A massive galaxy in its core formation phase three billion years after the Big Bang

Abstract

Most massive galaxies are thought to have formed their dense stellar cores at early cosmic epochs. [1][2][3] However, cores in their formation phase have not yet been observed. Previous studies have found galaxies with high gas velocity dispersions 4 or small apparent sizes 5-7 but so far no objects have been identified with both the stellar structure and the gas dynamics of a forming core. Here we present a candidate core in formation 11 billion years ago, at z = 2.3. GOODS-N-774 has a stellar mass of 1.0 × 10 11 M⊙, a half-light radius of 1.0 kpc, and a star formation rate of 90 +45 −20 M⊙/yr. The star forming gas has a velocity dispersion 317 ± 30 km/s, amongst the highest ever measured. It is similar to the stellar velocity dispersions of the putative descendants of GOODS-N-774, compact quiescent galaxies at z ∼ 2 8-11 and giant elliptical galaxies in the nearby Universe. Galaxies such as GOODS-N-774 appear to be rare; however, from the star formation rate and size of the galaxy we infer that many star forming cores may be heavily obscured, and could be missed in optical and near-infrared surveys.We identified the candidate forming core, GOODS-N-774, using the 3D-HST catalogs in the five CANDELS fields. 12 GOODS-N-774 has a circularized effective radius re = 1.0 kpc from HST F160W WFC3 imaging; 13 a stellar mass of 1.0 × 10 11 M⊙ 12,14 ; rest-frame U V J colors consistent with a star-forming galaxy; and a MIPS 24 µm flux of 104 µJy. Fig. 1 shows the stellar mass density profile derived from the observed H160 surface brightness profile corrected for the HST PSF. 15 The surface density profile is strikingly similar to the average profile of massive quiescent galaxies at z ≈ 2 (red line), and much more concentrated than the average profile of massive star forming galaxies at that redshift (light blue). 13 The near infrared spectrum of GOODS-N-774 is shown in Fig. 2. The continuum is clearly detected, along with emission lines that we identify as Hα and [N II] redshifted to z = 2.300. The gas velocity dispersion is σ = 317 ± 30 km/s, equivalent to a FWHM ≈ 750 km/s. Typically, objects with such large linewidths are mergers or dominated by active galactic nuclei (AGN). 4 If the line emission in GOODS-N-774 is partially or largely due to the presence of an AGN, its velocity dispersion, size, and stellar mass measurements would not be reliable. There is no evidence for the presence of an active nucleus in GOODS-N-774. It is not detected in the deep Chandra 2 Ms X-ray data in GOODS-North with an upper limit of LX < 1.2 × 10 42 ergs s −1 . While an AGN cannot be conclusively ruled out, this upper limit is consistent with the star formation rate of the galaxy. Also, the galaxy has line ratios [O III]/[O II]= 0.7 ± 0.5, [O III]/Hβ = 1.2 ± 0.9, and [NII]/Hα = 0.4 ± 0.1, indicating a low ionization state of the gas. Therefore stellar photoionization, and hence ultimately star formation, is the likely origin of the line emission. Finally, the observed infrared SED, shown in Fig. 3, requires strong PAH emission to simultan...

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“…Compact star-forming galaxies been found in small numbers at z = 2−3 (Barro et al 2014a,b;Nelson et al 2014), but many host AGN, complicating the interpretation of the observations. At the same time, rest-frame ultra-violet (UV) or optically measured sizes of star-forming galaxies may be affected by dust-obscured central regions, thereby increasing their effective radii.…”

SPATIALLY RESOLVED DUST MAPS FROM BALMER DECREMENTS IN GALAXIES AT z ∼ 1.4

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