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

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Science  17 Apr 2015:
Vol. 348, Issue 6232, pp. 314-317
DOI: 10.1126/science.1261094

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Eat your heart out, old galaxies

Most galaxies exceeding 100 billion solar masses are dense spheroids that exhibit no star-forming activity in the present day. Nevertheless, galaxies of the same size were actively forming stars when the universe was only a few billion years old. Tacchella et al. used integral-field spectroscopy and high-resolution imaging to map the distributions of star formation rates and stellar mass densities within ancient galaxies. Star formation apparently quenched first in the center, while remaining lively in the galactic outskirts, with quenching taking a few billion years to proceed outward.

Science, this issue p. 314


Most present-day galaxies with stellar masses ≥1011 solar masses show no ongoing star formation and are dense spheroids. Ten billion years ago, similarly massive galaxies were typically forming stars at rates of hundreds solar masses per year. It is debated how star formation ceased, on which time scales, and how this “quenching” relates to the emergence of dense spheroids. We measured stellar mass and star-formation rate surface density distributions in star-forming galaxies at redshift 2.2 with ~1-kiloparsec resolution. We find that, in the most massive galaxies, star formation is quenched from the inside out, on time scales less than 1 billion years in the inner regions, up to a few billion years in the outer disks. These galaxies sustain high star-formation activity at large radii, while hosting fully grown and already quenched bulges in their cores.

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