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galaxy dynamics

Page history last edited by lyndsay old 9 years, 3 months ago

Galaxy dynamics & substructure project


Background & motivation


Dynamical analysis of clusters using spectroscopic redshifts of member galaxies exposes substructure in the form of asymmetrical velocity distributions and dynamically distinct subgroups (e.g., Geller & Beers 1982; Dressler & Shectman 1988; Cohn 2012; Einasto et al. 2012; Hou et al. 2012). As most of our cluster mass estimation techniques rely on the assumption that clusters are relaxed, we want to know whether this assumption is valid or not. The presence of dynamical substructure is a strong indicator that clusters have recently undergone a merger and, as such, are not relaxed. In addition, if galaxies relax on longer timescales then the X-ray gas, we'd expect that the imprint of a merger to be visible for longer in the galaxy distribution compared to the X-ray gas.


More specifically, I'd like to explore the following questions:

  • Can the most common (& the most successful according to Hou et al. 2012) dynamical substructure test, the 'Dressler-Shectmann' test, detect substructure just before, during and after more major mergers occur in the DM?
  • How does the galaxy distribution and substructure correlate with other cluster properties? E.g., is galaxy dynamical substructure detected in the 3 X-ray unrelaxed and 3 X-ray relaxed clusters? 



Post-workshop project status


  • Due to the issues with the data, we have abandoned analysis on the SAM catalogues. We have moved on to applying dynamical substructure tests to the full physics run
  • 'Maps' of structure from the Dressler-Shectman test and Kappa test will be provided for cluster 00019 at redshift 0 and 0.43 for comparison with other properties e.g., lensing, X-ray. Galaxy member selection is performed by Matt Owers shifting gapper+caustic methodology on objects in the catalogue that have a stellar mass of > 10^9 Msolar.


The maps for the Dressler-Shectman (DS) test are run on the MUSIC, velociraptor catalogue taken from '/home/nifty2014/ClustersOfGalaxies/PLAYGROUND/pelahi/velociraptor_ahf_catalogs/RAD' (files 'music.rad.z0.000.AHF.VELOCIraptor_halos' and 'music.rad.z0.430.AHF.VELOCIraptor_halos').


PDFs of the DS maps (bubble plots) can be found at '/ClustersOfGalaxies/PerthClusters2015/users/lold/' and have the filenames 'cl_00019XY_velociraptor_MUSIC_ahf_z_0_XY.pdf' and 'cl_00019XY_velociraptor_MUSIC_ahf_z_0point43_XY.pdf'.

An ascii file of the DS maps can also be found at '/ClustersOfGalaxies/PerthClusters2015/users/lold/' and have the filenames 'cl_00019XY_velociraptor_MUSIC_ahf_z_0_XY.txt' and 'cl_00019XY_velociraptor_MUSIC_ahf_z_0point43_XY.txt'. These ascii files have the following format:



As with the velociraptor catalogues, values are in physical units, no little h, kpc and km/s




During-workshop: Rockstar + Galform Cluster 19






Pre-workshop: *Very preliminary* investigation for Rockstar + Galform Cluster 19


For some initial tests, I took the cluster and chose all galaxies according to the following criteria:
- their X and Y locations are within the halo R200c aperture 'on the sky'
- their z peculiar velocity is +/- 1000km/s
- they are not orphans
- they have a stellar mass => 1 x 10^9 Msolar

With this criteria, I performed the DS test as a function of redshift and scale factor (‘Galform_19_DS_bubbleplot_fixed_axis_original_selection.gif’ and, 'Galform_19_DS_z_a.pdf' where ‘original_selection’ in the filenames refer to the selection criteria above.)

I then compared the DS test detection to the DM bound mass as a function of z and a. See this an annotated plot 'Galform_19_DS_binary_Mbound_z_a_annotated.pdf‘.

I then had a look at how the stellar mass threshold one applies affects the DS substructure detection.

I took 10 linearly-spaced stellar mass thresholds between 1E8 - 1E10 Msolar (not linearly spaced in log Mstellar) and calculated the DS p-values. Note that I kept the rest of the selection criteria (e.g., radius, velocity, orphans)  the same. The results of this are shown here ‘Galform_19_DS_binary_z_a_Mstellar_subsampling.pdf’, where filled markers denote substructure detections (where p < 0.05) and the empty markers denote no substructure detected.

Throughout my analysis, I only performed the DS test when Ngal > 5, this is the reason why some DS points are missing in the above plot. At first, I was surprised that the DS test was calculated for some higher redshifts and not lower redshifts but after further inspection of various aspects of my selection, this is actually due to the combination of radial velocity, X Y position and Mstellar, rather then just the Mstellar itself.

*Very preliminary* thoughts so far:

  • With the ‘original selection’ criteria, the DS test picks out substructure for a decent amount of time before and after the apparent merger at z~1.4. However, the DS test does only detects substructure for a very short time for the (slightly weaker) merger at z~0.65. Detection seems sporadic at lower redshifts then this. To get down to why this is the case, I'll need to check what is happening with the galaxy/DM interactions in X, Y, Z space.
  • The DS test is very sensitive to the stellar mass cut! Take the z = 0 snapshot as an example, for the lowest stellar mass cuts, the DS test detections substructure but for the higher thresholds, no substructure is detection. I think this supports the following hypothesis: dynamically district subgroups, which have recently fell into the cluster, are most lightly fainter then cluster galaxies. When you take a higher stellar mass cut, galaxies disappear from the sample, so instead of having a subgroup, you may just have the main galaxy of that subgroup, which the DS test would not detect as a dynamically distinct subgroup.
  • However, there are some case where substructure is not detected at low stellar mass threshold, but then substructure is detected for a higher stellar mass threshold at the same snapshot.






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