Constraining star formation rates in cool-core brightest cluster galaxies

TL;DR

This study uses Bayesian stellar population synthesis to constrain star formation rates in cool-core brightest cluster galaxies, revealing their correlation with ICM cooling and evidence of periodic starbursts over billions of years.

Contribution

It provides the first robust Bayesian analysis of SFRs in BCGs, accounting for model uncertainties and revealing their connection to ICM cooling and feedback processes.

Findings

SFRs are 4 to 50 times lower than X-ray mass deposition rates.

Most BCGs have experienced starbursts over the past 6 Gyr.

Star formation episodes are periodic, with intervals of 20-200 Myr.

Abstract

We used broad-band imaging data for 10 cool-core brightest cluster galaxies (BCGs) and conducted a Bayesian analysis using stellar population synthesis to determine the likely properties of the constituent stellar populations. Determination of ongoing star formation rates (SFRs), in particular, has a direct impact on our understanding of the cooling of the intracluster medium (ICM), star formation and AGN-regulated feedback. Our model consists of an old stellar population and a series of young stellar components. We calculated marginalized posterior probability distributions for various model parameters and obtained 68% plausible intervals from them. The 68% plausible interval on the SFRs is broad, owing to a wide range of models that are capable of fitting the data, which also explains the wide dispersion in the star formation rates available in the literature. The ranges of possible SFRs are robust and highlight the strength in such a Bayesian analysis. The SFRs are correlated with the X-ray mass deposition rates (the former are factors of 4 to 50 lower than the latter), implying a picture where the cooling of the ICM is a contributing factor to star formation in cool-core BCGs. We find that 9 out of 10 BCGs have been experiencing starbursts since 6 Gyr ago. While four out of 9 BCGs seem to require continuous SFRs, 5 out of 9 seem to require periodic star formation on intervals ranging from 20 Myr to 200 Myr. This time scale is similar to the cooling-time of the ICM in the central (< 5 kpc) regions.