Structure and kinematics of edge-on galaxy discs - V. The dynamics of stellar discs

Abstract

In earlier papers in this series we determined the intrinsic stellar disc kinematics of 15 intermediate‐ to late‐type edge‐on spiral galaxies using a dynamical modelling technique. The sample covers a substantial range in maximum rotation velocity and deprojected face‐on surface brightness, and contains seven spirals with either a boxy or peanut‐shaped bulge. Here we discuss the structural, kinematical and dynamical properties. From the photometry we find that intrinsically more flattened discs tend to have a lower face‐on central surface brightness and a larger dynamical mass‐to‐light ratio. This observation suggests that, at a constant maximum rotational velocity, lower surface brightness discs have smaller vertical stellar velocity dispersions. Although the individual uncertainties are large, we find from the dynamical modelling that at least 12 discs are submaximal. The average disc contributes 53 ± 4 per cent to the observed rotation at 2.2 disc scalelengths (hR), with a 1σ scatter of 15 per cent. This percentage becomes somewhat lower when effects of finite disc flattening and gravity by the dark halo and the gas are taken into account. Since boxy and peanut‐shaped bulges are probably associated with bars, the result suggests that at 2.2 hR the submaximal nature of discs is independent of barredness. The possibility remains that very high surface brightness discs are maximal, as these discs are underrepresented in our sample. We confirm that the radial stellar disc velocity dispersion is related to the galaxy maximum rotational velocity. The scatter in this σ versus vmax relation appears to correlate with the disc flattening, face‐on central surface brightness and dynamical mass‐to‐light ratio. Low surface brightness discs tend to be more flattened and have smaller stellar velocity dispersions. The findings are consistent with the observed correlation between disc flattening and dynamical mass‐to‐light ratio and can generally be reproduced by the simple collapse theory for disc galaxy formation. Finally, the disc mass Tully–Fisher relation is offset from the maximum‐disc scaled stellar mass Tully–Fisher relation of the Ursa Major cluster. This offset, −0.3 dex in mass, is naturally explained if the discs of the Ursa Major cluster spirals are submaximal.

The DiskMass Survey

“…On average, after rejecting UGC 1862 which has no measurement of V c at 2.2h R , F 2.2h R b =0.57±0.07 (std). This result is consistent with what was found by Bottema (1993) and Kregel et al (2005), and based on the correlations with M K and μ i 0,K seen in Fig. 14, we conclude that small differences in the mean may be due to differences in the galaxy samples. However, also note that both Bottema (1993) and Kregel et al (2005) measured the disk mass fraction, excluding the contribution from the stellar bulge (see discussion in Sect.…”

“…Bottema (1993) found that disks contribute only 63 ± 10% to the observed rotation speed. Kregel et al (2005) found an even smaller average disk contribution of 58 ± 5%, with a 1σ scatter of 18% when including two outliers. Excluding the outliers, they find an average disk contribution of 53 ± 4%, with a 1σ scatter of 15%.…”

“…In the expected range of density distributions discussed by van der Kruit (1988), our adopted value of k will effectively maximize the measurement of Σ dyn and Υ dyn . The strategy of measuring the stellar velocity dispersion to obtain the dynamical mass surface density has been attempted before (van der Kruit & Freeman 1984, 1986Bottema 1993;Kregel et al 2005). These studies showed that the ratio of the maximum amplitude of the disk's rotation curve (calculated from the observed velocity dispersion) and the maximum of the observed rotation speed (V max ) was much lower than expected for a maximum disk.…”

“…However, observations have so far not lead to a consensus, and the maximality may depend on the galaxy type (Bottema 1993;Weiner et al 2001;Kranz et al 2003;Kregel et al 2005;Byrd et al 2006;Herrmann & Ciardullo 2009;Dutton et al 2011Dutton et al , 2013Barnabè et al 2012).…”

Comparing Dynamical and Stellar Population Mass-to-Light Ratio Estimates

“…Thus, to determine a disk mass from a measured vertical velocity dispersion in a face-on system we have to make assumptions about the (unobservable) vertical stellar distribution, while for edge-on systems, where we can measure the vertical stellar distribution, we have to relate the observed radial and tangential velocity dispersion to the (unobservable) vertical velocity dispersion (Bottema 1997). Recently, Kregel et al (2005) used velocity dispersions of a sample of 15 edge-on galaxies to determine a dynamical mass Tully-Fisher relation and compared it to the stellar population mass Tully-Fisher relation of Bell & de Jong (2001). They find an offset of about -0.24 dex, assuming a vertical-to-radial velocity dispersion ratio (σ z /σ R ) of 0.6.…”