The dense gas in the largest molecular complexes of the antennae: HCN and HCO+ observations of NGC 4038/39 using ALMA
File(s)
Published version
Author(s)
Schirm, MRP
Wilson, CD
Madden, SC
Clements, DL
Type
Journal Article
Abstract
We present observations of the dense molecular gas tracers $\mathrm{HCN}$, $\mathrm{HNC}$, and ${\mathrm{HCO}}^{+}$ in the $J=1-0$ transition using the Atacama Large Millimeter/submillimeter Array. We supplement our data sets with previous observations of $\mathrm{CO}$ $J=1-0$, which trace the total molecular gas content. We separate the Antennae into seven bright regions in which we detect emission from all three molecules, including the nuclei of NGC 4038 and NGC 4039, five super giant molecular complexes in the overlap region, and two additional bright clouds. We find that the ratio of ${L}_{\mathrm{HCN}}/{L}_{\mathrm{CO}}$, which traces the dense molecular gas fraction, is greater in the two nuclei (${L}_{\mathrm{HCN}}/{L}_{\mathrm{CO}}$ $\sim \quad 0.07-0.08$) than in the overlap region (${L}_{\mathrm{HCN}}/{L}_{\mathrm{CO}}$ $\lt 0.05$). We attribute this to an increase in pressure due to the stellar potential within the nuclei; a similar effect to what has been seen previously in the Milky Way and nearby spiral galaxies. Furthermore, the ratio of ${L}_{\mathrm{HNC}}/{L}_{\mathrm{HCN}}$ $\sim \quad 0.3-0.4$ does not vary by more than a factor of 1.5 between regions. By comparing our measured ratios to photon dominated region (PDR) models including mechanical heating, we find that the ratio of ${L}_{\mathrm{HNC}}/{L}_{\mathrm{HCN}}$ is consistent with mechanical heating contributing gsim5%–10% of the PDR surface heating to the total heating budget. Finally, the ratio of ${L}_{\mathrm{HCN}}/{L}_{\mathrm{HCO}+}$ varies from ~1 in the nucleus of NGC 4038 down to ~0.5 in the overlap region. The lower ratio in the overlap region may be due to an increase in the cosmic ray rate from the increased supernova rate within this region.
Date Issued
2016-05-25
Date Acceptance
2016-04-01
Citation
Astrophysical Journal, 2016, 823 (2)
ISSN
1538-4357
Publisher
American Astronomical Society
Journal / Book Title
Astrophysical Journal
Volume
823
Issue
2
Copyright Statement
© 2016 The American Astronomical Society. All rights reserved.
Subjects
Science & Technology
Physical Sciences
Astronomy & Astrophysics
galaxies: individual (NGC 4038, NGC 4039)
galaxies: interactions
galaxies: ISM
ISM: molecules
LUMINOUS INFRARED GALAXIES
STAR-FORMATION EFFICIENCY
PHOTON-DOMINATED REGIONS
PROTOTYPICAL MERGER
CO
ORIGIN
MASS
NGC-4038/4039
DIAGNOSTICS
STARBURSTS
0201 Astronomical And Space Sciences
0305 Organic Chemistry
0306 Physical Chemistry (Incl. Structural)
Publication Status
Published
Article Number
87