FIRSTP (E0): NRAO 20cm "FIRST" Preliminary Survey; 950106 ver. (19950218)
FILE: firstp.e0 (26892 sources)
CONTENTS:
NRAO FIRST (Faint Images of the Radio Sky at Twenty-cm) Survey Preliminary version of 950106
07h 14m < RA < 16h 32m +28 16' < Dec < 31 03'
DATASET: This file comes in HyperSky dataset E-20; version 19950218 Availability on HyperSky CD-ROM: Vol. 2 (1995)
DESCRIPTION: This datafile contains the 26892 sources in the NRAO 20cm (1400 MHz) "FIRST" Survey; preliminary version of 950106. FIRST (Faint Images of the Radio Sky at 20-cm) is a project designed to produce the radio equivalent of the Palomar Observatory Sky Survey over 10,000 square degrees of the North Galactic Cap. Using the NRAO Very Large Array (VLA) in its B-configuration, 3-minute snapshots were acquired in 1993 covering a hexagonal grid using 2W7 3-MHz frequency channels centered at 1365 and 1435 MHz. After processing, a final atlas of maps was produced by coadding the twelve images adjacent to each pointing center. These maps have 1.8" pixels and a resolution of 5". At the 0.75 mJy source detection threshold, there are about 110 sources per square degree. Of these, ~35% have resolved structure on scales from 2-30".
A source catalog including peak and integrated flux densities and morphological parameters was generated from the coadded images. The astrometric reference frame of the maps is accurate to 0.05", and individual sources have 90% confidence error circles of radius < 0.5" at the 3 mJy level and 1" at the survey threshold. Approximately 15% of the sources have optical counterparts at the limit of the POSS I plates (E~20.0); unambiguous optical identifications (<5% false rates) are achievable to m(v)~24. The survey area has been chosen to coincide with that of the Sloan Digital Sky Survey (SDSS). At the m(v)~24 limit of SDSS, about 50% of the optical counterparts to FIRST sources will be detected.
Lower radio frequencies such as 1400 MHz (20cm) are typically dominated by emissions from nonthermal sources. In the extragalactic realm, such sources are associated with active galaxies having suspected central black holes, emission jets, and the like.
Because of the density of sources in the FIRST catalog, it is best to use HyperSky fields at scales of 2 degrees or less to achieve adequate source separation.
The HyperSky datafile is based on a preliminary version of the FIRST catalog, available at the following World-Wide-Web site as of Feb 1995:
http://sundog.stsci.edu/first/catalogs.html
Additional documentation on the FIRST project is also available through this Web site.
DATA: The HyperSky datafile contains the following data from the catalog:
POSITION: Right Ascension and Declination are based on the catalog equinox 2000 positions. Catalog positions were provided to an accuracy of 0.001 second in Right Ascension and .01" in declination. Positions are stored in the HyperSky datafile to an accuracy of 0.1 second in RA and 1" in declination.
MAG: No magnitudes or equivalents are provided in the FIRST catalog. A set of artificial magnitudes is provided, however, in the HyperSky catalog strictly to allow use of HyperSky's limiting-magnitude controls. In this way, you can display only those sources above a desired threshold. The artificial magnitude system is based on the relation (from Pogson) used to define visual magnitudes:
m1 - m2 = 2.5 * log10(S1/S2)
where { m1, m2 } are magnitudes, and { S1, S2 } are source strengths. For the HyperSky catalog, integrated (instead of peak) source strengths are used to calculate magnitudes. The brightest integrated source strength (S1 = 15089.31 mJy) is arbitrarily assigned the magnitude m1 = 1.0. Given this, any other source S2 can have its relative magnitude, m2, derived using the relation:
m2 = 1.0 + 2.5 * log10 (15089.31/S2);
In this system, the faintest catalog source (0.34 mJy) has a magnitude of 12.6. The following table summarizes the relation between magnitude and source strength for catalog extremes and intermediate magnitudes:
Magnitude Source Strength (mJy)
----------|----------------------
1.0 15089.31 (brightest FIRST source)
3.2 1953.
4.0 950.
5.0 374.
6.0 147.
7.0 58.
8.0 23.5
9.0 9.5
10.0 3.6
11.0 1.5
12.0 0.6
12.6 0.34 (faintest FIRST source)
With the above table, a user can set HyperSky's limiting magnitude (for emission sources) to mask out catalog sources below a desired strength. Except for this use, the derived source magnitudes have NO significance. It should be emphasized again that the magnitudes are NOT provided in the original catalog; only the source strengths are fundamental catalog values.
As a note, the position of the brightest FIRST catalog source (at RA=13h31m08s, Dec=+30d30'32") matches that of quasar 3C286 (=4C+30.26; z=0.846) according to the 1983 Catalog of Extragalactic Radio Source Identifications (HyperSky catalog E-1). There are in fact a large number of FIRST catalog sources concentrated at this location.
IDENTIFICATION: No unique source identifications are provided in the FIRST catalog. The HyperSky name for the source is constructed from the equinox 2000 RA and Dec, with a 'J' prepended to indicate the equinox. The form for source names is thus Jhhmm+ddmm. These names are HyperSky constructions only, and are not necessarily unique for each source.
COMMENTS: The comment lines contain the following information, provided if available for each FIRST radio source:
* A line giving the 20cm flux density information , as follows:
A value followed by the letter 'p', giving peak flux density.
A value followed by the letter 'i', giving integrated flux density.
A value followed by the letter 'r', giving a local RMS noise estimate at the source position.
All values are in milli-janskys, or mJy. A jansky (Jy, originally flux units) equals 1E-26 watt per square meter per hertz.
* A line giving the major and minor axes (FWHM) and position angle (degrees east of north) of an elliptical Gaussian model for the source. This is not provided for all sources.
* A line giving the field name (the name of the coadded image).