Quasar Outflow Contribution to AGN Feedback: Observations of QSO SDSS J0838+2955
Summary (2 min read)
1. INTRODUCTION
- Quasar outflows have been increasingly invoked as a primary feedback mechanism to explain the formation and evolution of supermassive black holes, their host galaxies, the surrounding intergalactic medium (IGM), and cluster cooling flows (Elvis 2006).
- The latter lead to unreliable measurements of column densities in the observed troughs (Nion), which are crucial for determining almost every physical aspect of the outflows.
- The authors caution that Ω is not directly constrained by their observations, and therefore express the kinetic luminosity in terms of Ω0.2 (see Section 5).
- Gas column and number densities are measured in Section 3 after determining the inhomogeneity of the absorber across the continuum source.
2. DATA ACQUISITION AND REDUCTION
- Spectra of SDSS J0838+2955 were obtained on 2008 February 2 and 6 with the Dual Imaging Spectrograph (DIS) instrument on the Astrophysical Research Consortium (ARC) 3.5 m telescope at Apache Point Observatory (APO).
- Considering the outstanding S/N but rather moderate resolution, the spectrum was binned to one-half the FWHM resolution element for analysis except for the absorption lines from.
- Fe ii which were binned to one full FWHM resolution element to increase the S/N for these shallow troughs.
- The authors then added in quadrature the errors from normalization to the derived uncertainties of the column densities.
3. COLUMN AND NUMBER DENSITIES FOR COMPONENT C
- Ionic column densities were determined for all the transitions covered by the ARC spectrum as well as Mg ii which was only covered by SDSS.
- For the three optically thick ions of Al iii, Si iv, and C iv, the two different inhomogeneous absorber models were able to bracket the error due to saturation and constrain the column densities to within a factor of 2.
- The measured column densities assuming apparent optical depth and the various inhomogeneous absorber models are presented in Table 2.
- To ensure consistency when calculating number densities, the measured column densities are integrated over the same velocity region for each species instead of using the total column densities reported in Table 2.
4.1. Spectral Energy Distribution
- The photon SED incident on the outflow is important in determining the ionization and thermal structures of the absorbing plasma.
- This discrepancy can be due to either an inherent difference in the intrinsic quasar continuum or extinction due to dust grains either within the outflowing gas or somewhere within the host galaxy.
- Considering that the authors do not know whether the deviation from the theoretical SED is due to an inherent difference in the SED, dust extinction that occurs within the absorber, or extinction that occurs close to the AGN, they simply modified the MF87 SED to match the observed UV continuum .
4.2. Photoionization Modeling
- The authors now examine the formation of the various ionic species observed in their spectrum.
- Moreover, small variation of nH have negligible effects on the results of photoionization modeling.
- To determine the effects of varying the abundances of silicon and carbon, the authors scaled the column densities of these species from the solar abundance grid output and then compared this scaled grid model to the observed column densities.
- Note that all the displayed squares are within 0.2 dex of the estimated solution for NH .
- No absorption from lower ionization species can be measured on their own merits, suggesting this system is more ionized and/ or lower in hydrogen column density than component c.
5. RESULTS AND DISCUSSION
- See also Hall et al. (2003) for further discussion regarding this point.
- Indeed, this value of ∼1.4% is more likely a lower limit considering that the other outflow components a and b could further contribute to AGN feedback (see N. Arav et al. 2009, in preparation).
- Given the bolometric luminosity and assuming that the black hole is accreting near the Eddington limit, the authors find that the amount of mass being accreted is (Salpeter 1964): Ṁacc = LBol rc2 ≈ 60 M yr−1 (7) assuming an accretion efficiency r of 0.1.
- An upper limit on the distance of this component a of the outflow can be estimated assuming that the transverse velocity cannot exceed the Keplerian velocity.
6. CONCLUSIONS
- The apparent optical depth method underestimates the true column density by 0.2–0.8 dex for the optically thick species of Al iii, Si iv, and C iv.
- The derived column density is 0.2–0.3 dex smaller than the column density necessary to reach the hydrogen ionization front.
- There is a strong indication that outflow component c has super-solar metallicities, ranging in Z/Z = 1.6–3.1, consistent with other reliable abundance estimates of AGN outflows (Gabel et al. 2006; Arav et al. 2007).
- The authors acknowledge support from NSF grant AST 0507772.
- M.M. and N.A. thank the Astrophysical and Planetary Sciences Department at the University of Colorado for the use of the ARC 3.5 m telescope.
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Frequently Asked Questions (18)
Q2. Why is it important to accurately model this region of the SED?
Because the ions used in photoionization modeling have ionization potentials for destruction in the far ultraviolet (FUV), it is important to accurately model this region of the SED.
Q3. What are the parameters used for the ground and excited state profiles?
Four pairs of Gaussians were used for the ground and excited state profiles, each pair with three free parameters (line center, optical depth, and Doppler b parameter).
Q4. How do the authors determine the column densities of the absorber?
To determine the electron number density of the absorber, ne, the authors use the column densities of the ground and first excited levels of Si ii, 3s23p 2Po1/2 and 3s 23p 2Po3/2 at 287 cm −1 respectively, and of Fe ii, a6D9/2 and a6D7/2 at 385 cm−1 respectively.
Q5. How is the grid of models interpolated?
When minimizing χ2ν , as well as determining the solution using a pair of ions, the grid of models is interpolated using a cubic convolution technique so that the true solution may be estimated.
Q6. What are the main reasons why the QSO is being used as a primary feedback mechanism?
Quasar outflows have been increasingly invoked as a primary feedback mechanism to explain the formation and evolution of supermassive black holes, their host galaxies, the surrounding intergalactic medium (IGM), and cluster cooling flows (Elvis 2006).
Q7. What is the reason for the variation in the optical depth of component a?
The significant variation in optical depth of component a is most likely due to a change in flux of the ionizing source or transverse velocity of the outflow out of the line of site to the central AGN (Barlow 1994).
Q8. How many QSOs displayed the intrinsic absorption system?
Of the remaining ∼70 QSOs, ∼30 objects displayed the intrinsic absorption system containing the excited states at velocities 1000 km s−1 with respect to the systemic rest frame of the QSO and continuum flux levels 10−16 erg s−1 cm−2 Å−1 in the observed frame.
Q9. What is the first approach to determining the solution to NH and UH?
The first approach entails using pairs of ionic column densities from a single element, specifically C ii and C iv or Si ii and Si iv, to determine a solution to NH and UH .
Q10. What is the strength of the column densities extracted?
The robustness of these extracted column densities lends strength to their photoionization solutions for the outflow (see Section 4), which in turn are used to determine the mass flux and kinetic luminosity of the outflow (see Section 5).
Q11. What is the kinetic luminosity of the outflow observed in this object?
Based on the Hamann et al. (2001) analysis of QSO 3C 191, the kinetic luminosity of the outflow observed in this object is ∼ 9 × 1043Ω0.2 erg s−1 and its mass flux is ∼310Ω0.2 M yr−1 for an outflow situated at ∼28 kpc.
Q12. What is the kinetic luminosity of absorption outflows?
The kinetic luminosity of absorption outflows (observed as blueshifted troughs in the quasar spectrum) is directly proportional to the distance R of the outflow from the central AGN, the total hydrogen column density NH , and the fraction Ω of the solid angle subtended by the wind (see Section 5).
Q13. What is the error in due to photoionization modeling?
If the solution to NH and UH were assumed to be uncorrelated, then the error in Ṁ due to photoionization modeling would be 0.30 dex instead of the actual 0.19 dex.
Q14. What is the upper limit on the distance of the outflow?
An upper limit on the distance of this component a of the outflow can be estimated assuming that the transverse velocity cannot exceed the Keplerian velocity.
Q15. What is the kinetic luminosity of the outflow component c?
4. There is a strong indication that outflow component c has super-solar metallicities, ranging in Z/Z = 1.6–3.1, consistent with other reliable abundance estimates of AGN outflows (Gabel et al. 2006; Arav et al. 2007).
Q16. What is the ionic column density of the outflow component c?
In order to derive the true column densities of the ionic species present in outflow component c, one must carefully consider variations in optical depth τ across the continuum source.
Q17. How much is the kinetic luminosity of the outflow?
5. The authors measured the distance of this outflow to be 2.3–4.8 kpc with a kinetic luminosity of 0.8%–2.5% the bolometric luminosity of the QSO.
Q18. What is the spectral model of the outflowing absorbing gas?
For the present work, the authors use the photoionization modeling code Cloudy (Ferland et al. 1998) v08.00 to compute spectral models of the outflowing absorbing gas in SDSS J0838+2955.