Find quasar candidates in selected fields (developed by E. Hatziminaoglou)

Uses VODesktop, TOPCAT, VOSED, VOSpec and Aladin.

Find quasar candidates in the Lockman field, ELAIS N2 or ELAIS S1

1. launch VODesktop, Aladin, TOPCAT and VOSpec and make sure they "see" eachother
2. in VODesktop: New Smart List ->Any main field contains SDSS; Create
3. select e.g. the SDSS photometric catalogue, release 6; Query with Astroscope (10:45:00, +58:00:00, try up to 0.4 degrees radius; or 16:36:48,+41:01:45 or 16:11:00, +55:00:00) -> Search
4. Note: you can make the same query using TOPCAT: Load -> DataSources -> Keyword: SDSS DR6, using the same coordinates and search radius
5. send table to TOPCAT; select quasar candidates based on their morphology (cl=6) and u-g vs g-r colours (quasars have a UV excess)
6. create the observed SEDs of the selected quasar candidates using VOSED
7. if the number of your candidates is small, you can try the Single Object Search; unmark the Spectroscopic data; select 2MASS and SDSS from the Photometric Data, select a small search radius (typically 1-2") and Submit query
8. you can then Display SED; this will launch a new window of VOSpec; click on the list at the bottom right of the main VOSpec window and Retrieve
9. compare the observed SED to models SEDs from e.g. the Kurucz library (just click Query and select the Theoretical list of SSAs)
10. UVX quasars have similar colours to O and B stars; select models with Teff>10000K; logg 4.0; metallicity of 0.0 (example values) and overplot the model SEDs; scale model and observed SEDs and compare
11. in case the list of candidates is long select the Multi Object Search (VOSED main window) and repeat the previous steps; the results will come in a .zip file that will contain an xml file per object; you can visualise the individual SEDs by loading them one by one in VOSpec and compare with the pre-selected Kurucz models
12. once you decide on the the quasar candidates, create the correct subsample in TOPCAT and send the table to Aladin; in Aladin, query VizieR for a catalogue of known quasars in the fields and cross-correlate the two to find out if 1) the quasar candidates are indeed quasars and 2) how complete the selection criteria were

Variant:

• define the colour selection criteria based on a known quasar catalogue (e.g. download the SDSS DR3 or DR5 quasar catalogues using VODesktop and send it to TOPCAT); check their position on the u-g vs g-r diagram, also as a function of redshift (add a third column on the TOPCAT scatter plot); quasars of redshift typically less than 2.3 have a UV excess
• now query the SDSS DR6 photometric catalogue (again using VODesktop) in a random region of the sky (suggested regions: 242.8, 54.25; 163.0, 57.50; 158.4, 57.75; radius up to 0.4deg)
• select only point sources
from the colour cuts previously established, select quasar candidates based on their UV excess (or other colours, depending on the redshift)
• check for X-ray counterparts (e.g. 2XMMi catalogue)
• use VOSED to plot the SEDs of the candidates (follow steps 6-10 of the first variant)