De la conception instrumentale
à l’exploitation des observables
Accueil > Séminaires > The Sightline towards the Quasar (…)

The Sightline towards the Quasar PG1543+489 : Evidence for Cool Halo Gas at z=0.1

Brenda L Frye (University of Arizona)

We examine the line of sight to the quasar PG1543+489 at z=0.401. The
absorption line system at z=0.0749, ALS-1543, probes the outer halo of a
disk galaxy `G1’ at a distance of 45h^-1kpc. By fitting profiles to the HI damping
wings and to the NI lines in our HST/STIS spectroscopy, we measure a gas-phase
abundance of 12+log(N/H)=8.6 +/- 0.2, one-tenth that of the star forming disk of G1.
This low metallicity makes this ALS unlikely to be a part of a recent gas outflow event,
especially given the nearly anti-polar alignment with respect to the disk of G1.
From a joint analysis of the N I and Si II absorption lines we compute a kinetic gas
temperature of 3.7 x 10^4 K, lower than expected for halo gas. To test the infall
hypothesis, we construct a rotation curve for this edge-on galaxy. The UV metal
lines are all systematically offset from the systemic redshift of G1 in the direction
of rotation, and by an amount larger than the rotational velocity of the disk
(by 70 km/s) from which we infer ALS-1543 to be orbiting and co-rotating with G1.
If this is cool halo gas in the local universe then it appears to have escaped shock
heating to the virial temperature, possibly owing to a low total mass (<10^12 solar
masses). We report the discovery of the QSO host galaxy seen as a multi-line
emission source at an angular separation of 7.33 arcsec, equating to 40 (physical)
kpc, and discuss the implications of this extended emission line source for fueling
its central supermassive black hole.