Astron. Astrophys. 341, 44-57 (1999)

2. The GPS source B1144+352

The GPS nature of the radio source B1144+352 was first recognized by Snellen et al. (1995, hereafter S95), who find that the radio spectrum peaks at 2.4 GHz, with a spectral index of in the optical thick (low frequency) part of the radio spectrum, and a spectral index of -0.56 in the optical thin part. These values should be treated with care, however, because of the variability of the central source (see also Sect. 4.1) and the large time spanned by the observations used in obtaining them.

The radio source has been identified with the galaxy CGCG 186-048 (Zwicky et al. 1960-1968). The Gunn r-band magnitude of the host galaxy is (Snellen et al. 1996) and it has a redshift of 0.0630 (Colla et al. 1975a; Hewitt & Burbidge 1991). This makes it one of the nearest GPS galaxies known. The absolute magnitude in the Gunn r-band is , using the K-correction as given in Snellen et al. (1996). The optical spectrum shows bright narrow emission lines, typical for powerful radio sources, on top of a starlight dominated continuum (Marcha et al. 1996). There is no hint of a non-thermal continuum or of broad Balmer emission lines, hence the classification as a GPS galaxy. An optical R-band CCD image can be found in Snellen et al. (1996). Fig. 1 presents a contour plot of the host galaxy from the Digitized Sky Survey (DSS). The host galaxy appears to have a small companion galaxy at a projected distance of kpc () to the west. This is close to the median distance at which apparent companions were found in a sample of GPS galaxies by O'Dea et al. (1996a). From the DSS it is apparent that CGCG 186-048 is the brightest galaxy within a radius of at least 1 Mpc (). There is no indication that it is a member of a rich cluster, although there are several fainter (R-band magn. ) galaxies within a radius of a few hundred kpc. More likely, it is situated in a poor group of galaxies.

Fig. 1. Contourplot of the host galaxy of the GPS source B1144+352 from the DSS. The contours are drawn at logarithmic intervals of , starting at three times the background noise level. The cross at the center of the host galaxy gives the position of the GPS source. Its size is not related to the positional uncertainty which is only .

VLA maps of the GPS source at 1.4 GHz are presented by Parma et al. (1986) and S95. Both were obtained using the same configuration and have a resolution of arcsec. The GPS source appears as a strong unresolved point-source. However, the maps also show a long weak jet-like feature emanating from the radio core, and a shorter () and somewhat weaker counter-jet. The total linear size of this extended system is kpc. S95 note that the flux density from the 20-cm Green Bank survey (White & Becker 1992) is higher (by mJy) than that from their VLA observations. Since the Green Bank survey has a beamsize of (FWHM), they attribute this difference to a radio source component extending even beyond the observed jet-like feature. The fractional polarization of the core, obtained from high resolution 8.4-GHz VLA observations, is 3% at a -level (Marcha et al. 1996).

A VLBI map of the central source is presented by Henstock et al. (1995). It shows a pc large double radio structure, with one well resolved bright radio lobe-like component and an unresolved second component. The position angle of the radio axis is , counted counter-clockwise (CCW) from the North. Giovannini et al. (1995) find that the two components are separating from each other superluminally with an apparent velocity of . This is surprising since GPS galaxies in general do not show superluminal motion (Stanghellini et al. 1997).

Giovannini et al. (1990) report that the flux density of the peak in the radio spectrum (the peak flux density) has continuously increased from  mJy in 1974 to  mJy in 1990. S95 and Snellen et al. (1998) claim that it has decreased again somewhat thereafter. Snellen et al. (1998) further state that the brightest (eastern) VLBI component, which has a radio lobe-like morphology, must be largely responsible for the observed change in the peak flux density. Variability in the flux density has been found in other GPS sources, but mostly in GPS quasars (e.g. Stanghellini et al. 1998).

© European Southern Observatory (ESO) 1999

Online publication: November 26, 1998
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