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VLBI observations of high-opacity HI gas in NGC 5793
New Astronomy Reviews 43 (1999) 647–650 www.elsevier.nl / locate / newar
VLBI observations of high-opacity HI gas in NGC 5793 ¨ a , J.E. Conway a , R.S. Booth a , P.J. Diamond b , B. Koribalski c Y.M. Pihlstrom a
Onsala Space Observatory, S-439 92 Onsala, Sweden b NRAO, P.O. Box 0, Socorro, NM 87801, USA c ATNF, PO Box 76, Epping NSW 2121, Australia
Abstract We report on observations, with sub-parsec resolution, of neutral hydrogen seen in absorption in the l 521 cm line against the nucleus of the active spiral galaxy NGC 5793. The absorption line consists of three components separated in both location as well as velocity. We derive HI column densities of | 2 3 10 22 cm 22 assuming a gas spin temperature of 100 K. For the first time we are able to reliably estimate the HI cloud sizes ( ¯ 15 pc) and atomic gas densities ( ¯ 200 cm 23 ). Our results suggest that the HI gas is not associated with the , 10 pc region which presumably contains the H 2 O masers, but it is more distant from the nucleus, and is probably associated with the r | 1 kpc gas seen in CO. 1999 Elsevier Science B.V. All rights reserved. PACS: 95.85.Bh; 98.54.Cm Keywords: Galaxies: active; Galaxies: individual (NGC 5793); Radio lines: galaxies
1. Introduction NGC 5793 is a bright spiral galaxy at 70 Mpc (H0 5 50 km s 21 Mpc 21 ) seen almost edge-on with an optical inclination of 738. It has an optical magnitude of 14, and is classified as an Sb galaxy. From its excess of FIR radiation (LFIR $ 10 10.8 L( ) this object has been classified as a starburst galaxy (Soifer et al., 1987). Recent results (Hagiwara et al., 1997) reveal luminous water vapour megamasers at velocities 6250 km s 21 around the systemic velocity. In addition single dish observations show very strong HI absorption (t . 1) as well as OH absorption towards the very compact ( , 30 pc) central radio continuum source (Gardner & Whiteoak, 1986). This source is also known to harbour CO (J 5 1–0) in emission that extends up to 1.2 kpc from the centre (Hagiwara et al., 1997). Earlier EVN VLBI observations have been made of the HI absorbing gas (Gardner et al., 1992). Here we report
on new higher spatial and spectral resolution VLBA observations of the HI absorption features.
2. Observations The new 21 cm HI absorption observations used the VLBA centered at the redshifted frequency of 1404 MHz. A bandwidth of 4 MHz (correlated with 512 channels) corresponding to 850 km s 21 covered the range of known absorption at the optical redshift of the galaxy, cz 5 3491666 km s 21 (heliocentric frame and the optical velocity definition). The onsource integration time was 6.4 h and every hour one 20-minute scan of a calibrator was included. Data reduction was performed by using standard tasks in AIPS. After initial calibration, including bandpass corrections, a set of line-free continuum channels were selected on either side of the very deep absorption line. Those line-free channels were aver-
1387-6473 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S1387-6473( 99 )00071-8
648
¨ et al. / New Astronomy Reviews 43 (1999) 647 – 650 Y.M. Pihlstrom
Fig. 1. 1.4 GHz continuum map from VLBA observations.
aged and used to create a continuum map in DIFMAP, see Fig. 1. The spectral data were then amplitude and phase calibrated against this continuum map, the continuum subtracted using UVLIN, and finally the data were mapped with a channel separation of 3.3 km s 21 . For the uniformly weighted spectral cube the rms sensitivity in each channel was 2 mJy beam 21 , and in the continuum map the rms was 0.5 mJy beam 21 .
3. Results As seen in Fig. 1, the continuum structure is
somewhat unusual. The location of the nucleus is unclear; it may be associated with the continuum peak, lie between the two main continuum components, or be associated with the linear east-west structure within the western component. More detailed investigation of the spectral index for the different components is needed to find the core location. The image cube was searched in order to find the spatial and velocity locations of the HI absorption. We confirm the results in Gardner et al. (1992) of an absorption profile consisting of three components separated in velocity and space. In Fig. 2 we plot the line-to-continuum ratio (1 2 e 2t ) for three velocity intervals, which correspond approximately to these three components. Fig. 3 plots the integrated absorption spectra over the whole source, while Fig. 4 plots the integrated optical depths from selected regions of the continuum source. Fig. 2 shows that the lowest velocity component, A, appears to be centered between the two continuum structures, while a larger and even more opaque cloud corresponding to the intermediate velocity range, B, covers the whole continuum. The highest velocity component C seems to be located over the western part, although we can see from the spectra in Fig. 4 that it also has a more extended, low-opacity component. To parameterise the opacities and velocity widths of the three spectral components A, B, C and their spatial variation, we fitted three Gaussians to each of the spectra shown in Fig. 4. Each fitted Gaussian had similar FWHM linewidths in each of the component spectra, these values are listed in Table 1. Also listed are the peak opacities found for each component and the derived HI column densities assuming a spin
Fig. 2. Line to continuum ratio for the three velocity ranges A, B and C indicated in Fig. 3. Plotted greyscale ranges are for (a) 0.4–0.8, (b) 0.5–1.0, and (c) 0.4–1.0 (see text for full explanation). These plots indicate the spatial distribution of the gas. The low velocity gas appears to be confined to an area of , 15 pc in diameter. The tick marks are spaced every 20 mas.
¨ et al. / New Astronomy Reviews 43 (1999) 647 – 650 Y.M. Pihlstrom
649
Fig. 3. Total neutral atomic hydrogen absorption averaged over the whole continuum. The velocity intervals shown in Fig. 2 are indicated.
temperature of 100 K and a covering factor of unity. Assuming that the observed diameter of cloud A, 15 pc (see Fig. 2a), is typical, then the inferred HI densities are about 200(T / 100 K) cm 23 and the mass of HI . 10 4 (T / 100 K) M( per cloud.
4. Location of the absorbing gas An important question to address is the location of the HI absorbing clouds we detect. Are they, for instance, relatively close to the nucleus and due to
Fig. 4. Spectra of optical depths of three different locations. The dotted lines show the fits of three Gaussian spectral components to each of the observed spectra. These spectra show how the velocity ranges A, B, and C vary spatially in optical depth. The lower row plots optical depth versus velocity [km / s].
¨ et al. / New Astronomy Reviews 43 (1999) 647 – 650 Y.M. Pihlstrom
650 Table 1 Absorption feature parameters Comp
Velocity (km s 21 )
Peak opacity
FWHM (km s 21 )
NHI (T s / 100 K)21 (cm 22 )
A B C
3415-3486 3489-3549 3550-3584
3.0 3.2 2.7
20 34 34
1.1 3 10 22 2.0 3 10 22 1.7 3 10 22
atomic gas associated with the proposed H 2 O megamaser rotating disk (Hagiwara et al., 1997; Neufeld & Maloney, 1995)? The water masers in NGC 4258 are located within 1 pc of the nucleus (Miyoshi et al., 1995), and the H 2 O masers in NGC 5793 have similar satellite lines widely separated in velocity, which presumably occur on similar scales (say , 10 pc). Alternatively, the HI might be associated with the few-kpc scale CO emitting gas or simply be normal spiral galaxy disk ISM clouds that lie coincidently along the line of sight. We can rule out that the absorbing gas is very close ( , 10 pc) to the nucleus, since there is no simple relationship with the continuum at the pc scales, and we know that the clouds are $ 15 pc in size. Additionally, the observed line widths are much narrower than for the HI associated with TORUS gas in ellipticals, which have FWHM $ 100 km s 21 even for gas on scales up to r ¯ 100 pc (Conway, 1998). Koribalski (1996) also finds similar | 100 km s 21 linewidths for HI absorbing gas in spirals; this gas is argued to be circumnuclear and lie at 100 , r , 1000 pc. The alternative that the gas belongs to normal HI ISM clouds, similar to those in spiral galaxy disks, that lie coincidently along the line of sight, is possible but not very likely. Even though the densities and cloud sizes are reasonable, the velocity widths do not agree. Compared to the Galaxy, where the cloud linewidths are typically 5 km s 21 , the clouds in NGC 5793 must be significantly broadened due to turbulent or bulk motions within the gas. Our best guess is that the HI is associated with the
CO gas seen at r | 1 kpc. If this is the case, then our observations of NGC 5793 are very similar to the MERLIN HI absorption measurements against Mrk6 (Gallimore et al., 1998), where the gas was detected only against one of the continuum components at a projected distance of 380 pc (here H0 5 75 km s 21 Mpc 21 ). A maximum optical depth of 0.45 together with a linewidth of 30 km s 21 resulted in NHI 5 2.3 3 10 21 cm 22 . It was argued that the gas is in a kpc-scale ring or spiral arms associated with an observed dust lane. Another example is the FR II galaxy 3C293 that shows HI consistent with clouds in a 1.1 kpc radius CO disk rotating around the centre (Haschick & Baan, 1985; Evans et al., 1998). In the case of NGC 5793 this | 1 kpc gas could be part of an extended continuous TORUS structure (Conway, 1999) which feeds the central engine. Alternatively it might be part of a distinct dynamically decoupled ring or disk of starburst activity.
References Conway, J.E., 1998, in: Carilli, C., Menten, K., & Radford, S., (Eds.), Highly Redshifted Radio Lines, PASP, in press. Conway, J.E., 1999, These Proceedings, NewAR, 43, 509. Evans, A.S., Sanders, D.B., Surace, J.A., & Mazzarella, J.M., 1998, ApJ, submitted. Gallimore, J.F., Holloway, A.J., Pedlar, A., & Mundell, C.G., 1998, A&A, 333, 13. Gardner, F.F. & Whiteoak, J.B., 1986, MNRAS, 221, 537. Gardner, F.F., Whiteoak, J.B., Norris, R.P., & Diamond, P.J., 1992, MNRAS, 258, 296. Hagiwara, Y., Kohno, K., Kawabe, R., & Nakai, N., 1997, PASJ, 49, 171. Haschick, A.D. & Baan, W.A., 1985, ApJ, 289, 574. Koribalski, B., 1996, in: Skillman, E.D., (Ed.), The Minnesota Lectures on Extragalactic Neutral Hydrogen, ASPC, Vol 106, ASP, San Francisco, p. 238. Miyoshi, M., Moran, J., Herrnstein, J., Greenhill, L., Nakai, N., Diamond, P., & Inoue, M., 1995, Natur, 373, 127. Neufeld, D.A. & Maloney, P.R., 1995, ApJ, 447, L17. Soifer, B.T., Boehmer, L., Neugebauer, G., & Sanders, D.B., 1987, ApJ, 320, 238.
VLBI observations of high-opacity HI gas in NGC 5793 ¨ a , J.E. Conway a , R.S. Booth a , P.J. Diamond b , B. Koribalski c Y.M. Pihlstrom a
Onsala Space Observatory, S-439 92 Onsala, Sweden b NRAO, P.O. Box 0, Socorro, NM 87801, USA c ATNF, PO Box 76, Epping NSW 2121, Australia
Abstract We report on observations, with sub-parsec resolution, of neutral hydrogen seen in absorption in the l 521 cm line against the nucleus of the active spiral galaxy NGC 5793. The absorption line consists of three components separated in both location as well as velocity. We derive HI column densities of | 2 3 10 22 cm 22 assuming a gas spin temperature of 100 K. For the first time we are able to reliably estimate the HI cloud sizes ( ¯ 15 pc) and atomic gas densities ( ¯ 200 cm 23 ). Our results suggest that the HI gas is not associated with the , 10 pc region which presumably contains the H 2 O masers, but it is more distant from the nucleus, and is probably associated with the r | 1 kpc gas seen in CO. 1999 Elsevier Science B.V. All rights reserved. PACS: 95.85.Bh; 98.54.Cm Keywords: Galaxies: active; Galaxies: individual (NGC 5793); Radio lines: galaxies
1. Introduction NGC 5793 is a bright spiral galaxy at 70 Mpc (H0 5 50 km s 21 Mpc 21 ) seen almost edge-on with an optical inclination of 738. It has an optical magnitude of 14, and is classified as an Sb galaxy. From its excess of FIR radiation (LFIR $ 10 10.8 L( ) this object has been classified as a starburst galaxy (Soifer et al., 1987). Recent results (Hagiwara et al., 1997) reveal luminous water vapour megamasers at velocities 6250 km s 21 around the systemic velocity. In addition single dish observations show very strong HI absorption (t . 1) as well as OH absorption towards the very compact ( , 30 pc) central radio continuum source (Gardner & Whiteoak, 1986). This source is also known to harbour CO (J 5 1–0) in emission that extends up to 1.2 kpc from the centre (Hagiwara et al., 1997). Earlier EVN VLBI observations have been made of the HI absorbing gas (Gardner et al., 1992). Here we report
on new higher spatial and spectral resolution VLBA observations of the HI absorption features.
2. Observations The new 21 cm HI absorption observations used the VLBA centered at the redshifted frequency of 1404 MHz. A bandwidth of 4 MHz (correlated with 512 channels) corresponding to 850 km s 21 covered the range of known absorption at the optical redshift of the galaxy, cz 5 3491666 km s 21 (heliocentric frame and the optical velocity definition). The onsource integration time was 6.4 h and every hour one 20-minute scan of a calibrator was included. Data reduction was performed by using standard tasks in AIPS. After initial calibration, including bandpass corrections, a set of line-free continuum channels were selected on either side of the very deep absorption line. Those line-free channels were aver-
1387-6473 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S1387-6473( 99 )00071-8
648
¨ et al. / New Astronomy Reviews 43 (1999) 647 – 650 Y.M. Pihlstrom
Fig. 1. 1.4 GHz continuum map from VLBA observations.
aged and used to create a continuum map in DIFMAP, see Fig. 1. The spectral data were then amplitude and phase calibrated against this continuum map, the continuum subtracted using UVLIN, and finally the data were mapped with a channel separation of 3.3 km s 21 . For the uniformly weighted spectral cube the rms sensitivity in each channel was 2 mJy beam 21 , and in the continuum map the rms was 0.5 mJy beam 21 .
3. Results As seen in Fig. 1, the continuum structure is
somewhat unusual. The location of the nucleus is unclear; it may be associated with the continuum peak, lie between the two main continuum components, or be associated with the linear east-west structure within the western component. More detailed investigation of the spectral index for the different components is needed to find the core location. The image cube was searched in order to find the spatial and velocity locations of the HI absorption. We confirm the results in Gardner et al. (1992) of an absorption profile consisting of three components separated in velocity and space. In Fig. 2 we plot the line-to-continuum ratio (1 2 e 2t ) for three velocity intervals, which correspond approximately to these three components. Fig. 3 plots the integrated absorption spectra over the whole source, while Fig. 4 plots the integrated optical depths from selected regions of the continuum source. Fig. 2 shows that the lowest velocity component, A, appears to be centered between the two continuum structures, while a larger and even more opaque cloud corresponding to the intermediate velocity range, B, covers the whole continuum. The highest velocity component C seems to be located over the western part, although we can see from the spectra in Fig. 4 that it also has a more extended, low-opacity component. To parameterise the opacities and velocity widths of the three spectral components A, B, C and their spatial variation, we fitted three Gaussians to each of the spectra shown in Fig. 4. Each fitted Gaussian had similar FWHM linewidths in each of the component spectra, these values are listed in Table 1. Also listed are the peak opacities found for each component and the derived HI column densities assuming a spin
Fig. 2. Line to continuum ratio for the three velocity ranges A, B and C indicated in Fig. 3. Plotted greyscale ranges are for (a) 0.4–0.8, (b) 0.5–1.0, and (c) 0.4–1.0 (see text for full explanation). These plots indicate the spatial distribution of the gas. The low velocity gas appears to be confined to an area of , 15 pc in diameter. The tick marks are spaced every 20 mas.
¨ et al. / New Astronomy Reviews 43 (1999) 647 – 650 Y.M. Pihlstrom
649
Fig. 3. Total neutral atomic hydrogen absorption averaged over the whole continuum. The velocity intervals shown in Fig. 2 are indicated.
temperature of 100 K and a covering factor of unity. Assuming that the observed diameter of cloud A, 15 pc (see Fig. 2a), is typical, then the inferred HI densities are about 200(T / 100 K) cm 23 and the mass of HI . 10 4 (T / 100 K) M( per cloud.
4. Location of the absorbing gas An important question to address is the location of the HI absorbing clouds we detect. Are they, for instance, relatively close to the nucleus and due to
Fig. 4. Spectra of optical depths of three different locations. The dotted lines show the fits of three Gaussian spectral components to each of the observed spectra. These spectra show how the velocity ranges A, B, and C vary spatially in optical depth. The lower row plots optical depth versus velocity [km / s].
¨ et al. / New Astronomy Reviews 43 (1999) 647 – 650 Y.M. Pihlstrom
650 Table 1 Absorption feature parameters Comp
Velocity (km s 21 )
Peak opacity
FWHM (km s 21 )
NHI (T s / 100 K)21 (cm 22 )
A B C
3415-3486 3489-3549 3550-3584
3.0 3.2 2.7
20 34 34
1.1 3 10 22 2.0 3 10 22 1.7 3 10 22
atomic gas associated with the proposed H 2 O megamaser rotating disk (Hagiwara et al., 1997; Neufeld & Maloney, 1995)? The water masers in NGC 4258 are located within 1 pc of the nucleus (Miyoshi et al., 1995), and the H 2 O masers in NGC 5793 have similar satellite lines widely separated in velocity, which presumably occur on similar scales (say , 10 pc). Alternatively, the HI might be associated with the few-kpc scale CO emitting gas or simply be normal spiral galaxy disk ISM clouds that lie coincidently along the line of sight. We can rule out that the absorbing gas is very close ( , 10 pc) to the nucleus, since there is no simple relationship with the continuum at the pc scales, and we know that the clouds are $ 15 pc in size. Additionally, the observed line widths are much narrower than for the HI associated with TORUS gas in ellipticals, which have FWHM $ 100 km s 21 even for gas on scales up to r ¯ 100 pc (Conway, 1998). Koribalski (1996) also finds similar | 100 km s 21 linewidths for HI absorbing gas in spirals; this gas is argued to be circumnuclear and lie at 100 , r , 1000 pc. The alternative that the gas belongs to normal HI ISM clouds, similar to those in spiral galaxy disks, that lie coincidently along the line of sight, is possible but not very likely. Even though the densities and cloud sizes are reasonable, the velocity widths do not agree. Compared to the Galaxy, where the cloud linewidths are typically 5 km s 21 , the clouds in NGC 5793 must be significantly broadened due to turbulent or bulk motions within the gas. Our best guess is that the HI is associated with the
CO gas seen at r | 1 kpc. If this is the case, then our observations of NGC 5793 are very similar to the MERLIN HI absorption measurements against Mrk6 (Gallimore et al., 1998), where the gas was detected only against one of the continuum components at a projected distance of 380 pc (here H0 5 75 km s 21 Mpc 21 ). A maximum optical depth of 0.45 together with a linewidth of 30 km s 21 resulted in NHI 5 2.3 3 10 21 cm 22 . It was argued that the gas is in a kpc-scale ring or spiral arms associated with an observed dust lane. Another example is the FR II galaxy 3C293 that shows HI consistent with clouds in a 1.1 kpc radius CO disk rotating around the centre (Haschick & Baan, 1985; Evans et al., 1998). In the case of NGC 5793 this | 1 kpc gas could be part of an extended continuous TORUS structure (Conway, 1999) which feeds the central engine. Alternatively it might be part of a distinct dynamically decoupled ring or disk of starburst activity.
References Conway, J.E., 1998, in: Carilli, C., Menten, K., & Radford, S., (Eds.), Highly Redshifted Radio Lines, PASP, in press. Conway, J.E., 1999, These Proceedings, NewAR, 43, 509. Evans, A.S., Sanders, D.B., Surace, J.A., & Mazzarella, J.M., 1998, ApJ, submitted. Gallimore, J.F., Holloway, A.J., Pedlar, A., & Mundell, C.G., 1998, A&A, 333, 13. Gardner, F.F. & Whiteoak, J.B., 1986, MNRAS, 221, 537. Gardner, F.F., Whiteoak, J.B., Norris, R.P., & Diamond, P.J., 1992, MNRAS, 258, 296. Hagiwara, Y., Kohno, K., Kawabe, R., & Nakai, N., 1997, PASJ, 49, 171. Haschick, A.D. & Baan, W.A., 1985, ApJ, 289, 574. Koribalski, B., 1996, in: Skillman, E.D., (Ed.), The Minnesota Lectures on Extragalactic Neutral Hydrogen, ASPC, Vol 106, ASP, San Francisco, p. 238. Miyoshi, M., Moran, J., Herrnstein, J., Greenhill, L., Nakai, N., Diamond, P., & Inoue, M., 1995, Natur, 373, 127. Neufeld, D.A. & Maloney, P.R., 1995, ApJ, 447, L17. Soifer, B.T., Boehmer, L., Neugebauer, G., & Sanders, D.B., 1987, ApJ, 320, 238.