We investigate the relation between star formation (SF) and black hole accretion luminosities, using a sample of 492 type-2 active galactic nuclei (AGNs) at z < 0.22, which are detected in the far-infrared (FIR) surveys with AKARI and Herschel. We ad
opt FIR luminosities at 90 and 100 um as SF luminosities, assuming the proposed linear proportionality of star formation rate with FIR luminosities. By estimating AGN luminosities from [OIII]5007 and [OI]6300 emission lines, we find a positive linear trend between FIR and AGN luminosities over a wide dynamical range. This result appears to be inconsistent with the recent reports that low-luminosity AGNs show essentially no correlation between FIR and X-ray luminosities, while the discrepancy is likely due to the Malmquist and sample selection biases. By analyzing the spectral energy distribution, we find that pure-AGN candidates, of which FIR radiation is thought to be AGN-dominated, show significantly low-SF activities. These AGNs hosted by low-SF galaxies are rare in our sample (~ 1%). However, the low fraction of low-SF AGN is possibly due to observational limitations since the recent FIR surveys are insufficient to examine the population of high-luminosity AGNs hosted by low-SF galaxies.
We present an analysis of broad emission lines observed in moderate-luminosity active galactic nuclei (AGNs), typical of those found in X-ray surveys of deep fields, with the aim to test the validity of single-epoch virial black hole mass estimates.
We have acquired near-infrared (NIR) spectra of AGNs up to z ~ 1.8 in the COSMOS and Extended Chandra Deep Field-South Survey, with the Fiber Multi-Object Spectrograph (FMOS) mounted on the Subaru Telescope. These low-resolution NIR spectra provide a significant detection of the broad Halpha line that has been shown to be a reliable probe of black hole mass at low redshift. Our sample has existing optical spectroscopy which provides a detection of MgII, a broad emission line typically used for black hole mass estimation at z > 1. We carry out a spectral-line fitting procedure using both Halpha and MgII to determine the virial velocity of gas in the broad line region, the monochromatic continuum luminosity at 3000 A, and the total Halpha line luminosity. With a sample of 43 AGNs spanning a range of two decades in luminosity (i.e., L ~ 10^44-46 ergs/s), we find a tight correlation between the continuum and line luminosity with a distribution characterized by <log(L_3000/L_Halpha)> = 1.52 and a dispersion sigma = 0.16. There is also a close one-to-one relationship between the FWHM of Halpha and of MgII up to 10000 km/s with a dispersion of 0.14 in the distribution of the logarithm of their ratios. Both of these then lead to there being very good agreement between Halpha- and MgII-based masses over a wide range in black hole mass (i.e., M_BH ~ 10^7-9 M_sun). We do find a small offset in MgII-based masses, relative to those based on Halpha, of +0.17 dex and a dispersion sigma = 0.32. In general, these results demonstrate that local scaling relations, using MgII or Halpha, are applicable for AGN at moderate luminosities and up to z ~ 2.
We present a deep optical spectrum of TN J0924-2201, the most distant radio galaxy at z = 5.19, obtained with FOCAS on the Subaru Telescope. We successfully detect, for the first time, the CIV1549 emission line from the narrow-line region (NLR). In a
ddition to the emission-line fluxes of Ly alpha and CIV, we set upper limits on the NV and HeII emissions. We use these line detections and upper limits to constrain the chemical properties of TN J0924-2201. By comparing the observed emission-line flux ratios with photoionization models, we infer that the carbon-to-oxygen relative abundance is already [C/O] > -0.5 at a cosmic age of ~ 1.1 Gyr. This lower limit on [C/O] is higher than the ratio expected at the earliest phases of the galaxy chemical evolution, indicating that TN J0924-2201 has already experienced significant chemical evolution at z = 5.19.
Active galactic nuclei (AGNs) are characterized by a clear correlation between luminosity and metallicity (L_AGN-Z_AGN relation). The origin of this correlation is not clear. It may result from a relation between the black hole mass (M_BH) and metall
icity, or from a relation between the accretion rate (L/L_Edd) and metallicity. To investigate the origin of the L_AGN-Z_AGN relation, we use optical spectra of 2383 quasars at 2.3 < z < 3.0 from the Sloan Digital Sky Survey. By using this data set, we have constructed composite spectra of 33 subsamples in intervals of both M_BH and L/L_Edd. From these composite spectra we measure emission-line flux ratios that are sensitive to the metallicity of the broad line region (BLR); specifically, NV/CIV, NV/HeII, (SiIV+OIV])/CIV, and AlIII/CIV. We find that there is a significant correlation between M_BH and Z_BLR as inferred from all four metallicity-sensitive emission-line flux ratios. This result strongly suggests that the observed L_AGN-Z_AGN relation is mostly a consequence of the M_BH-Z_AGN relation. The relation between M_BH and Z_BLR is likely a consequence of both the M_BH-M_bul relation and of the mass-metallicity relation in the host galaxy. We also find that L/L_Edd correlates with the emission line flux ratios involving NV (more specifically, NV/CIV and NV/HeII), while it does not correlate with the other two metallicity sensitive emission line flux ratios, i.e., (SiIV+OIV])/CIV and AlIII/CIV. These correlations indicate that the emission-line flux ratios involving NV depend on both metallicity and relative abundance of nitrogen. We suggest that the relation between L/L_Edd and those line ratios involving nitrogen, is caused by a delay of the black hole accretion rate relative to the onset of nuclear star formation of about 10^8 years, which is the timescale required for the nitrogen enrichment.
We present new deep optical spectra of 9 high-z radio galaxies (HzRGs) at z > 2.7 obtained with FORS2 on VLT. These rest-frame ultraviolet spectra are used to infer the metallicity of the narrow-line regions (NLRs) in order to investigate the chemica
l evolution of galaxies in high-z universe. We focus mainly on the CIV/HeII and CIII]/CIV flux ratios that are sensitive to gas metallicity and ionization parameter. Although the NV emission has been widely used to infer the gas metallicity, it is often too weak to be measured accurately for NLRs. By combining our new spectra with data from the literature, we examine the possible redshift evolution of the NLR metallicity for 57 HzRGs at 1 < z < 4. Based on the comparison between the observed emission-line flux ratios and the results of our photoionization model calculations, we find no significant metallicity evolution in NLRs of HzRGs, up to z ~ 4. Our results imply that massive galaxies had almost completed their chemical evolution at much higher redshift (z > 5). Finally, although we detect strong NV emission lines in 5 HzRGs at z > 2.7, we point out that high NV/HeII ratios are not indicative of high metallicities but correspond to high ionization parameters of gas clouds in NLRs.
