Figure 1. Artist’s impression of a galaxy with a large-scale outflow.
Credit: ESA/ATG medialab.
Figure 2. An example object from the GMOS observations. The background
image is from the Sloan Digital Sky Survey. The cyan rectangle shows the
GMOS field of view. The red/yellow contours show the distribution of
high-‐velocity ionized gas. The inset shows an example oxygen
emission-‐line profile ([O III]5007) that was used to trace the
velocity of the gas.
Figure 3. Left: Distribution of the emission-line widths (a proxy for
gas velocity) of both the overall population of luminous quasars (yellow
histogram) and the sample studied here (red histogram). Around half the
luminous quasars in the parent population show very high gas velocities
(> 700 kilometers per second). The 16 objects studied here were
selected from this population. Right: Similar histograms show that the
radio luminosities of the quasars studied here are representative of the
parent population.
Observations using the Gemini Multi-Object Spectrograph on Gemini
South reveal that galaxy-wide high-velocity outflows are extremely
common among galaxies that host luminous quasars. These outflows may
represent a crucial stage in a galaxy’s evolution when the supermassive
black hole at its center begins injecting vast amounts of mass and
energy into the galaxy.
Supermassive black holes reside at the center of all massive galaxies
and they grow through mass accretion, taking on material from their
surroundings. During the most active periods they become luminous
quasars. The most successful models of galaxy evolution predict that
quasars play an integral role in the formation and evolution of massive
galaxies by injecting mass and energy into their host galaxies. Without
such feedback mechanisms, models are unable to accurately reproduce the
properties of local massive galaxies such as the distributions of
colors, star formation rates and stellar masses.
A popular idea for a feedback mechanism is that extremely powerful and
high-velocity outflows of gas are launched by quasars and consequently
propagate throughout the galaxy (see Figure 1). These outflows could
sweep up and heat material, reducing the star formation rates of the
host galaxies and add material to the larger scale environment. While
earlier observations have shown that high-velocity outflows exist in
some quasars, it has remained unclear what the spatial extent of these
outflows are and how often they occur.
Using the Gemini Multi-Object Spectrograph (GMOS) on Gemini South,
astronomers at Durham University, led by Chris Harrison, have begun to
answer these questions and show the properties and frequency of outflows
in quasars. Using GMOS’s integral field spectrograph, they traced the
properties of the gas across the host galaxies of 16 quasars.
Critically, GMOS on Gemini South enabled the astronomers to trace the
velocity of the gas over the full spatial extent of the quasars’ host
galaxies. By using emission lines produced by ionized hydrogen and
oxygen to trace the gas velocities the observations revealed that
outflows, reaching velocities of >1000 kilometers/second, were found
over the full spatial extent of the host galaxies of all of the quasars
observed (e.g., see Figure 2). Estimates of the masses involved in
these outflows indicate that they are removing significant amounts of
gas from the galaxies and their overall properties are in line with
model predictions.
A key aspect of this work is the quasars that were observed with GMOS
were initially selected from a parent sample of several thousand objects
with optical spectra (Figure 3). These observations can thus be placed
in the context of the overall population and reveal properties of
quasars generally. For example, these observations imply that at least
70% of the most luminous quasars (those predicted by models to drive the
feedback mechanisms) exhibit galaxy-wide outflows of this type. Further
work now needs to be done to pin-down exactly how the central black
holes are able to launch such large-scale outflows. Furthermore, while
models predict that outflows, such as those observed here, have a
dramatic impact upon the star formation in their host galaxies the
current observational evidence for these effects remains only indirect.
Astronomers continue to search for direct observational proof that these
outflows can have a profound influence the evolution of massive
galaxies.
This research is presented in the paper: Kiloparsec-scale outflows are
prevalent among luminous AGN: outflows and feedback in the context of
the overall AGN population by C. M. Harrison, D. M. Alexander, J. R.
Mullaney and A. M. Swinbank. The paper is published in the Monthly Notices of the Royal Astronomical Society (2014 441 3306) and is available on astro-ph.
Source: Gemini Observatory
