We present 0 ′′ .2-resolution Atacama Large Millimeter/submillimeter Array observations at 870 µm for 25 Hα-seleced star-forming galaxies around the main-sequence at z = 2.2 − 2.5. We detect significant 870 µm continuum emission in 16 (64%) of these galaxies. The high-resolution maps reveal that the dust emission is mostly radiated from a single region close to the galaxy center. Exploiting the visibility data taken over a wide uv distance range, we measure the half-light radii of the rest-frame far-infrared emission for the best sample of 12 massive galaxies with log(M * /M ⊙ ) >11. We find nine galaxies to be associated with extremely compact dust emission with R 1/2,870µm < 1.5 kpc, which is more than a factor of 2 smaller than their rest-optical sizes, R 1/2,1.6µm =3.2 kpc, and is comparable with optical sizes of massive quiescent galaxies at similar redshifts. As they have an exponential disk with Sérsic index of n 1.6µm =1.2 in the rest-optical, they are likely to be in the transition phase from extended disks to compact spheroids. Given their high star formation rate surface densities within the central 1 kpc of ΣSFR 1kpc = 40 M ⊙ yr −1 kpc −2 , the intense circumnuclear starbursts can rapidly build up a central bulge with ΣM * ,1kpc > 10 10 M ⊙ kpc −2 in several hundred Myr, i.e. by z ∼ 2. Moreover, ionized gas kinematics reveal that they are rotation-supported with an angular momentum as large as that of typical star-forming galaxies at z = 1 − 3. Our results suggest bulges are commonly formed in extended rotating disks by internal processes, not involving major mergers.
We report two secure (z = 3.775, 4.012) and one tentative (z ≈ 3.767) spectroscopic confirmations of massive and quiescent galaxies close to their quenching epoch through K-band observations with Keck/MOSFIRE and VLT/X-Shooter. The stellar continuum emission, the absence of strong nebular emission lines and the lack of significant far-infrared detections confirm the passive nature of these objects, disfavoring the alternative solution of low-redshift dusty star-forming interlopers. We derive stellar masses of log(M /M ) ∼ 11 and ongoing star formation rates placing these galaxies 1 − 2 dex below the main sequence at their redshifts. The adopted parametrization of the star formation history suggests that these sources experienced a strong ( SFR ∼ 1200 − 3500 M yr −1 ) and short (∼ 50 Myr) burst of star formation, peaking ∼ 150 − 500 Myr before the time of observation, all properties reminiscent of the characteristics of sub-millimeter galaxies (SMGs) at z > 4. We investigate this connection by comparing the comoving number densities and the properties of these two populations. We find a fair agreement only with the deepest sub-mm surveys detecting not only the most extreme 2 Valentino et al.starbursts, but also more normal galaxies. We support these findings by further exploring the Illustris-TNG cosmological simulation, retrieving populations of both fully quenched massive galaxies at z ∼ 3 − 4 and SMGs at z ∼ 4 − 5, with number densities and properties in broad agreement with the observations at z ∼ 3, but in increasing tension at higher redshift. Nevertheless, as suggested by the observations, not all the progenitors of quiescent galaxies at these redshifts shine as bright SMGs in their past and, similarly, not all bright SMGs quench by z ∼ 3, both fractions depending on the threshold assumed to define the SMGs themselves. This cautions against the blind application of the assumption of a univocal connection between the two populations at high redshift.
We present molecular gas reservoirs of eighteen galaxies associated with the XMMXCS J2215.9-1738 cluster at z = 1.46. From Band 7 and Band 3 data of the Atacama Large Millimeter/submillimeter Array (ALMA), we detect dust continuum emission at 870 µm and CO J = 2-1 emission line from 8 and 17 member galaxies respectively within a cluster-centric radius of R 200 . The molecular gas masses derived from the CO and/or dust continuum luminosities show that the fraction of molecular gas mass and the depletion time scale for the cluster galaxies are larger than expected from the scaling relations of molecular gas on stellar mass and offset from the main sequence of star-forming galaxies in general fields. The galaxies closer to the cluster center in terms of both projected position and accretion phase seem to show a larger deviation from the scaling relations. We speculate that the environment of galaxy cluster helps feed the gas through inflow to the member galaxies and also reduce the efficiency of star formation. The stacked Band 3 spectrum of 12 quiescent galaxies with M stellar ∼ 10 11 M within 0.5R 200 shows no detection of CO emission line, giving the upper limit of molecular gas mass and molecular gas fraction to be 10 10 M and 10%, respectively. Therefore, the massive galaxies in the cluster core quench the star formation activity while consuming most of the gas reservoirs.
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