Graphical User Interface

The Graphical User Interface (GUI) of the ESA Gaia Archive offers the possibility to carry out basic and advanced queries using ADQL (Astronomical Data Query Language). The outputs of these queries can be easily downloaded in a variety of formats, including VOTable, FITS and CSV. Below you can find a brief description of the main features offered by the Archive, as well as two video tutorials explaining how to use it.

Tutorial: Basic queries

Authors: Héctor Cánovas & Alcione Mora

The main function of the Gaia Archive is to provide data to the astronomers. The Search tab in the GUI landing page provides two different ways of accesing the Archive for Basic (default option) and Advanced (ADQL) queries. The main objectives for the Basic tab are:

• To ease the exploration of the Archive catalogues for simple use cases, and
• To help users into the transition towards the Advanced (ADQL) tab for complex use cases.

To that end, the Basic tab allows to perform two of the most common operations executed when exploring an astronomy archive in a very simple and intuitive manner. These operations are the ADQL cone search, which allows to search by coordinates for one or more sources in a given catalogue, and an ADQL query to retrieve all the sources encompassed by a circular or box-like region in the projected sky.

All Basic queries are synchronous, which means that they will time out after 60 and 90 seconds for non-registered and registered users, respectively (see this FAQ). Furthermore, the output of these queries is limited to 2000 sources. Therefore, we recommend to use the Advanced (ADQL) tab to execute complex and/or long queries. Through this tutorial you will learn to use the Basic tab to:

1. Retrieve data for a single source.
2. Retrieve data for multiple sources.
3. Search for data inside a sky region.
4. Understanding the query results.

1. single source data retrieval

The most basic use case could be formulated as "I want the most relevant Gaia results for a single object". It can be accomplished using the Basic > Position subtab. The first step is to fill in the "Name" box with either the object identifier (e.g. "UX Ori") or its ICRS coordinates. The accepted input formats are described in the pop-up window that appears when clicking on top of the "Name" tooltip (see Fig. 1). The single object search launches an ADQL cone search around the celestial coordinates of the input object, which are provided by the Sesame name resolver. The drop-down menu highlighted by the thin solid circle in Fig. 1 allows to choose the service that will be queried to obtain the object coordinates (by default the system tries Simbad, then NED, and then Vizier, but if the input name is a Gaia designation the system queries the Gaia Archive). Once the name or coordinates are successfully resolved the object box turns green and a confirmation message is shown. Note that epoch propagation (see this tutorial) is applied if the proper motions of the resolved source are included in the databases searched for by the name resolver. The cone search radius can be tuned using the "Radius" box, and its units can be adjusted using its associated drop-down menu (highlighted by the dashed circle in Fig. 1).

Figure 1: Content of the Basic > Position subtab (single source resolver). The vertical arrows highlight the tooltips with explanatory text, while the circles and the horizontal arrow highlight the drop-down menus and extra options available to customize the query, respectively.

The next step is to choose the catalogue that is going to be explored. The latest Gaia data release is the default one, but all the catalogues hosted by the Archive (e.g., previous Gaia data releases, external catalogues) containing geometric information in the form of celestial coordinates can be explored by clicking on the drop-down menu highlighted by the thick circle in Fig. 1. Registered users can also access to their user-uploaded tables provided that their tables contain indexed celestial coordinates (see this tutorial). By default, only a few pre-selected columns of the choosen catalogue are shown in the query outputs. Therefore, you may want to verify that the output of your search will contain the columns that you are interested in. To do so, simply click on the "Display columns" menu (indicated by the dashed horizontal arrow in Fig. 1) and mark the columns that you want to retrieve.

Now you are ready to hit the "Submit Query" button. If you are interested in learning how your query is expressed in the ADQL language, you can hit the "Show Query" button. The query results are shown in the Query Results tab, whose contents are explained in the Understanding the query results section below.

2. Multiple source data retrieval

Three common use cases are:

1. I want to look for Gaia counterparts on my list of known objects,
2. I want to look for the neighbours of my favourite source, and
3. I want to look for the neighbours of my list of favourite sources.

2.1 Search for the Gaia counterparts of my source list

This popular use case can be accomplished using the Basic > File subtab of the GUI. First of all, you must prepare a single column ascii file (without header) with the names or ICRS oordinates of the objects you are interested in. The input file must be formatted as described in the pop-up window that appears when clicking on top of the "Select a file with Target names" tooltip (see Fig. 2). Second, select the file that you want to upload using the wizard that appears when clicking on top of the "Choose file" button. Be aware that the query output is limitted to 2000 sources. If you aim to retrieve a larger dataset you should upload your target list to your user space as explained in this tutorial, and then use the Advanced (ADQL) tab to perform a cross-match as explained in this other tutorial.

Figure 2: Content of the Basic > File subtab (multi source resolver). The arrow highlight the tooltip with explanatory text.

2.2 Search for the neighbours of my favourite source

Imagine that we want to retrieve all the sources from the Gaia EDR3 catalogue that, on-sky, are separated by less than 5 arc minutes from UX Ori. This can easily be achieved by simply updating the cone search radius units (using the drop-down menu highlighted by the red dashed circle in Fig. 1). This query outputs 195 sources, including several sources without parallax data. The Basic > Position subtab allows to apply additional selection criteria to e.g., filter out the targets with parallax signal-to-noise below a given threshold. To do so simply click on the "Extra conditions" drop-down menu as highlighted by the horizontal solid arrow in Fig. 1. A menu that allows to apply and combine different filters will show up as illustrated by Fig. 3. Applying the condition exemplified in Fig. 3 reduces the output of the previous query to 30 sources.

Figure 3: Content of the "Extra conditions" menu of the Basic > Position subtab. The arrow indicate how to select the operators that will be included in the query. The solid and dashed circles highlight the drop-down menus allowing to select the column over which the operator is going to be applied and the filter combination, respectively.

2.3 Search for the neighbours of my source list

This use case is a combination of the previous two cases. If you are interested in retrieving the neighbours of a pre-computed source list you should 1) upload the target list as explained in Sect 2.1 and 2) then adjust the cone search radius as described in Sect 2.2. As before, you should keep in mind that the output catalogue of the query is limitted to 2000 sources so you may want to apply a selection criteria using the "Extra conditions" menu as described in Sect 2.2.

3. Retrieve sources in a region of the sky

A cone search with an arbitrary radius can be generated around any point on the celestial sphere by entering the target coordinates in the "Name" box and adjusting the cone search radius as explained in the Single source data retrieval section. Alternatively, it is also possible to use the "Equatorial" button under the Basic > Position subtab (see Fig. 1) and choosing the "Circle" option (see Fig. 4). The Basic > Position subtab also allows to retrieve the sources encompassed by a box region in the projected sky. To do so, simply select the "Box" option, as illustrated by Fig. 4. The accepted formats of the input coordinates are described in the RA and Dec tooltips of the coordinate boxes. As with the previous options, it is also possible to add different selection criteria to filter out the query output by means of the "Extra conditions" menu.

Figure 4: Content of the "Box" menu of the Basic > Position (Equatorial) subtab. The arrows indicate the tooltips with explanatory text.

4. Query Results

The query results are presented in tabular format in the Query Results subtab that is automatically opened once the query is finished, as shown by Fig. 5. The columns provide units when available. Further details on the meaning of each field can be found in the "<Target Catalogue> Data Model" accessible in the bottom part. Figure 5 shows the results of searching for the Gaia EDR3 counterparts of a list of sources (Sect. 2.1) and therefore the link points to the authoritative reference on the Gaia EDR3 contents. For this example only five columns ("source_id", "ra", "dec", "parallax", and "phot_g_mean_mag") were selected for displaying (using the "Displayed columns" menu as explained in Sect. 1). When the query consist in finding counterparts of an input source list in a given catalogue (as in this example), the output table contains addicional columns that are automatically added by the Archive to aid the user in finding the correspondence between the input targets and the query results. These columns are:

• "target_id": contains the input target name as provided by the user in the input target list.
• "target_id, target_ra, target_dec, target_parallax, target_pm_ra, target_pm_dec, target_radial_velocity": contain data provided by the Name resolver (see Sect. 1).
• "target_distance": contains the on-sky angular separation (in units of degrees) between the target coordinates provided by the Name resolver and the target coordinates of the selected catalogue (Gaia EDR3 in this example).

Figure 5: Query Results subtab (see text for detailed explanation). The arrows indicate the tooltips with explanatory text.

Clicking over the "Show query in ADQL form" button will open the Advanced (ADQL) tab to show the ADQL query that has been launched. This utility can be helpful for non-expert users aiming to learn the ADQL query language. Finally,the query output can be downloaded using the "Download results" menu (indicated by a vertical arrow in Fig. 5). The format of the output file can be set by means of the drop-down menu highlighted by the red circle in Fig. 5.

ADQL syntax

ADQL (Astronomical Data Query Language) is based on SQL (Structured Query Language) which is a language to retrieve information from tables stored in a database.

(References: ADQL 2.0and SQL 92)

A very concise introduction to ADQL is provided in the next sections. There are a number of tutorials and resources providing a gentler learning curve for newcomers, specially those without previous experience with SQL or relational data bases. A small selection is provided below

• The DPAC ADQL Cookbook
• GAVO ADQL reference card
• GAVO ADQL short course for Gaia
• The Gaia Archive White dwarfs exploration and Cluster analysis tutorials

Tutorial content:

1. BASIC SYNTAX
2. SELECTING COMPLEX DATA
3. ADDITIONAL FUNCTIONS AVAILABLE
4. ADQL MATHEMATICAL FUNCTIONS
5. ADQL TRIGONOMETRIC FUNCTIONS
6. DATA TYPE CASTING ADQL FUNCTIONS
7. ADQL EXTENSION: CONDITIONAL EXPRESSIONS
8. DEVIATIONS OF ADQL FUNCTIONS IMPLEMENTATION FROM STANDARD

1. Basic syntax

In ADQL/SQL you write queries to the database. A query is compound of the table columns you want to retrieve (the SELECT part), the table or tables that store the data (the FROM part) and the conditions to restrict the data you obtain (the WHERE part). E.g.

SELECT <columns> FROM <tables> WHERE <conditions>

So, if you want to obtain Right Ascension and Declination of all items from the table gaia_source, you may write:

SELECT ra, dec FROM gaiaedr2.gaia_source

ra is the column name of Right Ascension in gaia_source table.
dec is the column name of Declination in gaia_source table.
gaiadr1 is the database schema name where gaia_source table belongs to. It is a good practice to add schema names to tables to avoid name clashes.

Probably, you want to obtain also the object identification, so you can modify the query as follows:

SELECT source_id, ra, dec FROM gaiaedr3.gaia_source

If you want to know all the column names associated to a table, you may use GACS GUI, clicking on the plus sign next to a table name.

Or, in TAP+, you may obtain all the columns and descriptions of a table using the following syntax:

curl "https://gea.esac.esa.int/tap-server/tap/tables?tables=gaiadr1.gaia_source"

Now, suppose you are interested in an specific region (e.g. ra=266.41683, dec=-29.00781, radius=0.083333 arc.min.). So you want to restrict the results to that region. In order to do that, you may execute a 'cone search' to obtain all the objects where ra,dec are inside a cone:

SELECT source_id, ra, dec FROM gaiaedr3.gaia_source
   WHERE 1=CONTAINS(POINT(ra,dec),
                    CIRCLE(266.41683,-29.00781, 0.08333333))

(You may read the ADQL recommendation to obtain the list of functions that can be used).

One way to create a complex query could be to use the 'Simple Form' page to create the basic query graphically. Then you should press 'Show query' button to show the query as ADQL in the 'ADQL Form'. And then, you can modify it.

2. Selecting complex data

You are not restricted to obtain column names only. You can obtain complex values too.

For instance, you may want to obtain the distance of each source to the center of an specific region. Then, you may type:

SELECT source_id, ra, dec, DISTANCE(POINT(ra,dec),
       POINT(266.41683,-29.00781)) AS dist
   FROM gaiaedr3.gaia_source
   WHERE 1=CONTAINS(POINT(ra,dec),CIRCLE(266.41683,-29.00781, 0.08333333))

source_id, ra and dec are the Source Identifier, Right ascension and Declination of each item of the gaia_source table.

'DISTANCE(POINT(ra,dec), POINT(266.41683,-29.00781)) AS dist' is a created on the fly column, with the name 'dist' that contains the distance of the item to the specified point.

3. Additional functions available

Apart from the standard ADQL functions, ESA Gaia TAP+ service offers the next functions:

SELECT array_element(array,2) : 5						

SELECT array_ndims(array) : 2

SELECT array_dims(array) : [1:2][1:3]

SELECT array_length(array,1) : 3

SELECT cardinality(array) : 4

4. ADQL mathematical functions

ADQL defines some mathematical functions. Also, other mathematical functions have been defined in our service to support scientific queries

5. ADQL trigonometric functions

ADQL defines some trigonometric functions considered important for astronomical queries

6. Data type casting ADQL functions

Some User Defined Functions have been implemented in order to allow the casting of values between different data types. The casting functions relies in the underlying PostgreSQL functions so errors received during casting will be, in most of the cases, PosgreSQL errors.

7. ADQL extension: conditional expressions

Some conditional expressions have been implemented as User Defined Functions.

Note: 'is' and 'in' are reserved ADQL keywords. Conditions including them are possible, but need to be escaped within single quotes for a correct execution. See for example, the following queries:

SELECT TOP 10 random_index, parallax, if_then_else('parallax is null', 1, 0)
FROM gaiaedr3.gaia_source
ORDER BY random_index

SELECT TOP 10 random_index, parallax, if_then_else('random_index in (0, 2, 4, 6, 8)', 1, 0)
FROM gaiaedr3.gaia_source
ORDER BY random_index

CASE WHEN condition THEN result
[WHEN ...]
[ELSE result]
END

CASE a
WHEN 1 THEN 'one'
WHEN 2 THEN 'two'
ELSE 'other'
END

CASE WHEN condition THEN result
[ELSE result]
END

8. Deviations of ADQL functions implementation from standard

ADQL standard is implemented in compliance with IVOA ADQL 2.0 plus the ADQL Errata available by 16th April 2018. Some implementation limitations apply in the following functions:

1. BOX(coordsys, longitudeCentre, latitudeCentre, longitudeExtent, latitudeExtent): it is interpreted as follows:
   • As defined into the standard when arguments are fixed values (A cross at the central position with arms extending, parallel to the coordinate axes at the center position, for half the respective sizes on either side).
   • When the arguments are variable (e.g. table columns), the sides of the box are line segments or great circles intersecting the arms of the cross in its end points at right angles with the arms.