- Email: [email protected]
Chandra’s view of the X-ray jet and halo of the giant radio galaxy NGC 6251
New Astronomy Reviews 47 (2003) 447–449 www.elsevier.com / locate / newastrev
Chandra’s view of the X-ray jet and halo of the giant radio galaxy NGC 6251 J. Kerp a , K.-H. Mack b,c,a , * a
¨ Bonn, Auf dem Hugel ¨ 71, D-53121 Bonn, Germany Radioastronomisches Institut der Universitat b Istituto di Radioastronomia del CNR, Via P. Gobetti 101, I-40129 Bologna, Italy c ASTRON, Postbus 2, NL-7990 AA Dwingeloo, The Netherlands
Abstract We present a deep Chandra observation of the giant radio galaxy NGC 6251. We found evidence that the well-known radio jet is associated with X-ray excess emission and that the central galaxy is embedded within a large diffuse soft X-ray halo. The superior angular resolution of Chandra discloses for the first time the innermost part of the radio jet in X-ray emission. The diffuse X-ray halo appears to provide an additional pressure component which confines the radio jet. Beyond a radial distance of about 110 kpc, the diffuse X-ray emission is undetectable whereas the radio emission widens and traces the huge radio lobe. 2003 Elsevier B.V. All rights reserved. PACS: 98.54.Gr; 98.62.Nx; 95.85.Nv
1. Radio properties of NGC 6251 NGC 6251 belongs to the class of giant radio galaxies (GRGs). It is classified as E2 galaxy. Optical studies indicate a low Seyfert activity if any at all. NGC 6251 is famous because of its wellcollimated radio jet, which is one of the brightest observed. The jet reaches a radial distance of about 140 kpc (H0 5 75 km s 21 Mpc 21 ) from the central engine. Beyond this radial distance the radio emission is distributed across a huge radio lobe of more than 500 kpc in extent. The redshift of 0.0234
*Corresponding author. Tel.: 139-051-639-9373; fax: 139-051639-9431. E-mail addresses: [email protected] (J. Kerp), [email protected] (K.-H. Mack).
localizes NGC 6251 at a distance of 91 Mpc. Fig. 1 (left) shows the Westerbork 92-cm observation superposed onto the Chandra map of NGC 6251 and its environment.
2. Chandra’s view of NGC 6251 Chandra observed NGC 6251 with the ACIS-I array in focus for 40 ks. The data confirm the existence of X-ray emission from the radio jet of NGC 6251 as well as from the halo (Mack et al., 1997a). Fig. 1 (right) shows the X-ray intensity weighted energy map of NGC 6251 with an angular resolution of about 40. Both knots of the radio jet are clearly detected as excess X-ray emitting regions. The first knot is doubtlessly extended with its major axis along the jet direction. Both knots are enclosed by the radio continuum emission.
1387-6473 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S1387-6473(03)00069-1
448
J. Kerp, K.-H. Mack / New Astronomy Reviews 47 (2003) 447–449
Fig. 1. Left: Chandra data superposed by the 92-cm data (Mack et al., 1997b). Right: Chandra map superposed by the 21-cm VLA data. Knot 1 is well encircled by the 21-cm radio continuum data.
2.1. X-ray and radio data
2.2. Chandra and ROSAT
Superposed as contours on Fig. 1 (right) is the radio continuum emission at 21-cm (NRAO archive data of Perley et al., 1984). As Fig. 1 (right) shows, the galaxy core and knot 1 are associated with enhanced X-ray emission; knot 2 is also detected. The X-ray emission of knot 1 reaches its intensity maximum exactly centred within the peak radio continuum contours. This is a clear indication that the radio structure and the X-ray emission originate in the same volume of space. Low radio frequencies trace the extended radio emission of NGC 6251 far away from the core (Fig. 1, left). We superposed Westerbork 92-cm data as contours on top of the Chandra data. The radio jet is well confined as long as the jet is embedded within the diffuse extended X-ray plasma. At larger radial distances, the radio emission fills a much larger volume. This indicates that the X-ray plasma provides an additional pressure component which confines the radio emission up to a radial distance of 70 kpc.
Fig. 2 (left) shows the smoothed and exposuremap corrected Chandra ACIS-I data (0.3 keV,E , 8.0 keV). Superposed as contours are the ROSAT PSPC data (0.5 keV,E ,2.1 keV, Mack et al., 1997a). Chandra confirms not only the existence of the excess X-ray emitting regions associated with radio intensity maxima but also the extended diffuse soft X-ray halo emission. The scale length (radially averaged) of the soft X-ray plasma is about 30 kpc. Most of the detected X-ray plasma emission is produced at E ,2 keV which yields the excellent spatial correlation of Chandra and ROSAT data.
2.3. The nuclear region Focussing towards the very centre of NGC 6251, X-ray emission from the innermost part of the radio jet is detected. Fig. 2 (right) presents the Chandra data with an angular resolution of 10. Mack et al. (1997a) estimated the life-time of X-ray emitting relativistic electrons to about 3.5 3 10 4 years. With a
J. Kerp, K.-H. Mack / New Astronomy Reviews 47 (2003) 447–449
449
Fig. 2. Left: The greyscale map shows the Chandra diffuse soft X-ray emission of the halo of NGC 6251, while the contour lines mark the ROSAT PSPC data analysed by Mack et al. (1997b). Right: The central region of NGC 6251 at an angular resolution level of 10. An X-ray jet emerges from the AGN. The extent of this jet is about 3 kpc, consistent with the expected life-time of relatistic electrons emitting soft X-ray photons.
velocity of 0.3 c the maximum size of the expected X-ray synchrotron jet is 3 kpc. The extent of the innermost jet is about this linear size, suggesting that the X-ray emission is produced by synchrotron radiation.
3. Results Chandra discloses the existence of X-ray emission associated with the radio jet and the halo of the giant radio galaxy NGC 6251. The X-ray emission of the jet emerges directly from the central engine. Beyond a radial distance of . 3 kpc the X-ray emission becomes undetectable. The life-time of relativistic particles is long enough to produce X-ray synchrotron radiation in the innermost region of the radio jet. The X-ray emission re-appears at a radial distance of about 75 kpc from the central engine. The emission
is extended, oriented parallel to the jet, and narrowly confined within the extent of the radio jet. The second X-ray knot is located at a radial distance of about 110 kpc. The Chandra data also reveal the existence of diffuse X-ray emission around the active galactic nucleus. The radial extent is about 70 kpc. Most of the X-ray photons of this diffuse gas are emitted at E,2 keV. The thermal pressure of the plasma seems to confine the jet because outside the X-ray halo the radio jet expands into the northeastern radio lobe of NGC 6251.
References Mack, K.-H., Kerp, J., Klein, U., 1997a. A&A 324, 870. Mack, K.-H., Klein, U., O’Dea, C.P., Willis, A.G., 1997b. A&AS 123, 423. Perley, R.A., Bridle, A.H., Willis, A.G., 1984. ApJS 54, 291.
Chandra’s view of the X-ray jet and halo of the giant radio galaxy NGC 6251 J. Kerp a , K.-H. Mack b,c,a , * a
¨ Bonn, Auf dem Hugel ¨ 71, D-53121 Bonn, Germany Radioastronomisches Institut der Universitat b Istituto di Radioastronomia del CNR, Via P. Gobetti 101, I-40129 Bologna, Italy c ASTRON, Postbus 2, NL-7990 AA Dwingeloo, The Netherlands
Abstract We present a deep Chandra observation of the giant radio galaxy NGC 6251. We found evidence that the well-known radio jet is associated with X-ray excess emission and that the central galaxy is embedded within a large diffuse soft X-ray halo. The superior angular resolution of Chandra discloses for the first time the innermost part of the radio jet in X-ray emission. The diffuse X-ray halo appears to provide an additional pressure component which confines the radio jet. Beyond a radial distance of about 110 kpc, the diffuse X-ray emission is undetectable whereas the radio emission widens and traces the huge radio lobe. 2003 Elsevier B.V. All rights reserved. PACS: 98.54.Gr; 98.62.Nx; 95.85.Nv
1. Radio properties of NGC 6251 NGC 6251 belongs to the class of giant radio galaxies (GRGs). It is classified as E2 galaxy. Optical studies indicate a low Seyfert activity if any at all. NGC 6251 is famous because of its wellcollimated radio jet, which is one of the brightest observed. The jet reaches a radial distance of about 140 kpc (H0 5 75 km s 21 Mpc 21 ) from the central engine. Beyond this radial distance the radio emission is distributed across a huge radio lobe of more than 500 kpc in extent. The redshift of 0.0234
*Corresponding author. Tel.: 139-051-639-9373; fax: 139-051639-9431. E-mail addresses: [email protected] (J. Kerp), [email protected] (K.-H. Mack).
localizes NGC 6251 at a distance of 91 Mpc. Fig. 1 (left) shows the Westerbork 92-cm observation superposed onto the Chandra map of NGC 6251 and its environment.
2. Chandra’s view of NGC 6251 Chandra observed NGC 6251 with the ACIS-I array in focus for 40 ks. The data confirm the existence of X-ray emission from the radio jet of NGC 6251 as well as from the halo (Mack et al., 1997a). Fig. 1 (right) shows the X-ray intensity weighted energy map of NGC 6251 with an angular resolution of about 40. Both knots of the radio jet are clearly detected as excess X-ray emitting regions. The first knot is doubtlessly extended with its major axis along the jet direction. Both knots are enclosed by the radio continuum emission.
1387-6473 / 03 / $ – see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016 / S1387-6473(03)00069-1
448
J. Kerp, K.-H. Mack / New Astronomy Reviews 47 (2003) 447–449
Fig. 1. Left: Chandra data superposed by the 92-cm data (Mack et al., 1997b). Right: Chandra map superposed by the 21-cm VLA data. Knot 1 is well encircled by the 21-cm radio continuum data.
2.1. X-ray and radio data
2.2. Chandra and ROSAT
Superposed as contours on Fig. 1 (right) is the radio continuum emission at 21-cm (NRAO archive data of Perley et al., 1984). As Fig. 1 (right) shows, the galaxy core and knot 1 are associated with enhanced X-ray emission; knot 2 is also detected. The X-ray emission of knot 1 reaches its intensity maximum exactly centred within the peak radio continuum contours. This is a clear indication that the radio structure and the X-ray emission originate in the same volume of space. Low radio frequencies trace the extended radio emission of NGC 6251 far away from the core (Fig. 1, left). We superposed Westerbork 92-cm data as contours on top of the Chandra data. The radio jet is well confined as long as the jet is embedded within the diffuse extended X-ray plasma. At larger radial distances, the radio emission fills a much larger volume. This indicates that the X-ray plasma provides an additional pressure component which confines the radio emission up to a radial distance of 70 kpc.
Fig. 2 (left) shows the smoothed and exposuremap corrected Chandra ACIS-I data (0.3 keV,E , 8.0 keV). Superposed as contours are the ROSAT PSPC data (0.5 keV,E ,2.1 keV, Mack et al., 1997a). Chandra confirms not only the existence of the excess X-ray emitting regions associated with radio intensity maxima but also the extended diffuse soft X-ray halo emission. The scale length (radially averaged) of the soft X-ray plasma is about 30 kpc. Most of the detected X-ray plasma emission is produced at E ,2 keV which yields the excellent spatial correlation of Chandra and ROSAT data.
2.3. The nuclear region Focussing towards the very centre of NGC 6251, X-ray emission from the innermost part of the radio jet is detected. Fig. 2 (right) presents the Chandra data with an angular resolution of 10. Mack et al. (1997a) estimated the life-time of X-ray emitting relativistic electrons to about 3.5 3 10 4 years. With a
J. Kerp, K.-H. Mack / New Astronomy Reviews 47 (2003) 447–449
449
Fig. 2. Left: The greyscale map shows the Chandra diffuse soft X-ray emission of the halo of NGC 6251, while the contour lines mark the ROSAT PSPC data analysed by Mack et al. (1997b). Right: The central region of NGC 6251 at an angular resolution level of 10. An X-ray jet emerges from the AGN. The extent of this jet is about 3 kpc, consistent with the expected life-time of relatistic electrons emitting soft X-ray photons.
velocity of 0.3 c the maximum size of the expected X-ray synchrotron jet is 3 kpc. The extent of the innermost jet is about this linear size, suggesting that the X-ray emission is produced by synchrotron radiation.
3. Results Chandra discloses the existence of X-ray emission associated with the radio jet and the halo of the giant radio galaxy NGC 6251. The X-ray emission of the jet emerges directly from the central engine. Beyond a radial distance of . 3 kpc the X-ray emission becomes undetectable. The life-time of relativistic particles is long enough to produce X-ray synchrotron radiation in the innermost region of the radio jet. The X-ray emission re-appears at a radial distance of about 75 kpc from the central engine. The emission
is extended, oriented parallel to the jet, and narrowly confined within the extent of the radio jet. The second X-ray knot is located at a radial distance of about 110 kpc. The Chandra data also reveal the existence of diffuse X-ray emission around the active galactic nucleus. The radial extent is about 70 kpc. Most of the X-ray photons of this diffuse gas are emitted at E,2 keV. The thermal pressure of the plasma seems to confine the jet because outside the X-ray halo the radio jet expands into the northeastern radio lobe of NGC 6251.
References Mack, K.-H., Kerp, J., Klein, U., 1997a. A&A 324, 870. Mack, K.-H., Klein, U., O’Dea, C.P., Willis, A.G., 1997b. A&AS 123, 423. Perley, R.A., Bridle, A.H., Willis, A.G., 1984. ApJS 54, 291.