Star Formation in Galaxies at z ∼ 4–5 from the SMUVS Survey: A Clear Starburst/Main-sequence Bimodality for Hα Emitters on the SFR–M Plane

• Published in: The Astrophysical journal Vol. 849; no. 1; p. 45
• Main Authors: Caputi, K. I., Deshmukh, S., Ashby, M. L. N., Cowley, W. I., Bisigello, L., Fazio, G. G., Fynbo, J. P. U., Fèvre, O. Le, Milvang-Jensen, B., Ilbert, O.
• Format: Journal Article
• Language: English
• Published: Philadelphia IOP Publishing 01-11-2017; American Astronomical Society
• Subjects: Active galactic nuclei; Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; DENSITY; DISTRIBUTION; EMISSION; Emitters; Fluxes; GALACTIC EVOLUTION; GALAXIES; Galaxy distribution; GALAXY NUCLEI; H alpha line; MAIN SEQUENCE STARS; MASS; Physics; Polls & surveys; RED SHIFT; Star & galaxy formation; STAR EVOLUTION; Star formation; Stars & galaxies; STATISTICS; UNIVERSE; Wavelengths; galaxies: starburst; infrared: galaxies; galaxies: evolution; galaxies: high-redshift; galaxies: star formation
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa901e

Abstract
We study a large galaxy sample from the Spitzer Matching Survey of the UltraVISTA ultra-deep Stripes (SMUVS) to search for sources with enhanced
3.6
μ
m
fluxes indicative of strong H
α
emission at
z
=
3.9
–
4.9
. We find that the percentage of “H
α
excess” sources reaches 37%–40% for galaxies with stellar masses
log
10
(
M
*
/
M
⊙
)
≈
9
–
10
and decreases to
<
20
%
at
log
10
(
M
*
/
M
⊙
)
∼
10.7
. At higher stellar masses, however, the trend reverses, although this is likely due to active galactic nucleus contamination. We derive star formation rates (SFR) and specific SFR (sSFR) from the inferred H
α
equivalent widths of our “H
α
excess” galaxies. We show, for the first time, that the “H
α
excess” galaxies clearly have a bimodal distribution on the SFR–
M
* plane: they lie on the main sequence of star formation (with
log
10
(
sSFR
/
yr
−
1
)
<
−
8.05
) or in a starburst cloud (with
log
10
(
sSFR
/
yr
−
1
)
>
−
7.60
). The latter contains
∼
15
%
of all the objects in our sample and accounts for
>
50
%
of the cosmic SFR density at
z
=
3.9
–
4.9
, for which we derive a robust lower limit of
0.066
M
⊙
yr
−
1
Mpc
−
3
. Finally, we identify an unusual
>
50
σ
overdensity of
z
=
3.9
–
4.9
galaxies within a
0.20
×
0.20
arcmin
2
region. We conclude that the SMUVS unique combination of area and depth at mid-IR wavelengths provides an unprecedented level of statistics and dynamic range that are fundamental to revealing new aspects of galaxy evolution in the young universe.