We present deep near-infrared spectroscopy of six quasars at 6.1 ≤ z ≤ 6.7 with Very Large Telescope/X-Shooter and Gemini-N/GNIRS. Our objects, originally discovered through a wide-field optical ...survey with the Hyper Suprime-Cam (HSC) Subaru Strategic Program (HSC-SSP), have the lowest luminosities (−25.5 mag ≤ M1450 ≤ −23.1 mag) of the z > 5.8 quasars with measured black hole (BH) masses. From single-epoch mass measurements based on Mg ii λ2798, we find a wide range in BH masses, from MBH = 107.6 to 109.3 M . The Eddington ratios Lbol/LEdd range from 0.16 to 1.1, but the majority of the HSC quasars are powered by MBH ∼ 109 M supermassive black holes (SMBHs) accreting at sub-Eddington rates. The Eddington ratio distribution of the HSC quasars is inclined to lower accretion rates than those of Willott et al., who measured the BH masses for similarly faint z ∼ 6 quasars. This suggests that the global Eddington ratio distribution is wider than has previously been thought. The presence of MBH ∼ 109 M SMBHs at z ∼ 6 cannot be explained with constant sub-Eddington accretion from stellar remnant seed BHs. Therefore, we may be witnessing the first buildup of the most massive BHs in the first billion years of the universe, the accretion activity of which is transforming from active growth to a quiescent phase. Measurements of a larger complete sample of z 6 low-luminosity quasars, as well as deeper observations with future facilities, will enable us to better understand the early SMBH growth in the reionization epoch.
Abstract The physical quantity that directly controls the feedback of active galactic nuclei (AGNs) in elliptical galaxies remains to be determined. The discovery of molecular gas around the AGNs ...suggests that the gas is fueling the AGNs. Therefore, we analyze Atacama Large Millimeter/submillimeter Array data for the CO line ( J = 1–0, 2–1, 3–2) emission and estimate the mass of molecular gas within 500 pc of the center of 12 noncentral elliptical galaxies (NCEGs) and 10 of the brightest cluster galaxies. We find that the mass ( M mol ∼ 10 5 –10 9 M ☉ ) is correlated with the jet power of their AGNs, which is represented by P cav ≈ 4.1 × 10 42 ( M mol / 10 7 M ☉ ) 1.3 erg s − 1 , although NCEGs alone do not show the correlation. We also find that M mol is correlated with the AGN continuum luminosities at ∼1.4 GHz ( L 1.4 ) and ∼100–300 GHz ( L con ). Since P cav reflects galactic-scale, long-term AGN activity, while the continuum luminosities reflect local (≲500 pc), short-term AGN activity, our results suggest that AGN activity depends on the amount of gas, regardless of its timescale. On the other hand, we cannot find a clear correlation between the mass of the black holes in the AGNs ( M BH ) and P cav . This suggests that M mol , rather than M BH , is the main factor controlling AGN activity. We confirm that the origin of the continuum emission from the AGNs at ∼1.4–300 GHz is mostly synchrotron radiation.
Abstract
We present the results of our ALMA ≲0.5 kpc resolution dense molecular line (HCN and HCO
+
J
= 2–1,
J
= 3–2, and
J
= 4–3) observations of 12 nearby (ultra)luminous infrared galaxies ...(ULIRGs). After matching beam sizes of all molecular line data to the same values in all (U)LIRGs, we derive molecular line flux ratios by extracting spectra in the central 0.5, 1, and 2 kpc circular regions and in 0.5–1 and 1–2 kpc annular regions. Based on non–local thermal equilibrium model calculations, we quantitatively confirm that the innermost (≲0.5 kpc) molecular gas is very dense (≳10
5
cm
−3
) and warm (≳300 K) in ULIRGs, and that in one LIRG, it is also modestly dense (10
4–5
cm
−3
) and warm (∼100 K). We then investigate the spatial variation of the HCN-to-HCO
+
flux ratios and high-
J
to low-
J
flux ratios of HCN and HCO
+
. A subtle sign of a decreasing trend in these ratios from the innermost (≲0.5 kpc) to the outer nuclear (0.5–2 kpc) region is discernible in a significant fraction of the observed ULIRGs. For two ULIRGS hosting an active galactic nucleus (AGN), which display the trend most clearly, we find based on a Bayesian approach that the HCN-to-HCO
+
abundance ratio and gas kinetic temperature systematically increase from the outer nuclear to the innermost region. We suggest that this trend comes from potential AGN effects because no such spatial variation is found in a starburst-dominated LIRG.
We present ALMA ∼0 02 resolution observations of the nucleus of the nearby (∼14 Mpc) type 2 active galactic nucleus NGC 1068 at HCN/HCO+/HNC J = 3-2 lines, as well as at their 13C isotopologue and ...vibrationally excited lines, to scrutinize the morphological, dynamical, chemical, and physical properties of dense molecular gas in the putative dusty molecular torus around a mass-accreting supermassive black hole. We confirm almost east-west-oriented dense molecular gas emission both morphologically and dynamically, which we regard as coming from the torus. Bright emission is compact ( 3 pc), and low-surface-brightness emission extends out to 5-7 pc. These dense molecular gas properties are not symmetric between the eastern and western torus. The HCN J = 3-2 emission is stronger than the HCO+ J = 3-2 emission within the ∼7 pc torus region, with an estimated dense molecular mass of (0.4-1.0) × 106 M . We interpret that HCN abundance is enhanced in the torus. We detect signatures of outflowing dense molecular gas and a vibrationally excited HCN J = 3-2 line. Finally, we find that in the innermost ( 1 pc) part of the torus, the dense molecular line rotation velocity, relative to the systemic velocity, is the opposite of that in the outer ( 2 pc) part, in both the eastern and western torus. We prefer a scenario of counter-rotating dense molecular gas with innermost almost Keplerian rotation and outer slowly rotating (far below Keplerian) components. Our high-spatial-resolution dense molecular line data reveal that torus properties of NGC 1068 are much more complicated than the simple axisymmetrically rotating torus picture in the classical active galactic nucleus unification paradigm.
We present the results of our Atacama Large Millimeter Array HCN J = 3-2 and HCO+ J = 3-2 line observations of a uniformly selected sample (>25) of nearby ultraluminous infrared galaxies (ULIRGs) at ...z < 0.15. The emission of these dense molecular gas tracers and continuum are spatially resolved in the majority of observed ULIRGs for the first time with achieved synthesized beam sizes of 0 2 or 500 pc. In most ULIRGs, the HCN-to-HCO+ J = 3-2 flux ratios in the nuclear regions within the beam size are systematically higher than those in the spatially extended regions. The elevated nuclear HCN J = 3-2 emission could be related to (a) luminous buried active galactic nuclei, (b) the high molecular gas density and temperature in ULIRG's nuclei, and/or (c) mechanical heating by spatially compact nuclear outflows. A small fraction of the observed ULIRGs display higher HCN-to-HCO+ J = 3-2 flux ratios in localized off-nuclear regions than those of the nuclei, which may be due to mechanical heating by spatially extended outflows. The observed nearby ULIRGs are generally rich in dense (>105 cm−3) molecular gas, with an estimated mass of >109 M within the nuclear (a few kpc) regions, and dense gas can dominate the total molecular mass there. We find a low detection rate (<20%) regarding the possible signature of a vibrationally excited (v2 = 1f) HCN J = 3-2 emission line in the vicinity of the bright HCO+ J = 3-2 line that may be due, in part, to the large molecular line widths of ULIRGs.
ABSTRACT We present the results of our ALMA Cycle 2 high angular resolution (0 1-0 2) observations of the nuclear region of the nearby well-studied type-2 active galactic nucleus (AGN), NGC 1068, at ...HCN J = 3-2 and HCO+ J = 3-2 emission lines. For the first time, due to a higher angular resolution than previous studies, we clearly detected dense molecular gas emission at the putative AGN location, identified as a ∼1.1 mm (∼266 GHz) continuum emission peak, by separating this emission from brighter emission located at 0 5-2 0 on the eastern and western sides of the AGN. The estimated intrinsic molecular emission size and dense molecular mass, which are thought to be associated with the putative dusty molecular torus around an AGN, were ∼10 pc and ∼several × 105M , respectively. HCN-to-HCO+ J = 3-2 flux ratios substantially higher than unity were found throughout the nuclear region of NGC 1068. The continuum emission displayed an elongated morphology along the direction of the radio jet located at the northern side of the AGN, as well as a weak spatially-resolved component at ∼2 0 on the southwestern side of the AGN. The latter component most likely originated from star formation, with the estimated luminosity more than one order of magnitude lower than the luminosity of the central AGN. No vibrationally excited (v2 = 1f) J = 3-2 emission lines were detected for HCN and HCO+ across the field of view.
Abstract
We studied the absorption features of CO lines against the continuum originating from the heated dust in the obscuring tori around active galactic nuclei (AGNs). We investigated the ...formation of absorption lines corresponding to the CO rotational transitions using three-dimensional non-LTE line transfer simulations considering the dust thermal emission. As in Papers I–III of this series, we performed post-processed radiative transfer calculations using the “radiation-driven fountain model” (Wada et al. 2016), which yields a geometrically thick obscuring structure around the nucleus. This model is consistent with the spectral energy distribution of the nearest type-2 Seyfert galaxy, the Circinus galaxy. We found that the continuum-subtracted channel maps of
J
= 4−3 and higher transitions show absorption regions along the disk midplane for an edge-on viewing angle. The spectra consist of multiple absorption and emission features, reflecting the internal inhomogeneous and turbulent structure of the torus. The deepest absorption feature is caused by the gas on the near side of the torus between
r
= 10 and 15 pc, which is located in front of the AGN-heated dust inside
r
≃ 5 pc. We also found that a spatial resolution of 0.5–1.0 pc is necessary to resolve the absorption features. Moreover, the inclination angle must be close to the edge-on angle (i.e., ≳85°) to observe the absorption features. The findings of the present study imply that combining our radiation-hydrodynamic model with high-resolution observations of CO (7–6) by ALMA can provide new information about the internal structure of the molecular tori in nearby AGNs.
We present our ALMA multi-transition molecular line observational results for the ultraluminous infrared galaxy IRAS 20551−4250, which is known to contain a luminous buried active galactic nucleus ...and shows detectable vibrationally excited (v2 = 1f) HCN and HNC emission lines. The rotational J = 1-0, 4-3, and 8-7 of HCN, , and HNC emission lines were clearly detected at a vibrational ground level (v = 0). Vibrationally excited (v2 = 1f) J = 4-3 emission lines were detected for HCN and HNC, but not for . Their observed flux ratios further support our previously obtained suggestion, based on J = 3-2 data, that (1) infrared radiative pumping plays a role in rotational excitation at v = 0, at least for HCN and HNC, and (2) HCN abundance is higher than and HNC. The flux measurements of the isotopologue H13CN, , and HN13C J = 3-2 emission lines support the higher HCN abundance scenario. Based on modeling with collisional excitation, we constrain the physical properties of these line-emitting molecular gases, but find that higher HNC rotational excitation than HCN and is difficult to explain, due to the higher effective critical density of HNC. We consider the effects of infrared radiative pumping using the available 5-30 m infrared spectrum and find that our observational results are well-explained if the radiation source is located at 30-100 pc from the molecular gas. The simultaneously covered very bright CO J = 3-2 emission line displays a broad emission wing, which we interpret as being due to molecular outflow activity with the estimated rate of .
Hydrogen recombination lines at the submillimetre band (submm-RLs) can serve as probes of ionized gas without dust extinction. One therefore expects to probe the broad-line region (BLR) of an ...obscured (type 2) active galactic nucleus (AGN) with those lines. However, admitting the large uncertainty in the continuum level, here we report on the non-detection of both broad and narrow H26 α emission line (rest frequency = 353.62 GHz) towards the prototypical type 2 AGN of NGC 1068 with the Atacama Large Millimeter/submillimeter Array (ALMA). We also investigate the nature of BLR clouds that can potentially emit submm-RLs with model calculations. As a result, we suggest that clouds with an electron density (N
e) of ∼109 cm−3 can mainly contribute to broad submm-RLs in terms of the line flux. On the other hand, line flux from other density clouds would be insignificant considering their too large or too small line optical depths. However, even for the case of N
e ∼ 109 cm−3 clouds, we also suggest that the expected line flux is extremely low, which is impractical to detect even with ALMA.
ABSTRACT We present the results of ALMA Cycle 2 observations of the ultraluminous infrared galaxy IRAS 20551−4250 at HCN/HCO+/HNC J = 3-2 lines at both vibrational ground (v = 0) and vibrationally ...excited (v2 = 1) levels. This galaxy contains a luminous buried active galactic nucleus (AGN), in addition to starburst activity, and our ALMA Cycle 0 data revealed a tentatively detected vibrationally excited HCN v2 = 1f J = 4-3 emission line. In our ALMA Cycle 2 data, the HCN/HCO+/HNC J = 3-2 emission lines at v = 0 are clearly detected. The HCN and HNC v2 = 1f J = 3-2 emission lines are also detected, but the HCO+ v2 = 1f J = 3-2 emission line is not. Given the high energy level of v2 = 1 and the resulting difficulty of collisional excitation, we compared these results with those of the calculation of infrared radiative pumping, using the available infrared 5-35 m spectrum. We found that all of the observational results were reproduced if the HCN abundance was significantly higher than that of HCO+ and HNC. The flux ratio and excitation temperature between v2 = 1f and v = 0, after correction for possible line opacity, suggests that infrared radiative pumping affects rotational (J-level) excitation at v = 0 at least for HCN and HNC. The HCN-to-HCO+ v = 0 flux ratio is higher than those of starburst-dominated regions, and will increase even more when the derived high HCN opacity is corrected. The enhanced HCN-to-HCO+ flux ratio in this AGN-hosting galaxy can be explained by the high HCN-to-HCO+ abundance ratio and sufficient HCN excitation at up to J = 4, rather than the significantly higher efficiency of infrared radiative pumping for HCN than HCO+.
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