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, the s 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, with spectral coverage from 0.85 to 2.5 mu m, to obtain sigma measurements from the 2.29 mu m CO band heads (sigma(CO)) and the 0.85 mu m 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 per cent. 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.