A Dimensional study of Disk Galaxies

We present a highly simplified model of the dynamical structure of a disk galaxy where only two parameters fully determine the solution, mass and angular momentum. We show through simple physical scalings that once the mass has been fixed, the angular momentum parameter $\lambda$ is expected to regulate such critical galactic disk properties as colour, thickness of the disk and disk to bulge ratio. It is hence expected to be the determinant physical ingredient resulting in a given Hubble type. A simple analytic estimate of $\lambda$ for an observed system is provided. An explicit comparison of the distribution of several galactic parameters against both Hubble type and $\lambda$ is performed using observed galaxies. Both such distributions exhibit highly similar characteristics for all galactic properties studied, suggesting $\lambda$ as a physically motivated classification parameter for disk galaxies.Comment: 10 pages including 11 figures, Final version, MNRAS in pres

On the galactic spin of barred disk galaxies

We present a study of the connection between the galactic spin parameter $\lambda_{d}$ and the bar fraction in a volume-limited sample of 10,674 disk galaxies drawn from the Sloan Digital Sky Survey Data Release 7. The galaxies in our sample are visually classified into galaxies hosting long or short bars, and non-barred galaxies. We find that the spin distributions of these three classes are statistically different, with galaxies hosting long bars with the lowest $\lambda_{d}$ values, followed by non-barred galaxies, while galaxies with short bars present typically high spin parameters. The bar fraction presents its maximum at low to intermediate $\lambda_{d}$ values for the case of long bars, while the maximum for short bars is at high $\lambda_{d}$. This bi-modality is in good agreement with previous studies finding longer bars hosted by luminous, massive, red galaxies with low content of cold gas, while short bars are found in low luminosity, low mass, blue galaxies, usually gas rich. In addition, the rise and fall of the bar fraction as a function of $\lambda_{d}$, within the long-bar sample, shown in our results, can be explained as a result of two competing factors: the self-gravity of the disk that enhances bar instabilities, and the support by random motions instead of ordered rotational motion, that prevents the formation/growth of bars.Comment: 10 pages, 6 figures,1 table. Accepted for publication in Ap

Empirical distributions of galactic λ\lambda spin parameters from the SDSS

Using simple dimensional arguments for both spiral and elliptical galaxies, we present formulas to derive an estimate of the halo spin parameter $\lambda$ for any real galaxy, in terms of common observational parameters. This allows a rough estimate of $\lambda$, which we apply to a large volume limited sample of galaxies taken from the SDSS data base. The large numbers involved (11,597) allow the derivation of reliable $\lambda$ distributions, as signal adds up significantly in spite of the errors in the inferences for particular galaxies. We find that if the observed distribution of $\lambda$ is modeled with a log-normal function, as often done for this distribution in dark matter halos that appear in cosmological simulations, we obtain parameters $\lambda_{0}=0.04 \pm 0.005$ and $\sigma_{\lambda}=0.51 \pm 0.05$, interestingly consistent with values derived from simulations. For spirals, we find a good correlation between empirical values of $\lambda$ and visually assigned Hubble types, highlighting the potential of this physical parameter as an objective classification tool.Comment: 8 pages, 6 figures, expanded final version, MNRAS (in press