A 10 kpc SCALE SEYFERT GALAXY OUTFLOW: HST/COS OBSERVATIONS OF IRAS F22456–5125
Abstract: We present analysis of the UV spectrum of the low-z AGN IRAS F22456?5125 obtained with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. The spectrum reveals six main kinematic components, spanning a range of velocities of up to 800?km?s?1, which for the first time are observed in troughs associated with C II, C IV, N V, Si II, Si III, Si IV, and S IV. We also obtain data on the O VI troughs, which we compare to those available from an earlier Far Ultraviolet Spectroscopic Explorer epoch. Column densities measured from these ions allow us to derive a well-constrained photoionization solution for each outflow component. Two of these kinematic components show troughs associated with transitions from excited states of Si II and C II. The number density inferred from these troughs, in combination with the deduced ionization parameter, allows us to determine the distance to these outflow components from the central source. We find these components to be at a distance of ~10?kpc. The distances and the number densities derived are consistent with the outflow being part of a galactic wind.
Summary (4 min read)
- In Section 3, the authors detail the computation of the column densities associated with every species.
- The reduced spectrum with its original 5 Details on CALCOS can be found in the COS Data Handbook.
- In Figure 1, the authors show the majority of the spectrum on which they identified major intrinsic absorption features associated with the outflow.
2.1. Identification of Spectral Features
- These components, spanning a total velocity range of 800 km s−1, were detected in O vi, C iii, and several lines of the Lyman series (Lyβ to Lyη).
- While the absorption troughs associated with the higherionization lines generally exhibit broader profiles, the authors observe a 1:1 kinematic correspondence between the core of these components and the narrower features associated with the lowerionization species of the outflow.
- Given the significantly broad range of velocities covered by the components and their net kinematic separations, such a match is not likely to occur by chance.
- This argues in favor of a scenario where the troughs of the different ionic species detected in a given kinematic component are generated in the same region.
- Nevertheless, given the selfblending of these features in the strongest lines (e.g., O vi) and the absence of apparent change between the FUSE and COS observations, the authors will use the labeling of components as defined in Dunn et al. (2010).
2.2. Deconvolution of the COS Spectrum
- Detailed analysis of the on-orbit COS line-spread function (LSF) revealed the presence of broadened wings that scatter a significant part of the continuum flux inside the absorption troughs (see Kriss et al. 2011 for details).
- This continuum leaking is particularly strong for narrow absorption troughs (FWHM ∼50 km s−1) in which this effect may significantly affect the estimation of the true column density by artificially increasing the residual intensity observed inside the troughs.
- Adopting the procedure described in Kriss et al. (2011), the authors deconvolve the spectrum obtained for each grating in 50 Å intervals using the wavelength-dependent LSFs and an IDL implementation of the stsdas Richardson–Lucy (RL) “lucy” algorithm (G. Schneider & B. Stobie 2011, private communication).
- The main effect of the deconvolution is illustrated in Figure 2, in which the authors clearly see that the deconvolved spectrum shows significantly deepened intrinsic Lyα absorption troughs and produces a square, black bottom for the saturated interstellar line C ii λ1334.532.
- In order to decrease these effects, the authors modified the RL algorithm by forcing the deconvolved spectrum to have an LSF satisfying the sampling theorem.
2.3. Unabsorbed Emission Model
- The unabsorbed emission model F0(λ) of IRAS F22456− 5125 is constructed in a similar manner to the one described in detail for IRAS F04250−5718 in Paper I, in which the authors consider three main sources of emission: a continuum, a broad emission line (BEL) component, and a narrow emission line (NEL) component.
- The NEL component of each line of a doublet is fit by a single narrower Gaussian (FWHM ∼ 600 km s−1) centered around the rest wavelength of each line, with the separation of the two Gaussians fixed to the velocity difference between the doublet lines.
- A normalized spectrum is then obtained by dividing the data with the emission model.
- The column density associated with a given ionic species detected in the outflow is determined by modeling the residual intensity in the normalized data of the absorption troughs.
- The authors use these three models in order to account for possible inhomogeneities in the absorber (see Section 6), which cause the apparent strength ratio Ra = τi/τj of two lines i, j from a given ion to deviate from the expected laboratory ratio 1995; Hamann 1997; Arav et al. 1999).
- For singlet lines the authors will generally only derive a lower limit on the column density using the AOD method.
- In the following subsections, the authors use the term (non-black) saturation to qualify Notes.
3.2. Column Density Measurements
- Computed ionic column densities are determined using the deconvolved line profiles presented in Figure 3 and the ionic transition properties reported in Table 1.
- The computed column densities are reported in Table 2 for the three absorber models when possible.
- The adopted values shown in the last column of Table 2 are the ones used in the photoionization analysis.
- When available, the authors choose to use the value reported in the PC column as the measurement and use the PL measurement and error as the upper error in order to account for the possible inhomogeneities in the absorbing material distribution.
- If only the AOD determination is available, the authors will consider the reported value minus the error as a lower limit unless they have evidence suggesting a high covering.
3.2.8. The Density Diagnostic Lines
- Using the oscillator strengths from NIST for the quoted transitions (rated either B+ or C in the database), the authors find that the relative strength order of the lines matches the observed residual flux for the λλ1190.42, 1193.28, and 1260.37 lines and the weak detection of the λ1304.37 transition.
- While this could be due to a blend, the narrowness of the trough and its location away from any known ISM lines do not support this scenario.
- The column density derived using the PL absorber model is 2.5 times larger than the one assuming the PC model, potentially suggesting an underestimation of the column density when using the PC model.
- While the authors observe a small increase of the derived columns using this PC model, the ratio of column density between the resonance and excited states remains identical (as expected given the similar residual flux inside the C ii and C ii* troughs), strengthening the density diagnostic obtained from these lines.
4. PHOTOIONIZATION ANALYSIS OF THE ABSORBERS
- In order to derive the physical properties of each kinematic component of the outflow, the authors solve the ionization equilibrium equations using version c08.00 of the spectral synthesis code Cloudy (last described by Ferland et al. 1998).
- The authors model each absorber by a plane-parallel slab of gas of constant hydrogen number density (nH) and assume solar elemental abundances as given in Cloudy.
- The COS observations show a wealth of absorption lines compared to the earlier FUSE observations discussed in Dunn et al. (2010).
- The authors prefer this formalism to the traditional definition of χ2 since it preserves the multiplicative nature of the errors when dealing with logarithmic values.
4.1. Troughs T 2 and T 3
- The physical parameters of component T 2 are constrained by 10 ionic column densities, eight from COS data along with H i and C iii from FUSE data (keeping in mind that the latter have been obtained at a different epoch).
- For component T 3, the authors have column density measurements for seven ions in the COS spectrum, along with H i and C iii from FUSE data and an upper limit on Si ii due to non-detection of the stronger lines in the COS spectrum (see Section 3.2.8).
- This solution fits all the lines within a factor of ∼3 (see Table 4).
- The constraints on the (NH, UH) parameter space for trough T 4 are presented in Figure 11.
- While the saturation observed in the troughs of several ions limits the analysis of the physical properties of the gas, the estimated (NH, UH) solution is able to reproduce most of the ionic columns to within a factor of two.
5. ABSORBER DISTANCE AND ENERGETICS
- The label SI corresponds to the single-ionization model, while TIlo and TIhi are the low- and high-ionization phases of the two-ionization model of the absorber.
- To these two kinematic components from the central source.
- Using the derived ionization parameter of that phase, this density implies a distance of R 10.3+5.1−1.6 kpc, where the errors are conservatively computed from the ne range allowed by the Si ii*/Si ii ratio and the error on the ionization parameter.
- This situation is nonphysical since for the inferred temperature of the absorbing gas (T ∼ 104 K) the velocity width of the outflow (Δv 50 km s−1) is at least 10 times larger than the sound speed, and therefore the outflowing material cannot Note.
- The authors note, however, that, using the definition of the filling factor (f = NH/(nHΔR)), this instantaneous mass flow rate relates to the average mass flow rate defined in Equation (7) by the relation ṀTi,ins = ṀTi /(ΔR/R).
6. DISCUSSION AND CONCLUSIONS
- The authors analyzed the physical properties of the UV outflow of IRAS F22456−5125 based on high-S/N COS observations.
- Dunn et al. (2010) analyzed that ASCA and XMM-Newton spectra of IRAS F22456−5125 do not reveal any evidence for an X-ray warm absorption edge; however, the limited S/N in these data can still allow the presence of a warm phase with significant column density.
- These observations suggest a model where the low-ionization phase is formed by relatively small, discrete clumps of denser material embedded in a lowerdensity, higher-ionization phase as suggested by Hamann (1998) and Gabel et al. (2005b).
- Comparing the properties of the outflow present in IRAS F22456−5125 and the bona fide AGN outflow observed in NGC 3783 reveals a more complex situation.
- B.B. also thanks S. Penton for the introduction to the HST/COS pipeline.
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Frequently Asked Questions (2)
Q1. What have the authors contributed in "C: " ?
The authors present analysis of the UV spectrum of the low-z AGN IRAS F22456−5125 obtained with the Cosmic Origins Spectrograph on board the Hubble Space Telescope.
Q2. What future works have the authors mentioned in the paper "C: " ?
The authors investigated the possibility that the absorber is collisionally ionized by producing grid models of NH versus temperature with a fixed ionization parameter of 10−5. The authors thank the anonymous referee for a careful reading of the manuscript and suggestions that helped to improve the paper.