Abstract

We present high resolution HIFI spectroscopy of the nucleus of the archetypical starburst galaxy M82. Six 12CO lines, 2 13CO lines and 4 fine-structure lines are detected. Besides showing the effects of the overall velocity structure of the nuclear region, the line profiles also indicate the presence of multiple components with different optical depths, temperatures and densities in the observing beam. The data have been interpreted using a grid of PDR models. It is found that the majority of the molecular gas is in low density (n=10^3.5 cm^-3) clouds, with column densities of N_H=10^21.5 cm^-2 and a relatively low UV radiation field (GO = 10^2). The remaining gas is predominantly found in clouds with higher densities (n=10^5 cm^-3) and radiation fields (GO = 10^2.75), but somewhat lower column densities (N_H=10^21.2 cm^-2). The highest J CO lines are dominated by a small (1% relative surface filling) component, with an even higher density (n=10^6 cm^-3) and UV field (GO = 10^3.25). These results show the strength of multi-component modeling for the interpretation of the integrated properties of galaxies.

Highlights

  • The CO emission in M 82 has two main velocity components (the southwest (SW, vhel ∼ 160 km s−1) and northeast (NE, vhel ∼ 300 km s−1) lobes; see e.g., Fig. 2 and Wild et al 1992), which can be seen in our data

  • The CO and fine-structure lines provide an excellent tool for determining the physical parameters of the nuclear interstellar medium (ISM) of M 82

  • While the 160 and 300 km s−1 features reflect the large-scale kinematics of M 82, the 100 km s−1 feature shows an excitation different from the other components, which is only revealed by the velocity-resolved HIFI data

Read more

Summary

Introduction

M 82 is one of the best studied starburst galaxies in the local universe. Its short distance (3.9 Mpc, Sakai & Madore 1999) makes it a superb candidate for detailed studies of the physical processes related to star formation and their effects on the galaxy. M 82 has been studied in many atomic and molecular species These observations show a complex environment where multiple components with different excitation, temperatures, densities, and filling factors coexists (e.g., Wild et al 1992; Lord et al 1996; Mao et al 2000; Weiß et al 2001; Ward et al 2003; Spaans & Meijerink 2007; Fuente et al 2008). Due to the large spectral coverage available with Herschel, we can observe a large number of lines, enabling comprehensive study of the excitation of the different ISM components. We combine these observations with detailed modelling to derive the physical conditions and excitation mechanisms of the nuclear ISM of M 82.

Data reduction
Spectra
Analysis and discussion
CO excitation
Findings
Fine-structure lines
Summary and outlook

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.