On the star-formation rate - brightest cluster relation: estimating the peak SFR in post-merger galaxies

We further the recent discussion on the relation between the star-formation rate (SFR) of a galaxy and the luminosity of its brightest star-cluster (SFR vs. M V ). We first show that the observed trend between SFR vs. M V is due to the brightest cluster in a galaxy being preferentially young (6 15 Myr for a constant SFR) and hence a good tracer of the current SFR, although we give notable exceptions to this rule. Archival HST imaging of high-SFR galaxies, as well as additional galaxies/clusters from the literature, are used to further confirm the observed trend. Using a series of Monte Carlo simulations we show that a pure power-law mass function with index, α = 2, is ruled out by the current data. Instead we find that a Schechter function (i.e. a power-law with an exponential truncation at the high mass end) provides an excellent fit to the data. Additionally, these simulations show that bound cluster formation (in M⊙/yr) represents only ∼ 8 ± 3% of the total star-formation within a galaxy, independent of the star-formation rate. From this we conclude that there is only a single mode of cluster formation which operates over at least six orders of magnitude in the SFR. We provide a simple model of star/cluster formation feedback within dwarf galaxies (and star-forming complexes within spirals) which highlights the strong impact that a massive cluster can have on its surroundings. Using this relation, we can extrapolate backwards in time in order to estimate the peak SFR of major merger galaxies, such as NGC 7252, NGC 1316, and NGC 3610. The derived SFRs for these galaxies are between a few hundred and a few thousand solar masses per year. The inferred far infrared luminosity of the galaxies, from the extrapolated SFR, places them well within the range of Ultra-luminous galaxies (ULIRGs) and for NGC 7252 within the Hyper-luminous infrared galaxy regime. Thus, we provide evidence that these post merger galaxies passed through a ULIRG/HLIRG phase and are now evolving passively. Using the current and extrapolated past SFR of NGC 34, we infer that the ULIRG phase of this galaxy has lasted for at least 150 Myr.