FRESCO: The Paschen-$\alpha$ Star Forming Sequence at Cosmic Noon

TL;DR

This study uses JWST observations to analyze the star forming sequence of galaxies at redshifts 1.0 to 1.7, employing Paschen-$\alpha$ emission to measure star formation rates and comparing these with stellar masses, revealing lower SFRs than previous studies and low scatter in the SFR-M* relation.

Contribution

First spectroscopic measurement of galaxy star formation rates at cosmic noon using Paschen-$\alpha$, providing a more consistent SFR-M* relation and insights into galaxy classification.

Findings

SFRs are lower than in earlier studies by up to 0.6 dex.

The Paschen-$\alpha$ luminosity correlates tightly with rest-frame H-band magnitude.

Some galaxies classified as quiescent show significant Paschen-$\alpha$ emission.

Abstract

We present results from the JWST First Reionization Epoch Spectroscopically Complete Observations survey (FRESCO) on the star forming sequence of galaxies at $1.0<z<1.7$, around the peak of the cosmic star formation history. Star formation rates (SFRs) are measured from the redshifted, nearly dust-insensitive Paschen-$\alpha$ emission line, and stellar mass measurements include the F444W (4.4 $\mu$m; rest-frame H) band. We find SFRs of galaxies with $M*>9.5 M_\odot$ that are lower than found in many earlier studies by up to 0.6 dex, but in good agreement with recent results obtained with the Prospector fitting framework. The difference log(SFR(Pa$\alpha$)-SFR(Prospector)) is -0.09 $\pm$ 0.04 dex at $10^{10-11} M_\odot$. We also measure the empirical relation between Paschen-$\alpha$ luminosity and rest-frame H band magnitude and find that the scatter is only 0.04 dex lower than that of the SFR-M* relation and is much lower than the systematic differences among relations in the literature due to various methods of converting observed measurements to physical properties. We additionally identify examples of sources -- that, with standard cutoffs via the UVJ diagram, would be deemed quiescent -- with significant, typically extended, Paschen-$\alpha$ emission. Our results may be indicative of the potential unification of methods used to derive the star forming sequence with careful selection of star forming galaxies and independent star formation rate and stellar mass indicators.