Spanish Virtual Observatory


Primer Curso Práctico de la Red Temática SVO.

27-28 Octubre 2009

Caso 3: Find quasar candidates in the Lockman hole

  • Introduction: A patch of the sky with an area of 15 square degrees, that is almost free from absorption by neutral hydrogen gas in the Galaxy. Having the minimum column density of galactic hydrogen, it allows the most sensitive searches for extragalactic objects.
  • Workflow
  • Launch VODesktop (java -Xmx512m -jar, TOPCAT (Webstar / java -Xmx512m -jar topcat-full.jar) and VOSpec (
  • In VOExplorer:
    • New Smart List ->Any main field contains SDSS DR7; Create. --> 2 catalogues. Choose the catalogue available in Vizier
    • Actions / Query --> In Astroscope: 161.25,58.0, 0.1degrees radius / Search --> 1225 results
    • New Smart List --> Any main field contains SDSS quasar; Create. --> 57 catalogues. Choose DR3QSO. SDSS Quasar Catalog (LEDAS)
    • Actions / Query --> In Astroscope: 161.25,58.0, 5degrees radius / Search --> 1163 results
  • Send the two tables to TOPCAT via PLASTIC ("Send Tables to TOPCAT in Astroscope)
  • Note: you can make the same query using TOPCAT only : Load / DataSources / ConeSearch / Keyword: SDSS DR7 / SDSS quasar and with the same coordinates and search radius
    • Select quasar candidates based on their morphology (cl=6) --> 309 results
    • Plot the quasar candidates (309) and the quasar catalog (1163) in the (u-g) vs (g-r ) diagram
    • Choose a subset of quasar candidates in the -0.2 < (u-g) < 0.4 -0.2 (g-r) < 0.4 region with the option Subsets / Draw subset region. To finish --> Subsets/ Finish drawing region --> 5 results.
    • Save table with the 5 candidates (only RA and DEC) in ASCII format
  • In VOSED (
    • Choose "Multiple Object Search"
    • Create the observed SEDs of the selected quasar candidates using VOSED. Use a small search radius (typically 1-2")
  • In VOSpec
    • Load the xml generated by VOSED for the different candidates
    • Compare the observed SED to models SEDs from e.g. the Kurucz library (just click Query and select the Theoretical list of SSAs). 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
    • Once you decide on the the quasar candidates based on colors and SED make a cross-correlation between the candidates and the quasar catalog using TOPCAT (Join/ Pair Match / algoritm= sky) to find out if your candidates are indeed quasars.