Recent stellar mass assembly of low-mass star-forming galaxies at redshifts 0.3 < z < 0.9*

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The epoch when low-mass star-forming galaxies (LMSFGs) form the bulk of their stellar mass is uncertain. While some models predict an early formation, others favor a delayed scenario until later ages of the universe. We present constraints on the star formation histories (SFHs) of a sample of LMSFGs obtained through the analysis of their spectral energy distributions using a novel approach that (1) consistently combines photometric (broadband) and spectroscopic (equivalent widths of emission lines) data, and (2) uses physically motivated SFHs with non-uniform variations of the star formation rate (SFR) as a function of time. The sample includes 31 spectroscopically confirmed LMSFGs (7.3 <= logM(*)/M circle dot <= 8.0), at 0.3 < z(spec) < 0.9, in the Extended-Chandra Deep Field-South field. Among them, 24 were selected with photometric stellarmass logM(*)/M circle dot < 8.0, 0.3< z(phot) < 1.0, and m(NB816, AB) < 26 mag; the remaining 7 were selected as blue compact dwarfs within the same photometric redshift and magnitude ranges. We also study a secondary sample of 43 more massive spectroscopically confirmed galaxies (8.0 < logM(*)/M circle dot <= 9.1), selected with the same criteria. The SFRs and stellar masses derived for both samples place our targets on the standard main sequence of star-forming galaxies. The median SFH of LMSFGs at intermediate redshifts appears to form 90% of the median stellar mass inferred for the sample in the similar to 0.5-1.8 Gyr immediately preceding the observation. These results suggest a recent stellar mass assembly for LMSFGs, consistent with the cosmological downsizing trends. We find similar median SFH timescales for the more massive secondary sample.
© 2015. The American Astronomical Society. All rights reserved. We are grateful to the anonymous referee for a very thorough report which led to a substantial improvement of the paper. We acknowledge support from the Spanish Programa Nacional de Astronomía y Astrofísica: Project AYA2009-10368 and AYA2012-30717. This work has used the Rainbow Cosmological Surveys Database, which is operated by the Universidad Complutense de Madrid (UCM), partnered with the University of California Observatories at Santa Cruz (UCO/Lick, UCSC). L.R.M. thanks Pablo G. Pérez González for his technical advice on Rainbow usage and enriching discussions, and Roger Griffith for providing the latest version of the morphological catalogs on the E-CDF-S field. C.P. acknowledges fundings by the KASI-Yonsei Joint Research Program for the Frontiers of Astronomy and Space Science funded by the Korea Astronomy and Space Science Institute. S.C. acknowledges support from the European Research Council via an advanced grant under grant agreement No. 321323-NEOGAL.
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