2002 PHD PROJECT :- Alan Pedlar
STUDIES OF NEUTRAL GAS IN ACTIVE GALAXIES
Introduction
Observational studies of active galactic nuclei (AGN) and starbursts have largely concentrated on studies of ionised (eg emission lines) and relativistic plasma (eg synchrotron emission). However, it has also emerged that neutral gas is intimately involved in both AGN and starbursts and plays a crucial role in their physics.
Atomic Hydrogen Absorption in Seyferts
The properties of neutral gas in the central few hundred parsecs of AGN has only recently become accessible to study. Although initially its very existence was surprising (considering the presence of shocks and copious UV emission from the AGN) the properties of the neutral gas are now proving vital in constraining the physics involved in the collimation and fuelling of the nuclear activity.
MARKARIAN 6 continuuum :- Click on the image to see H1 absorption
NGC5929 Continuum : Click on image to see H1 absorption
NGC7674
We have pioneered the use of MERLIN to image the H1 absorption from Seyfert nuclei with typically 0.1 arcsec resolution which correponds to linear scales of a few parsecs in nearby objects such as NGC4151. At present (and until the advent of large mm arrays- eg ALMA) MERLIN is the only instrument which can image neutral hydrogen on scales directly comparably to Hubble Space Telescope resolution. These observations constrain models involving the fuelling of the nuclear activity, test current unified schemes and set upper limits to the mass of the central engine. One of the main drivers for this research was investigating the 'standard model' in which the active galactic nucleus is surrounded by a dusty torus which neatly accounts for collimated UV emission, hidden broad lines and fuelling. Molecular hydrogen gas in this torus is dissociated to form atomic hydrogen which we can detect by absorption of the 21cm radio line.
For further reading :- typical papers which have arisen from earlier Phd projects include
NGC4151 :- MNRAS 272 355
Markarian 6:- A&A 333, 13
NGC5929:- MNRAS 301 782
For a good summary of H1 absorption in Seyferts see Gallimore et al. ApJ
524, 684
Atomic Hydrogen in Radio Galaxies
3C305 - Click on image to see absorption spectra
3C293 - 20cm Radio Continuum, HST image & selected H1 spectra
The science case is similar to the Seyfert case, although the radio
emission is stronger which allows lower column densities of hydrogen
to be investigated. Also many of these galaxies are at higher
redshifts than typical Seyferts. Only a few radio galaxies have been
observed, many remain to be investigated. One of the most impressive
studies - 3C 293 (see above) has just been completed as part of Rob
Beswick's PhD project.
(MNRAS - in press) 
Click for more
information on 3C293
Atomic Hydrogen Absorption in Starbursts
As well as work on Seyferts & Radio galaxies the PhD project can also
include work on starbursts. In the the case of starbursts we can
determine the absorption spectra against individual supernova remnants
which not only allows us to probe the interstellar medium on parsec
scales, but also is crucial in determining the dynamics of the neutral
gas which is fuelling the starburst.
Click on the image to enlarge
This figure shows the MERLIN/VLA 20cm image with the Shen & Lo CO emission superimposed as contours
Click HERE to see
some of the H1 absorption spectra seen against supernova remnants in M82.
Examples of work already completed can be found in
M82:- MNRAS 298 347
NGC3628:- MNRAS 300 656 (1998) Click here to see the spectra
NGC6240:- MNRAS 325 151 Click here to see image &
spectra
Arp 220:- Ap.J. 560 168
THE PHD PROJECT The 2002-2005 PhD work
would continue to investigate H1 absorption against all three types of
active galaxy. One area which work is required is in surveys of complete
samples of each type of object as much of the earlier work has been
heavily focussed on individual objects rather than statistics. The whole
area is fairly open ended, and, as with earlier projects much of the final
shape depends on the students particular interests. There is some overlap
with the starburst PhD project also proposed for 2002- which may mean that
some areas (eg M82 OH etc) may be required for this- however this would
not be a major problem given the amount of data available.
A number of observations have already been taken which would form the
basis of the project. Further projects will be proposed during the PhD
project.
a) A search for atomic hydrogen absorption in Seyfert nuclei and
nearby radio galaxies using MERLIN & VLA
b) EVN & MERLIN studies of atomic hydrogen in individual objects
c)Studies and searches for OH and H2CO absorption (NGC3079, NGC3628, M82).
d) VLA(New Mexico) & GMRT(India) observations of H1 emission
e) CO & mm continuum work using Onsala 20m (Sweden) & SEST 15m (Chile)
dishes
Previous Jodrell PhD students who have completed projects involving H1
absorption
Rob Beswick rbeswick@jb.man.ac.uk
(To submit in 2002)
Gordon Cole gordonhjcole@hotmail.com
Karen Wills K.Wills@sheffield.ac.uk
Carole Mundell cgm@astro.livjm.ac.uk
Standard Model 
