Radio and far-IR emission associated with a massive star-forming galaxy candidate atz≃ 6.8: a radio-loud AGN in the reionization era?
Ryan EndsleyDaniel P. StarkXiaohui FanRenske SmitFeige WangJinyi YangKevin HainlineJianwei LyuRychard BouwensSander Schouws
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We report the identification of radio (0.144-3 GHz), mid-IR, far-IR, and sub-mm (24-850$\mu$m) emission at the position of one of 41 UV-bright (M$_\mathrm{UV}^{}\lesssim-21.25$) $z\simeq6.6-6.9$ Lyman-break galaxy candidates in the 1.5 deg$^2$ COSMOS field. This source, COS-87259, exhibits a sharp flux discontinuity (factor $>$3) between two narrow/intermediate bands at 9450 and 9700 Angstroms and is undetected in all nine bands blueward of 9600 Angstroms, as expected from a Lyman-alpha break at $z\simeq6.8$. The full multi-wavelength (X-ray through radio) data of COS-87529 can be self-consistently explained by a very massive (M$_{\ast}=10^{10.8}$ M$_{\odot}$) and extremely red (rest-UV slope $\beta=-0.59$) $z\simeq6.8$ galaxy with hyperluminous infrared emission (L$_{\mathrm{IR}}=10^{13.6}$ L$_{\odot}$) powered by both an intense burst of highly-obscured star formation (SFR$\approx$1800 M$_{\odot}$ yr$^{-1}$) and an obscured ($\tau_{\mathrm{9.7\mu m}}=7.7\pm2.5$) radio-loud (L$_{\mathrm{1.4\ GHz}}\approx10^{25.4}$ W Hz$^{-1}$) AGN. The radio emission is compact (1.04$\pm$0.12 arcsec) and exhibits an ultra-steep spectrum between 1.32-3 GHz ($\alpha=-1.57^{+0.22}_{-0.21}$) that flattens at lower frequencies ($\alpha=-0.86^{+0.22}_{-0.16}$ between 0.144-1.32 GHz), consistent with known $z>4$ radio galaxies. We also demonstrate that COS-87259 may reside in a significant (11$\times$) galaxy overdensity at $z\simeq6.6-6.9$, as common for systems hosting radio-loud AGN. Nonetheless, a spectroscopic redshift will ultimately be required to establish the true nature of COS-87259 as we cannot yet completely rule out low-redshift solutions. If confirmed to lie at $z\simeq6.8$, the properties of COS-87259 would be consistent with a picture wherein AGN and highly-obscured star formation activity are fairly common among very massive (M$_{\ast}>10^{10}$ M$_{\odot}$) reionization-era galaxies.21 cm intensity mapping (IM) has the potential to be a strong and unique probe of cosmology from redshift of order unity to redshift potentially as high as 30. For post-reionization 21 cm observations, the signal is modulated by the thermal and dynamical reaction of gas in the galaxies to the passage of ionization fronts during the Epoch of Reionization. In this work, we investigate the impact of inhomogeneous reionization on the post-reionization 21 cm power spectrum and the induced shifts of cosmological parameters at redshifts $3.5 \lesssim z \lesssim 5.5$. We make use of hydrodynamics simulations that could resolve small-scale baryonic structure evolution to quantify HI abundance fluctuation, while semi-numerical large box 21cmFAST simulations capable of displaying inhomogeneous reionization process are deployed to track the inhomogeneous evolution of reionization bubbles. We discussed the prospects of capturing this effect in two post-reionization 21 cm intensity mapping experiments: SKA1-LOW and PUMA. We find the inhomogeneous reionization effect could impact the HI power spectrum up to tens of percent level and shift cosmological parameters estimation from sub-percent to tens percent in the observation of future post-reionization 21 cm intensity mapping experiments such as PUMA, while SKA1-LOW is likely to miss this effect at the redshifts of interest given the considered configuration. In particular, the shift is up to 0.0206 in the spectral index $n_s$ and 0.0192 eV in the sum of the neutrino masses $\sum m_\nu$ depending on the reionization model and the observational parameters. We discuss strategies to mitigate and separate these biases.
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Simulations of galaxy growth need to invoke strong negative feedback from active galactic nuclei (AGN) to suppress the formation of stars and thus prevent the over-production of very massive systems. While some observations provide evidence for such negative feedback, other studies find either no feedback, or even positive feedback, with increased star formation associated with higher AGN luminosities. Here we report an analysis of several hundred AGN and their host galaxies in the Chandra Deep Field South using X-ray and radio data for sample selection. Combined with archival far infrared data as a reliable tracer of star formation activity in the AGN host galaxies, we find that AGN with pronounced radio jets exhibit a much higher star formation rate than the purely X-ray selected ones, even at the same X-ray luminosities. This difference implies that positive AGN feedback plays an important role, too, and therefore has to be accounted for in all future simulation work. We interpret this to indicate that the enhanced star formation rate of radio selected AGN arises because of jet-induced star formation, as is hinted by the different jet powers among our AGN samples, while the suppressed star formation rate of X-ray selected AGN is caused by heating and photo-dissociation of molecular gas by the hot AGN accretion disc.
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This is the fourth in our series of papers discussing the nature of radio-excess galaxies, which have radio emission associated with an active nucleus but do not fit into the traditional categories of either radio-loud or radio-quiet active galaxies. In this paper we present optical spectra of our sample of far-infrared (FIR)-luminous radio-excess galaxies. Optical emission-line ratio diagnostics are used to determine the dominant source of the ionizing radiation. We find that radio excess is an excellent indicator of the presence of an active nucleus. The radio-excess sample contains a much higher fraction of active galactic nuclei (AGNs) than samples selected on FIR luminosity alone or using other criteria such as warm FIR colors. Several objects have ambiguous classifications and are likely to be composite objects with mixed excitation. The type of optical spectrum appears to be associated with the radio loudness: our results suggest that radio-loud objects may be more "pure" AGNs than radio-intermediate objects. We find strong evidence for interaction between the radio plasma and the surrounding gas. Broad, structured optical emission lines are observed, and a relative blueshift is measured between the [O III] λ5007 and Hα lines in several sources. Jet energy fluxes are inferred from the [O III] λ5007 luminosities using a shock model for the interaction between the radio jet and the line-emitting gas. The jet energy fluxes of the radio-excess objects are lower than in powerful radio sources, consistent with our previous results. We conclude that the jets of radio-intermediate sources are intrinsically weaker than those in sources with more powerful radio emission. A significant fraction of the sample spectra show poststarburst stellar continua with A star absorption lines. Poststarburst stellar populations are consistent with the large fraction of merging or disturbed host galaxies in the sample. The ages of the radio sources are significantly less than those of A stars, indicating that, if the radio sources are associated with merging activity, there is a delay between the interaction and the initiation of the radio activity.
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Powerful extended radio galaxies in the 3CR sample are observed out to redshifts of about 2. For redshifts greater than 0.3, the average lobe-lobe size of these sources decreases monotonically with redshift for all reasonable cosmological parameter choices. This suggests that the characteristic time for which an active galactic nucleus (AGN) produces highly collimated outflows that power radio emission is shorter for high-redshift sources than it is for low-redshift sources. The analysis presented here supports this conclusion.
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Intergalactic medium temperature is a powerful probe of the epoch of reionisation, as information is retained long after reionisation itself. However, mean temperatures are highly degenerate with the timing of reionisation, with the amount heat injected during the epoch, and with the subsequent cooling rates. We post-process a suite of semi-analytic galaxy formation models to characterise how different thermal statistics of the intergalactic medium can be used to constrain reionisation. Temperature is highly correlated with redshift of reionisation for a period of time after the gas is heated. However as the gas cools, thermal memory of reionisation is lost, and a power-law temperature-density relation is formed, $T = T_0(1+\delta)^{1-\gamma}$ with $\gamma \approx 1.5$. Constraining our model against observations of electron optical depth and temperature at mean density, we find that reionisation likely finished at $z_{\rm{reion}} = 6.8 ^{+ 0.5} _{-0.8}$ with a soft spectral slope of $\alpha = 2.8 ^{+ 1.2} _{-1.0}$. By restricting spectral slope to the range $[0.5,2.5]$ motivated by population II synthesis models, reionisation timing is further constrained to $z_{\rm{reion}} = 6.9 ^{+ 0.4} _{-0.5}$. We find that, in the future, the degeneracies between reionisation timing and background spectrum can be broken using the scatter in temperatures and integrated thermal history.
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ABSTRACT 21-cm intensity mapping (IM) has the potential to be a strong and unique probe of cosmology from redshift of order unity to redshift potentially as high as 30. For post-reionization 21-cm observations, the signal is modulated by the thermal and dynamical reaction of gas in the galaxies to the passage of ionization fronts during the epoch of reionization. In this work, we investigate the impact of inhomogeneous reionization on the post-reionization 21-cm power spectrum and the induced shifts of cosmological parameters at redshifts 3.5 ≲ z ≲ 5.5. We make use of hydrodynamics simulations that could resolve small-scale baryonic structure evolution to quantify H i abundance fluctuation, while seminumerical large box 21cmfast simulations capable of displaying inhomogeneous reionization process are deployed to track the inhomogeneous evolution of reionization bubbles. We discussed the prospects of capturing this effect in two post-reionization 21-cm IM experiments: SKA1-LOW and PUMA. We find the inhomogeneous reionization effect could impact the H i power spectrum up to tens of per cent level and shift cosmological parameters estimation from sub-per cent to tens per cent in the observation of future post-reionization 21-cm IM experiments such as PUMA, while SKA1-LOW is likely to miss this effect at the redshifts of interest given the considered configuration. In particular, the shift is up to 0.0206 in the spectral index ns and 0.0192 eV in the sum of the neutrino masses ∑mν depending on the reionization model and the observational parameters. We discuss strategies to mitigate and separate these biases.
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There have been a multitude of observational findings supporting a co-evolution of the central BH and its host galaxy. This co-evolution is expected to be driven by a connection between the two growing mechanisms of active galactic nuclei (AGN; accretion onto the central super-massive black hole), and star formation. In an effort to find more direct evidence for a connection between the two mechanisms of AGN and star formation I investigate the star formation rates (SFRs) of galaxies hosting X-ray, optical, and radio AGN. For the analysis of these samples I have developed methods to calculate the mean and individual SFRs of distant AGN and improve on previous work by simultaneously taking into account redshift and mass when interpreting the results. I use infrared photometry from WISE, Spitzer and Herschel to decompose the infrared (IR) spectral en- ergy distributions (SEDs) into AGN and star formation components and undertake careful treatment of the upper limits in the SED analyses and average SFR calculations. Using these methods I have calculated the mean SFRs of ~2000 X-ray and ∼3000 optically selected AGN spanning the AGN luminosity range of 10^43
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We have formed a new sample which consists of extended extragalactic radio sources without obvious active galactic nuclei (AGNs) in them. Most of these sources appear to be dead double radio sources. These sources with steep spectra (α < −1.8; S ∝ να) were identified using the 74 (VLSS) and the 1400 MHz (NVSS) surveys and further imaged using the Very Large Array (VLA) and the Giant Meterwave Radio Telescope (GMRT). The radio morphologies of these sources are rather unusual in the sense that no obvious cores and jets are detected in these sources, but two extended lobes are detected in most. The mean redshift of 4 of the 10 sources reported here is ∼ 0.2. At a redshift of 0.2, the linear extents of the sources in the current sample are ∼ 250 kpc with their spectral luminosities at 1.4 GHz in the range 2–25 ×1023 W Hz−1. The steep spectra of these sources are a result of the cessation of AGN activities in them about 15–100 million years ago. Before the cessation of AGN activity, the radio luminosities of these galaxies were ∼ 1000 times brighter than their current luminosities and would have been comparable to those of the brightest active radio galaxies detected in the local universe (L1.4 ∼ 1027 W Hz−1). The dead radio galaxies reported here represent the "tip of the iceberg" and quantifying the abundance of such a population has important implications to the life cycle of the AGN.
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We have recently shown that X-ray observations of the population of ‘low-excitation’ radio galaxies, which includes most low-power, Fanaroff–Riley class I sources as well as some more powerful Fanaroff–Riley class II objects, are consistent with a model in which the active nuclei of these objects are not radiatively efficient at any waveband. In another recent paper, Allen et al. have shown that Bondi accretion of the hot, X-ray emitting phase of the intergalactic medium (IGM) is sufficient to power the jets of several nearby, low-power radio galaxies at the centres of clusters. In this paper, we combine these ideas and suggest that accretion of the hot phase of the IGM is sufficient to power all low-excitation radio sources, while high-excitation sources are powered by accretion of cold gas that is in general unrelated to the hot IGM. This model explains a number of properties of the radio-loud active galaxy population, and has important implications for the energy input of radio-loud active galactic nuclei into the hot phase of the IGM: the energy supply of powerful high-excitation sources does not have a direct connection to the hot phase.
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