Radio Sources in the 2dF Galaxy Redshift Survey. I. Radio source populations1

Elaine M. Sadler , V.J. McIntyre , C.A. Jackson , R.D. Cannon, PASA, 16 (3), 247.
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Two kinds of radio source: AGN/SF classification

We classified each matched galaxy as either `AGN' or `star-forming' (SF) based on its 2dF spectrum. `AGN' galaxies have either a pure absorption-line spectrum like that of a giant elliptical galaxy, or a stellar continuum plus nebular emission lines such as [OII] and [OIII] which are strong compared with any Balmer-line emission. Some of the emission-line AGN have spectra which resemble Seyfert galaxies. `SF' galaxies are those where strong, narrow emission lines of H$\alpha $ and (usually) H$\beta $ dominate the spectrum. They include both nearby spirals and more distant IRAS galaxies. Figure 1 shows examples of spectra we classified as AGN, Seyfert and SF. Note that in this classification scheme, `AGN' may simply denote the presence of radio emission, with no obvious optical signature. The origin of radio emission in the AGN and SF galaxies is believed to be quite different (e.g. Condon 1989), arising from non-thermal processes related to a central massive object in the AGN galaxies and from processes related to star formation (including supernova remnants, HII regions, etc.) in the SF galaxies.

We are confident that this simple `eyeball' classification of the 2dF spectra allows us to separate the AGN and SF classes accurately. Jackson & Londish (1999) measured several emission-line ratios (including [OIII, $\lambda $5007]/H$\beta $, [NII, $\lambda $6584]/H$\alpha $, [OI, $\lambda $6300]/H$\alpha $ and [SII,

$\lambda\lambda$6716,6731]/H$\alpha $) for most of the galaxies studied here and plotted them on the diagnostic diagrams of Veilleux & Osterbrock (1987). They found that the `eyeball' classifications and line-ratio based classifications agreed more than 95% of the time, and hence that `eyeball' classifications can be used with confidence to analyse large samples of 2dF spectra.

Most of the 2dF spectra are of impressively good quality. However, of the 98 spectra we examined (one galaxy was not actually observed by the 2dFGRS), nine had such a low signal-to-noise ratio that we were unable to classify the spectrum. One other object appeared to be a Galactic star. We were therefore left with 88 good-quality 2dF spectra of candidate radio matches with an offset D<10arcsec. Of these 88 galaxies, 36 (41%) were classified as SF and 52 (59%) as AGN. One galaxy classified as SF had an emission-line spectrum which resembled an AGN, but was also detected as an IRAS source at 60$\mu $m. This may be a genuinely composite object. Table 2 lists the matched galaxies, their spectral classification, 1.4GHz radio continuum flux density, apparent magnitude and redshift. A more quantitative spectral classification using diagnostic emission-line ratios will be presented in the forthcoming paper by Jackson and Londish (1999).

Figure 2: Distribution of the star-forming and AGN galaxies in apparent magnitude. Note that star-forming galaxies dominate at the bright end (

$b_{\rm J} < 17$ mag.), while most of the fainter galaxies have AGN spectra.
\begin{figure} \begin{center} \psfig{file=2df_fig2.eps,width=9.35cm,angle=0}\end{center}\end{figure}

Figure 3: The redshift distribution of the AGN and star-forming galaxies, and of the whole sample. Most of the star-forming galaxies are relatively nearby, though some extend out to redshifts of 0.25. The AGN galaxies are a more distant population, with a median redshift almost three times that of the star-forming galaxies.
\begin{figure} \begin{center} \psfig{file=2df_fig3.eps,width=9.35cm,angle=0}\end{center}\end{figure}

Figures 2 and 3 show the distribution of AGN and SF classes in apparent magnitude and redshift respectively. There is a clear segregation in apparent magnitude: most galaxies brighter than about $b_{\rm J}$ = 16.5-17 magnitude fall into the star-forming (SF) class, while the AGN class dominates the population fainter than

$b_{\rm J}\sim17$. This reflects strong differences in the global properties of the two classes as well as the radio and optical flux limits of the NVSS and 2dFGRS. The AGN galaxies are typically more distant than the SF galaxies (by about a factor of 3: Figure 3), and more luminous both optically and in radio power (see Figures 4 and 5). We know that the SF galaxies continue to large redshifts and to very faint optical magnitudes (e.g. Benn et al. 1993), but these galaxies quickly drop out of our sample because of the 2-3mJy limit of the NVSS in radio flux density. Similarly, we know that the AGN galaxies extend to much higher redshifts than probed by the 2dFGRS, but these distant AGN galaxies will be fainter than the

$b_{\rm J}=19.4$ mag optical limit of the 2dFGRS. Figure 6 shows plots of radio power and optical luminosity versus redshift for the AGN and SF classes -- the solid lines correspond to the survey completeness limits of 3.5mJy and 19.4 mag for the NVSS and 2dFGRS respectively. Galaxies below these lines will be excluded from our sample. Note that most of the SF galaxies are weak radio sources, lying close to the NVSS cutoff at all redshifts, while most of the AGN galaxies lie well above the radio limit but start to drop below the optical cutoff at redshifts above 0.15.

Figure 4: Absolute magnitude histograms for the AGN and star-forming galaxies. The AGN spectra are typically found in luminous optical galaxies, while the star-forming galaxies span a much wider range in optical luminosity.
\begin{figure} \begin{center} \psfig{file=2df_fig4.eps,width=10.2cm,angle=0}\end{center}\end{figure}

Figure 5: Histograms of radio power for the AGN and star-forming galaxies.
\begin{figure} \begin{center} \psfig{file=2df_fig5.eps,width=8.5cm,angle=0}\end{center}\end{figure}

Figure 6: Plots of radio power (bottom) and optical luminosity (top) versus redshift for AGN (shown by open circles) and SF (shown by stars) galaxies. The solid line in each plot shows the radio and optical limits of the NVSS and 2dFGRS respectively -- galaxies below these lines will drop out of our sample.
\begin{figure} \centering\psfig{file=2df_fig6.eps,width=8.5cm,angle=0}\end{figure}

Figure 7: Comparison of radio (1.4GHz) and IRAS far-infrared (60$\mu $m) flux density for the radio-detected star-forming galaxies in the 2dFGRS. The dotted line is for

$S_{60\mu {\rm m}} = 100\, S_{\rm 1.4\,GHz}$. Of the nine SF galaxies in Table 2 which lack IRAS data, two lie in regions not observed by IRAS. For the remaining seven we show (open triangles) upper limits of 0.28Jy at 60$\mu $m, corresponding to the limits of the IRAS Faint Source Catalogue.
\begin{figure} \centering\psfig{file=2df_fig7.eps,width=10.2cm,angle=0}\end{figure}

Next Section: Matches with IRAS sources
Title/Abstract Page: Radio Sources in the
Previous Section: The radio data
Contents Page: Volume 16, Number 3

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