For a sample including both CO targets, we also find faint [N II] 205 µm emission relative to [C II] in all but the most IR-luminous "normal" galaxies at z = 5-6, implying more intense or harder radiation fields in the ionized gas relative to lower redshift. These radiation properties suggest that low metallicity may be common in typical ∼1010 M☉ galaxies at z = 5-6. While a fraction of main-sequence star formation in the first billion yr may take place in conditions not dissimilar to lower redshift, lower metallicity may affect the remainder of the population.
© 2019. The American Astronomical Society. All rights reserved.
cosmology: observations - galaxies: formation - galaxies: high-redshift - galaxies: ISM - galaxies: star formation - radio lines: galaxies
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