- Email: [email protected]
HI and OH absorption toward NGC 6240
New Astronomy Reviews 51 (2007) 58–62 www.elsevier.com/locate/newastrev
HI and OH absorption toward NGC 6240 Yoshiaki Hagiwara b
a,*
, Willem A. Baan b, Peter Hofner
c
a National Astronomical Observatory of Japan, 2-21, Mitaka, Tokyo, Japan ASTRON, Westerbork Observatory, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands c Physics Department, New Mexico Tech, Socorro, NM 87801, Mexico
Abstract We present results of HI and OH absorption imaging of the merging galaxy of NGC 6240 using the very large array at 1 arcsec resolution. HI absorption is found across the extended radio continuum structure with a significant concentration towards the two nuclei, while the OH absorption is confined mostly between the nuclei. The OH velocity gradients around the nuclei confirm earlier results of radio molecular emission lines in defining the central gas peak between the nuclei and the kinematics of the nuclear region. The HI velocity gradients might trace remnants of the two interacting galaxies and the characteristics of a symmetric superwind outflow. The absorbing gas provides a key to solve kinematics of two merging galaxies. 2006 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . Observations . Results . . . . . Discussion. . . References . .
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1. Introduction NGC 6240 is a prototypical Ultra-Luminous Infrared Galaxy housing a very complex merger structure. NGC 6240 (D = 97 Mpc with H0 = 75 km s1 Mpc1) has an IR luminosity of L8–1000 lm = 6 · 1011 Lx (Sanders et al., 1988). X-ray observations with the Chandra revealed that the galaxy is resolved into the double nuclei system that overlays the known double-peaked radio source, thus pro-
*
Corresponding author. E-mail address: [email protected] (Y. Hagiwara).
1387-6473/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2006.10.005
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58 59 60 61 62
viding an evidence of the binary Active Galactic Nuclei (AGN) (Komossa et al., 2003). The AGN-activity is more dominant in the southern nucleus where the strongest 6.4 keV Fe-line and X-ray obscuration are detected (Komossa et al., 2003). Also, these nuclei have been resolved at near-infrared wavelengths using the Keck II and the HST, showing an elongation with some considerable structures for each nucleus (Max et al., 2005). The CO(2–1) emission studies at 1 arcsec resolution show that the molecular emission follows the H2 emission and peaks between the two nuclei. Most of the CO flux is concentrated in a thick and turbulent disk-like structure between the double nuclei (Tacconi et al., 1999). The galaxy exhibits
Y. Hagiwara et al. / New Astronomy Reviews 51 (2007) 58–62
HI and OH absorption against the nuclear continuum (Baan et al., 1985). High-resolution synthesis map of HI absorption at 0.2 arcsec resolution distinguished separate nuclear structures (Beswick et al., 2001), but these nuclei are not connected since most of the absorbing structures between them are resolved out. In this report, we present synthesis radio imaging of HI/OH absorption at 1 arcsec angular resolution towards NGC 6240.
59
2. Observations Observations of neutral hydrogen (HI) absorption and hydroxyl (OH) absorption at 1667/1665 MHz toward NGC 6240 were conducted on 1 September 1995 with NRAO very large array (VLA) in A-Configuration. The HI line was observed using the two IF mode, with each IF having a bandwidth of 6.25 MHz subdivided into 32
Fig. 1. (upper): HI 1st moment map. Contours are plotted at 7200, 7250, 7300, 7350, 7400, 7450, 7500, 7550, and 7600 km s1. (lower): HI 2nd moment map, showing HI velocity dispersion. Contours are plotted at 10, 17, 30, 51, 90, 156, 270, and 466 km s1. Crosses mark the locations of the twin-nuclei.
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Y. Hagiwara et al. / New Astronomy Reviews 51 (2007) 58–62
150 km s1 and peaking at 200 km s1 running along a ridge from the southern part below N1 to the western extension. Fig. 2 shows the HI absorption spectra at each component and the HI column density map. From these maps, an east-to-west velocity gradient in the northern region is seen, which is clearly distinguished from the south-to-north gradient. The gradient continues into the north-west region but is interrupted by a perpendicular velocity gradient from V = 7000 km s1 to 7800 km s1 in the ‘Dust lane’ region. This gradient traces a foreground dust lane passing west of the nuclei in optical images. Southwest of N1, there is a velocity gradient at a lower velocity range entering from the west. The systemic velocities at the nuclei of N1 and N2 are V(N1) = 7235 km s1 and V(N2) = 7305 km s1, which are not consistent with those at 0.2 arcsec resolution with MERLIN (Beswick et al., 2001). The distribution of the HI absorption column density presented in Fig. 2 is determined using the absorption line strengths and the associated continuum data at 1420 MHz. The hydrogen column density is estimated by the standard R expression: NH/TS = 1.823 · 1018 sðV Þ dV cm2, where
channels of width 195.3 kHz. This setup yields a velocity coverage of 1298 km s1 and a velocity resolution of 43.3 km s1. The OH transitions were covered using a two partially overlapping IFs with 6.25 MHz width. The total OH absorption velocity coverage is 1326 km s1 and a velocity resolution of 36.9 km s1. Before processing absorption maps, continuum subtraction was performed by using line-free channels. Due to the uncertainty of the baseline at the edges of the spectrum, we estimate flux uncertainties of 10–20%. The resultant synthesized beam sizes are 1.95’’ · 1.79’’ at 21-cm and 1.11’’ · 1.08’’ at 18-cm. RMS noise levels per channel are 0.49 mJy/beam in HI maps and 0.41 mJy/beam in OH maps. 3. Results The first moment HI map in Fig. 1 (upper) displays the dominant south-to-north velocity gradient between the double nuclei of N1 and N2, in which the gradient value is 0.2 km s1 pc1. Fig. 1 (lower) displays the second moment HI map with large velocity widths of
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Fig. 2. HI absorption spectra at seven components in NGC 6240 and HI column density map obtained with the VLA. Crosses in the map mark the locations of the twin-nuclei.
Y. Hagiwara et al. / New Astronomy Reviews 51 (2007) 58–62
TS is the hydrogen spin temperature, and s is absorption optical depth. The column density map (Fig. 2) shows the highest column density of NH = 1.28 · 1022 cm2 at nucleus N1, assuming TS = 100 K. The optical depth varies from 0.15 at N1 to 0.11 at N2 and values of about 0.05 in the other components. The OH columnR density is estimated using NOH = 2.35 · 1014 T ex sðV Þ dV , where Tex is the excitation temperature with a typical value of 20 K. The first moment map of 1667 MHz OH (Fig. 3 (left)) shows a velocity gradient that resembles that of HI in the central region. The velocity gradient starts in the south-east region close to N1 as part of the southern galaxy and then moves from N1 to N2 and into north-east direction. The gradient connects the kinematics between the nuclei, which can not be observed in the earlier HI MERLIN map. There is some other evidence of kinematics. In Fig. 3 (right), the line width in the 1667 MHz line is largest in between the two nuclei but with a value of about 75 km s1, which is significantly smaller than the width found in HI. These OH velocity fields and the dispersion nearly confirm the earlier CO work at 2 arcsec resolution (Tacconi et al., 1999). The highest OH column density lies in between N1 and N2 with NOH67 = 2.2 · 1016 cm2. The values at N1 and N2 are a factor of about three lower.
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4. Discussion The central gas structure between the nuclei has been deposited during the interaction/merger process. The HI absorption column density peaks close to the southern nucleus N1, while the OH column density peaks about 0.9’’ north of N1 (closer to N2) and not on the N1–N2 connecting line. The OH absorption is confined to the nuclear region itself and shows rough agreement with the distribution of thermal molecular emissions. The HI absorbing gas is very extended as it samples a larger volume than the OH and picks up more structural radio continuum components. The velocity gradients in the OH and HI absorption lines of 0.31 km s1 pc1 and of the CO (J=2–1) emission line of 0.74 km s1 pc1 are different, suggesting a different structural scale size. If the velocity gradient in the central molecular structure is tied to the orbital motion of the two nuclei, the CO gradient would be closer to the HI and OH absorption values. The uniformly large line widths found in the HI and OH data confirm a large-scale contribution to at least the HI absorption in the central absorption. Given the values of 200 km s1 for HI and 76 km s1 for OH, the HI values are consistent with the stellar velocity dispersion peaking at 270 km s1 close to the H2 and CO J = 2–1 emission peaks (Tecza et al., 2000). Line widths in HI and OH are
Fig. 3. (left): First moment map of the OH 1667 MHz line. Crosses indicate the twin-nuclei. The contours are spaced by 20 km s1 between 7160 km s1 and 7320 km s1. (right): Second moment map of the OH 1667 MHz. Contours are at 50, 55, 60, 65, 70, and 75 km s1.
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quite large and may trace multiple structures in front of the two nuclei and particularly near N1 (Beswick et al., 2001). The HI and OH column densities suggest that the obscured southern nucleus N1 lies behind the less obscured northern nucleus N2. The total HI column density at the southern nucleus N1 of 1.28 · 1022 cm2 agrees with the column density estimated from the X-ray nuclear emission (Komossa et al., 2003). Also the lower HI column at N2 of 1.01 · 1022 cm2 agrees with the X-ray estimate. These nuclei have AGN characteristics in the radio and X-rays (Hagiwara et al., 2003; Komossa et al., 2003). However, the extended radio continuum seen in the nuclear region might be associated with intense star-formation resulting
from the close encounter/merger of the nuclei. The redshifted velocities of HI absorption towards the western arc structures trace the merging remnants. References Baan et al., 1985. ApJ 293, 294. Beswick et al., 2001. MNRAS 325, 151. Hagiwara et al., 2003. A & A 400, 457. Komossa et al., 2003. ApJ 582, L15. Max et al., 2005. ApJ 621, 738. Sanders et al., 1988. ApJ 328, 35. Tacconi et al., 1999. ApJ 524, 732. Tecza et al., 2000. ApJ 537, 178.
HI and OH absorption toward NGC 6240 Yoshiaki Hagiwara b
a,*
, Willem A. Baan b, Peter Hofner
c
a National Astronomical Observatory of Japan, 2-21, Mitaka, Tokyo, Japan ASTRON, Westerbork Observatory, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands c Physics Department, New Mexico Tech, Socorro, NM 87801, Mexico
Abstract We present results of HI and OH absorption imaging of the merging galaxy of NGC 6240 using the very large array at 1 arcsec resolution. HI absorption is found across the extended radio continuum structure with a significant concentration towards the two nuclei, while the OH absorption is confined mostly between the nuclei. The OH velocity gradients around the nuclei confirm earlier results of radio molecular emission lines in defining the central gas peak between the nuclei and the kinematics of the nuclear region. The HI velocity gradients might trace remnants of the two interacting galaxies and the characteristics of a symmetric superwind outflow. The absorbing gas provides a key to solve kinematics of two merging galaxies. 2006 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . Observations . Results . . . . . Discussion. . . References . .
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1. Introduction NGC 6240 is a prototypical Ultra-Luminous Infrared Galaxy housing a very complex merger structure. NGC 6240 (D = 97 Mpc with H0 = 75 km s1 Mpc1) has an IR luminosity of L8–1000 lm = 6 · 1011 Lx (Sanders et al., 1988). X-ray observations with the Chandra revealed that the galaxy is resolved into the double nuclei system that overlays the known double-peaked radio source, thus pro-
*
Corresponding author. E-mail address: [email protected] (Y. Hagiwara).
1387-6473/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2006.10.005
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58 59 60 61 62
viding an evidence of the binary Active Galactic Nuclei (AGN) (Komossa et al., 2003). The AGN-activity is more dominant in the southern nucleus where the strongest 6.4 keV Fe-line and X-ray obscuration are detected (Komossa et al., 2003). Also, these nuclei have been resolved at near-infrared wavelengths using the Keck II and the HST, showing an elongation with some considerable structures for each nucleus (Max et al., 2005). The CO(2–1) emission studies at 1 arcsec resolution show that the molecular emission follows the H2 emission and peaks between the two nuclei. Most of the CO flux is concentrated in a thick and turbulent disk-like structure between the double nuclei (Tacconi et al., 1999). The galaxy exhibits
Y. Hagiwara et al. / New Astronomy Reviews 51 (2007) 58–62
HI and OH absorption against the nuclear continuum (Baan et al., 1985). High-resolution synthesis map of HI absorption at 0.2 arcsec resolution distinguished separate nuclear structures (Beswick et al., 2001), but these nuclei are not connected since most of the absorbing structures between them are resolved out. In this report, we present synthesis radio imaging of HI/OH absorption at 1 arcsec angular resolution towards NGC 6240.
59
2. Observations Observations of neutral hydrogen (HI) absorption and hydroxyl (OH) absorption at 1667/1665 MHz toward NGC 6240 were conducted on 1 September 1995 with NRAO very large array (VLA) in A-Configuration. The HI line was observed using the two IF mode, with each IF having a bandwidth of 6.25 MHz subdivided into 32
Fig. 1. (upper): HI 1st moment map. Contours are plotted at 7200, 7250, 7300, 7350, 7400, 7450, 7500, 7550, and 7600 km s1. (lower): HI 2nd moment map, showing HI velocity dispersion. Contours are plotted at 10, 17, 30, 51, 90, 156, 270, and 466 km s1. Crosses mark the locations of the twin-nuclei.
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Y. Hagiwara et al. / New Astronomy Reviews 51 (2007) 58–62
150 km s1 and peaking at 200 km s1 running along a ridge from the southern part below N1 to the western extension. Fig. 2 shows the HI absorption spectra at each component and the HI column density map. From these maps, an east-to-west velocity gradient in the northern region is seen, which is clearly distinguished from the south-to-north gradient. The gradient continues into the north-west region but is interrupted by a perpendicular velocity gradient from V = 7000 km s1 to 7800 km s1 in the ‘Dust lane’ region. This gradient traces a foreground dust lane passing west of the nuclei in optical images. Southwest of N1, there is a velocity gradient at a lower velocity range entering from the west. The systemic velocities at the nuclei of N1 and N2 are V(N1) = 7235 km s1 and V(N2) = 7305 km s1, which are not consistent with those at 0.2 arcsec resolution with MERLIN (Beswick et al., 2001). The distribution of the HI absorption column density presented in Fig. 2 is determined using the absorption line strengths and the associated continuum data at 1420 MHz. The hydrogen column density is estimated by the standard R expression: NH/TS = 1.823 · 1018 sðV Þ dV cm2, where
channels of width 195.3 kHz. This setup yields a velocity coverage of 1298 km s1 and a velocity resolution of 43.3 km s1. The OH transitions were covered using a two partially overlapping IFs with 6.25 MHz width. The total OH absorption velocity coverage is 1326 km s1 and a velocity resolution of 36.9 km s1. Before processing absorption maps, continuum subtraction was performed by using line-free channels. Due to the uncertainty of the baseline at the edges of the spectrum, we estimate flux uncertainties of 10–20%. The resultant synthesized beam sizes are 1.95’’ · 1.79’’ at 21-cm and 1.11’’ · 1.08’’ at 18-cm. RMS noise levels per channel are 0.49 mJy/beam in HI maps and 0.41 mJy/beam in OH maps. 3. Results The first moment HI map in Fig. 1 (upper) displays the dominant south-to-north velocity gradient between the double nuclei of N1 and N2, in which the gradient value is 0.2 km s1 pc1. Fig. 1 (lower) displays the second moment HI map with large velocity widths of
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Fig. 2. HI absorption spectra at seven components in NGC 6240 and HI column density map obtained with the VLA. Crosses in the map mark the locations of the twin-nuclei.
Y. Hagiwara et al. / New Astronomy Reviews 51 (2007) 58–62
TS is the hydrogen spin temperature, and s is absorption optical depth. The column density map (Fig. 2) shows the highest column density of NH = 1.28 · 1022 cm2 at nucleus N1, assuming TS = 100 K. The optical depth varies from 0.15 at N1 to 0.11 at N2 and values of about 0.05 in the other components. The OH columnR density is estimated using NOH = 2.35 · 1014 T ex sðV Þ dV , where Tex is the excitation temperature with a typical value of 20 K. The first moment map of 1667 MHz OH (Fig. 3 (left)) shows a velocity gradient that resembles that of HI in the central region. The velocity gradient starts in the south-east region close to N1 as part of the southern galaxy and then moves from N1 to N2 and into north-east direction. The gradient connects the kinematics between the nuclei, which can not be observed in the earlier HI MERLIN map. There is some other evidence of kinematics. In Fig. 3 (right), the line width in the 1667 MHz line is largest in between the two nuclei but with a value of about 75 km s1, which is significantly smaller than the width found in HI. These OH velocity fields and the dispersion nearly confirm the earlier CO work at 2 arcsec resolution (Tacconi et al., 1999). The highest OH column density lies in between N1 and N2 with NOH67 = 2.2 · 1016 cm2. The values at N1 and N2 are a factor of about three lower.
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4. Discussion The central gas structure between the nuclei has been deposited during the interaction/merger process. The HI absorption column density peaks close to the southern nucleus N1, while the OH column density peaks about 0.9’’ north of N1 (closer to N2) and not on the N1–N2 connecting line. The OH absorption is confined to the nuclear region itself and shows rough agreement with the distribution of thermal molecular emissions. The HI absorbing gas is very extended as it samples a larger volume than the OH and picks up more structural radio continuum components. The velocity gradients in the OH and HI absorption lines of 0.31 km s1 pc1 and of the CO (J=2–1) emission line of 0.74 km s1 pc1 are different, suggesting a different structural scale size. If the velocity gradient in the central molecular structure is tied to the orbital motion of the two nuclei, the CO gradient would be closer to the HI and OH absorption values. The uniformly large line widths found in the HI and OH data confirm a large-scale contribution to at least the HI absorption in the central absorption. Given the values of 200 km s1 for HI and 76 km s1 for OH, the HI values are consistent with the stellar velocity dispersion peaking at 270 km s1 close to the H2 and CO J = 2–1 emission peaks (Tecza et al., 2000). Line widths in HI and OH are
Fig. 3. (left): First moment map of the OH 1667 MHz line. Crosses indicate the twin-nuclei. The contours are spaced by 20 km s1 between 7160 km s1 and 7320 km s1. (right): Second moment map of the OH 1667 MHz. Contours are at 50, 55, 60, 65, 70, and 75 km s1.
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quite large and may trace multiple structures in front of the two nuclei and particularly near N1 (Beswick et al., 2001). The HI and OH column densities suggest that the obscured southern nucleus N1 lies behind the less obscured northern nucleus N2. The total HI column density at the southern nucleus N1 of 1.28 · 1022 cm2 agrees with the column density estimated from the X-ray nuclear emission (Komossa et al., 2003). Also the lower HI column at N2 of 1.01 · 1022 cm2 agrees with the X-ray estimate. These nuclei have AGN characteristics in the radio and X-rays (Hagiwara et al., 2003; Komossa et al., 2003). However, the extended radio continuum seen in the nuclear region might be associated with intense star-formation resulting
from the close encounter/merger of the nuclei. The redshifted velocities of HI absorption towards the western arc structures trace the merging remnants. References Baan et al., 1985. ApJ 293, 294. Beswick et al., 2001. MNRAS 325, 151. Hagiwara et al., 2003. A & A 400, 457. Komossa et al., 2003. ApJ 582, L15. Max et al., 2005. ApJ 621, 738. Sanders et al., 1988. ApJ 328, 35. Tacconi et al., 1999. ApJ 524, 732. Tecza et al., 2000. ApJ 537, 178.