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When the Universe was just 3 billion years old, half of the most massive galaxies had already ceased star formation, and such a galaxy has now been observed using gravitational lensing, unexpectedly turning out to be a compact, fast-spinning disk galaxy rather than a proto-bulge galaxy. Dead disk galaxy formed by incoming gas When the Universe was only three billion years old, half of the most massive galaxies were already 'dead', meaning that few new stars would form in them. It is believed that these galaxies grew into the massive local elliptical galaxies seen today. Sune Toft et al . report an analysis of a galaxy that has been strongly gravitationally lensed. This means that they can observe spatial scales that are far smaller than those accessible by any other means. They find that, surprisingly, the galaxy is a fast-spinning disk and that its stars formed in situ rather than in a nuclear starburst. They conclude that the gas out of which the stars formed was accreted from outside the galaxy in cold streams of gas. 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 1 , 2 , 3 , 4 . 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 5 , 6 , 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 7 . 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 8 . 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 9 . This result confirms previous indirect indications 10 , 11 , 12 , 13 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.

At z=2, when the Universe was just 3 Gyr old, half of the most massive galaxies were extremely compact and had already exhausted their fuel for star formation1–4. 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 companions5,6, but validating this scenario requires higher resolution observations of their centers than currently possible, even from space. Magnification due to gravitational lensing offers a unique opportunity to resolve their inner regions, as demonstrated in a recent study of a z=2.6 compact spheroidal galaxy which revealed a bulge, rotating at velocities comparable to the fastest rotating local ellipticals7. Following the same approach, here we map the stellar populations and kinematics of a lensed z=2.1478 compact galaxy, which surprisingly turn out to be a fast spinning, rotationally supported disk galaxy. Rather than in a merger-driven nuclear starburst8, its stars must thus have formed in a disk, likely 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 halo9. This result unambiguously confirm indications from a growing body of indirect evidence10–13 that the first galaxies to cease star formation must go through major changes not just in their structure, but also in their kinematics to evolve into present day ellipticals.

When the Universe was just 3 billion years old, half of the most massive galaxies had already ceased star formation, and such a galaxy has now been observed using gravitational lensing, unexpectedly turning out to be a compact, fast-spinning disk galaxy rather than a proto-bulge galaxy.

Quiescent galaxy candidates are typically identified by their low unobscured star formation rates from deep field photometric surveys. However, their selection technique relies on the assumption of a universal dust attenuation curve. It is important to verify the selection through independent SFR indicators at longer wavelengths. Current mid-, far-infrared and radio surveys are limited to detecting only galaxies with very strong star formation or AGN activity. Here, I present the first comprehensive stacking results across mid-, far-infrared and radio wavelengths using Spitzer, Herschel and VLA data in the COSMOS field (Man et al. 2014). We find that the rest-frame NUV-r and r-J color criteria, combined with low 24 μm emission, provides a robust selection of truly quiescent galaxies out to z = 3. Additionally, we find evidence of radio emission in excess of the expected total star formation in quiescent galaxies at z ~ 0-1.5, indicative of a ubiquitous presence of low-luminosity radio AGN among them.

The role of galaxy mergers in the evolution of massive galaxies remains debated. While deep near-infrared surveys have enabled several independent merger rate measurements out to z~3, they are limited to small samples and results are discrepant at z=2–3. In Man et al., we use the UltraVISTA and CANDELS surveys to obtain the largest sample of photometric galaxy pairs at z>1 for measuring the galaxy merger fraction and rate of massive galaxies. We find that the discrepancy of previous studies is due to selection effect. Defining galaxy pairs by stellar mass ratio leads to a flat z-evolution of the merger fraction, while defining by flux ratio leads to an increasing trend. The implications on the evolution of massive galaxies are summarized here.