[Illustris-1] FITS pixels for higher-resolution photometry
mauro sciarratta
13 Nov '17
Hello,
I am asking the following questions, regarding computing surface brightness and other photometrical quantities using FITS files of galaxies, because I didn't find an answer through the FAQs and the whole forum, but I apologize if they have been already discussed.
In Torrey+15, at subsection 2.2.4 it is written:
"Images are produced by projecting the stellar light using pinhole cameras with a resolution of Npixels × Npixels pixels. We use a fiducial value of Npixels = 256 for the full image catalog. However, we have used higher pixel resolution (Npixels = 512) for many of the individual images included in this paper."
So I'm asking:
given that camera images into accessible FITS files are at fixed resolution 256 x 256, is it possible to get a more detailed set of camera images (i.e. with higher pixel resolution), perhaps only of a restricted set of galaxies at z=0?
also the thumbnail images in the Galaxy Observatory seem to be obtained with such higher resolution images, are them?
lastly, if I use particle information for the galaxies I am interested in, can I already get a reliable information?
I can (or you can directly) ask Paul Torrey if this is possible, do you have already a set of subhalo/halo IDs that you would be interested in?
As far as I know the Galaxy Observatory images are derived from the FITS. Indeed they are 256x256 pixels (for example). This set of python codes should help you transform from a FITS into such an image.
I am not sure what you mean exactly? Do you mean, can you yourself make a surface brightness projection using the stellar particles of a given subhalo? This is definitely possible and not too difficult, particularly if you want one of the eight bands stored in the GFM_StellarPhotometrics field. The only complexity is the choice of the "smoothing", i.e. what size to assign to each star particle, e.g. the discussion in Torrey+15.
mauro sciarratta
14 Nov '17
Hello Dylan,
thanks for your answer, it actually helped more than expected.
Yes I have the set, but I am already working with particles so I will evaluate in the next days whether to ask for higher resolution images. Thank you!
OK for Galaxy Observatory
OK also for stellar photometrics, which is what I am using by now; thanks for reminding me about smoothing: although I am working on surface brightness profiles and so smoothing should not be an important problem, I will possibly decide to smooth stellar light in order to take it into account.
Thanks a lot, kind regards.
MS
Dylan Nelson
14 Nov '17
Hi Mauro,
If you are making 1D radial surface brightness profiles, then you're right this should be doable and no need to worry about smoothing until you get to quite large distances from the galaxy center - there you will start to see noise in the profiles due to the small number statistics of the star particles in each bin. You can then either smooth these over multiple bins, truncate the profile, or do an adaptive/increasing bin size.
Hello,
I am asking the following questions, regarding computing surface brightness and other photometrical quantities using FITS files of galaxies, because I didn't find an answer through the FAQs and the whole forum, but I apologize if they have been already discussed. In Torrey+15, at subsection 2.2.4 it is written:
"Images are produced by projecting the stellar light using pinhole cameras with a resolution of Npixels × Npixels pixels. We use a fiducial value of Npixels = 256 for the full image catalog. However, we have used higher pixel resolution (Npixels = 512) for many of the individual images included in this paper."
So I'm asking:
Thank you, regards.
MS
Hi Mauro,
GFM_StellarPhotometricsfield. The only complexity is the choice of the "smoothing", i.e. what size to assign to each star particle, e.g. the discussion in Torrey+15.Hello Dylan,
thanks for your answer, it actually helped more than expected.
Thanks a lot, kind regards.
MS
Hi Mauro,
If you are making 1D radial surface brightness profiles, then you're right this should be doable and no need to worry about smoothing until you get to quite large distances from the galaxy center - there you will start to see noise in the profiles due to the small number statistics of the star particles in each bin. You can then either smooth these over multiple bins, truncate the profile, or do an adaptive/increasing bin size.