The suppression of star formation by powerful active galactic nuclei (1310.4147v1)

Abstract: The old, red stars which constitute the bulges of galaxies, and the massive black holes at their centres, are the relics of a period in cosmic history when galaxies formed stars at remarkable rates and active galactic nuclei (AGN) shone brightly from accretion onto black holes. It is widely suspected, but unproven, that the tight correlation in mass of the black hole and stellar components results from the AGN quenching the surrounding star formation as it approaches its peak luminosity. X-rays trace emission from AGN unambiguously, while powerful star-forming galaxies are usually dust-obscured and are brightest at infrared to submillimetre wavelengths. Here we report observations in the submillimetre and X-ray which show that rapid star formation was common in the host galaxies of AGN when the Universe was 2-6 Gyrs old, but that the most vigorous star formation is not observed around black holes above an X-ray luminosity of 10^44 erg/s. This suppression of star formation in the host galaxies of powerful AGN is a key prediction of models in which the AGN drives a powerful outflow, expelling the interstellar medium of its host galaxy and transforming the galaxy's properties in a brief period of cosmic time.

The Suppression of Star Formation by Powerful Active Galactic Nuclei

In the paper titled "The suppression of star formation by powerful active galactic nuclei," the authors explore the intricate relationship between active galactic nuclei (AGN) and star formation in the universe's early epochs. Their key focus is the potential quenching effect that AGN might have on star formation, which aligns with theoretical models predicting that AGN feedback mechanisms significantly influence galaxy evolution.

Summary of Findings

The paper leverages submillimetre and X-ray observations to analyze star formation rates in AGN host galaxies when the universe was 2–6 Gyrs old. Through the examination of deep X-ray data from the Chandra Deep Field North (CDF-N) and infrared/submillimetre data from SPIRE observations, the authors present definitive evidence indicating that vigorous star formation is markedly absent near black holes exceeding X-ray luminosities of 10<sup\>44</sup> erg s^-1.

Data and Methodology

The research hinges on a substantial dataset from the CDF-N and SPIRE/HerMES program. The authors adopt strict selection criteria, focusing on X-ray sources in the 2–8 keV band to mitigate the influence of obscuration and employing spectroscopic redshift data to ensure accurate analysis. The sample includes AGN luminous enough to exclude sources with 2–8 keV luminosities below 10<sup\>42</sup> erg s^-1. Luminosity calculations assume standard power law spectra.

Results

Detection statistics reveal that for AGN within the redshift range 1 < z < 3, 11 out of 44 AGN with intermediate X-ray luminosities (10<sup\>43</sup> erg s^-1 < L_X < 10<sup\>44</sup> erg s^-1) were detected at 250 µm, implying ongoing star formation. In stark contrast, none of the 21 AGN with X-ray luminosities above 10<sup\>44</sup> erg s^-1 were detected at this wavelength, underpinning the notion that AGN luminosity suppresses star formation at these scales.

The spectral energy distributions of detected galaxies indicate significant star formation rates, ranging from 50 to 1750 M⊙ per year, mostly unaffected by interference from the AGN itself which typically constituted less than 10% of infrared luminosity.

Implications

The findings challenge models where AGN feedback acts solely through radio-mode suppression, typically posited in semi-analytical models to limit galaxy mass and luminosity. Observations instead affirm quasar-mode feedback scenarios, predicting that AGN luminosity peaks after star formation rates, consistent with the observed suppression at high AGN luminosities. This also suggests residual star formation persists during peak AGN activity.

The researchers speculate on the significance of these dynamics in galaxy formation. While quasar-mode feedback emerges as a plausible mechanism for terminating star formation, the paper does not discriminate between potential triggers for initial starburst episodes, such as major mergers or gas accretion into massive halos.

Future Directions

Further exploration into the mechanisms and exact temporal dynamics of AGN feedback in galaxy evolution could refine these models and enhance our understanding of cosmic history. Continued observations, particularly with more sensitive instruments capable of reducing spectral energy confusion and improving detection accuracy, will be critical in deciphering the complex interplay between star formation and AGN activity.

This paper represents a significant step towards elucidating the multifaceted role of AGN in shaping galaxy evolution, providing empirical constraints crucial for advancing theoretical models and simulations in astrophysics.