The spectral characteristics of the 2dFGRS-NVSS galaxies
C A Jackson, D M Londish, PASA, 17 (3), 234.
Next Section: Emission-line diagnostic analysis
Title/Abstract Page: The spectral characteristics of
Previous Section: Introduction
Contents Page: Volume 17, Number 3

The sample of 2dFGRS-NVSS galaxies

The sample of 88 2dFGRS-NVSS galaxies has been extracted from the 2dFGRS and NVSS surveys, selecting sources whose radio-optical offset is less than 10 arcsec. Complete details of the selection criteria, cross-matching and classification procedures are described by Sadler et al (1999).

FITS files containing the reduced 2dF spectra for these galaxies were provided by the 2dFGRS project.

Each galaxy has an assigned spectral type of either `S'=starburst or `A'=AGN, defined as follows:

AGNs have either a pure absorption-line spectrum, characteristic of an early-type galaxy or an emission-line signature similar to optically-selected Seyferts, with dominant nebular emission lines (e.g. [OIII] and [OII]) relative to any Balmer-line emission. As Figure 1 shows, the majority of these objects have much lower radio powers than usually associated with radio-loud AGN ( $\log_{10}(P_{1.4 \rm\thinspace GHz}) \sim 10^{21} - 10^{24}$ W Hz^-1 sr^-1).

We can describe these AGN in terms of two sub-classes:

The absorption-line AGN have radio powers typical of FRI radio galaxies and optical spectra typical of early-type host galaxies. These sources show none of the characteristic emission lines associated with starformation processes.

The emission-line AGN could be either (i) `radio-quiet' Seyfert 2s - late-type hosts with sub-pc scale radio core-jet structures, (ii) emission-line FRI radio galaxies - rare but not unknown or (iii) emission-line FRII radio galaxies although this is unlikely as these are usually more powerful, $\log_{10}(P_{1.4 \rm\thinspace GHz}) \ge 10^{24}$ W Hz^-1 sr^-1, and are rare at z < 0.3.

Starburst galaxies have emission-line signatures similar to optically-selected star-forming galaxies, with strong Balmer line emission relative to any other emission features. Figure 2 shows that most of these objects are of low radio powers, $\log_{10}(P_{1.4 \rm\thinspace GHz}) \le 10^{22}$ W Hz^-1 sr^-1, typical of known starburst galaxies.

Our sample comprises 36 starburst galaxies and 52 AGNs. At the time of this analysis, FITS data was unavailable for 3 of the 36 starburst galaxies. Of the remaining 33, 22 have both H$\alpha$ and H$\beta$ in emission within the 2dFGRS wavelength range (i.e. they lie at z < 0.2). The 52 AGN can be sub-divided into 1 broad-line Seyfert 1, 40 absorption-line (early-type) galaxies, 1 galaxy at z > 0.2 and 10 with both H$\alpha$ and H$\beta$ in emission (and z < 0.2). The 22 starburst galaxies and 10 AGNs with Balmer line emission form our `narrow-line galaxy sample'.

Figure 2 shows the distribution of radio power with redshift for the sample of galaxies. The starburst galaxies cluster at low redshifts due to the flux density limit of the NVSS survey. The AGNs generally lie at higher redshifts and radio powers. The highest-power objects are absorption-line AGN.

$\parbox{8cm}{{\bf Figure 1.} Luminosity distributions by type for the 88 2dFGRS--NVSS galaxies. }$

Figure 2. Radio power as a function of redshift for the sample of 88 2dFGRS-NVSS galaxies. Symbols show the initial galaxy classification types: $\star$ Starburst galaxy, $\odot$ emission-line AGN and + absorption-line AGN.

Next Section: Emission-line diagnostic analysis
Title/Abstract Page: The spectral characteristics of
Previous Section: Introduction
Contents Page: Volume 17, Number 3