Study Of Stellar Formation And Evolution In Spiral Galactic Disks
Creators: name: Gil, Carmen Sánchez
name: Navarro, Emilio J. Alfaro
name: Jiménez, Enrique Perez
name: Cerviño, Miguel
name: Bland-Hawthorn, Joss
name: Jones, Heath
Publisher: name: Zenodo
Year: 2018

description: Spiral arms formation is still an open problem. Even in our Galaxy, the number of spiral arms and its origin is not well-known. It could even coexist different or several modes of formation in the same galaxy. 

On the one hand, we present hierarchical bayesian inference models to determine the age in disks of nearby and nearly face-on, spiral galaxies. For a sample of 9 galaxies H\(\alpha\) data are taken with Taurus Tunable Filter, (TTF), ultraviolet band far UV or FUV from GALEX, and infrared bands at 24, 70 and 160 microns from Spitzer surveys. This is, we present the burst ages for young stellar populations in a sample of nearby and nearly face-on galaxies, proving enough spatial resolution of the galactic disks. And H\(\alpha\) to FUV flux ratio is a good relative indicator of the very recent star formation history (SFH). Through synthetic stellar populations model, we obtain theoretical H\(\alpha\)/FUV ratios to compare with our observed flux ratios, and thus to estimate the ages of the observed regions. Due to the nature of the problem, it is necessary to take into account the mean uncertainties, and the interrelationships between parameters when the Halpha/FUV flux is obtained. 

We propose a Bayesian hierarchical model, where a joint probability distribution is defined to determine the parameters (age, metallicity, fraction of ionizing photons.), from the observed data. The joint distribution of the parameters is described through an i.i.d.(independent and identically distributed random variables), generated through MCMC (Markov Chain Monte Carlo) techniques. 

The sample associated shows a variety of morphologies, as spiral arms and rings among others. The age patterns obtained for these galaxies indicate the great diversity of physical mechanisms that seem to act in the modeling of the star-to-large formation process scale. We analyze with more detail the posible origin of such structures as a function of spatial patterns from their age maps. The comparison of the age patterns obtained with those predicted by Dobbs &amp; Pringle (2010) for the distribution of young clusters allows us to conjecture about the different physical mechanisms that excite and maintain the spiral arms. The casuistry is varied: training stellar spiral arms mainly induced by density waves (M100, M74 and M51), circumnuclear rings (M94), age gradients affecting the entire galaxy (from West to East, M63), or radial age gradient (M74), or dominated by HII regions. N1068 seems to be a good and clear example, as outcome of our results, that N1068 seems to be a good and clear example, as outcome of our results, that AGN interaction with gas can reach larger scales (up to 7-8 kpc from the nucleus).

Finally, we address the study of the detection in Halpha of a radial corrugation in the vertical velocity field in disks of nearly face-on, spiral galaxies. With the aim of studying the corrugated velocity patterns in terms of the star formation processes, we describe the geometry of the problem and establish its fundamental relationships. In particular, the relationship between density of the ionized gas and the amplitude of the corrugation in speed. Two large groups have been found. In one of them the velocity and gas density show a out of phase correlation by a distance of a few hundred pc. The another class does not seem to show a well-defined pattern. These two classes show different physical properties of the ionized gas defined from different diagnostic diagrams (Sánchez-Gil et. al 2015).

The origin of the corrugations is still matter of debate. Corrugations are closely link, as cause/effect, to the large scale star formation processes: density waves, tidal interactions, collisions of high velocity clouds with the disk, or a galactic bore generated by the interaction of a spiral density wave with a thick gaseous disk, as modeled by [Martos &amp; Cox 1998, Martos et al. 1999], etc. Which mechanism is the origin of disk corrugations is still an open problem.