The challenging task of determining star formation rates: the case of a massive stellar burst in the brightest cluster galaxy of Phoenix galaxy cluster

TL;DR

This study estimates the star formation rate in the brightest galaxy of the Phoenix cluster using Bayesian SED fitting, revealing a range of plausible rates and emphasizing the importance of priors in such measurements.

Contribution

Introduces BAYESCOOL, a Bayesian SED-fitting code that explores extensive parameter space to more accurately estimate star formation rates in galaxy centers.

Findings

Estimated SFR range without prior: 2230-2890 solar masses/yr.

Estimated SFR range with extinction prior: 454-494 solar masses/yr.

Highlights the impact of priors on SFR estimates.

Abstract

Star formation in galaxies at the center of cooling-flow galaxy clusters is an important phenomenon in the context of formation and evolution of massive galaxies in the Universe. Yet, star formation rates (SFRs) in such systems continue to be elusive. We use our Bayesian-motivated spectral energy distribution (SED)-fitting code, BAYESCOOL, to estimate the plausible SFR values in the brightest cluster galaxy of a massive, X-ray luminous galaxy cluster, Phoenix. Previous studies of Phoenix have resulted in the highest measurement of SFR for any galaxy, with the estimates reaching up to 1000 solar masses/yr. However, a very small number of models have been considered in those studies. BAYESCOOL allows us to probe a large parameter space. We consider two models for star formation history, instantaneous bursts and continuous star formation, a wide range of ages for the old and the young stellar population, along with other discrete parameters, such as the initial mass function, metallicities, internal extinction and extinction law. We find that in the absence of any prior except that the maximum cooling rate < 3000 solar masses/yr, the SFR lies in the range (2230-2890) solar masses/yr. If we impose an observational prior on the internal extinction, E(B-V) < 0.6, the best-fit SFR lies in (454-494) solar masses/yr, and we consider this as the most probable range of SFR values for Phoenix. The SFR dependence on the extinction is a reflection of the standard age-extinction degeneracy, which can be overcome by using a prior on one of the two quantities in question.