Evidence for Mature Bulges and an Inside-out Quenching Phase 3 Billion Years After the Big Bang (1504.04021v1)

Abstract: Most present-day galaxies with stellar masses $\geq10^{11}$ 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 timescales, 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 $\sim1$ kiloparsec resolution. We find that, in the most massive galaxies, star formation is quenched from the inside out, on timescales 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.

Inside-out Quenching of Massive Galaxies at High Redshift

The paper documents a comprehensive paper of the cessation, or "quenching," of star formation in massive galaxies approximately 10 billion years ago, exploring the role of such processes in the evolution of these galaxies into the dense, spheroidal systems observed in the present universe. The paper employs high-resolution measurements of stellar mass and star-formation rate (SFR) surface densities in star-forming galaxies at a redshift of 2.2, utilizing adaptive-optics kinemetric spectroscopy from the SINFONI instrument on the Very Large Telescope, accompanied by Hubble Space Telescope imaging.

Key Findings

1. Inside-out Quenching Observations: The research evidences that quenching occurs from the central regions outward in the most massive galaxies. The inner regions exhibit cessation of star formation on timescales less than 1 billion years, while the outer disks continue star formation over several billion years. This inside-out quenching mechanism is critical to explaining the transition from star-forming galaxies to quiescent, dense spheroids by z=0.
2. Star Formation and Surface Densities: Star formation is sustained at large radial distances in these galaxies while significant stellar growth is completed in the central bulges. The inner regions become star-formation inactive, with the specific star-formation rate (sSFR) being two orders of magnitude lower than in the outer disk regions. Despite their current active star-forming state, these galaxies already possess the central stellar mass densities characteristic of today's non-star-forming spheroids.
3. Sérsic Profile Analysis: Surface SFR density profiles conform to a disk-like Sérsic profile, with indices reflecting the progressive central concentration of stellar mass with increasing total mass. This provides a structural understanding of galaxy morphology evolution over cosmic time.
4. Implications for Bulge Formation: The stellar densities present in these high-redshift cores suggest that today's massive spheroids are their evolutionary descendants. The observations align with gas-rich, dissipative processes preceding the current epoch. Despite notable quenching in centers, star formation at the peripheries sustains the stellar mass increase, reaffirming past merger or instability theories driving early bulge compaction.

Implications and Future Directions

The paper clarifies key facets of galaxy evolution, particularly regarding the quenching processes necessary for transitioning between active star-forming galaxies to quiescent spheroidal systems. Notably, the findings bolster models involving gas-rich environments and internal feedback mechanisms to explain early cessation of star formation. These insights can guide simulations and theoretical work exploring galaxy evolution, especially addressing the balance between internal dynamics and external factors like supermassive black hole feedback.

Future research might focus on broadening the sample size to verify trends across a variety of galaxy masses and environments or employ higher sensitivity instruments to capture finer structural details of the quenching processes. Further integration with cosmological simulations could test the causality between high central stellar mass densities and star formation cessation, examining whether internal mechanisms or broader cosmic conditions primarily drive this evolutionary path.

Overall, this intricate paper advances the astrophysical understanding of galactic evolution at a pivotal epoch, offering solutions to long-standing questions concerning how galaxies develop structural and morphological attributes recognized today.