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Register a new userCalibration and Limitations of the Mg II line-based Black Hole Masses
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Huynh Anh Le Nguyen
Publication date
2018
fields
Physics
and research's language is
English
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We present the single-epoch black hole mass (M$_{rm BH}$) calibrations based on the rest-frame UV and optical measurements of Mg II 2798AA and H$beta$ 4861AA lines and AGN continuum, using a sample of 52 moderate-luminosity AGNs at z$sim$0.4 and z$sim$0.6 with high-quality Keck spectra. We combine this sample with a large number of luminous AGNs from the Sloan Digital Sky Survey to increase the dynamic range for a better comparison of UV and optical velocity and luminosity measurements. With respect to the reference M$_{rm BH}$ based on the line dispersion of H$beta$ and continuum luminosity at 5100AA, we calibrate the UV and optical mass estimators, by determining the best-fit values of the coefficients in the mass equation. By investigating whether the UV estimators show systematic trend with Eddington ratio, FWHM of H$beta$, the Fe II strength, and the UV/optical slope, we find no significant bias except for the slope. By fitting the systematic difference of Mg II-based and H$beta$-based masses with the L$_{3000}$/L$_{5100}$ ratio, we provide a correction term as a function of the spectral index as $Delta$C = 0.24 (1+$alpha_{lambda}$) + 0.17, which can be added to the Mg II-based mass estimators if the spectral slope can be well determined. The derived UV mass estimators typically show $>$$sim$0.2 dex intrinsic scatter with respect to H$beta$-based M$_{rm BH}$, suggesting that the UV-based mass has an additional uncertainty of $sim$0.2 dex, even if high quality rest-frame UV spectra are available.
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Using a compiled sample of 34 broad-line active galactic nuclei (AGNs) with measured H$beta$ time lags from the reverberation mapping (RM) method and measured bulge stellar velocity dispersions $sigma_*$, we calculate the virial factor $f$ by assuming that the RM AGNs intrinsically obey the same $M_{rm BH}-sigma_*$ relation as quiescent galaxies, where $M_{rm BH}$ is the mass of the supermassive black hole (SMBH). Considering four tracers of the velocity of the broad-line regions (BLRs), i.e., the H$beta$ line width or line dispersion from the mean or rms spectrum, there are four kinds of the factor $f$. Using the hb Full-width at half-maximum (FWHM) to trace the BLRs velocity, we find significant correlations between the factor $f$ and some observational parameters, e.g., FWHM, the line dispersion. Using the line dispersion to trace the BLRs velocity, these relations disappear or become weaker. It implies the effect of inclination in BLRs geometry. It also suggests that the variable $f$ in $M_{rm BH}$ estimated from luminosity and FWHM in a single-epoch spectrum is not negligible. Using a simple model of thick-disk BLRs, we also find that, as the tracer of the BLRs velocity, H$beta$ FWHM has some dependence on the inclination, while the line dispersion $sigma_{rm Hbeta }$ is insensitive to the inclination. Considering the calibrated FWHM-based factor $f$ from the mean spectrum, the scatter of the SMBH mass is 0.39 dex for our sample of 34 low redshift RM AGNs. For a high redshift sample of 30 SDSS RM AGNs with measured stellar velocity dispersions, we find that the SMBH mass scatter is larger than that for our sample of 34 low redshift RM AGNs. It implies the possibility of evolution of the $M_{rm BH}-sigma_*$ relation from high-redshift to low-redshift AGNs.
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We present the results from a spectroscopic monitoring campaign to obtain reverberation-mapping measurements and investigate the broad-line region kinematics for active galactic nuclei (AGN) of Mrk~817 and NGC~7469. This campaign was undertaken with the Lijiang 2.4-meter telescope, the median spectroscopic sampling is 2.0 days for Mrk~817 and 1.0 days for NGC~7469. We detect time lags of the broad emission lines including H$beta$, H$gamma$, He~{sc ii} and He~{sc i} for both AGNs, and including Fe~{sc ii} for Mrk~817 with respect to the varying AGN continuum at 5100~AA. Investigating the relationship between line widths and time lags of the broad emission lines, we find that the BLR dynamics of Mrk~817 and NGC~7469 are consistent with the virial prediction. We estimate the masses of central supermassive black hole (SMBH) and the accretion rates of both AGNs. Using the data of this campaign, we construct the velocity-resolved lag profiles of the broad H$gamma$, H$beta$, and He~{sc i} lines for Mrk~817, which show almost the same kinematic signatures that the time lags in the red wing are slightly larger than the time lags in the blue wing. For NGC~7469, we only clearly construct the velocity-resolved lag profiles of the broad H$gamma$ and H$beta$, which show very similar kinematic signatures to the BLR of Mrk~817. These signatures indicate that the BLR of Keplerian motion in both AGNs seemingly has outflowing components during the monitoring period. We discuss the kinematics of the BLR and the measurements including SMBH mass and accretion rates.
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We re-examine claims of redshift evolution in black hole-bulge scaling relations based on lensed quasars. In particular, we refine the black hole mass estimates using measurements of Balmer lines from near-infrared spectroscopy obtained with Triplespec at Apache Point Observatory. In support of previous work, we find a large scatter between Balmer and UV line widths, both MgII 2796, 2803 and CIV 1548, 1550. There is tentative evidence that CIII] 1909, despite being a blend of multiple transitions, may correlate well with MgII, although a larger sample is needed for a real calibration. Most importantly, we find no systematic changes in the estimated BH masses for the lensed sample based on Balmer lines, providing additional support to the interpretation that black holes were overly massive compared to their host galaxies at high redshift.
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