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Tuesday, October 28, 2014

SZ effect detected from Active Galactic Nuclei (AGN) or "Dark Holes" (DH in short).

This is the very first time the thermal Sunyaev–Zel’dovich effect associated with the halos that host the AGN is detected at the 5σ level through its spectral signature – typically used to study large galaxy clusters – can also be used to learn a great deal about smaller formations.

The SZ effect occurs when high-energy electrons in hot gas interact with faint light in the cosmic microwave background, light left over from earliest times when the universe was a thousand times hotter and a billion times denser than today.

The SZ is usually used to study clusters of hundreds of galaxies but the galaxies they were observing are much smaller and have just a companion or two.

Thus research on the SZ effect has entered a new era and implications are profound in the theory of galaxy formation.

In space, hot gas drawn into a galaxy can cool and condense, forming stars. Some gas also funnels down into the galaxy’s dark hole, which grows together with the stellar population. This cycle can repeat continuously; more gas is pulled in to cool and condense, more stars begin to shine and the central black hole grows more massive.

But in nearly all mature galaxies – the big galaxies called “elliptical” because of their shape – that gas doesn’t cool any more. If gas is kept hot, it can’t collapse. When that happens: No new stars.

This research found that the elliptical galaxies with radio-frequency feedback – relativistic radio-frequency-emitting particles shooting from the massive central black holes at their center at close to the speed of light – all contain hot gas and a dearth of infant stars. That provides crucial evidence for their hypothesis that this radio-frequency feedback is the “off switch” for star-making in mature galaxies.

However it is still not known just why black holes in mature elliptical galaxies begin to emit radio-frequency feedback. The exact mechanism behind this is not fully understood and there are still debates.

In the paper linked below is investigated the ensemble millimeter properties of 1.4 GHz selected radio sources by stacking 148, 218, 277, 600, 857 and 1200 GHz data from ACT and Herschel on the positions of radio sources selected from two joint catalogs of FIRST and NVSS 1.4 GHz sources.

Although most of the radio sources are AGN, whose spectra are expected to fall with increasing frequency at millimeter wavelengths, they see evidence for an average rising spectrum. This observed spectral inversion in the ACT bands is attributed to the SZ effect of the halos that host the AGN. They construct SEDs for a sample of radio sources (Best & Heckman 2012) that have optical counterparts and thus known redshifts and classifications as either AGN or SFGs. In order to constrain the contribution to the SED from dust emission, we also stack data from Herschel surveys, and to better constrain the synchrotron spectrum we include 5 GHz data from PMN and a GBT survey. The AGN SED is well fit by a model that includes synchrotron emission, the SZ effect and dust emission. The SFG SED is well fit by a model that includes synchrotron emission, free-free emission, CO line emission and dust emission.Using a larger catalog enables the construction of SEDs for sources binned in 1.4 GHz flux density. The radio source sample from Kimball & Ivezi ́c (2008) was constructed with- out optical counterparts, and while spectroscopic redshifts are lacking, the sample includes greater numbers of sources (4,344 used in this study) and extends to higher redshifts. Even though we exclude bright ACT sources, the binned 1.4 GHz selected radio sources are detected in the ACT data. The ensemble averaged millimeter synchrotron spec- tral index (S ∝ ν−α) for AGN is 0.95 ± 0.01 for the bright (∼ 100 mJy) sources, with evidence for flattening toward fainter 1.4 GHz source flux densities. The best-fit model includes an SZ contribution detected at the 5σ level. No relativistic SZ effect was needed to adequately describe the data.
The detection of the SZ effect in the radio galaxies’ millimetre wavelength SED provides evidence that, on average, galaxies that host radio AGN support hot gaseous halos. This is important in the context of the growing support for the overall picture that most radio AGN are powered by radio-mode accretion, where AGN are fueled by the accretion of gas ultimately from the hot hydrostatically supported halo. In this picture, the radio jets from the AGN in turn provide heat that counters the cooling of the gas, shutting off the AGN fuel supply by establishing a feedback loop.

We have also calculated the contribution of these radio sources to the CMB power spectrum. At 148 GHz, after correcting for the SZ effect, the sources contribute l(l+1)CA×A/(2π) = 0.37±0.03 μK2 at l = 3000, where the uncertainty is dominated by the uncertainty on the contribution to the SZ effect to the measured flux densities. The cross-power between the SZ effect and the radio source emission at 148 GHz is l(l + 1)CA×SZ/(2π) = −0.29 ± 0.07 μK2.

First image is an elliptical galaxy NGC 1132, as seen by NASA’s Chandra X-Ray Observatory; the blue/purple in the image is the X-ray glow from hot, diffuse gas that is not forming into stars. (Credit: NASA, ESA, M. West (ESO, Chile), and CXC/Penn State University/G. Garmire, et al.)


We present a statistical analysis of the millimeter-wavelength properties of 1.4 GHz- selected sources and a detection of the Sunyaev-Zel’dovich effect associated with the halos that host them. The Atacama Cosmology Telescope (ACT) has conducted a survey at 148 GHz, 218 GHz and 277 GHz along the celestial equator. Using samples of radio sources selected at 1.4 GHz from the Faint Images of the Radio Sky at Twenty- Centimeters (FIRST) Survey and the National Radio Astronomy Observatory Very Large Array Sky Survey (NVSS), we measure the stacked 148, 218 and 277 GHz flux densities for sources with 1.4 GHz flux densities ranging from 5 to 200 mJy. At these flux densities, the radio source population is dominated by active galactic nuclei (AGN), with both steep and flat spectrum populations, which have combined radio-to- millimeter spectral indices ranging from 0.5 to 0.95, reflecting the prevalence of steep spectrum sources at high flux densities and the presence of flat spectrum sources at lower flux densities. The thermal Sunyaev-Zel’dovich (SZ) effect associated with the halos that host the AGN is detected at the 5σ level through its spectral signature. When we compare the SZ effect with weak lensing measurements of radio galaxies, we find that the relation between the two is consistent with that measured by Planck for local bright galaxies. We present a detection of the SZ effect in some of the lowest mass≈ 1013 M h−1) studied to date. This detection is particularly 200 ⊙ 70 halos (average M important in the context of galaxy evolution models, as it confirms that galaxies with radio AGN also typically support hot gaseous halos. With Herschel⋆ observations, we show that the SZ detection is not significantly contaminated by dusty galaxies or by dust associated with the AGN or galaxies hosting the AGN. We show that 5 mJy < S1.4 < 200 mJy radio sources contribute l(l + 1)Cl/(2π) = 0.37 ± 0.03 μK2 to the angular power spectrum at l = 3000 at 148 GHz, after accounting for the SZ effect associated with their host halos.