Galactic powerhouses

by Dr. Rebecca McElroy

It is generally believed that at the centre of most galaxies like our own there is a supermassive black hole. This supermassive black hole can be up to a billion times the mass of the Sun and generally lives within the central star cluster of its host galaxy. Though incredibly massive these objects typically make up less than 0.1% of the mass of the whole galaxy.

Since black holes are intrinsically black, they can only be studied if the matter surrounding them is emitting light. When matter falls onto a supermassive black hole it lights up, and in some cases these objects can greatly outshine their host galaxy. We call such objects quasars or active galactic nuclei (AGN).

Centaurus A, a nearby active galaxy whose central supermassive black hole is driving a jet out of the galaxy. Image: ESO

The huge amounts of energy emitted by AGN are thought to have a powerful effect on how galaxies evolve over time. In particular, it is thought that the jets and outflows of gas driven by AGN suppress the number of stars that a galaxy can form thereby limiting its growth.

Many methods can be used to study how AGN effect the galaxies they live in, but here I will highlight one. Integral field spectroscopy (IFS) combines spectroscopy and imaging, allowing us to probe the chemical composition of the gas and stars throughout a galaxy.

So what does this look like? Well, something like this:

Integral field spectroscopy provides imaging, but also spectral (wavelength) information resulting in a 3D data product.

This kind of data can be thought of as an image at many wavelengths, or an image where every pixel also has a spectral (colour) dimension. This technique is particularly useful in the study of AGN as it allows us to look at both the conditions close to the supermassive black hole and further away. We can then use this information to map the influence of the AGN on the galaxy as a whole.

Using this technique to study the brightest AGN in the nearby Universe we were able to show that the galaxies are significantly effected by the presence of the AGN. In particular, we found evidence that these galaxies had significant outflows of gas being driven by the AGN. Due to the detailed chemical information in our data we were able to show that these outflows are colliding with the interstellar gas of the galaxies. This is direct evidence that energy from the AGN is influencing the galaxy as a whole.