How to use the 2SLAQ QSO catalogue search

General comments

This search engine for the 2SLAQ QSO catalogue and spectral database is based on the 2QZ search engine. Much of the text below is taken from this, with modifications where appropriate.

With the exception of RA,dec position (see below), all search criteria and combined with an AND operation. That is, the returned objects will satisfy all the specified criteria.

There are no compulsory criteria which must be filled in. You could if you wished run a search with no criteria set at all and it would return all objects in the catalogue. This is strongly discouraged though, as it would take a very long time and when the results finally came back, your browser probably would not be able to display it all anyway.  The only constraint made on the parameters you fill in is that if a search around a position is made, all three parameters, RA, DEC and RADIUS must be used.

Target position:

All RA and dec positions may be input in either decimal degrees (ddd.ddd -dd.ddd) or hours, minutes, seconds (hh mm -dd mm format.

There are two positional search mechanisms.

By position and radius:

A position must be specified in RA,dec coordinates and all objects within a certain radius of that point are returned. Radius is given in minutes of arc.

RA format: hours, minutes and seconds (hh mm or decimal degrees (d.ddd) e.g.:
12 30 00 or 187.6380

DEC format: degrees, minutes and seconds (+/-dd mm ss.s) or decimal degrees (+/-d.ddd). e.g.:
-30 30 00 or -30.50

RADIUS format: this is always in arcminutes. e.g.:

By maximum/minimum RA and/or declination:

Objects are selected if they have positions within the specified limits. J2000 coordinates are assumed.  Any number of the limits may be input (none to all).  There are special cases to consider when using RA minimum and/or maximum.

If only one RA limit (minimum or maximum) is entered this is assume to be bounded at
RA=0/24h, so  a search with only:
ramax=00 30 0
will be interpreted as ramin=00 00 0 ramax=00 30 0

If both RA limits are used (minimum and maximum) then the search is done between the limits.  In the special case of a search spanning RA=0h, the search algorithm will also perform a valid search.  For example
ramin=23 30 0 ramax=00 30 0
will result in sources around RA=0h being selected.

Object Names

Here you can search for an specific 2SLAQ object name (IAU format) or some substring of a name. For example searching for J235935.4-313344 would find just this object, while searching for J2359 would find all objects which have names beginning with this string. The 2SLAQ at the beginning of the full name is not required.

Object identification

There are six different object classes listed in the 2SLAQ catalogue:
QSO spectrum with one of more broad lines (>1000 km/s)
NELG spectrum with one or more narrow lines (<1000 km/s)
gal galaxy spectrum with no emission lines
star galactic star spectrum
cont high signal to noise spectrum (S/N>10) with no identifiable emission or absorption features
?? unclassifiable spectrum.

There are also a number of sub classes, which are placed after the main classification in parentheses:
BAL broad absorption line QSO
DA DA white dwarf (hydrogen Balmer line dominated)
DB DB white dwarf (neutral helium dominated)
DO DO white dwarf (singly ionized helium dominated)
DZ DZ white dwarf (calcium H,K dominated)
CV Cataclysmic variable
DA/M DA - M dwarf binary
DB/M DB - M dwarf binary

All these classifications are the same as used in the 2QZ survey. For a fuller description of the classification scheme see the 2QZ papers Croom et al. (2001) - Paper V. or Croom et al. (2003) - Paper XII.

Magnitudes and colours

You may also select objects based on both their apparent magnitudes and colours.  The survey is flux limited in the SDSS g-band with 18.0<g<21.85.  Therefore a selection in other bands (i.e. u,r,i,z) will not be complete to any given flux limits, but will depend on the colour distribution of the sources.

Extinction correction

All of the magnitudes may be searched based on either their observed values, or their values after correction for galactic extinction (based on the work of Schlegel et al. (1998)). If the extinction correction is applied, it is these magnitudes that are displayed in the output table (but NOT in the plain text catalogue file).


Specify redshift range to select.

Data quality

Two data quality criteria are available:

Quality Flag: The quality flag contains information on both the reliability of the object's type identification and the assigned redshift.
Quality = 10 × ID_quality + redshift_quality
ID_quality and redshift_quality are 1, 2 or 3 meaning `Good', `Poor' and `Too poor to classify' respectively.  Therefore the best quality objects have a 11 quality flag, while the worst have a 33 quality flag.

Signal-to-noise ratio: Signal-to-noise is calculated as an average over the entire spectrum.

Data output format

The data are available in a number of formats. The catalogue is a single ASCII file with ~50000 lines, one for each object in the catalogue. The search tool will output a page containing the basic details of each object selected (name, position, object type, redshift). There are also links provided for the following: These links are from the coloured balls on the left hand side of the results table. In a number of cases we have repeat observation of an object. There will then be two or more coloured balls for each data type with the numbers 1, 2... printed on them. The spectra are sorted by observation date, but the redshift listed in the table is from the best spectrum (which is listed in the main catalogue file). If a coloured ball has a red cross through it, that data/image is not available on-line. This is typically the case for non-2SLAQ data, or unobserved objects.

Lastly, when setting the maximum number of records, we recommend that the user doesn't set this value to be too large (> 1000), as the browser will struggle to display the table. In all cases the catalogue file produced as output from the search (there is a link to this at the bottom of the results page) contains all the selected objects, not only those up to the maximum number of rows.

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The 2SLAQ team (Oct 2008)