The effects of global galaxy properties on star-forming regions

2015 • 2015PhDT.......141C

Authors • Cook, David O.

Abstract • This work aims to study the effects of global galaxy properties on star formation processes that occur on sub-galactic scales. The properties of star-forming regions (star clusters, HII regions, etc.) have been studied many times before, but this work derives from an unprecedented combination of sample size, proximity, and diversity. Thousands of star-forming regions were identified from a sample of 258 nearby galaxies spanning a wide range of mass, luminosity, morphology, and metal abundance. Initial efforts focused on robustly and uniformly characterizing the global properties of the galaxies (M_B, M_{star}, A_{FUV}, etc.). In addition, the optical portion of the galaxies' panchromatic spectral energy distributions was utilized to derive the first empirical transformation between the Sloan Digital Sky Survey ugri and Johnson UBVR filter systems for galaxies. Two types of star-forming regions are studied: 1) star clusters via high-resolution optical imaging which represent gravitationally bound groups of stars that formed roughly at the same time and in the same physical location; and 2) star-forming regions via lower-resolution ultraviolet imaging which represent groups of stars that formed over a short range in time (∼ 0-100∼Myr) and in relative spatial proximity (∼10s to a few 100s of pc). We study the relationship between the field star formation and star cluster formation properties in a sub-sample of nearby dwarf galaxies with available high-resolution Hubble Space Telescope (HST) imaging. We derive the ages and masses of the young (t_{age} ≤sssim 100 Myr) cluster sample from the HST and from ground-based telescopes. Our data provides the first constraints on two proposed relationships between the star formation rate of galaxies and the properties of their cluster systems in the low star formation rate regime. The data show broad agreement with these relationships, but significant galaxy-to-galaxy scatter exists. In part, this scatter can be accounted for by simulating the small number of clusters detected from stochastically sampling the cluster mass function. However, this stochasticity does not fully account for the observed scatter in our data suggesting there may be true variations in the fraction of stars formed in clusters in dwarf galaxies. Comparison of the cluster formation and the brightest cluster in our sample galaxies also provide constraints on cluster destruction models. We have used the GALEX FUV images of 258 nearby galaxies to identify thousands of star-forming regions and measured each galaxy's luminosity function. We characterize the relationships between the luminosity function slope (alpha) and several global galaxy properties. These relationships are based on 79 galaxies with reliable luminosity functions, and represent the largest sample of galaxies to study the connection between luminosity function properties and galaxy environment. We find that alpha strongly correlates with global star formation properties, where the star formation rate density (Sigma_{SFR}) shows the strongest relationship. In addition, we find that neither stochastic sampling of the luminosity function in galaxies with low-number statistics nor the effects of blending due to distance can fully account for these relationships. We conclude that the relationship between alpha and Sigma_{SFR} is, in part, due to the star formation environments found in the host galaxies. Furthermore, we speculate that this trend is due to higher gas densities and higher star formation efficiencies in galaxies with higher Sigma_{SFR}. We also create a composite luminosity function composed of star-forming regions from many galaxies to characterize a truncation in the luminosity function at brighter luminosities. The composite luminosity function shows a clear truncation, however, we find that this truncation is an artifact of varying luminosity limits for galaxies at different distances.


IPAC Authors

David Cook

Assistant Scientist