Differences between CO- and calcium triplet-derived velocity dispersions in spiral galaxies: evidence for central star formation?

et al.
We examine the stellar velocity dispersions (σ) 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 σ 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 μm, to obtain σ measurements from the 2.29 μm CO band heads (σCO) and the 0.85 μm calcium triplet (σCaT). For the spiral galaxies in the sample, we found that σCO is smaller than σCaT, with a mean fractional difference of 14.3 per cent. The best fit to the data is given by σopt = (46.0 ± 18.1) + (0.85 ± 0.12)σCO. This ‘σ-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 σ-discrepancy in dust-poor early-type galaxies, and a much larger discrepancy in dusty merger remnants and ULIRGs. That σCO is lower than σopt may also indicate the presence of a dynamically cold stellar population component. This would agree with the spatial correspondence between low-σCO and young/intermediate-age stellar populations that has been observed in spatially resolved spectroscopy of a handful of galaxies.
RAR acknowledges support from FAPERGS (project no. 12/1209-6) and CNPq (project no. 470090/2013-8). LCH acknowledges support from the Kavli Foundation, Peking University, and grant no. XDB09030102 (Emergence of Cosmological Structures) from the Strategic Priority Research Program of the Chinese Academy of Sciences. ARA acknowledges CNPq for partial support to this work through grant 307403/2012-2. LM thanks CNPq through grant 305291/2012-2. LC acknowledges support from the Special Visiting Researcher Fellowship (PVE 313945/2013-6) under the Brazilian Scientific Mobility Program ‘Ciencias sem Fronteiras’. RR acknowledges funding from FAPERGs (ARD 11/1758-5) and CNPq (PeP 304796/2011-5). CRA is supported by a Marie Curie Intra European Fellowship within the 7th European Community Framework Programme (PIEF-GA-2012-327934) and by the Spanish Ministry of Science and Innovation (MICINN) through project PN AYA2010-21887-C04.04.