Abstract
One of the fundamental goals of modern Astronomy is to estimate the physical parameters of galaxies from images in different spectral bands. We present a hierarchical Bayesian model for obtaining age maps from images in the Ha line (taken with Taurus Tunable Filter (TTF)), ultraviolet band (far UV or FUV, from GALEX) and infrared bands (24, 70 and 160 microns (μm), from Spitzer). As shown in [1], we present the burst ages for young stellar populations in the nearby and nearly face on galaxy M74.As it is shown in the previous work, the Hα to FUV flux ratio gives a good relative indicator of very recent star formation history (SFH). As a nascent star-forming region evolves, the Ha line emission declines earlier than the UV continuum, leading to a decrease in the HαFUV ratio. Through a specific star-forming galaxy model (Starburst 99, SB99), we can obtain the corresponding theoretical ratio Hα / FUV 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 propose a model of high complexity to take into account the mean uncertainties, and the interrelationship between parameters when the Hα / FUV flux ratio mentioned above is obtained. To address the complexity of the model, we propose a Bayesian hierarchical model, where a joint probability distribution is defined to determine the parameters (age, metallicity, IMF), from the observed data, in this case the observed flux ratios Hα / FUV. 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.
Highlights
The study of the star formation history (SFH) and star formation rate (SFR) in galaxies provide vital information on the evolutionary properties of galaxies and the physical processes which drive that evolution
The measured Hα/far-ultraviolet band (FUV) ratios are compared with the stellar population synthesis (SPS) models to study the dynamics of stellar populations of different ages within the galaxies; we use Starburst99, SB99 ( [Leitherer et al (1999)]; [Vazquez & Leitherer (2005)]
We address the problem of deriving the galaxy age map from Hα/FUV flux ratio images by establishing a probabilistic framework which explain the relationship of the random variables involved in the problem
Summary
The study of the star formation history (SFH) and star formation rate (SFR) in galaxies provide vital information on the evolutionary properties of galaxies and the physical processes which drive that evolution. The measured Hα/FUV ratios are compared with the stellar population synthesis (SPS) models to study the dynamics of stellar populations of different ages within the galaxies; we use Starburst, SB99 ( [Leitherer et al (1999)]; [Vazquez & Leitherer (2005)]). We address the problem of deriving the galaxy age map from Hα/FUV flux ratio images by establishing a probabilistic framework which explain the relationship of the random variables involved in the problem. This relationship will be formulated in terms of a joint probability distribution given the observations and their uncertainties. We explore the parameters of interest (age) marginalizing nuisance parameters in the posterior distribution defined by HBM model
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