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Register a new userDeep ALMA photometry of distant X-ray AGN: improvements in star formation rate constraints, and AGN identification
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We present the star formation rates (SFRs) of a sample of 109 galaxies with X-ray selected active galactic nuclei (AGN) with moderate to high X-ray luminosities (L(2-8keV)= 10^42-10^45 erg/s), at redshifts 1 < z < 4.7, that were selected to be faint or undetected in the Herschel bands. We combine our deep ALMA continuum observations with deblended 8-500{mu}m photometry from Spitzer and Herschel, and use infrared (IR) SED fitting and AGN - star formation decomposition methods. The addition of the ALMA photometry results in an order of magnitude more X-ray AGN in our sample with a measured SFR (now 37%). The remaining 63% of the sources have SFR upper limits that are typically a factor of 2-10 times lower than the pre-ALMA constraints. With the improved constraints on the IR SEDs, we can now identify a mid-IR (MIR) AGN component in 50% of our sample, compared to only ~1% previously. We further explore the F870{mu}m/F24{mu}m-redshift plane as a tool for the identification of MIR emitting AGN, for three different samples representing AGN dominated, star formation dominated, and composite sources. We demonstrate that the F870{mu}m/F24{mu}m-redshift plane can successfully split between AGN and star formation dominated sources, and can be used as an AGN identification method.
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In this study we investigate the relationship between the star formation rate, SFR, and AGN luminosity, L(AGN), for ~2000 X-ray detected AGN. The AGN span over three orders of magnitude in X-ray luminosity (10^(42) < L(2-8keV) < 10^(45.5) erg/s) and are in the redshift range z = 0.2 - 2.5. Using infrared (IR) photometry (8 - 500um), including deblended Spitzer and Herschel images and taking into account photometric upper limits, we decompose the IR spectral energy distributions into AGN and star formation components. Using the IR luminosities due to star formation, we investigate the average SFRs as a function of redshift and AGN luminosity. In agreement with previous studies, we find a strong evolution of the average SFR with redshift, tracking the observed evolution of the overall star forming galaxy population. However, we find that the relationship between the average SFR and AGN luminosity is flat at all redshifts and across all the AGN luminosities investigated; in comparison to previous studies, we find less scatter amongst the average SFRs across the wide range of AGN luminosities investigated. By comparing to empirical models, we argue that the observed flat relationship is due to short timescale variations in AGN luminosity, driven by changes in the mass accretion rate, which wash out any underlying correlations between SFR and L(AGN). Furthermore, we show that the exact form of the predicted relationship between SFR and AGN luminosity (and its normalisation) is highly sensitive to the assumed intrinsic Eddington ratio distribution.
The JWST MIRI instrument will revolutionize extragalactic astronomy with unprecedented sensitivity and angular resolution in mid-IR. Here, we assess the potential of MIRI photometry to constrain galaxy properties in the Cosmic Evolution Early Release Science (CEERS) survey. We derive estimated MIRI fluxes from the spectral energy distributions (SEDs) of real sources that fall in a planned MIRI pointing. We also obtain MIRI fluxes for hypothetical AGN-galaxy mixed models varying the AGN fractional contribution to the total IR luminosity ($rm frac_{AGN}$). Based on these model fluxes, we simulate CEERS imaging (3.6-hour exposure) in 6 bands from F770W to F2100W using MIRISIM, and reduce these data using JWST PIPELINE. We perform PSF-matched photometry with TPHOT, and fit the source SEDs with X-CIGALE, simultaneously modeling photometric redshift and other physical properties. Adding the MIRI data, the accuracy of both redshift and $rm frac_{AGN}$ is generally improved by factors of $gtrsim 2$ for all sources at $zlesssim 3$. Notably, for pure-galaxy inputs ($rm frac_{AGN}=0$), the accuracy of $rm frac_{AGN}$ is improved by $sim 100$ times thanks to MIRI. The simulated CEERS MIRI data are slightly more sensitive to AGN detections than the deepest X-ray survey, based on the empirical $L_{rm X}$-$L_{rm 6mu m}$ relation. Like X-ray observations, MIRI can also be used to constrain the AGN accretion power (accuracy $approx 0.3$ dex). Our work demonstrates that MIRI will be able to place strong constraints on the mid-IR luminosities from star formation and AGN, and thereby facilitate studies of the galaxy/AGN co-evolution.
We present high resolution (0.3) Atacama Large Millimeter Array (ALMA) 870um imaging of five z~1.5-4.5 X-ray detected AGN with luminosities of L(2-8keV)>10^42 erg/s. These data provide a >~20x improvement in spatial resolution over single-dish rest-frame FIR measurements. The sub-millimetre emission is extended on scales of FWHM~0.2-0.5, corresponding to physical sizes of 1-3 kpc (median value of 1.8 kpc). These sizes are comparable to the majority of z=1-5 sub-millimetre galaxies (SMGs) with equivalent ALMA measurements. In combination with spectral energy distribution analyses, we attribute this rest-frame far-infrared (FIR) emission to dust heated by star formation. The implied star-formation rate surface densities are ~20-200 Mo/yr/kpc^2, which are consistent with SMGs of comparable FIR luminosities (i.e., L(IR)~[1-5]x10^(12)Lo). Although limited by a small sample of AGN, which all have high FIR luminosities, our study suggests that the kpc-scale spatial distribution and surface density of star formation in high-redshift star-forming galaxies is the same irrespective of the presence of X-ray detected AGN.
This paper estimates the specific accretion-rate distribution of AGN using a sample of 4821 X-ray sources from both deep and shallow surveys. The specific accretion-rate distribution is defined as the probability of a galaxy with a given stellar mass and redshift hosting an active nucleus with a certain specific accretion rate. We find that the probability of a galaxy hosting an AGN increases with decreasing specific accretion rate. There is evidence that this trend reverses at low specific accretion rates, $lambda<10^{-4}-10^{-3}$ (in Eddington units). There is also a break close to the Eddington limit, above which the probability of an accretion event decreases steeply. The specific accretion-rate distribution evolves such that the fraction of AGN among galaxies drops toward lower redshifts. This decrease in the AGN duty cycle is responsible for the strong evolution of the accretion density of the Universe from redshift $zapprox1-1.5$ to the present day. Our analysis also suggests that this evolution is accompanied by a decoupling of accretion events onto black holes from the formation of stars in galaxies. There is also evidence that at earlier times the relative probability of high vs low specific accretion-rate events among galaxies increases. We argue that this differential redshift evolution of the AGN duty cycle with respect to $lambda$ produces the AGN downsizing trend, whereby luminous sources peak at earlier epochs compared to less luminous ones. Finally, we also find a stellar-mass dependence of the specific accretion-rate distribution, with more massive galaxies avoiding high specific accretion-rate events.
We investigate the balance of power between stars and AGN across cosmic history, based on the comparison between the infrared (IR) galaxy luminosity function (LF) and the IR AGN LF. The former corresponds to emission from dust heated by stars and AGN, whereas the latter includes emission from AGN-heated dust only. We find that at all redshifts (at least up to z~2.5), the high luminosity tails of the two LFs converge, indicating that the most infrared-luminous galaxies are AGN-powered. Our results shed light to the decades-old conundrum regarding the flatter high-luminosity slope seen in the IR galaxy LF compared to that in the UV and optical. We attribute this difference to the increasing fraction of AGN-dominated galaxies with increasing total infrared luminosity (L_IR). We partition the L_IR-z parameter space into a star-formation and an AGN-dominated region, finding that the most luminous galaxies at all epochs lie in the AGN-dominated region. This sets a potential `limit to attainable star formation rates, casting doubt on the abundance of `extreme starbursts: if AGN did not exist, L_IR>10^13 Lsun galaxies would be significantly rarer than they currently are in our observable Universe. We also find that AGN affect the average dust temperatures (T_dust) of galaxies and hence the shape of the well-known L_IR-T_dust relation. We propose that the reason why local ULIRGs are hotter than their high redshift counterparts is because of a higher fraction of AGN-dominated galaxies amongst the former group.
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