### surface brightness, subhalos

Moritz Haslbauer
• 1
• 27 Sep '17

Dear Dr. Dylan Nelson,

I am currently studying the properties of galaxies in the Illustris simulation. Therefore some questions arises and I would be glad if I can ask you some of them.

1a) Could it be that a single subhalo contains more than one galaxy? Or in other words: How can I identify dwarf (or satellite) galaxies and distinguish them from their host galaxies? (only by there different masses(distances)?) 1b) What is the difference between central and satellite subhalos?

2) I want to calculate the mean surface brightness within the circle of a given radius. Unfortunately, I am not able to find the data for the surface brightness (mag/arcsec**2) in the Illustris data. Can I find them maybe in the fits file? Since I want to calculate the mean surface brightness within a given radius, I would be interested in getting the surface brightness map over a given area, which contains all my selected galaxies (and not just the mean surface brightness from a single galaxy).

Sincere regards, Moritz Haslbauer

Dylan Nelson
• 1
• 6 Oct '17

Hi Moritz,

1a. A "halo" can contain many "subhalos". The most massive subhalo we call the central, and any others are satellites. Therefore, if you wanted to find e.g. satellites with stellar mass between 10^9 and 10^10 Msun around clusters with a central BCG of stellar mass between 10^11.4 and 10^11.6 Msun, you could first search and find all central galaxies matching the high mass criterion, then look through the other subhalos (i.e. satellites) of each halo hosting such a central.

1b. As above, the central is the most massive subhalo of the group/halo. See also the first question of the Millennium-II FAQ.

2a. In the Illustris catalog data, the `SubhaloStellarPhotometrics` field is the only one which gives the stellar luminosities. This includes all stars in the subhalo (i.e. no well defined radius). If you assumed a radius, you could then calculate a surface brightness. This is pretty imprecise though, I would suggest instead that you go to the particle data. For a given subhalo, you can then compute the 2D distance to all its star particles, select those within the radius you want, and sum up their luminosities from the above field, then calculate the SB (converting the radius into arcsec^2 at the appropriate distance).

You could, alternatively, use the FITS files, if you would be more comfortable learning the details/units of these files, than working with the particle data.

At the end of your question, I assume you mean that the computation should not include just the stars of a single galaxy, but all the stars in that region of space. The FITS files by default include all stars in the parent halo - so this would work to e.g. capture the light from nearby satellites of other satellites. If you wanted to capture the light of multiple nearby central galaxies (or halos themselves), over larger distances, you should again use the particle data, and just make sure to compute the distance to all stars in the box, to the position you specify, before selecting.

Moritz Haslbauer
• 16 Oct '17

Dear Dr. Dylan Nelson, thank you very much for your answer. I have got another question: In order to compare the Illustris Simulation with observational data (for example the SDSS and an all sky survey), I want to create a light cone from a given point in the box at redshift 0 up to redshift 0.5. Do you know if there exists a routine which creates such light cones from Illustris automatically? Or do I have to add these simulation boxes together by myself ?

Dylan Nelson
• 16 Oct '17

Hi Moritz,

The only lightcones currently available from Illustris are mocking 'deep field' type geometries. Please see

https://archive.stsci.edu/prepds/illustris/

also here you will find a GitHub link and further information (all from Greg Snyder) on the process.

There isn't a SDSS lightcone available - note that the volume of Illustris is much too small to make a (non-repetitive) all-sky lightcone out to z=0.5.

Wentao Luo
• 3 Dec '22

Dear Dr. Dylan Nelson,

I am trying to use HI mass to deduce HI intensity for SKA interferometry galaxy survey.
Is there a paper to specifically describing the relation between HI mass and HI intensity?

Thank you!

Best regards,
Wentao Luo

Dylan Nelson
• 3 Dec '22

It would be useful to understand both directions, converting from a simulation to a mock observation e.g. the MARTINI code paper, and converting from observed flux to HI mass, e.g. the ALFALFA survey paper.

Wentao Luo
• 4 Dec '22

Thank you!

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