.. sectionauthor:: Stephanie Juneau <stephanie.juneau@noirlab.edu>
.. index::
   pair: science example; large-scale structure
   single: legacy survey
   single: DECaLS
   single: SDSS
   
.. _sec_ProbeLargeScaleStructureInDECaLS:

*************************************
Probe large-scale structure in DECaLS
*************************************

.. _sec_ProbeLargeScaleStructureInDECaLS_background:

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 Jypter notebook file and a rendered HTML version are available on GitHub:

`Jupyter notebook <https://github.com/noaodatalab/notebooks-latest/blob/master/03_ScienceExamples/LargeScaleStructure/LargeScaleStructureSdssLs.ipynb>`_

`Rendered html file <https://github.com/noaodatalab/notebooks-latest/blob/master/03_ScienceExamples/LargeScaleStructure/LargeScaleStructureSdssLs.html>`_

You will also find them on your `Data Lab notebook server
<https://datalab.noirlab.edu/devbooks>`_, in the ``notebooks-latest/``
folder. [It's best to first :ref:`fetch the newest notebooks
<sec_updatenotebooks>`].

.. _sec_ProbeLargeScaleStructureInDECaLS_goal:

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.

.. _sec_ProbeLargeScaleStructureInDECaLS_data:

Data
====

We will use both a Legacy Survey (DECaLS) table and a SDSS specObj
table. 

`The columns from the DECaLS table used
<http://datalab.noirlab.edu/query.php?name=ls_dr3.dr3_specobj_dr13>`_  (Tractor, pre-matched to
specObj DR13)


The columns from the SDSS/BOSS DR13 table are listed 
`here <http://datalab.noirlab.edu/query.php?name=sdss_dr13.specobj>`_.


.. _sec_ProbeLargeScaleStructureInDECaLS_technique:

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 Survey (e.g., DECaLS).

.. _sec_ProbeLargeScaleStructureInDECaLS_workflow:

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_dr3.dr13_specobj* table (L) of the
LS DR3 database, which was pre-matched to SDSS DR13 by the DECaLS
team jointly with the sdss_dr13.specobj table (S). The query is writte
below, for a limited region of the sky of 5x5 square degrees.

.. code-block:: python

  L.ra,L.dec,L.type,
           S.z,S.plug_ra,S.plug_dec,S.class, 
           S.vdisp,S.vdisp_err 
         FROM ls_dr3.dr3_specobj_dr13 as L JOIN sdss_dr13.specobj as S 
         ON L.specObjId = 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

The RA, dec positions from the DECaLS table (L) will be compared to
the plug_RA, plug_dec positions of the SDSS table (S). We can compare
them as a sanity check that the matches correspond. The main quantity
of interest here will be the spectroscopic redshift (*z*).

.. _sec_ProbeLargeScaleStructureInDECaLS_output:

Output
======

.. _sec_ProbeLargeScaleStructureInDECaLS_links:

.. figure:: RaDec_LSS.jpg
   :scale: 40 %
   :alt: Sky region with thin redshift slice in blue

   **Figure**: RA and Dec position for galaxies in both DECaLS and
   SDSS (light red). Overplotted in blue are galaxies in a thin
   redshift slice.

In the figure above, the red points correspond to objects within the SDSS
specObj table. They do not include, for example, DECaLS photometric
sources that lack a entry in the SDSS specObj table. 
Filled blue symbols mark galaxies in a narrow redshift slice (here,
0.105<*z*<0.125). We can see by eye possible filaments along which
galaxies are located. Further work would be needed to confirm the
nature of these filaments.


Links to resources
==================

`Legacy Survey at Data Lab <http://datalab.noirlab.edu/decals/ls.php>`_

`DECaLS DR3 at Data Lab <http://datalab.noirlab.edu/decals/decals.php>`_

`DECaLS DR3 <http://legacysurvey.org/dr3>`_

`SDSS DR13 <http://www.sdss.org/dr13/>`_