Paper detail

Bayesian hierarchical modelling of the $\mathrm{M_{\star}}$-SFR relation from 1<z<6 in ASTRODEEP

The Hubble Frontier Fields represent the opportunity to probe the high-redshift evolution of the main sequence of star-forming galaxies to lower masses than possible in blank fields thanks to foreground lensing of massive galaxy clusters. We use the BEAGLE SED-fitting code to derive stellar masses, $\mathrm{M_{\star}}=\log(M/\mathrm{M_{\odot}})$, SFRs, $Ψ=\log(ψ/\mathrm{M_{\odot}}\,\mathrm{yr}^{-1})$ and redshifts from galaxies within the ASTRODEEP catalogue. We fit a fully Bayesian hierarchical model of the main sequence over $1.25<z<6$ of the form $Ψ= α_\mathrm{9.7}(z) + β(\mathrm{M_{\star}}-9.7) + \mathcal{N}(0,σ^2)$ while explicitly modelling the outlier distribution. The redshift-dependent intercept at $\mathrm{M_{\star}}=9.7$ is parametrized as $α_\mathrm{9.7}(z) = \log[N (1+z)^γ] + 0.7$. Our results agree with an increase in normalization of the main sequence to high redshifts that follows the redshift-dependent rate of accretion of gas onto dark matter halos with $γ=2.40^{+0.18}_{-0.18}$. We measure a slope and intrinsic scatter of $β=0.79^{+0.03}_{-0.04}$ and $σ=0.26^{+0.02}_{-0.02}$. We find that the sampling of the SED provided by the combination of filters (Hubble + ground-based Ks-band + Spitzer 3.6 and 4.5 $\mathrm{μm}$) is insufficient to constrain $\mathrm{M_{\star}}$ and $Ψ$ over the full dynamic range of the observed main sequence, even at the lowest redshifts studied. While this filter set represents the best current sampling of high-redshift galaxy SEDs out to $z>3$, measurements of the main sequence to low masses and high redshifts still strongly depend on priors employed in SED fitting (as well as other fitting assumptions). Future data-sets with JWST should improve this.