The outflowing ionised gas of I Zw 1 observed by HST COS
arxiv(2024)
Abstract
We present an analysis of the HST COS spectrum of IZw1 aiming to probe the
absorbing medium associated with the active galactic nucleus (AGN). We fitted
the emission spectrum and performed spectral analysis of the identified
absorption features to derive the corresponding ionic column densities and
covering fractions of the associated outflows. We employed photoionisation
modelling to constrain the total column density and the ionisation parameter of
four detected kinematic components. By investigating the implications of the
results together with the observed kinematic properties of both emission and
absorption features, we derived constraints on the structure and geometry of
the absorbing medium in the AGN environment. We find and characterise
absorption line systems from outflowing ionised gas in four distinct kinematic
components, located at -60, -280, -1950, and -2900 km/s with respect to the
source rest frame. While the two slower outflows are consistent with a full
covering of the underlying radiation source, the well-constrained doublet line
ratios of the faster two, higher column density, outflows suggest partial
covering, with a covering fraction of C_f 0.4. The faster outflows show also
line-locking in the NV doublet, a signature of acceleration via line
absorption. This makes IZw1 possibly the closest object that shows evidence for
hosting line-driven winds. The observed -1950 km/s absorption is likely due to
the same gas as an X-ray warm absorber. Furthermore, the behaviour in UV and
X-ray bands implies that this outflow has a clumpy structure. We find that the
highly asymmetric broad emission lines in IZw1, indicative of a collimated,
outflowing broad line region, are covered by the absorbing gas. Finally, the
strongest UV–X-ray absorber may be connected to some of the blueshifted line
emission, indicative of a more spatially extended structure of this ionised
medium.
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