Searching for AGN
Student: Joshua Tiffany; Queen Mary, University of London
Supervisor: Dr Marc Sarzi; University of Hertfordshire
My summer project consisted of working alongside Dr. Marc Sarzi doing research into galaxies hosting an Active Galactic Nucleus, or AGN-host galaxies in short. AGNs
emission is powered by the accretion onto central supermassive black holes, which are now believed to lurk in almost every galaxy [ref]. The energy produced by an AGN
is in principle sufficient to empty a galaxy of its gas, so that AGNs could
effectively shut down the star formation in galaxies and therefore be a key ingredient in our models for their formation and evolution.
In this project we analysed 480 galaxies with non-thermal radio emission detection with the Very Large Array (VLA), which could signal
the presence of a central AGN or of diffuse star-formation activity. These objects were selected to also have Very Large Baseline
Interferometry (VBLI), which we use to check whether the radio signal is nuclear, and central optical spectrum using data from the Sloan
Digital Sky Survey (SDSS), which we use to check for the presence of outflows. For the latter, we studied the Na I interstellar absorption
lines at 5890 and 5896 Angstrom and look whether they appear systematically blue-shifted in with respect to all other stellar absorption features in the SDSS spectra.
The finding of outflows in galaxies with nuclear radio emission detected with VLBI would indicate the presence of AGN feedback in action.
To measure the Na I velocity offset we used data from the SDSS Data Release 9 (DR9) and the GandALF code written by Dr. Sarzi to analyse
spectra from the previous DR7 release, which I adapted to fit the DR9 data. This code fits simultaneously the stellar spectrum and the
nebular emission of a galaxy, through a superposition of stellar templates and Gaussian functions meant to represent the various
ionised-gas emission lines. In this context, GandALF can fit also the interstellar Na I absorption features also by means of two Gaussian
functions, albeit with negative amplitude. While fitting the SDSS spectra, GandALF also adjust for the redshift of the stellar and
Gaussian templates to account to their mean velocity with respect to the observer. This also allows us to check for a shift between the
stellar and interstellar Na I absorption lines.
The figure below show our GandALF fit for the SDSS spectrum of one galaxy from our sample, over the its entirety on the left or just in
the wavelength region around the Na I doublet on the right. The black line shows the data, which are indeed well fitted by our model that is shown by the red line. The
stellar component is also shown by the green line, whereas the blue line shows both nebular emission and
interstellar absorption that we model with Gaussian functions. In this example there a clear excess of Na I absorption due to interstellar
medium, although this does not appear particularly shifted with respect to the stellar lines.
Our NaD results are summarised in the next two figures, where we split our sample between galaxies with a nuclear radio detection with VLBI (left) and objects when no
such a nuclear emission was found (right). The plots show the strength of the NaD absorption, parameterised in terms of the A/N ratio between the mean amplitudes of
the lines and the noise level in the fit residuals, against the difference between the velocity of the NaD lines and the bulk of the stars in the galaxy. A/N values above 3.5
indicate a secure detection of the NaD lines, in which case we colour the data points
depending on whether the nebular emission arise from star-forming regions (blue), from
optical AGN activity (red and yellow) or likely combination of the two (green). The plot
on the left shows that objects with a central radio AGN are mostly devoid of interstellar
medium and do not display sign of outflows (i.e. a blue-shift) when this is present. On
the other hand, galaxies with optical AGNs and star-forming regions but no radio central
source (right) show more often (unsurprisingly) a rich gas environment and that when
outflows occur these are likely to be driven by star-formation activity (as observed in the
nearby galaxy M82).
This study thus indicates that optical AGN activity does not drive outflows in the kind of regular galaxies that have targeted, and
that by the time radio AGN occurs these objects have already been emptied of most of their gas. If AGN feedback occurs, it must either act very
quickly at the onset of radio AGN activity, or it must take place during an evolutionary phase that we have not surveyed here, such as
galaxy mergers and interactions.
Working with Dr Sarzi in this area was an amazing opportunity to do research into an area of astrophysics I have yet to study. I learnt so much, not only about galactic physics and how galaxies are far more
complex than I originally though but also about the world of academic research and I got to experience how research was done on a day to day level.
I would like to thank SEPnet for arranging this Internship and allowing me an insight into physics research, Dr Marc Sarzi and Dr Sugata Kaviraj for their patience, kind words and constant help over this summer and the University of Hertfordshire for hosting me and allowing me to do my work there.