ALMA CO Observations of the Host Galaxies of Long-duration Gamma-ray Bursts. I: Molecular Gas Scaling Relations. (arXiv:2002.09121v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hatsukade_B/0/1/0/all/0/1">Bunyo Hatsukade</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ohta_K/0/1/0/all/0/1">Kouji Ohta</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hashimoto_T/0/1/0/all/0/1">Tetsuya Hashimoto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kohno_K/0/1/0/all/0/1">Kotaro Kohno</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nakanishi_K/0/1/0/all/0/1">Kouichiro Nakanishi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Niino_Y/0/1/0/all/0/1">Yuu Niino</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tamura_Y/0/1/0/all/0/1">Yoichi Tamura</a>
We present the results of CO observations toward 14 host galaxies of
long-duration gamma-ray bursts (GRBs) at z = 0.1-2.5 by using the Atacama Large
Millimeter/submillimeter Array. We successfully detected CO(3-2) or CO(4-3)
emission in eight hosts (z = 0.3-2), which more than doubles the sample size of
GRB hosts with CO detection. The derived molecular gas mass is $M_{rm gas} =
(0.2-6) times 10^{10}$ $M_{odot}$ assuming metallicity-dependent CO-to-H$_2$
conversion factors. By using the largest sample of GRB hosts with molecular gas
estimates (25 in total, of which 14 are CO-detected) including results from the
literature, we compared molecular gas properties with those of other
star-forming galaxies (SFGs). The GRB hosts tend to have a higher molecular gas
mass fraction ($mu_{rm gas}$) and a shorter gas depletion timescale ($t_{rm
depl}$) as compared with other SFGs at similar redshifts especially at $z
lesssim 1$. This could be a common property of GRB hosts or an effect
introduced by the selection of targets which are typically above the
main-sequence line. To eliminate the effect of selection bias, we analyzed
$mu_{rm gas}$ and $t_{rm depl}$ as a function of the distance from the
main-sequence line ($delta$MS). We find that the GRB hosts follow the same
scaling relations as other SFGs, where $mu_{rm gas}$ increases and $t_{rm
depl}$ decreases with increasing $delta {rm MS}$. No molecular gas deficit is
observed when compared to other SFGs of similar SFR and stellar mass. These
findings suggest that the same star-formation mechanism is expected to be
happening in GRB hosts as in other SFGs.
We present the results of CO observations toward 14 host galaxies of
long-duration gamma-ray bursts (GRBs) at z = 0.1-2.5 by using the Atacama Large
Millimeter/submillimeter Array. We successfully detected CO(3-2) or CO(4-3)
emission in eight hosts (z = 0.3-2), which more than doubles the sample size of
GRB hosts with CO detection. The derived molecular gas mass is $M_{rm gas} =
(0.2-6) times 10^{10}$ $M_{odot}$ assuming metallicity-dependent CO-to-H$_2$
conversion factors. By using the largest sample of GRB hosts with molecular gas
estimates (25 in total, of which 14 are CO-detected) including results from the
literature, we compared molecular gas properties with those of other
star-forming galaxies (SFGs). The GRB hosts tend to have a higher molecular gas
mass fraction ($mu_{rm gas}$) and a shorter gas depletion timescale ($t_{rm
depl}$) as compared with other SFGs at similar redshifts especially at $z
lesssim 1$. This could be a common property of GRB hosts or an effect
introduced by the selection of targets which are typically above the
main-sequence line. To eliminate the effect of selection bias, we analyzed
$mu_{rm gas}$ and $t_{rm depl}$ as a function of the distance from the
main-sequence line ($delta$MS). We find that the GRB hosts follow the same
scaling relations as other SFGs, where $mu_{rm gas}$ increases and $t_{rm
depl}$ decreases with increasing $delta {rm MS}$. No molecular gas deficit is
observed when compared to other SFGs of similar SFR and stellar mass. These
findings suggest that the same star-formation mechanism is expected to be
happening in GRB hosts as in other SFGs.
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