Astron. Astrophys. 336, 479-489 (1998)

Appendix A: notes on individual objects and comparison with previous NIR photometry

PKS 0048-097. Our H-band photometry agrees well with previous studies (Table 6). High resolution optical imaging by Falomo (1996) revealed a faint (m(R)22.5) companion object 2.5" to the E of the BL Lac nucleus. The host galaxy of PKS 0048-097 remained unresolved by Falomo (1996), but assuming an elliptical host of M(R) = -23.5, he derived a lower limit to the redshift of z0.5. The host galaxy remains unresolved in the NIR. Some hint of the eastern companion object is visible in NIR.

Table 6. H-band photometry compared to previous literature photometry.
Notes:
Aperture photometry in a 6 " diameter aperture.
A74 = Andrews, Glass & Hawarden (1974); A82 = Allen, Ward & Hyland (1982); B86 = Brindle et al. (1986); B92 = Bersanelli et al. (1992); G79 = Glass (1979); G93 = Gear (1993); M90 = Mead et al. (1990); T86 = Tanzi et al. (1986)

PKS 0118-272. Our H-band magnitude is slightly fainter than that found in the literature (Table 6). The host galaxy remains unresolved in the optical (Falomo 1996), however, this is consistent with the presence of a luminous host (M(R) = -23.5) at the proposed redshift of z0.559. The host galaxy remains unresolved in the NIR.

PKS 0521-365 is at redshift z = 0.055. Our H-band photometry agrees well with literature values (Table 6). The host galaxy has been imaged in the optical by Falomo (1994), who found best fit for a giant elliptical (M(R) = -23.2 and R(e) = 9 kpc) with a faint stellar disk, and by Wurtz et al. (1996), who fit the host with an elliptical galaxy having M(R) = -23.0 and R(e) = 5.4 kpc. These values are in good agreement with those found in this study (M(H) = -25.4, R(e) = 5.8 kpc).

PKS 0537-441 is a high redshift BL Lac (z = 0.896; Peterson et al. 1976). Our H-band magnitude agrees well with previous studies (Table 6). The existence and nature of extended emission around this BL Lac, and its relevance to gravitational lensing, has been debated in the literature (e.g. Stickel, Fried & Kühr 1988; Falomo, Melnick & Tanzi 1992). No nebulosity surrounding this high redshift BL Lac is detected in the NIR, in agreement with Falomo et al. (1992), who conclude that there is no evidence for a foreground lensing galaxy.

PKS 0548-32 is the dominant member of a rich cluster of galaxies at redshift z = 0.069 (Fosbury & Disney 1976). Our H-band photometry is slightly fainter than found in previous studies (Table 6). Falomo et al. (1995) fit the host galaxy with a giant elliptical (M(R) = -24.2 and R(e) = 51 kpc) and a faint stellar disk. Wurtz et al. (1996) found for the host galaxy an elliptical fit, with M(R) = -23.2 and R(e) = 14 kpc. The values derived by us in the NIR are M(H) = -25.7, and R(e) = 8.3 kpc. Note that this BL Lac is surrounded by an extended halo, which was detected in the optical by Falomo et al. (1995), but not in the NIR. Therefore, the real colour of the host is redder than that derived from difference of total magnitudes in the two bands (see table 3).

PKS 1514-241 = AP Lib is at redshift z = 0.0486 (Disney, Peterson & Rodgers 1974). Our H-band magnitude agrees reasonably well with literature photometry (Table 6). The elliptical host galaxy has been studied extensively in the optical. Baxter et al. (1987) obtained M(V) = -22.8, Abraham et al. (1991) M(R) = -22.8 and R(e) = 7.5 kpc, and Stickel et al. (1993) derived M(R) = -23.5 and R(e) = 11.5 kpc. Finally, we have analysed our unpublished optical images of this BL Lac, obtained at the ESO 2.2m telescope, and find M(R) = -22.9 and R(e) = 4.3 kpc. The host galaxy parameters derived by us in the NIR (M(H) = -25.1, R(e) = 3.3 kpc) are in reasonable agreement with those found in the previous studies.

PKS 1538+149 is at redshift z = 0.605 (Stickel et al. 1993). Our H-band photometry agrees well with those found in the literature (Table 6). Stickel et al. (1993) could not resolve the host galaxy, while Wurtz et al. (1996) found a marginal fit for the host, with M(R) = -24.2 and R(e) = 12 kpc. Falomo et al. (1997b) derived for the host from HST imaging M(I) = -25.2. The host galaxy remains unresolved in the NIR.

PKS 2005-489 is at redshift z = 0.071 (Falomo et al. 1987). Our H-band magnitude agrees well with previous studies (Table 6). Stickel et al. (1993) derived for the host galaxy M(R) = -24.2 and R(e) = 5.2 kpc, while Falomo (1996) found the host to be an elliptical with M(R) = -23.7 and R(e) = 11 kpc. The absolute magnitude derived by us in the NIR is: M(H) = -26.3.

MS 2143.4+07 is at redshift z = 0.237. Our H-band photometry is slightly fainter than found in previous studies (Table 6). Wurtz et al. (1996) derived for the elliptical host galaxy M(R) = -23.4 and R(e) = 12 kpc, while Jannuzi et al. (1997) derived from HST imaging M(I) = -24.0 and R(e) = 9.0 kpc. These values are in excellent agreement with those derived by us in the NIR (M(H) = -25.9, R(e) = 5.4 kpc).

PKS 2155-305 is at redshift z = 0.116 (Falomo, Pesce & Treves 1993) and it is one of the brightest and most studied BL Lacs and is often considered the prototype of X-ray selected BL Lacs. Our H-band magnitude is in good agreement with literature values (Table 6). For the host galaxy, Falomo et al. (1991) derived M(R) = -24.4 and R(e) = 13 kpc, while Wurtz et al. (1996) could not resolve the host, with M(R)-23.1. The NIR properties of the host (M(H) = -26.8, R(e) = 5.7 kpc) are in good agreement with Falomo et al. (1991). The companion galaxy at 4.2" E of PKS 2155-305, previously detected in the optical (Falomo et al. 1991), is clearly seen in the NIR image (see Fig. 1).

PKS 2254+074 is at redshift z = 0.190 (Stickel et al. 1993). We obtained only a short exposure of this source under poor sky conditions. We also note that the BL Lac was situated in a bad area of the array during the observations. Consequently, both the NIR photometry (slightly fainter than found in literature; Table 6), and the host properties are less accurate than for the other resolved BL Lacs. Optical determinations of the host galaxy have yielded M(R) = -24.1 and R(e) = 14.5 kpc (Stickel et al. 1993), M(R) = -23.9 and R(e) = 15 kpc (Falomo 1996), M(R) = -23.9 and R(e) = 17 kpc (Wurtz et al. 1996) and M(I) = -24.8 and R(e) = 15 kpc (Falomo et al. 1997b). The absolute magnitude derived by us in the NIR (M(H) = -25.5) is 1 mag fainter than implied by the optical studies, assuming normal elliptical colours.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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