Galaxy evolution. Evidence for mature bulges and an inside-out quenching phase 3 billion years after the Big Bang

Spiral morphology in an intensely star-forming disk galaxy more than 12 billion years ago

Spiral galaxies have distinct internal structures, including a stellar bulge, a disk, and spiral arms. It is unknown when in cosmic history these structures formed. In this study, we analyzed observations of BRI 1335-0417, an intensely star-forming galaxy in the distant Universe, at a redshift of 4.41. The [C ii] gas kinematics shows a steep velocity rise near the galaxy center and has a two-armed spiral morphology, which extends from about 2 to 5 kiloparsecs in radius. We interpret these features as due to a central compact structure such as a bulge, a rotating gas disk, and either spiral arms or tidal tails. These features had formed within 1.4 billion years after the Big Bang, long before the peak of cosmic star formation.

GALAXY STRUCTURE, STELLAR POPULATIONS, AND STAR FORMATION QUENCHING AT 0.6 ≲ z ≲ 1.2

We use both photometric and spectroscopic data from the Hubble Space Telescope to explore the relationships among 4000 Å break (D4000) strength, colors, stellar masses, and morphology, in a sample of 352 galaxies with log(M */M ⊙) > 9.44 at 0.6 ≲ z ≲ 1.2. We have identified authentically quiescent galaxies in the UVJ diagram based on their D4000 strengths. This spectroscopic identification is in good agreement with their photometrically-derived specific star formation rates (sSFR). Morphologically, most (that is, 66 out of 68 galaxies, ~ 97 %) of these newly identified quiescent galaxies have a prominent bulge component. However, not all of the bulge-dominated galaxies are quenched. We found that bulge-dominated galaxies show positive correlations among the D4000 strength, stellar mass, and the Sérsic index, while late-type disks do not show such strong positive correlations. Also, bulge-dominated galaxies are clearly separated into two main groups in the parameter space of sSFR vs. stellar mass and stellar surface density within the effective radius, Σe, while late-type disks and irregulars only show high sSFR. This split is directly linked to the 'blue cloud' and the 'red sequence' populations, and correlates with the associated central compactness indicated by Σe. While star-forming massive late-type disks and irregulars (with D4000 < 1.5 and log(M */M ⊙) ≳ 10.5) span a stellar mass range comparable to bulge-dominated galaxies, most have systematically lower Σe ≲ 109 M ⊙kpc-2. This suggests that the presence of a bulge is a necessary but not sufficient requirement for quenching at intermediate redshifts.

A massive, dead disk galaxy in the early Universe

At redshift z = 2, when the Universe was just three billion years old, half of the most massive galaxies were extremely compact and had already exhausted their fuel for star formation. It is believed that they were formed in intense nuclear starbursts and that they ultimately grew into the most massive local elliptical galaxies seen today, through mergers with minor companions, but validating this picture requires higher-resolution observations of their centres than is currently possible. Magnification from gravitational lensing offers an opportunity to resolve the inner regions of galaxies. Here we report an analysis of the stellar populations and kinematics of a lensed z = 2.1478 compact galaxy, which-surprisingly-turns out to be a fast-spinning, rotationally supported disk galaxy. Its stars must have formed in a disk, rather than in a merger-driven nuclear starburst. The galaxy was probably fed by streams of cold gas, which were able to penetrate the hot halo gas until they were cut off by shock heating from the dark matter halo. This result confirms previous indirect indications that the first galaxies to cease star formation must have gone through major changes not just in their structure, but also in their kinematics, to evolve into present-day elliptical galaxies.