High-spectral resolution M-band observations of CO Rot-Vib absorption
  lines towards the Galactic center
  Context. In the near- to mid-infrared wavelength domain, bright continuum
sources in the central parsec of the Galactic center (GC) are subject to
foreground absorption. These sources therefore represent ideal probes of the
intervening material that is responsible for the absorption along the line of
sight. Aims. Our aim is to shed light on the location and physics of the
absorbing clouds. We try to find out which of the gaseous absorbing materials
is intimately associated with the GC and which one is associated with clouds at
a much larger distance. Methods. We used the capabilities of CRIRES
spectrograph located at ESO Very Large Telescope in Chile to obtain absorption
spectra of individual lines at a high spectral resolution of R=65000, that is,
5 km/s. We observed the 12CO R(0), P(1), P(2), P(3), P(4), P(5), P(6), P(7) and
P(9) transition lines, applied standard data reduction, and compared the
results with literature data. Results. We present the results of CRIRES
observations of 13 infrared sources located in the central parsec of the
Galaxy. The data provide direct evidence for a complex structure of the
interstellar medium along the line of sight and in the close environment of the
central sources. In particular we find four cold foreground clouds at radial
velocities vLS R of the order of -145,-85, -60, and -40+-15 km/s that show
absorption in the lower transition lines from R(0) to P(2) and in all the
observed spectra.We also find in all sources an absorption in velocity range of
50-60 km/s, possibly associated with the so-called 50 km/s cloud and suggesting
an extension of this cloud in front of the GC. Finally, we detect individual
absorption lines that are probably associated with material much closer to the
center and with the sources themselves, suggesting the presence of cold gas in
the local region.
