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Measuring the Average Molecular Gas Content of Star-forming Galaxies at z = 3-4

L. A. Boogaard, R. J. Bouwens, D. A. Riechers, P. van der Werf, R. Bacon, J. Matthee, M. Stefanon, A. Feltre, M. V. Maseda, H. Inami, M. Aravena, J. Brinchmann, C. L. Carilli, T. Contini, R. Decarli, J. González-López, T. Nanayakkara, F. Walter

We study the molecular gas content of 24 star-forming galaxies at z = 3-4, with a median stellar mass of 109.1 M, from the MUSE Hubble Ultra Deep Field (HUDF) Survey. Selected by their Lyα λ1216 emission and HF160W-band magnitude, the galaxies show an average ⟨EW0Lyα⟩≈20 Å, below the typical selection threshold for Lyα emitters ( EW0Lyα>25 Å), and a rest-frame UV spectrum similar to Lyman-break galaxies. We use rest-frame optical spectroscopy from KMOS and MOSFIRE, and the UV features observed with MUSE, to determine the systemic redshifts, which are offset from Lyα by ⟨Δv(Lyα)⟩ = 346 km s-1, with a 100 to 600 km s-1 range. Stacking 12CO J = 4 → 3 and [C I]3P13P0 (and higher-J CO lines) from the ALMA Spectroscopic Survey of the HUDF, we determine 3σ upper limits on the line luminosities of 4.0 × 108 K km s-1pc2 and 5.6 × 108 K km s-1pc2, respectively (for a 300 km s-1 line width). Stacking the 1.2 mm and 3 mm dust-continuum flux densities, we find a 3σ upper limits of 9 μJy and 1.2 μJy, respectively. The inferred gas fractions, under the assumption of a "Galactic" CO-to-H2 conversion factor and gas-to-dust ratio, are in tension with previously determined scaling relations. This implies a substantially higher αCO ≥ 10 and δGDR ≥ 1200, consistent with the subsolar metallicity estimated for these galaxies ( 12+log(O/H)≈7.8±0.2 ). The low metallicity of z ≥ 3 star-forming galaxies may thus make it very challenging to unveil their cold gas through CO or dust emission, warranting further exploration of alternative tracers, such as [C II].

Molecular gas; High-redshift galaxies; Interstellar medium; CO line emission; Dust continuum emission; Spectroscopy; 1073; 734; 847; 262; 412; 1558

The Astrophysical Journal
Volume 916, Number 12, Page 19
2021 July

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia