ALMA Multiple-transition Molecular Line Observations of the Ultraluminous Infrared Galaxy IRAS 20551-4250: Different HCN, HCO+, and HNC Excitation, and Implications for Infrared Radiative Pumping

作者：Imanishi Masatoshi; Nakanishi Kouichiro; Izumi Takuma

来源：Astrophysical Journal, 2017, 849(1): 29.

DOI：10.3847/1538-4357/aa7ff9

<jats:title>Abstract</jats:title>
<jats:p>We present our ALMA multi-transition molecular line observational results for the ultraluminous infrared galaxy IRAS 20551−4250, which is known to contain a luminous buried active galactic nucleus and shows detectable vibrationally excited (<jats:italic>v</jats:italic>
<jats:sub>2</jats:sub> = 1<jats:italic>f</jats:italic>) HCN and HNC emission lines. The rotational <jats:italic>J</jats:italic> = 1–0, 4–3, and 8–7 of HCN, <jats:inline-formula>
<jats:tex-math>
<?CDATA ${\mathrm{HCO}}^{+}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaa7ff9ieqn1.gif" xlink:type="simple" />
</jats:inline-formula>, and HNC emission lines were clearly detected at a vibrational ground level (<jats:italic>v</jats:italic> = 0). Vibrationally excited (<jats:italic>v</jats:italic>
<jats:sub>2</jats:sub> = 1<jats:italic>f</jats:italic>) <jats:italic>J</jats:italic> = 4–3 emission lines were detected for HCN and HNC, but not for <jats:inline-formula>
<jats:tex-math>
<?CDATA ${\mathrm{HCO}}^{+}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaa7ff9ieqn2.gif" xlink:type="simple" />
</jats:inline-formula>. Their observed flux ratios further support our previously obtained suggestion, based on <jats:italic>J</jats:italic> = 3–2 data, that (1) infrared radiative pumping plays a role in rotational excitation at <jats:italic>v</jats:italic> = 0, at least for HCN and HNC, and (2) HCN abundance is higher than <jats:inline-formula>
<jats:tex-math>
<?CDATA ${\mathrm{HCO}}^{+}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaa7ff9ieqn3.gif" xlink:type="simple" />
</jats:inline-formula> and HNC. The flux measurements of the isotopologue H<jats:sup>13</jats:sup>CN, <jats:inline-formula>
<jats:tex-math>
<?CDATA ${{\rm{H}}}^{13}{\mathrm{CO}}^{+}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaa7ff9ieqn4.gif" xlink:type="simple" />
</jats:inline-formula>, and HN<jats:sup>13</jats:sup>C <jats:italic>J</jats:italic> = 3–2 emission lines support the higher HCN abundance scenario. Based on modeling with collisional excitation, we constrain the physical properties of these line-emitting molecular gases, but find that higher HNC rotational excitation than HCN and <jats:inline-formula>
<jats:tex-math>
<?CDATA ${\mathrm{HCO}}^{+}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaa7ff9ieqn5.gif" xlink:type="simple" />
</jats:inline-formula> is difficult to explain, due to the higher effective critical density of HNC. We consider the effects of infrared radiative pumping using the available 5–30 <jats:italic>μ</jats:italic>m infrared spectrum and find that our observational results are well-explained if the radiation source is located at 30–100 pc from the molecular gas. The simultaneously covered very bright CO <jats:italic>J</jats:italic> = 3–2 emission line displays a broad emission wing, which we interpret as being due to molecular outflow activity with the estimated rate of <jats:inline-formula>
<jats:tex-math>
<?CDATA $\sim 150\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$?>
</jats:tex-math>
<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaa7ff9ieqn6.gif" xlink:type="simple" />
</jats:inline-formula>.</jats:p>

• 出版日期2017-11-1