BULGE-FORMING GALAXIES WITH AN EXTENDED ROTATING DISK AT z ∼ 2

Abstract

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.

Stellar Mass Function of Active and Quiescent Galaxies via the Continuity Equation

“…under heavily dust-enshrouded conditions (e.g., Scoville et al 2014Scoville et al , 2016Simpson et al 2015;Ikarashi et al 2015;Straatman et al 2015;Spilker et al 2016;Tadaki et al 2017). A duration of the main star formation episode τ sphe 0.5 − 1 Gyr in high-redshift dusty starforming galaxies, which are the candidate progenitors of massive spheroids, is also confirmed by local observations of the α−enhancement, i.e., iron underabundance compared to α elements.…”

“…In fact, galaxies endowed with star formation ratesṀ ⋆ a few tens M ⊙ yr −1 at redshift z 2 were largely missed by rest-frame optical/UV surveys because of heavy dust obscuration, difficult to correct for with standard techniques based only on UV spectral data (e.g., Bouwens et al 2016Bouwens et al , 2017Mancuso et al 2016a;Pope et al 2017;Ikarashi et al 2017;Simpson et al 2017). High-resolution, follow-up observations of these galaxies in the far-IR/sub-mm/radio band via ground-based interferometers, such as SMA, VLA, PdBI, and recently ALMA, have revealed star formation to occur in a few collapsing clumps distributed over spatial scales smaller than a few kpcs (see Simpson et al 2015;Ikarashi et al 2015;Straatman et al 2015;Spilker et al 2016;Barro et al 2016;Tadaki et al 2017). A strongly baryon-dominated stellar core with high ongoing SFR is often surrounded out to 15 kpc by a clumpy, unstable gaseous disk in nearly keplerian rotation (e.g., Swinbank et al 2017).…”

A Radio-to-mm Census of Star-forming Galaxies in Protocluster 4C23.56 at Z = 2.5: Gas Mass and Its Fraction Revealed with ALMA

“…Massive galaxies with  >  M M 10 11 (HAE1, HAE2) that have high SFRs from the Hα emission and other galaxies with high SFRs (HAE12, HAE13) are notdetected in either the CO(3-2) or 1.1 mm emissions, as opposed to our expectation that these galaxies would be detected if the normal KS relation applies. Furthermore, recent observations reported the detection of the massive mainsequence galaxies (e.g., Decarli et al 2016b;Tadaki et al 2017). There are several reasons that may apply for the non-detection.…”

Growth of Bulges in Disk Galaxies Since z ∼ 1

“…There is a growing number of observational evidences that disc galaxies have settled into rotational equilibrium before redshift z ∼ 1 (Kassin et al 2007;Buitrago et al 2014;Huertas-Company et al 2016;Margalef-Bentabol et al 2016;Simons et al 2016;Tadaki et al 2017); but the epoch at which such a transition happened remain obscured -making it harder to pinpoint exactly when bulges started forming inside them. Galaxies in the high redshift (z > 1) are clumpy and these clumps might have played an important role in developing the central bulge (Bournaud 2016;Tadaki et al 2017).…”

“…Galaxies in the high redshift (z > 1) are clumpy and these clumps might have played an important role in developing the central bulge (Bournaud 2016;Tadaki et al 2017). Whether they produce pseudo bulge or classical bulge remains under debate.…”