Differences between CO- and calcium triplet-derived velocity dispersions
  in spiral galaxies: evidence for central star formation?
  We examine the stellar velocity dispersions (sigma) of a sample of 48
galaxies, 35 of which are spirals, from the Palomar nearby galaxy survey. It is
known that for ultra-luminous infrared galaxies (ULIRGs) and merger remnants
thesigma derived from the near-infrared CO band-heads is smaller than that
measured from optical lines, while no discrepancy between these measurements is
found for early-type galaxies. No such studies are available for spiral
galaxies - the subject of this paper. We used cross-dispersed spectroscopic
data obtained with the Gemini Near-Infrared Spectrograph (GNIRS), with spectral
coverage from 0.85 to 2.5um, to obtain sigma measurements from the 2.29 $\mu$m
CO band-heads (sigma_{CO}), and the 0.85 um calcium triplet (sigma_{CaT}). For
the spiral galaxies in the sample, we found that sigma_{CO} is smaller than
sigma_{CaT}, with a mean fractional difference of 14.3%. The best fit to the
data is given by sigma_{opt} = (46.0+/-18.1) + (0.85+/-0.12)sigma_{CO}. This
"sigma discrepancy" may be related to the presence of warm dust, as suggested
by a slight correlation between the discrepancy and the infrared luminosity.
This is consistent with studies that have found no sigma-discrepancy in
dust-poor early-type galaxies, and a much larger discrepancy in dusty merger
remnants and ULIRGs. That sigma_{CO}$ is lower than sigma_{opt} may also
indicate the presence of a dynamically cold stellar population component. This
would agree with the spatial correspondence between low sigma_{CO} and
young/intermediate-age stellar populations that has been observed in
spatially-resolved spectroscopy of a handful of galaxies.
