Bibcode
Gowardhan, A.; Riechers, D. A.; Daddi, E.; Pavesi, R.; Dannerbauer, H.; Carilli, C.
Bibliographical reference
The Astrophysical Journal, Volume 838, Issue 2, article id. 136, 15 pp. (2017).
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2017
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Description
We report observations of dense molecular gas in the star-forming galaxy
EGS 13004291 (z = 1.197) using the Plateau de Bure Interferometer. We
tentatively detect HCN and HNC J=2\to 1 emission when stacked together
at 4σ significance, yielding line luminosities of
{L}{HCN(J=2\to 1)}\prime =(9+/- 3)×
{10}9 K km s‑1 pc2 and
{L}{HNC(J=2\to 1)}\prime =(5+/- 2)×
{10}9 K km s‑1 pc2, respectively.
We also set 3σ upper limits of <7–8 ×109
K km s‑1 pc2 on the
{{HCO}}+(J=2\to 1),
{{{H}}}2{{O}}({3}13\to {2}20), and
HC3N(J = 20 → 19) line luminosities. We serendipitously
detect CO emission from two sources at z∼ 1.8 and z∼ 3.2 in the
same field of view. We also detect CO(J=2\to 1) emission in EGS
13004291, showing that the excitation in the previously detected
CO(J=3\to 2) line is subthermal ({r}32=0.65+/- 0.15). We find
a line luminosity ratio of {L}{HCN}\prime
/{L}{CO}\prime = 0.17 ± 0.07, as an
indicator of the dense gas fraction. This is consistent with the median
ratio observed in z> 1 galaxies ({L}{HCN}\prime
/{L}{CO}\prime = 0.16 ± 0.07) and
nearby ULIRGs ({L}{HCN}\prime
/{L}{CO}\prime = 0.13 ± 0.03), but
higher than that in local spirals ({L}{HCN}\prime
/{L}{CO}\prime = 0.04 ± 0.02).
Although EGS 13004291 lies significantly above the galaxy main sequence
at z∼ 1, we do not find an elevated star formation efficiency
(traced by {L}{FIR}/{L}{CO}\prime ) as
in local starbursts, but a value consistent with main-sequence galaxies.
The enhanced dense gas fraction, the subthermal gas excitation, and the
lower than expected star formation efficiency of the dense molecular gas
in EGS 13004291 suggest that different star formation properties may
prevail in high-z starbursts. Thus, using
{L}{FIR}/{L}{CO}\prime as a simple
recipe to measure the star formation efficiency may be insufficient to
describe the underlying mechanisms in dense star-forming environments
inside the large gas reservoirs.
Based on observations carried out under project ID U030 with the IRAM
NOEMA Interferometer. IRAM is supported by INSU/CNRS (France), MPG
(Germany), and IGN (Spain).
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