Galaxy Hunting: Level 4.1


I have been trying to investigate the properties of the galaxies we have found. The production of emission lines due to recombination requires the presence of ionised gas. Two main sources of ionisation are high-temperature (O/B type) stars and active galactic nuclei (AGN). AGN are small, incredibly luminous regions of a galaxy, powered by matter heating up as it accretes around the galaxy’s supermassive black hole.

The primary source of ionisation in a galaxy may be distinguished using a BPT (Baldwin, Phillips and Terlevich) diagram. The two lines represent two different ways of differentiating the two types – the solid line is from theoretical modelling (Kewley et al., 2001), the dashed line is based on observations (Kauffmann et al. 2003) Galaxies with ionisation from hot stars appear under the line; AGNs above.

BPT diagram of where our sources lie. Galaxies with ionisation from hot stars appear under the line; AGNs above.

It took a surprisingly long time to get here – lots of wrangling with code, then realising that some my sources were contaminated by other sources! In the end only five sources remained, as shown above. I think the code is being over-fussy about what counts as a peak, which I should look further at.

To compare the properties we found to some other source, you need some other galaxies. The CLOUDY simulations provide this. Essentially they are computer-generated stars and AGN. With these, you don’t have to correct for dust absorption, distance etc., so you can obtain all sorts of information.

From the diagram above, you can see four galaxies appear to be dominated by star-formation (one of them is a bit odd, I’ll come back to it later). To see if our estimated metallicities (how much of the stuff isn’t hydrogen or helium) are reasonable, I compared our values to the CLOUDY simulated stars. The metallicities of all of them, bar PIG-7354-1408, fit in nicely. The closest match to the CLOUDY data is a set of galaxies dominated by O and B type stars (the hottest and most massive).

BPT diagram of PIG sources (grey circles) versus CLOUDY models (coloured), showing the ages of CLOUDY ‘stars’. Age_stars is logarithmic (e.g. Age_Stars = 7 means 10^7 (10 million) years old).

Hot stars die young (still tens of millions of years old, but that’s peanuts to space), so their presence indicates that the galaxy is still forming stars. If  galaxy stopped forming stars, the O and B type stars would die quickly, leaving dimmer stars to dominate the spectrum. So if you see the hot stars you would expect the star formation rate to be quite high. However, I am finding very low values for the star formation rate of our sources (<0.01 solar mass per year). I am still trying to work out what is going on here!

The blue galaxy is quite interesting. From its position on the BPT diagram, it clearly contains an active galactic nucleus of some sort. As I mentioned earlier, these are visible due to a supermassive black hole driving jets of matter out into space towards us. There are different types of AGN, which might be just to do with the orientation of the jets relative to us. A beam that has to pass through dust on its way to us appears different to one that does not.

Looking at the properties (such as the relative fluxes of different spectral lines and the galaxy’s luminosity), I have concluded that this galaxy is most likely a ‘LINER’. This is a fairly weak type of AGN.

The final source (PIG-7404-1368) is a real muddle. Depending on which features you look at, it appears to be star-forming, a LINER or a Seyfert galaxy (a more powerful type of AGN). Looking at the galaxy in Hubble, I think it is contaminated by its massive neighbour, so this could be the cause of some of the confusion! See the image below, where PIG-7404-1368 is shown by the thick blue circle, and other sources with thin circles.

Screen Shot 2018-07-24 at 16.38.59.png
Picture of PIG-7404-1368 (thick blue circle) from Hubble. Its spectrum is probably contaminated by its neighbour, PIG-7395-1374, to its right. This may explain its odd properties.

While difficult, I have found classifying these galaxies very satisfying. I am still unsure about some things – whether I can classify more sources, the strange-looking sources and low star formation rates, but I am happy to have achieved some things. It has made me really appreciate how difficult it is to classify galaxies properly, and amazed at how we manage to do it with so little information.


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