How to Access the Survey Data

The Sloan Digital Sky Survey (SDSS) is an ambitious collaboration of several institutions to create a digital map of one quarter of the sky. The database includes images, multicolor photometry, and spectra of selected objects in these fields. Remarkably, the observations are released for general use and are accessible to anyone through high speed Internet access to the SDSS servers.

Images, spectra, redshifts and other derived data are available over several slices of the sky. While imaging and detailed spectra have enormous data storage requirements, the redshift catalogs are small enough to handle locally. Furthermore, the redshift catalogs can be transformed into an $(x,y,z)$ coordinate system centered on the Sun for display with Partiview. On our computer systems use the command line

partiview_extragalactic

to start Partiview with Sloan Digital Sky Survey databases for galaxies and quasars loaded.

In this version of Partiview there are two separate catalogs:

• All galaxies (sdssgals.speck)
• Quasi-stellar objects (sdssqsos.speck)

which correspond to separate buttons on the Partiview control panel. The catalogs are available in text form in the directory /home/data/sdss with the names shown above. Here ``speck'' is the default extension used for Partiview files. Other versions of the catalogs may also be present in the directory. The most recent released data in SDSS are referred to as ``Data Release 4'' at this time, and you may see ``dr4'' in some file names. The Partiview data are based on an earlier release. There will be notes in the directory if there have been recent updates to the files. The speck files are useful for retrieving RA and Dec coordinates for each object visible in Partiview.

A center-click (press mouse wheel down), or a ``p'' keypress, with the cursor on an object will prompt Partiview to list data that are in the speck file. These are

• Coordinates $x,y,z$ in megaparsecs (Mpc). The coordinate system is centered on the Sun with the $xy$ plane the equatorial plane of the Earth.
• Red magnitude ($m_R$). This is a measurement through the optical fiber that selected the individual galaxy for spectroscopy.
• Redshift $z$ where $\lambda/\lambda_{rest} = 1 + z$
• Lookback time $t_{lookback}$ as a fraction of the Hubble time. This is the time it took for light from this object to reach us.
• comoving distance $d$ in Mpc. This is the distance to the object at this moment, that is, the distance we would have to travel at infinite speed to reach the object now.
• comoving distance $d$ in light years.
• Right ascension $\alpha$ in degrees.
• Declination $\delta$ in degrees.

In reverse, use less sdssgals.speck to scroll through the galaxy speck file. Both RA and Dec are given in decimal degrees, and may be entered in the web tools on the Sloan site to access additional data and images for the selected object.

If you know the RA and Dec in decimal degrees, then you can use the SDSS data server to find processed images, spectra, and derived data about the object in question. The most direct and visual access to these data is through the link

http://skyserver.sdss.org

which at the present time offers four choices for the latest data:

• Finding Chart
• Navigate
• Image List
• Explore

The ``Navigate'' option will show an image of the region of the sky around the object and allow you to query about different parts of the image. The ``Explore'' option brings up many measured quantities about an object at a particular location in the sky.