NARROW-BAND OPTICAL IMAGING AND PHYSICAL CONDITIONS

Carrillo, R.¹, Cruz-González, I.¹
and Guichard, J.²

¹ Instituto de Astronomía, UNAM, México

² Instituto Nacional de Astrofísica, Optica y Electrónica, México

Abstract

Results of a search for extended optical emission-line gas of 26 weak radio galaxies is presented. Emission is detected in all galaxies and extended regions are resolved in 81%, with mean sizes of 4.6 kpc. Morphologies are predominantly oval or elliptical with a central condensation of kpc in size and in some cases there are filamentary structures at several kpc from the nucleus. The estimated luminosities, density and mass of gas of the weak radio galaxies are similar to those of powerful ones. It is proposed that also in weak radio galaxies the central source is responsible of the gas ionization and that the observed gas has been acquired by fusion or interaction of galaxies, with at least one rich in gas.

1. Introduction

It is now well known that some radio galaxies and some quasars have extended emission-line regions (heron, EELR) which is extended on scales of tens of kpc. In some radio galaxies there is convincing evidence that the kinematics and excitation of the very EELR is governed by its interaction with the radio jet. However, in other radio galaxies the evidence for an interaction is much weaker, and the ionization of the EELR may be predominantly produced by the nuclear UV continuum and the kinematics of the gas by the gravitational potential of the host galaxy.

We have obtained optical broad-band (V, R, and I), narrow-band (H$\alpha$+[NII] and/or [OIII]), and infrared broad-band (J, H, and K'), images of a sample of weak radio galaxies (WRG). We also used data available in the literature to study the association between the EELR and the radio structure.

The details of our study of EELR in WRG are presented in Carrillo, Cruz-González & Guichard 1997, 1998.

2. Observations

The observations were carried out at the 2.12 m telescope of the Observatorio Astronómico Nacional at San Pedro Mártir, B. C., México. CCD images were taken through narrow-band filter centered on either H$\alpha$+[NII] $\lambda\lambda6548,6583$, and/or [OIII] $\lambda5007$. Two Thompson CCD detectors were used: 384$\times$576 and 1024$\times$1024, which with the f/7.5 secondary yield an image scale in the focal plane of 0.3''/pixel and 0.25''/pixel, respectively, so that the field of view is 1.93'$\times$2.89', and 4.25$\times$4.25' in each case.

Typical observation times were 30 min to 1 hr. A set of standard stars from Oke's list, were acquired each night for photometric calibrations. The reduction of the two-dimensional CCD frames follows the standard procedures. The Image Reduction and Analysis Facility (IRAF) software was used.

3. Results

In this work, we present the results of a program of optical observations in narrow-band filters, H$\alpha$+[N II] $\lambda \lambda$6548,6583 and [O III]$\lambda$5007, of a sample of 26 weak radio galaxies. The main results can be summarized as follows:

• Spatially extended emission-line gas is quite common in weak radio galaxies. Line emission is detected in all the sources in the sample and 81% of them have resolved emission-line nebulae.

• The mean size of the EELR is $\sim$4.6 kpc and the mean emission-line luminosity in H$\alpha$+[N II] or [O III] is $\sim 1.1 \times 10^{41}$ erg/s. The latter is one order of magnitude greater than the luminosity of emission-line nebulae in normal early-type galaxies, and similar to that of powerful radio galaxies.

• The emission-line nebulae of WRGs have morphologies similar to those in PRGs. In some sources we observe only small, centrally condensed, kpc scale regions with elliptical or oval forms, in others we detect external distortions as tongues or filaments and in few much more extended filaments several kpc from the host galaxy nucleus are detected. Just in B2 1346+26 two lobules are detected that spatially coincide with those observed in radio frequencies (Fig. 1), in the remaining sources the EELR is associated to the radio core.

• Estimates of the density and total mass of the emission-line gas, assuming case B recombination, yield values from 0.02 to 0.26 cm^-3 and 1.16 $\times 10^8$ to 2.63 $\times 10^9$ M$_{\odot}$, respectively. These values are similar to those of powerful radio galaxies.

• We confirmed the correlations of the emission-line luminosity with both the total radio luminosity and the radio core power for radio-AGNs. In Fig. 2, we present a diagram taken from Baum & Heckman 1989b, L _{Total  Radio} and L $_{H\alpha}$, for powerful radio galaxies, radio-loud steep-spectrum QSOs, high-redshift radio sources, Seyfert galaxies, and our sample of WRGs. With the addition of the WRGs the sequence becomes even stronger and a tight correlation (r=0.82) can be obtained with slope m = 0.59$\pm$0.14 and b = 15.96$\pm$1.57.

• From the correlation found between H$\alpha$ and radio luminosities, we propose that the central source is also the source of ionization of the EELR. The dispersion in this correlation may be produced by shocks between radio jets and the inter galactic gas from the host galaxy.

• Since most galaxies have interaction features and their properties (n_e, M_gas, U, and $L_{H \alpha}$) are enhanced by the interaction degree, we suggest that the emission gas in WRGs was acquired by fusion/collision between galaxies, where at least one galaxy was rich in gas.

• With these results, we proposed that the central source (maybe a black hole), in both weak and powerful radio galaxies, is the direct responsible for the ionization up to scales of 10 kpc, and that the properties of the host galaxy and the galactic environment play a secondary but important role, since they provide the ambient gas.

References

Baum S.A. & Heckman, T. 1989a, ApJ, 336, 681

Baum S.A. & Heckman, T. 1989b, ApJ, 336, 702

Carrillo, R., Cruz-González, I. & Guichard, J., 1997, RevMexAA, 33, 31

Carrillo, R., Cruz-González, I. & Guichard, J., 1998, submitted to RevMexAA

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Fig. 1. B2 1346+26 color representation of the [O III]$\lambda$5007 line emission overlayed with the radio emission (contours) at 6 cm (van Breugel, Heckman & Miley 1984). North is up and east is to the left. The bar corresponds to 5''

Fig. 2. Plot of log L _Total Radio and log L $_{H\alpha}$ for different types of AGNs: $\Box$ our sample of WRG; $\triangle$ PRG (Baum & Heckman 1989a); (A) region Seyfert galaxies (Whittle 1985; de Bruy & Wilson 1978); (B) the zone of steep-spectrum quasars (Stockton & Mac Kenty 1987); and (C) region of high redshift radio galaxies (Spinrad 1987).