Diffuse Atomic and Molecular Gas in the Interstellar Medium of M82
  toward SN 2014J
  We present a comprehensive analysis of interstellar absorption lines seen in
moderately-high resolution, high signal-to-noise ratio optical spectra of SN
2014J in M82. Our observations were acquired over the course of six nights,
covering the period from ~6 days before to ~30 days after the supernova reached
its maximum B-band brightness. We examine complex absorption from Na I, Ca II,
K I, Ca I, CH+, CH, and CN, arising primarily from diffuse gas in the
interstellar medium (ISM) of M82. We detect Li I absorption over a range in
velocity consistent with that exhibited by the strongest Na I and K I
components associated with M82; this is the first detection of interstellar Li
in a galaxy outside of the Local Group. There are no significant temporal
variations in the absorption-line profiles over the 37 days sampled by our
observations. The relative abundances of the various interstellar species
detected reveal that the ISM of M82 probed by SN 2014J consists of a mixture of
diffuse atomic and molecular clouds characterized by a wide range of
physical/environmental conditions. Decreasing N(Na I)/N(Ca II) ratios and
increasing N(Ca I)/N(K I) ratios with increasing velocity are indicative of
reduced depletion in the higher-velocity material. Significant
component-to-component scatter in the N(Na I)/N(Ca II) and N(Ca I)/N(Ca II)
ratios may be due to variations in the local ionization conditions. An apparent
anti-correlation between the N(CH+)/N(CH) and N(Ca I)/N(Ca II) ratios can be
understood in terms of an opposite dependence on gas density and radiation
field strength, while the overall high CH+ abundance may be indicative of
enhanced turbulence in the ISM of M82. The Li abundance also seems to be
enhanced in M82, which supports the conclusions of recent gamma-ray emission
studies that the cosmic-ray acceleration processes are greatly enhanced in this
starburst galaxy.
