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The ALMaQUEST Survey. XII. Dense Molecular Gas as Traced by HCN and HCO+ in Green Valley Galaxies

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Peer-reviewed

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Abstract

jats:titleAbstract</jats:title> jats:pWe present Atacama Large Millimeter/submillimeter Array (ALMA) observations of two dense gas tracers, HCN (1−0) and HCOjats:sup+</jats:sup> (1-0) for three galaxies in the green valley and two galaxies on the star-forming main sequence with comparable molecular gas fractions as traced by the CO (1−0) emissions, selected from the ALMaQUEST survey. We investigate whether the deficit of molecular gas star formation efficiency (SFEjats:submol</jats:sub>) that leads to the low specific star formation rate (sSFR) in these green valley galaxies is due to a lack of dense gas (characterized by the dense gas fraction jats:italicf</jats:italic> jats:subdense</jats:sub>) or the low star formation efficiency of dense gas (SFEjats:subdense</jats:sub>). We find that SFEjats:submol</jats:sub> as traced by the CO emissions, when considering both star-forming and retired spaxels together, is tightly correlated with SFEjats:subdense</jats:sub> and depends only weakly on jats:italicf</jats:italic> jats:subdense</jats:sub>. The sSFR on kiloparsec scales is primarily driven by SFEjats:submol</jats:sub> and SFEjats:subdense</jats:sub>, followed by the dependence on jats:italicf</jats:italic> jats:submol</jats:sub>, and is least correlated with jats:italicf</jats:italic> jats:subdense</jats:sub> or the dense-gas-to-stellar mass ratio (jats:italicR</jats:italic> jats:subdense</jats:sub>). When compared with other works in the literature, we find that our green valley sample shows lower global SFEjats:submol</jats:sub> and lower SFEjats:subdense</jats:sub> while exhibiting similar dense gas fractions when compared to star-forming and starburst galaxies. We conclude that the star formation of the three green valley galaxies with a normal abundance of molecular gas is suppressed, mainly due to the reduced SFEjats:subdense</jats:sub> rather than the lack of dense gas.</jats:p>

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Keywords

5109 Space Sciences, 51 Physical Sciences

Journal Title

Astrophysical Journal

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Journal ISSN

0004-637X
1538-4357

Volume Title

963

Publisher

American Astronomical Society
Sponsorship
National Science and Technology Council (NSTC) (MOST 111-2112-M-001-044 -)
National Science and Technology Council (NSTC) (NSTC 112-2112-M-001-062 -)
Academia Sinica (AS) (Career Development Award CDA-107-M03)