6 HST-COS Results: Evidence for a Corresponding UV Absorber

Our HST-COS observations reveal a previously unknown, weak, broad, blueshifted Ly$\alpha$ absorption feature consisting of two blended components, as shown in Figure 9. Kriss et al. (2018) discuss in detail the analysis of the UV data in general and this feature specifically. They find an average outflow velocity of $v_{\rm out} = -16\,980 \pm 40~\rm km~s^{-1}$ ( $z_{\rm out} = -0.0551$) with a FWHM of $1080 \pm 80~\rm km~s^{-1}$. Both the HST -detected outflow velocity and width are consistent with the strongest X-ray absorption features, giving additional credence to the reality of such a high velocity outflow. In addition to the Ly$\alpha$ feature in the G130M spectrum, we also detect Ly$\beta$ at the same velocity in the G140L spectrum, albeit at lower significance. No other UV ions are detected, including high-ionization species such as CIV, NV or OVI.

Several other narrow absorption lines appear in the UV spectrum of PG1211+143  embedded within the profile of the newly detected broad Ly$\alpha$ absorption feature. These include foreground interstellar absorption in the NV doublet, as well as the previously known features from the foreground intergalactic medium (IGM) (Tilton et al., 2012). We can safely conclude that these narrow features are not associated with the outflow from PG1211+143 because they do not share any of the characteristics typical of absorption lines associated with AGN outflows- they are not variable in strength, and they fully cover the source. The Ly$\alpha$ to Ly$\beta$ optical depth ratios (Danforth & Shull, 2008; Tilton et al., 2012) are consistent with their Doppler widths, which at $\sim 35$ km s$^{-1}$ are typical of other IGM Ly$\alpha$ absorbers.

The similar depths of the broad Ly$\alpha$ and Ly$\beta$ features in our spectra indicate that they are saturated, so that the UV absorber only partially covers the continuum source. Using the depth at the center of the Ly$\alpha$ absorption feature, we measure a covering fraction of $C_f = 0.30 \pm 0.04$. Since the lines are saturated, we cannot measure an accurate column density, but we can set a lower limit assuming a covering factor of unity and integrating across the apparent optical depth of the line profile (e.g., Hamann et al., 1997). This gives $\log N_{\text{H\,{\sc i}}} \gtrsim 14.5$ cm$^{-2}$.

Figure: HST-COS G130M spectrum of PG1211+143 in the wavelength range covering the broad Ly$\alpha$ absorption line profile, the IGM Ly$\alpha$ lines (Penton et al., 2004) and interstellar NV and SII lines, adapted from Kriss et al. (2018). The lower horizontal axis is the observed wavelength in Ångstrom. The upper horizontal axis is the outflow velocity of Ly$\alpha$ relative to a systemic redshift of $z = 0.0809$.

Despite the high ionization of the X-ray absorber detected at the same outflow velocity, our photoionization models predict a small residual column density of neutral hydrogen. At our best-fit $\log \xi = 2.87$, the ionization fraction of HI is $2.99_{-2.40}^{+1.49} \times 10^{-8}$. For our best-fit total hydrogen column density of $\log N_{\rm H} = 21.47_{-0.58}^{+0.18}$ cm$^{-2}$, we predict $\log N_{\text{H\,{\sc i}}} = 13.95_{ }^{+0.26}$ cm$^{-2}$, roughly consistent with our measured lower limit of $\log N_{\text{H\,{\sc i}}} \gtrsim 14.5$ cm$^{-2}$. As Kriss et al. (2018) show, the Ly$\alpha$ profile actually consists of two blended components, so it is possible that the X-ray UFO is associated with only a portion of the Ly$\alpha$ absorption profile. This would also explain the narrow turbulent velocity we find for the X-ray absorber compared to the Ly$\alpha$ absorber.

All other UV ions are predicted to have column densities at least three orders of magnitude lower, rendering them undetectable even in our high S/N HST-COS spectra.