The shape of the velocity ellipsoid in NGC 488

Theories of stellar orbit diffusion in disk galaxies predict different rates of increase of the velocity dispersions parallel and perpendicular to the disk plane, and it is therefore of interest to measure the different velocity dispersion components in galactic disks of different types. We show that it is possible to extract the three components of the velocity ellipsoid in an intermediate-inclination disk galaxy from measured line-of-sight velocity dispersions on the major and minor axes. On applying the method to observations of the Sb galaxy NGC 488, we find evidence for a higher ratio of vertical to radial dispersion in NGC 488 than in the solar neighbourhood of the Milky Way (the only other place where this quantity has ever been measured). The difference is qualitatively consistent with the notion that spiral structure has been relatively less important in the dynamical evolution of the disk of NGC 488 than molecular clouds.Comment: 5 pages LaTex, including 2 figures, mn.sty, submitted to MNRA

Disc heating in NGC 2985

Various processes have been proposed to explain how galaxy discs acquire their thickness. A simple diagnostic for ascertaining this ``heating'' mechanism is provided by the ratio of the vertical to radial velocity dispersion components. In a previous paper we have developed a technique for measuring this ratio, and demonstrated its viability on the Sb system NGC 488. Here we present follow-up observations of the morphologically similar Sab galaxy NGC 2985, still only the second galaxy for which this ratio has been determined outside of the solar neighbourhood. The result is consistent with simple disc heating models which predict ratios of $\sigma_z / \sigma_R$ less than oneComment: 5 pages, 4 figures. Accepted for publication in MNRA

Kinematic detection of the double nucleus in M31

Using a spectrum obtained under moderate (of order 1 arcsecond) seeing, we show that the double nucleus in M31 produces a strong kinematic signature even though the individual components are not spatially resolved. The signature consists of a significant asymmetric wing in the stellar velocity distribution close to the center of the system. The properties of the second nucleus derived from this analysis agree closely with those measured from high-spatial resolution Hubble Space Telescope images. Even Space Telescope only has sufficient resolution to study the structure of very nearby galactic nuclei photometrically; this spectroscopic approach offers a tool for detecting structure such as multiple nuclei in a wider sample of galaxy cores.Comment: 4 pages of uuencoded compressed postscript, figures included. Accepted for publication in MNRA

p3d: a general data-reduction tool for fiber-fed integral-field spectrographs

The reduction of integral-field spectrograph (IFS) data is demanding work. Many repetitive operations are required in order to convert raw data into, typically a large number of, spectra. This effort can be markedly simplified through the use of a tool or pipeline, which is designed to complete many of the repetitive operations without human interaction. Here we present our semi-automatic data-reduction tool p3d that is designed to be used with fiber-fed IFSs. Important components of p3d include a novel algorithm for automatic finding and tracing of spectra on the detector, and two methods of optimal spectrum extraction in addition to standard aperture extraction. p3d also provides tools to combine several images, perform wavelength calibration and flat field data. p3d is at the moment configured for four IFSs. In order to evaluate its performance we have tested the different components of the tool. For these tests we used both simulated and observational data. We demonstrate that for three of the IFSs a correction for so-called cross-talk due to overlapping spectra on the detector is required. Without such a correction spectra will be inaccurate, in particular if there is a significant intensity gradient across the object. Our tests showed that p3d is able to produce accurate results. p3d is a highly general and freely available tool. It is easily extended to include improved algorithms, new visualization tools and support for additional instruments. The program code can be downloaded from the p3d-project web site http://p3d.sourceforge.netComment: 18 pages, 15 figures, 3 tables, accepted for publication in A&

Disk heating agents across the Hubble sequence

We measure the shape of the velocity ellipsoid in two late-type spiral galaxies (Hubble types Sc and Scd) and combine these results with our previous analyses of six early-type spirals (Sa to Sbc) to probe the relation between galaxy morphology and the ratio of the vertical and radial dispersions. We confirm at much higher significance (99.9 percent) our prior detection of a tight correlation between these quantities. We explore the trends of the magnitude and shape of the velocity ellipsoid axes with galaxy properties (colour, gas surface mass density, and spiral arm structure). The observed relationships allow for an observational identification of the radial and vertical disk heating agents in external disk galaxies.Comment: 11 pages, 8 figures, 4 tables. Accepted for publication in MNRA

Observational Constraints on Disk Heating as a Function of Hubble Type

Current understanding of the secular evolution of galactic disks suggests that this process is dominated by two or more heating mechanisms, which increase the random motions of stars in the disk. In particular, the gravitational influence of giant molecular clouds and irregularities in the spiral potential have been proposed to explain the observed velocity dispersions in the solar neighborhood. Each of these mechanisms acts on different components of the stellar velocities, which affects the ratio of the vertical and radial components of the stellar velocity dispersion since the relative strengths of giant molecular clouds and spiral irregularities vary with Hubble type. A study of this ratio as function of Hubble type has the potential to provide strong constraints on disk heating mechanisms. We present major and minor axis stellar kinematics for four spiral galaxies of Hubble type from Sa to Sbc, and use the data to infer the ratios sigma_z/sigma_R in the galaxy disks. The results combined with two galaxies studied previously and with Milky Way data show that the ratio is generally in the range 0.5 - 0.8. There is a marginally significant trend of decreasing ratio with advancing Hubble type, consistent with the predictions of disk heating theories. However, the errors on individual measurements are large, and the absence of any trend is consistent with the data at the 1-sigma level. As a byproduct of our study, we find that three of the four galaxies in our sample have a central drop in their stellar line-of-sight velocity dispersion, a phenomenon that is increasingly observed in spiral galaxies. [ABRIDGED]Comment: 24 pages, LaTeX, 5 Postscript figures, to appear in AJ (Dec 2003

Stellar Kinematics of the Double Nucleus of M31

We report observations of the double nucleus of M31 with the f/48 long-slit spectrograph of the HST Faint Object Camera. We obtain a total exposure of 19,000 sec. over 7 orbits, with the 0.063-arcsec-wide slit along the line between the two brightness peaks (PA 42). A spectrum of Jupiter is used as a spectral template. The rotation curve is resolved, and reaches a maximum amplitude of ~250 km/s roughly 0.3 arcsec either side of a rotation center lying between P1 and P2, 0.16 +/- 0.05 arcsec from the optically fainter P2. We find the velocity dispersion to be < 250 km/s everywhere except for a narrow ``dispersion spike'', centered 0.06 +/- 0.03 arcsec on the anti-P1 side of P2, in which sigma peaks at 440 +/- 70 km/s. At much lower confidence, we see local disturbances to the rotation curve at P1 and P2, and an elevation in sigma at P1. At very low significance we detect a weak asymmetry in the line-of-sight velocity distribution opposite to the sense usually encountered. Convolving our V and sigma profiles to CFHT resolution, we find good agreement with the results of Kormendy & Bender (1998, preprint), though there is a 20% discrepancy in the dispersion that cannot be attributed to the dispersion spike. Our results are not consistent with the location of the maximum dispersion as found by Bacon et al. We find that the sinking star cluster model of Emsellem & Combes (1997) does not reproduce either the rotation curve or the dispersion profile. The eccentric disk model of Tremaine (1995) fares better, and can be improved somewhat by adjusting the original parameters. However, detailed modeling will require dynamical models of significantly greater realism.Comment: 29 pages, Latex, AASTeX v4.0, with 7 eps figures. To appear in The Astronomical Journal, February 199