Jekyll & Hyde: quiescence and extreme obscuration in a pair of massive galaxies 1.5 Gyr after the Big Bang

Abstract

We obtained ALMA spectroscopy and deep imaging to investigate the origin of the unexpected sub-millimeter emission toward the most distant quiescent galaxy known to date, ZF-COSMOS-20115 at z = 3.717. We show here that this sub-millimeter emission is produced by another massive (M * ∼ 10 11 M ), compact (r 1/2 = 0.67 ± 0.14 kpc) and extremely obscured galaxy (A V ∼ 3.5), located only 0.43 (3.1 kpc) away from the quiescent galaxy. We dub the quiescent and dusty galaxies Jekyll and Hyde, respectively. No dust emission is detected at the location of the quiescent galaxy, implying SFR < 13 M /yr which is the most stringent upper limit ever obtained for a quiescent galaxy at these redshifts. The two sources are spectroscopically confirmed to lie at the same redshift thanks to the detection of [C ii] 158 in Hyde (z = 3.709), which provides one the few robust redshifts for a highly-obscured "H-dropout" galaxy (H − [4.5] = 5.1 ± 0.8). The [C ii] line shows a clear rotating-disk velocity profile which is blueshifted compared to the Balmer lines of Jekyll by 549 ± 60 km/s, demonstrating that it is produced by another galaxy. Careful de-blending of the Spitzer imaging confirms the existence of this new massive galaxy, and its non-detection in the Hubble images requires extremely red colors and strong attenuation by dust. Full modeling of the UV-to-far-IR emission of both galaxies shows that Jekyll has fully quenched at least 200 Myr prior to observation and still presents a challenge for models, while Hyde only harbors moderate star-formation with SFR 120 M /yr, and is located at least a factor 1.4 below the z ∼ 4 main sequence. Hyde could also have stopped forming stars less than 200 Myr before being observed; this interpretation is also suggested by its compactness comparable to that of z ∼ 4 quiescent galaxies and its low [C ii]/FIR ratio, but significant on-going star-formation cannot be ruled out. Lastly, we find that despite its moderate SFR, Hyde hosts a dense reservoir of gas comparable to that of the most extreme starbursts. This suggests that whatever mechanism has stopped or reduced its star-formation must have done so without expelling the gas outside of the galaxy. Because of their surprisingly similar mass, compactness, environment and star-formation history, we argue that Jekyll and Hyde can be seen as two stages of the same quenching process, and provide a unique laboratory to study this poorly understood phenomenon.

Quiescent Galaxies 1.5 Billion Years after the Big Bang and Their Progenitors

“…Similarly, COS-466654 is not detected in any of the mid-infrared (24 µm) to radio (1.4 GHz) bands collected in the "superdeblended" catalog of the COSMOS field by Jin et al (2018), resulting in a combined infrared signal-to-noise ratio of SN IR = 1.8. At the current sensitivity and spatial resolution limits, this further confirms the quiescence of the two galaxies and excludes the presence of bright dusty star-forming companions in their immediate proximity, at odds with at least one previously reported case (Glazebrook et al 2017;Simpson et al 2017;Schreiber et al 2018a).…”

“…This evolutionary scheme has been recently challenged by the spectroscopic confirmation of quiescent systems with M ∼ 10 11 M above z > 3 and up to z = 3.717 (Gobat et al 2012;Glazebrook et al 2017;Simpson et al 2017;Schreiber et al 2018a,b, S18b hereafter, C. D'Eugenio et al in preparation), as part of a substantial population of photometrically selected red galaxies (e.g., Ilbert et al 2013;Muzzin et al 2013;Straatman et al 2014;Mawatari et al 2016;Davidzon et al 2017;Merlin et al 2018, to mention recent results). In at least one case, their quiescent nature has been initially challenged by sub-millimeter observations (Simpson et al 2017), but later confirmed with a high spatial resolution follow-up, necessary to disentangle the emission of these galaxies from nearby companions (Schreiber et al 2018a). Systematic studies of larger samples of z > 3 photometric candidates in the sub-millimeter further support their average quiescence (Santini et al 2019).…”

The VANDELS survey: the stellar metallicities of star-forming galaxies at $\mathbf {2.5\,\, \lt\,\, z\,\, \lt\,\, 5.0}$

“…Stellar masses for the full VANDELS sample were derived from SED fitting using Bruzual & Charlot (2003) templates with solar metallicity and assuming exponentially declining star-formation histories as described in McLure et al (2018b). We also derived stellar masses for our 2.5 < z < 5.0 sample using fast++, a rewrite of fast (Kriek et al 2009) described in Schreiber et al (2018). We adopted the same set of parameters as McLure et al (2018b), but allowed for rising star-formation histories using a delayed exponentially declining model (M * ∝ te t/τ ) and a range of metallicities values (0.3 − 2.5 × Z ).…”

An Extremely Massive Quiescent Galaxy at z = 3.493: Evidence of Insufficiently Rapid Quenching Mechanisms in Theoretical Models*

“…For a consistent comparison with the sample from S18, we utilize the FAST++ code 2 (Schreiber et al 2018a, S18) to model the SEDs of our galaxies. FAST++ is a rewrite of FAST (Kriek et al 2009) for C++ which allows for flexible star-formation history (SFH) parameterizations as well as spectroscopic data of different wavelength resolutions.…”