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Host galaxies and the unification of radio-loud AGN
New Astronomy Reviews 46 (2002) 349–351 www.elsevier.com / locate / newar
Host galaxies and the unification of radio-loud AGN C. Megan Urry a , *, Riccardo Scarpa a , Matthew O’Dowd a ,1 , Mauro Giavalisco a , Renato Falomo b , Joseph E. Pesce c , Aldo Treves d a b
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA Osservatorio Astronomico di Padova, Vicolo dell’ Osservatorio 5, 35122 Padova, Italy c George Mason University, 4400 Fairfax Drive, Fairfax, VA 20124, USA d Universita’ dell’ Insubria, via Lucini 3, 22100 Como, Italy
Abstract Our HST WFPC2 survey of 110 BL Lac objects, from six complete X-ray-, radio-, and optically-selected catalogs, probes the host galaxies of low-luminosity radio sources in the redshift range 0 , z , 1.35. The host galaxies are luminous ellipticals, well matched in radio power and galaxy magnitude to FR I radio galaxies. Similarly, the host galaxies of high luminosity quasars occupy the same region of this plane as FR II radio galaxies (matched in redshift). This strongly supports the unification of radio-loud AGN, and suggests that studying blazars at high redshift is a proxy for investigating less luminous (to us) but intrinsically identical radio galaxies, which are harder to find at high z. Accordingly, the difference between low-power jets in BL Lac objects and high-power jets in quasars can then be related to the FR I / FR II dichotomy; and the evolution of blazar host galaxies or their nuclei (jets) should correspond to the evolution of radio galaxies. 2002 Published by Elsevier Science B.V. Keywords: BL Lacertae objects: General; Galaxies: Elliptical; Galaxies: Jets; Radio continuum: Galaxies; Quasars: General
1. Sample and observations BL Lacertae objects (BL Lacs) are AGN of the blazar class—highly luminous, polarized and variable AGN. Unified Schemes suggest these properties are due to relativistic beaming of jets aligned with the line of sight. BL Lacs are characterized by a near-absence of emission lines, and are intrinsically less luminous than quasars. Our well-defined survey sample of 132 snapshot targets included X-ray-, radio-, and optically-selected *Corresponding author. E-mail address: [email protected] (C.M. Urry). 1 Also at School of Physics, University of Melbourne, Parkville, Victoria, Australia 3052.
BL Lacs spanning the full range of BL Lac types (Padovani and Giommi, 1995; Sambruna et al., 1996). We obtained WFPC2 images in a sensitive red broad-band filter, F702W. A total of 109 BL Lac objects were observed, spanning the redshift range 0.031 # z # 1.34, with a median redshift kzl 5 0.25 and 22 having z . 0.5.
2. Comparison to radio galaxies According to unified schemes for radio-loud AGN, BL Lac objects are FR I radio galaxies whose jets are aligned along the line of sight (Urry and Padovani, 1995). This implies BL Lac host galaxies should be statistically indistinguishable from FR I
1387-6473 / 02 / $ – see front matter 2002 Published by Elsevier Science B.V. PII: S1387-6473( 01 )00206-8
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C.M. Urry et al. / New Astronomy Reviews 46 (2002) 349 – 351
host galaxies. It has conversely been suggested that the parent population of BL Lacs might instead be FR IIs, or a subset thereof (Wurtz et al., 1996; Laing et al., 1994; Urry and Padovani, 1995). The original division between FR I and FR II galaxies was morphological—whether hot spots occurred at the inner or outer edges of the radio source, respectively—and the excellent correlation of morphology with radio luminosity was noted at the same time (Fanaroff and Riley, 1974). For lowfrequency radio luminosities below (above) P178 5
2 3 10 25 W Hz 21 sr 21 , almost all radio sources were type I (II). Owen and Ledlow (1994) showed that FR I / II division depends on both radio and optical power, with a diagonal line dividing FR Is from IIs. FR Is, while on average less luminous than FR IIs at radio frequencies, have systematically brighter host galaxies. Fig. 1 shows how the BL Lac objects in this survey, as well as quasars from the literature, sit with respect to the FR I / II division.
Fig. 1. Extended radio power versus host galaxy magnitude for radio galaxies and AGN (after Owen and Ledlow, 1994). Because these are unbeamed quantities, relativistic beaming in the BL Lac nuclei has no effect, and thus allowing a direct comparison between BL Lac objects and radio galaxies. FR I and II galaxies are from the 2 Jy sample (Wall and Peacock, 1985), which has similar depth and selection criteria as the 1 Jy BL Lac sample (Stickel et al., 1991); morphological classifications are from Morganti et al. (1993). The BL Lacs (open squares: high frequency peaked, filled squares: low frequency peaked) overlap extremely well with the FR I galaxies (1 s) as the quasars (stars) do with the FR IIs (2 s)(quasars from Taylor et al., 1996; McLeod and Rieke, 1995; Bahcall et al., 1997; Boyce et al., 1998; Hutchings et al., 1989). Diagonal lines represent the theoretical division caused by jet deceleration in the host galaxy’s ISM (Bicknell, 1996). Thus the present data strongly support the unification picture, with FR I and FR II galaxies constituting the parent populations of BL Lacs and quasars, respectively.
C.M. Urry et al. / New Astronomy Reviews 46 (2002) 349 – 351
351
3. Evolution of host galaxies
References
Given the unification of radio-loud AGN, and the fact that blazars are easily found at high redshift (unlike FR I radio galaxies), the evolution of radio galaxies can be probed via blazars and their host galaxies. The luminosity density function for normal galaxies is relatively flat out to | z 5 0.75 (Lilley et al., 1996). For FR Is, complete samples exist to redshifts of only | 0.2. Complete samples of BL Lacs extend to redshifts * 1, near the peak of the star-formation luminosity density function (Madau et al., 1998). As yet little is known about the evolution of blazar hosts. Our study shows that for z , 0.6 no evolution is detectable in the hosts of BL Lacs. The host properties are consistent with their being a subpopulation of brightest cluster elliptical galaxies, both in their luminosities (kMR l BLLac 5 2 23.5 mag vs. kMR l BrightestCluster 5 2 23.9 mag), and in their morphologies. The absence of strong evolution is also consistent with BL Lac hosts forming such a sub-population. With the short exposure times of our HST snapshot survey, only upper limits were found for the luminosities of most BL Lac hosts with z . 0.6. Expanding BL Lac host galaxy studies to redshifts z * 1 will better test this idea.
Bahcall, J.N., Kirhakos, S., Saxe, D.H., 1997. ApJ 479, 642. Bicknell, G.V., 1996. In: Hardee, P.E., Bridle, A.H., Zensus, J.A. (Eds.), Energy Transport in Radio Galaxies and Quasars, ASP Conf. Series, ASP, San Francisco, 100, p. 253. Boyce, P.J. et al., 1998. MNRAS 298, 121. Fanaroff, B.L., Riley, J.M., 1974. MNRAS 167, 31P. Hutchings, J.B., Janson, T., Neff, S.G., 1989. ApJ 342, 660. Laing, R.A. et al., 1994. In: Bicknell, G.V., Dopita, M.A., Quinn, P.J. (Eds.), The Physics of Active Galaxies, ASP Conf. Series 54, ASP, San Francisco, p. 201. Lilley, S.J., Le Fevre, O., Hammer, F., Crampton, D., 1996. ApJ 460, L1. Madau, P., Pozzetti, L., Dickinson, M., 1998. ApJ 498, 106. McLeod, K.K., Rieke, G.H., 1995. ApJ 454, L77. Morganti, R., Killeen, N.E.B., Tadhunter, C.N., 1993. MNRAS 263, 1023. Owen, F.N., Ledlow, M.J., 1994. In: Bicknell, G.V., Dopita, M.A., Quinn, P.J. (Eds.), The Physics of Active Galaxies, ASP Conf. Series 54, ASP, San Francisco, p. 31. Padovani, P., Giommi, P., 1995. MNRAS 277, 1477. Sambruna, R., Maraschi, L., Urry, C.M., 1996. ApJ 463, 444. ¨ H., Padovani, P., Urry, C.M., 1991. Stickel, M., Fried, J.W., Kuhr, ApJ 374, 431. Taylor, G.L., Dunlop, J.S., Hughes, D.H., Robson, E.I., 1996. MNRAS 283, 930. Urry, C.M., Padovani, P., 1995. PASP 107, 803. Wall, J.V., Peacock, J.A., 1985. MNRAS 216, 173. Wurtz, R., Stocke, J.T., Yee, H.K.C., 1996. ApJS 103, 109.
Host galaxies and the unification of radio-loud AGN C. Megan Urry a , *, Riccardo Scarpa a , Matthew O’Dowd a ,1 , Mauro Giavalisco a , Renato Falomo b , Joseph E. Pesce c , Aldo Treves d a b
Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA Osservatorio Astronomico di Padova, Vicolo dell’ Osservatorio 5, 35122 Padova, Italy c George Mason University, 4400 Fairfax Drive, Fairfax, VA 20124, USA d Universita’ dell’ Insubria, via Lucini 3, 22100 Como, Italy
Abstract Our HST WFPC2 survey of 110 BL Lac objects, from six complete X-ray-, radio-, and optically-selected catalogs, probes the host galaxies of low-luminosity radio sources in the redshift range 0 , z , 1.35. The host galaxies are luminous ellipticals, well matched in radio power and galaxy magnitude to FR I radio galaxies. Similarly, the host galaxies of high luminosity quasars occupy the same region of this plane as FR II radio galaxies (matched in redshift). This strongly supports the unification of radio-loud AGN, and suggests that studying blazars at high redshift is a proxy for investigating less luminous (to us) but intrinsically identical radio galaxies, which are harder to find at high z. Accordingly, the difference between low-power jets in BL Lac objects and high-power jets in quasars can then be related to the FR I / FR II dichotomy; and the evolution of blazar host galaxies or their nuclei (jets) should correspond to the evolution of radio galaxies. 2002 Published by Elsevier Science B.V. Keywords: BL Lacertae objects: General; Galaxies: Elliptical; Galaxies: Jets; Radio continuum: Galaxies; Quasars: General
1. Sample and observations BL Lacertae objects (BL Lacs) are AGN of the blazar class—highly luminous, polarized and variable AGN. Unified Schemes suggest these properties are due to relativistic beaming of jets aligned with the line of sight. BL Lacs are characterized by a near-absence of emission lines, and are intrinsically less luminous than quasars. Our well-defined survey sample of 132 snapshot targets included X-ray-, radio-, and optically-selected *Corresponding author. E-mail address: [email protected] (C.M. Urry). 1 Also at School of Physics, University of Melbourne, Parkville, Victoria, Australia 3052.
BL Lacs spanning the full range of BL Lac types (Padovani and Giommi, 1995; Sambruna et al., 1996). We obtained WFPC2 images in a sensitive red broad-band filter, F702W. A total of 109 BL Lac objects were observed, spanning the redshift range 0.031 # z # 1.34, with a median redshift kzl 5 0.25 and 22 having z . 0.5.
2. Comparison to radio galaxies According to unified schemes for radio-loud AGN, BL Lac objects are FR I radio galaxies whose jets are aligned along the line of sight (Urry and Padovani, 1995). This implies BL Lac host galaxies should be statistically indistinguishable from FR I
1387-6473 / 02 / $ – see front matter 2002 Published by Elsevier Science B.V. PII: S1387-6473( 01 )00206-8
350
C.M. Urry et al. / New Astronomy Reviews 46 (2002) 349 – 351
host galaxies. It has conversely been suggested that the parent population of BL Lacs might instead be FR IIs, or a subset thereof (Wurtz et al., 1996; Laing et al., 1994; Urry and Padovani, 1995). The original division between FR I and FR II galaxies was morphological—whether hot spots occurred at the inner or outer edges of the radio source, respectively—and the excellent correlation of morphology with radio luminosity was noted at the same time (Fanaroff and Riley, 1974). For lowfrequency radio luminosities below (above) P178 5
2 3 10 25 W Hz 21 sr 21 , almost all radio sources were type I (II). Owen and Ledlow (1994) showed that FR I / II division depends on both radio and optical power, with a diagonal line dividing FR Is from IIs. FR Is, while on average less luminous than FR IIs at radio frequencies, have systematically brighter host galaxies. Fig. 1 shows how the BL Lac objects in this survey, as well as quasars from the literature, sit with respect to the FR I / II division.
Fig. 1. Extended radio power versus host galaxy magnitude for radio galaxies and AGN (after Owen and Ledlow, 1994). Because these are unbeamed quantities, relativistic beaming in the BL Lac nuclei has no effect, and thus allowing a direct comparison between BL Lac objects and radio galaxies. FR I and II galaxies are from the 2 Jy sample (Wall and Peacock, 1985), which has similar depth and selection criteria as the 1 Jy BL Lac sample (Stickel et al., 1991); morphological classifications are from Morganti et al. (1993). The BL Lacs (open squares: high frequency peaked, filled squares: low frequency peaked) overlap extremely well with the FR I galaxies (1 s) as the quasars (stars) do with the FR IIs (2 s)(quasars from Taylor et al., 1996; McLeod and Rieke, 1995; Bahcall et al., 1997; Boyce et al., 1998; Hutchings et al., 1989). Diagonal lines represent the theoretical division caused by jet deceleration in the host galaxy’s ISM (Bicknell, 1996). Thus the present data strongly support the unification picture, with FR I and FR II galaxies constituting the parent populations of BL Lacs and quasars, respectively.
C.M. Urry et al. / New Astronomy Reviews 46 (2002) 349 – 351
351
3. Evolution of host galaxies
References
Given the unification of radio-loud AGN, and the fact that blazars are easily found at high redshift (unlike FR I radio galaxies), the evolution of radio galaxies can be probed via blazars and their host galaxies. The luminosity density function for normal galaxies is relatively flat out to | z 5 0.75 (Lilley et al., 1996). For FR Is, complete samples exist to redshifts of only | 0.2. Complete samples of BL Lacs extend to redshifts * 1, near the peak of the star-formation luminosity density function (Madau et al., 1998). As yet little is known about the evolution of blazar hosts. Our study shows that for z , 0.6 no evolution is detectable in the hosts of BL Lacs. The host properties are consistent with their being a subpopulation of brightest cluster elliptical galaxies, both in their luminosities (kMR l BLLac 5 2 23.5 mag vs. kMR l BrightestCluster 5 2 23.9 mag), and in their morphologies. The absence of strong evolution is also consistent with BL Lac hosts forming such a sub-population. With the short exposure times of our HST snapshot survey, only upper limits were found for the luminosities of most BL Lac hosts with z . 0.6. Expanding BL Lac host galaxy studies to redshifts z * 1 will better test this idea.
Bahcall, J.N., Kirhakos, S., Saxe, D.H., 1997. ApJ 479, 642. Bicknell, G.V., 1996. In: Hardee, P.E., Bridle, A.H., Zensus, J.A. (Eds.), Energy Transport in Radio Galaxies and Quasars, ASP Conf. Series, ASP, San Francisco, 100, p. 253. Boyce, P.J. et al., 1998. MNRAS 298, 121. Fanaroff, B.L., Riley, J.M., 1974. MNRAS 167, 31P. Hutchings, J.B., Janson, T., Neff, S.G., 1989. ApJ 342, 660. Laing, R.A. et al., 1994. In: Bicknell, G.V., Dopita, M.A., Quinn, P.J. (Eds.), The Physics of Active Galaxies, ASP Conf. Series 54, ASP, San Francisco, p. 201. Lilley, S.J., Le Fevre, O., Hammer, F., Crampton, D., 1996. ApJ 460, L1. Madau, P., Pozzetti, L., Dickinson, M., 1998. ApJ 498, 106. McLeod, K.K., Rieke, G.H., 1995. ApJ 454, L77. Morganti, R., Killeen, N.E.B., Tadhunter, C.N., 1993. MNRAS 263, 1023. Owen, F.N., Ledlow, M.J., 1994. In: Bicknell, G.V., Dopita, M.A., Quinn, P.J. (Eds.), The Physics of Active Galaxies, ASP Conf. Series 54, ASP, San Francisco, p. 31. Padovani, P., Giommi, P., 1995. MNRAS 277, 1477. Sambruna, R., Maraschi, L., Urry, C.M., 1996. ApJ 463, 444. ¨ H., Padovani, P., Urry, C.M., 1991. Stickel, M., Fried, J.W., Kuhr, ApJ 374, 431. Taylor, G.L., Dunlop, J.S., Hughes, D.H., Robson, E.I., 1996. MNRAS 283, 930. Urry, C.M., Padovani, P., 1995. PASP 107, 803. Wall, J.V., Peacock, J.A., 1985. MNRAS 216, 173. Wurtz, R., Stocke, J.T., Yee, H.K.C., 1996. ApJS 103, 109.