Section author: Stephanie Juneau <stephanie.juneau@noirlab.edu>
1.2.2. Probe large-scale structure in DECaLS¶
1.2.2.1. Background¶
Galaxies are not randomly distributed in space. Instead, they are organized in groups, clusters and/or filaments. In this science case, we do visual inspection of large-scale structures using galaxies with spectroscopic redshifts from the SDSS/BOSS survey.
The Juypter notebook file and a rendered HTML version are available on GitHub:
You will also find them on your Data Lab notebook server, in the notebooks-latest/
folder. [It’s best to first fetch the newest notebooks].
1.2.2.2. Goal¶
The goal is to probe large scale structures of galaxies with spectroscopic redshifts from SDSS/BOSS. We also use this science case to demonstrate a joint query between a DECaLS photometric table pre-matched to SDSS and the SDSS specObj table.
1.2.2.3. Data¶
We will use both a Legacy Surveys (LS) table and a SDSS specObj table.
The columns from the LS table used (Tractor)
The columns from the pre-crossmatched table can be seen here
The columns from the SDSS DR17 used for the 3D plot can be found here
1.2.2.4. Technique¶
This short notebook is a starting point for visual inspection of large-scale structures of galaxies. The main technique is to plot the positions of galaxies in narrow slides of redshift in order to find structures that may be physically associated such as filaments or clusters of galaxies.
Several extensions of this work would be possible, such as probing the large-scale structures as a function of galaxy type or other photometric property from the Legacy Surveys (e.g., LS).
1.2.2.5. Data Lab Workflow¶
The first steps are to import libraries that will be needed, and to authenticate.
Then, we query relevant columns from the ls_dr10.tractor table (L) of the LS DR10 database, the pre-crossmatched ls_dr10.x1p5__tractor__sdss_dr17__specobj table (X), and the sdss_dr17.specobj table (S). The query is written below, for a limited region of the sky of 5x5 square degrees.
query = ("""
SELECT L.ra, L.dec, L.type, L.sersic, L.g_r, L.r_z,
S.z, S.ra as plug_ra, S.dec as plug_dec, S.class, S.veldisp, S.veldisperr
FROM ls_dr10.tractor AS L
JOIN ls_dr10.x1p5__tractor__sdss_dr17__specobj AS X ON L.ls_id=X.id1
JOIN sdss_dr17.specobj AS S ON X.id2 = S.specobjid
WHERE L.ra BETWEEN %s AND %s AND L.dec BETWEEN %s AND %s AND (L.ra_ivar > 0)
LIMIT 100000
"") % (126,131,7.,12.) # small region
As a sanity check, we overplot the RA, Dec positions from the SDSS table and from the LS table to visually confirm that they overlap as expected. The main quantity of interest here will be the spectroscopic redshift (z).
1.2.2.6. Output¶
Figure: RA and Dec position for galaxies in both LS and SDSS (light red). Overplotted in blue are galaxies in a thin redshift slice.¶
Above, the left-hand panel shows a thin redshift slice (0.105 < z < 0.125, black symbols) among objects with redshifts from the spectroscopic SDSS DR17 sample (red symbols). We can see by eye some large-scale filamentary structures and overdensities. The blue rectangle shows a selected region where we zoom in the right-hand panel. On the latter, we further select a smaller region, which we will use in the next cell below.
