分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The existence of high-redshift supermassive black holes (SMBHs) -- shining brightly as quasars during the first billion years of our universe -- presents a conundrum in astrophysics. A broad variety of physical mechanisms have been proposed for the formation and rapid growth of these early SMBHs. Promising diagnostics are the relative properties of the black hole and its host galaxy. However, up to now, the detection of stars in quasar host galaxies has been elusive beyond $z>2$, even with deep HST observations. Here, we report the first detections of the stellar component of the host galaxies of two relatively low-luminosity quasars at $z>6$ observed with JWST using NIRCam. After modeling and subtracting the glare from the quasar itself, we find that the host galaxies are massive (stellar mass of $2.5\times$ and $6.3\times10^{10}$ M$_{\odot}$), compact, and disk-like. Unlike most SMBHs in the nearby universe, these quasars are displaced from the centers of their host galaxies in the rest-frame optical, in one case by $0.9$ kpc. These first positive detections of quasar hosts at $z>6$ are a pivotal milestone; we can now assess the stellar environment along with star formation and black hole accretion to determine the physical conditions that govern the formation and evolution of the first SMBHs.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Photoionization modeling of active galactic nuclei (AGN) predicts that diffuse continuum (DC) emission from the broad-line region makes a substantial contribution to the total continuum emission from ultraviolet through near-infrared wavelengths. Evidence for this DC component is present in the strong Balmer jump feature in AGN spectra, and possibly from reverberation measurements that find longer lags than expected from disk emission alone. However, the Balmer jump region contains numerous blended emission features, making it difficult to isolate the DC emission strength. In contrast, the Paschen jump region near 8200 \r{A} is relatively uncontaminated by other strong emission features. Here, we examine whether the Paschen jump can aid in constraining the DC contribution, using Hubble Space Telescope STIS spectra of six nearby Seyfert 1 nuclei. The spectra appear smooth across the Paschen edge, and we find no evidence of a Paschen spectral break or jump in total flux. We fit multi-component spectral models over the range $6800-9700$ \r{A} and find that the spectra can still be compatible with a significant DC contribution if the DC Paschen jump is offset by an opposite spectral break resulting from blended high-order Paschen emission lines. The fits imply DC contributions ranging from $\sim$10% to 50% at 8000 \r{A}, but the fitting results are highly dependent on assumptions made about other model components. These degeneracies can potentially be alleviated by carrying out fits over a broader wavelength range, provided that models can accurately represent the disk continuum shape, Fe II emission, high-order Balmer line emission, and other components.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The AGN STORM 2 collaboration targeted the Seyfert 1 galaxy Mrk 817 for a year-long multiwavelength, coordinated reverberation mapping campaign including HST, Swift, XMM-Newton, NICER, and ground-based observatories. Early observations with NICER and XMM revealed an X-ray state ten times fainter than historical observations, consistent with the presence of a new dust-free, ionized obscurer. The following analysis of NICER spectra attributes variability in the observed X-ray flux to changes in both the column density of the obscurer by at least one order of magnitude ($N_\mathrm{H}$ ranges from $2.85\substack{+0.48\\ -0.33} \times 10^{22}\text{ cm}^{-2}$ to $25.6\substack{+3.0\\ -3.5} \times 10^{22} \text{ cm}^{-2}$) and the intrinsic continuum brightness (the unobscured flux ranges from $10^{-11.8}$ to $10^{-10.5}$ erg s$^{-1}$ cm$^{-2}$ ). While the X-ray flux generally remains in a faint state, there is one large flare during which Mrk 817 returns to its historical mean flux. The obscuring gas is still present at lower column density during the flare but it also becomes highly ionized, increasing its transparency. Correlation between the column density of the X-ray obscurer and the strength of UV broad absorption lines suggests that the X-ray and UV continua are both affected by the same obscuration, consistent with a clumpy disk wind launched from the inner broad line region.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We derive a distance of $D = 16.6 \pm 0.3$~Mpc ($\mu=31.10\pm0.04$~mag) to the archetypal narrow-line Seyfert 1 galaxy NGC 4051 based on Cepheid Period--Luminosity relations and new Hubble Space Telescope multiband imaging. We identify 419 Cepheid candidates and estimate the distance at both optical and near-infrared wavelengths using subsamples of precisely-photometered variables (123 and 47 in the optical and near-infrared subsamples, respectively). We compare our independent photometric procedures and distance-estimation methods to those used by the SH0ES team and find agreement to 0.01~mag. The distance we obtain suggests an Eddington ratio $\dot{m} \approx 0.2$ for NGC 4051, typical of narrow-line Seyfert 1 galaxies, unlike the seemingly-odd value implied by previous distance estimates. We derive a peculiar velocity of $-490\pm34$~km~s$^{-1}$ for NGC 4051, consistent with the overall motion of the Ursa Major Cluster in which it resides. We also revisit the energetics of the NGC 4051 nucleus, including its outflow and mass accretion rates.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We study the demographics of z ~ 6 broad-line quasars in the black hole (BH) mass-luminosity plane using a sample of more than 100 quasars at 5.7 10^8.5 M_Sun, with a median 1-sigma uncertainty of ~0.5 dex in abundance. The intrinsic Eddington ratio distribution of M_BH > 10^8.5 M_Sun quasars can be approximated by a mass-dependent Schechter model, with a broad peak around log(L_bol/L_Edd}) ~ -0.9. We also find that, at 4.5 < z < 6, the number densities of more massive BHs tend to decline more rapidly with increasing redshift, contrary to the trend at 2.5 < z < 4.5 reported previously.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We study the demographics of z ~ 6 broad-line quasars in the black hole (BH) mass-luminosity plane using a sample of more than 100 quasars at 5.7 10^8.5 M_Sun, with a median 1-sigma uncertainty of ~0.5 dex in abundance. The intrinsic Eddington ratio distribution of M_BH > 10^8.5 M_Sun quasars can be approximated by a mass-dependent Schechter model, with a broad peak around log(L_bol/L_Edd}) ~ -0.9. We also find that, at 4.5 < z < 6, the number densities of more massive BHs tend to decline more rapidly with increasing redshift, contrary to the trend at 2.5 < z < 4.5 reported previously.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Mrk 590 is a known changing-look AGN which almost turned off in 2012, and then in 2017 partially re-ignited into a repeat flaring state, unusual for an AGN. Our \emph{Swift} observations since 2013 allow us to characterise the accretion-generated emission and its reprocessing in the central engine of a changing-look AGN. The X-ray and UV variability amplitudes are higher than those typically observed in `steady-state' AGN at similar moderate accretion rates; instead, the variability is similar to that of highly accreting AGN. The unusually strong X-ray to UV correlation suggests that the UV-emitting region is directly illuminated by X-ray outbursts. We find evidence that the X-rays are reprocessed by two UV components, with the dominant one at $\sim$3 days and a faint additional reprocessor at near-zero lag. However, we exclude a significant contribution from diffuse broad line region continuum, known to contribute for bona-fide AGN. A near-zero lag is expected for a standard `lamp-post' disk reprocessing model with a driving continuum source near the black hole. That the overall UV response is dominated by the $\sim$3-day lagged component suggests a complicated reprocessing geometry, with most of the UV continuum not produced in a compact disk, as also found in recent studies of NGC 5548 and NGC 4151. Nonetheless, the observed flares display characteristic timescales of $\sim$100 rest-frame days, consistent with the expected thermal timescale in an accretion disk.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present reverberation mapping measurements for the prominent ultraviolet broad emission lines of the active galactic nucleus Mrk817 using 165 spectra obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. Our ultraviolet observations are accompanied by X-ray, optical, and near-infrared observations as part of the AGN Space Telescope and Optical Reverberation Mapping Program 2 (AGN STORM 2). Using the cross-correlation lag analysis method, we find significant correlated variations in the continuum and emission-line light curves. We measure rest-frame delayed responses between the far-ultraviolet continuum at 1180 A and Ly$\alpha$ $\lambda1215$ A ($10.4_{-1.4}^{+1.6}$ days), N V $\lambda1240$ A ($15.5_{-4.8}^{+1.0}$days), SiIV + OIV] $\lambda1397$ A ($8.2_{-1.4}^{+1.4}$ days), CIV $\lambda1549$ A ($11.8_{-2.8}^{+3.0}$ days), and HeII $\lambda1640$ A ($9.0_{-1.9}^{+4.5}$ days) using segments of the emission-line profile that are unaffected by absorption and blending, which results in sampling different velocity ranges for each line. However, we find that the emission-line responses to continuum variations are more complex than a simple smoothed, shifted, and scaled version of the continuum light curve. We also measure velocity-resolved lags for the Ly$\alpha$, and CIV emission lines. The lag profile in the blue wing of Ly$\alpha$ is consistent with virial motion, with longer lags dominating at lower velocities, and shorter lags at higher velocities. The CIV lag profile shows the signature of a thick rotating disk, with the shortest lags in the wings, local peaks at $\pm$ 1500 $\rm km\,s^{-1}$, and a local minimum at line center. The other emission lines are dominated by broad absorption lines and blending with adjacent emission lines. These require detailed models, and will be presented in future work.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Broad line regions (BLRs) in high-redshift quasars provide crucial
information of chemical enrichment in the early universe. Here we present a
study of BLR metallicities in 33 quasars at redshift $5.7