Tutorial "Gaia in the CDS services"Gaia data workshop @ Heidelberg – June 19, 2018Sébastien Derriere (adapted from Thomas Boch)

Each section (numbered 1. to 6.) can be done independently.

1. Explore Gaia DR2 in Aladin Lite: HiPS

Open http://cds.unistra.fr/Gaia/DR2/AL-visualisation.gml in your browser. By default, it shows a color flux map of Gaia DR2 sources, obtained from catalogue magnitudes. You can zoom in, zoom out and pan around.

For now, switch off the layer Gaia DR2 sources.

Notice the high extinction regions in the galactic plane (and some green glitches when some magnitudes are missing in some regions).

You can switch the image layer to Gaia DR2 density map (number or catalogue sources per sky area). The satellite scanning pattern becomes visible :

Click on the icon to jump to a given target. Type in Andromeda and submit. Zoom in if needed. The Andromeda galaxy is clearly visible.

Repeat the same process for objects M 33 and NGC 55.You can switch the base image layer to DSS or 2MASS and see how these galaxies look like in optical or infrared.

Switch to the DSS image layer, and jump to LMC (Large Magellanic Cloud). Adjust zoom to frame the whole galaxy.

Switch on the Gaia DR2 sources HiPS progressive catalogue. When the field of view is large, only the brightest sources are displayed.

Click on one of the source and see the associated measurements displayed in a dedicated panel.As you zoom in, fainter stars appear.

Click on another object and select More details to see the VizieR page with the whole set of measurements.

NB: You can access the flux and density maps from Aladin Desktop : Collections > Ancillary > Gaia DR2. And the progressive catalogue for gaia2

2. Query Gaia tables in VizieR

2.1 Query table I/345/gaia2

Open VizieR homepage http://vizier.u-strasbg.fr/viz-bin/VizieR in your browser, type Gaia in the first text field and submit.

Click on I/345/gaia2

Type M44 as the target name and 20 arcmin for the search radius and submit :

By default, VizieR outputs the result as an HTML table, limited to 50 rows.This limit, as well as the output format, can be modified from the left sidebar shown below:

Change the maximum to 9999 and resubmit to get the whole list of rows in the requested cone.

Clicking on will show the sky positions ofthe rows in Aladin Lite.

Clicking on will let you visualize 2D scatter plots on catalogue attributes.

In our result, only default columns are visible. Click on Modify query, select a few more columns and Submit. Check that the selected columns are visible in the output.

We are now going to add a constraint on our positional query, in order to keep only sources with parallax measurement greater than 5 mas.Click again on Modify query and add the constraint:

Submit. You should retrieve 114 sources matching the constraint.

This result can be easily transmitted to Virtual Observatory tools:

• Fire up a SAMP-compatible application (either Topcat or Aladin).

• Click on the antennae at the top right corner of the page

• Accept connection

• Click on Broadcast

2.2 Query table I/345/cepheidGo back to http://vizier.u-strasbg.fr/viz-bin/VizieR , search for Gaia and select the Cepheids table I/345/cepheid

Select objects with Gmag<11, change the max. number of rows to Unlimited, and submit : you should retrieve 347 rows.

Click twice on the arrows below the Pf label to sort the rows according to the period, inascending order.

Click on to display the light curve of the star with the shortest period Pf=0.82062508 d.

The light curve is expressed as a function of the phase, given the object's period.

• click on the options of the x-axis to view the details

3. Retrieve Gaia measurements for stars in globular clusters, selected from Simbad

Open http://simbad.u-strasbg.fr/ and click on Queries by criteria

Type the following search expression :otype = '*inCl' & Bmag>11 & Bmag<12 & nbref>5

This query will search for stars in cluster having magnitude B between 11 and 12 and cited in more than 5 papers. Submit query. 1881 objects match the criteria.

Go back to the query page, select 'display' and resubmit the query.

Go to the bottom of the result page, and store the result as a VOTable (save to disk, for example as

simbad­stars­cluster.vot).

We will use this simbad file as an input for the cross-match service.

Go to http://cdsxmatch.u-strasbg.fr/At the top of the page, click on Table management and add the file to your store.

Go back to the tab, and choose simbad-stars-cluster.vot from My Store as the 1st table and GAIA DR2 from VizieR as the 2nd one.

Open the Options panel, change Radius to 1 arcsec and submit by clicking on Begin the X-MatchYou can follow computation progress in the bottom table. Once the job is completed, click on the info icon to get a summary of the result and click on the histogram icon to see the distribution of distances for the associations.

You can download the result as aVOTable, and load it in TOPCATfor example (File —> Load table)

4. Gaia data in Aladin Desktop

In this part, we will learn how to access to and visualize Gaia data from Aladin Desktop.

Launch Aladin with the following command:java -Xmx1024M -jar Aladin.jar -beta

4.1 Proper motion of Gaia stars in Aladin Desktop

In the Command/location bar at the top of the main Aladin window, type Praesepe and submit. Zoom out to see the whole open cluster.You can search « Gaia » in the collections data tree and access it like any other dataset.

Gaia DR2 can also be accessed from a dedicated tab from the Aladin server selector : menu File —> Open server selector (or CTRL+L).Click on the Gaia tab, change radius to 1°, select table "I/345/gaia2" and limit the output to the first million source, then Submit.

From menu Catalog, select Create a scatter plot and plot pmRA vs pmDE.Stars which are member of Praesepe are clustered around (pmRA=-36mas/yr, pmDE=-13mas/yr)Select these points in the scatter plot and notice they are automatically selected in the spatial view.

Right-click on the catalogue plane and create a new plane with the selected sources.Go back to a single panel (menu View —> Panels —> 1 panel).

Create a filter by clicking on the icon and select the predefined filter Draw proper motions of stars.To make the arrows bigger, switch to Advanced mode edit the filter as below and Apply:{

  draw white plus

  draw pm(10*$[pos.pm;pos.eq.ra], 10*$[pos.pm;pos.eq.dec])

}

Size of arrows can also be controlled with a slider available at the bottom of the data planes stack.

Select a few sources by dragging a rectangle. Associated measurements appear at the bottom of the window:

As the catalogue contains positions, proper motions along with the epoch of observation, Aladin cancompute the positions at a different epoch, using the J2000 slider below the data planes stack, or :

• select the Gaia plane and click on the icon to open the Properties window

• move the slider to visualize stars position at the corresponding epoch

4.2. Retrieve Gaia sources within some complex coverage

Goal: we will learn to use MOCsto retrieve Gaia data which are ina complex sky area intersectingseveral surveys.

Search in the Collections tree :Image>Optical>HST >HST-RImage>Optical>SDSS>SDSS9 CDScolor compositionImage>X-ray>XMM>False colorImage>Infrared>Spitzer>IRAC color

Load the corresponding HiPSprogressive surveys, and thecoverage maps (MOCs).

Example for loading HST-R HiPS (progressive) images and MOC (coverage) :

Open menu Coverage —> Logical operations , select the 4 MOCs and compute their intersection.

The resulting MOC plane is the common coverage between the 4 different surveys.Note that you could create it directly by CTRL-clicking the 4 planes in the tree, and computing the intersection directly.

VizieR tables can be queried to retrieve only sources within a given MOC (by region & MOC):

Activate the intersection MOC in the stack of planes, and use it to query the gaia2 table.Zoom in and pan around to verify that the returned rows fall into the MOC coverage.

5. CDS cross-match service from Topcat : find Gaia couterparts for a list of sources and draw a color-color diagram

Launch topcat:java ­Xmx1024M ­jar topcat­full.jar

We will load a first table in TOPCAT (here a catalogue published by Massey in the SMC), and then cross-match it against the Gaia DR2 to find counterparts for each source of the input catalogue.

Open the VO>Cone Search menu, andsearch for « Massey » keyword. Selectcatalogue II/236, and the table with theAccessURL ending in "smc?". Define a 3degrees cone centered on SMC andSubmit.

Now we will cross-match this data against GaiaDR2.Open the CDS X-Match window from VO —> CDS Upload X-MatchSelect I/345/gaia2 as VizieR table ID.Select SMC-II_236 as the input table.Set the match radius to 1.5 arcsec and submit.

Once the cross-match is done, open the result table and notice that each row contains the fields of the original file plus the fields coming from the Gaia data.

Click on icon and create the histogram for column angDist (last column -> separation in arcsec between II/236 source and Gaia source) as to assess the quality of the match.

You can create color-magnitude or color-color diagrams combining data from Massey and Gaia DR2.

6. More complex queries with TAP VizieR

Goal: visualize the rotation pattern of the Large Magellanic Cloud

You can do TAP queries (written in ADQL) on Gaia DR2 data in different ways :• On the TAP VizieR web interface

• From TOPCAT

• From Aladin ("by criteria")

Step 1 : find the average proper motions in the LMC area

Go to TAP VizieR web interface: http://tapvizier.u-strasbg.fr/adql/

Search tables with the Gaia keyword.Select I/345/gaia2 and click on Construct your query at the top right.

Enter the following ADQL query to retrieve the average proper motions within 1° of LMC.

SELECT avg(pmra) as avpmra, avg(pmdec) as avpmdec

FROM "I/345/gaia2"

WHERE CONTAINS(POINT('ICRS',ra,dec),CIRCLE('ICRS',80.8942,­69.7561,1.))=1

AND sqrt(pmra*pmra+pmdec*pmdec)<30

You can run the query. Or just click "Preview" for this simple case :

Step 2 : proper motions in different regions of the LMCWe will try to do a map of proper motions values in the LMC, averaging values on different Healpixcells. In Aladin, locate the Gaia DR2 catalogue, and query by criteria :

This will open a window where you can type a TAP query :

Make sure to run the query in ASYNC mode !SELECT avg(ra) as ra, avg(dec) as dec, avg(pmra) as pmra, avg(pmdec) as pmdec, avg(parallax) as parallax, avg(radial_velocity) as radial_velocity, healpix(ra, dec, 8) AS hFROM "I/345/gaia2"WHERE CONTAINS(POINT('ICRS',ra,dec),CIRCLE('ICRS',80.8942,­69.7561,2.5))=1 and 

sqrt(pmra*pmra+pmdec*pmdec)<30group by h

This may take a few minutes to execute. While the query is being processed, you can prepare the last step below.

Step 3 : visualize it in AladinCreate a Filter in Aladin. Go to advanced mode, and use the above values as average pmra and pmdec, yielding the following syntax :${pmra}*${pmra}+${pmdec}*${pmdec}<6{ 

  draw pm(2000*(${pmra}­1.8697), 2000*(${pmdec}­0.4273)) rainbow(${pmra}*${pmra}

+${pmdec}*${pmdec}, 2, 5)

} 

Congratulations, you've reached the end of this tutorial !