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On the origin of cosmic rays in the spiral galaxy NGC 3310
Adv. Space Res. Vol.3, No.10-12, pp.91-93, 1984 Printed in Great Britain. All rights reserved.
0273-1177/84 $0.00 + .50 Copyright © COSPAR
O N T H E O R I G I N OF C O S M I C R A Y S IN T H E S P I R A L G A L A X Y N G C 3310 N. Duric University of Toronto, Toronto, Canada
ABSTRACT The problem of cosmic ray production in the spiral galaxy NGC 3310 is addressed by analysing and comparing optical and radio continuum data. Tentative results indicate that on global scales relativistic electrons may be produced in the shock front associated with the density wave while on local scales extreme population I objects may be producing them. It is inferred that the same conclusions apply to all cosmic rays produced in the disk. KEYWORDS Cosmic rays; relativistic electrons; spiral galaxies; radio continuum emission; synchrotron emission; density waves; shock fronts.
INTRODUCTION NGC 3310 is a high surface brightness galaxy whose spiral structure may be of recent origin. The inner region consists of 2 very open and symmetric arms which become tightly wound within 1 kpc of the nucleus. The fainter outer regions are very assymetric and appear to be highly disturbed. Kinematical studies (van der Kruit, 1976) point to the presence of a spiral density wave and indicate that the optical nucleus is offset from the dynamical center of the galaxy. Balick and Heckman (1981) verify the offset and in addition observe that the inner region has an anomolously low metal abundance. They propose that all these observations are indicative of a merger with another galaxy which they suggest was a metal poor, gas rich dwarf and that the merger occurred less than l0 s years ago. Schweizer (1982) also favours a merger. The merger idea is consistent with the suggestion by Kormendy and Norman (1979) that the spiral arms are young and probably tansient. Studying a young, recently established, spiral density wave would allow us to address many interesting problems relating to the dynamical and chemical evolution of a galaxy. For the purposes of the symposium, we restrict our discussion to the role such a density wave would play in the production of relativistic electrons. Although it is tempting to associate all cosmic ray particles with the electrons recent work by Hermsen (1983, private communication) suggests that this is not the case. He shows that not all cosmic ray nuclei in our galaxy follow the distribution of relativistic electrons and that some may in fact be extragalactic. Thus, throughout our talk we will restrict our discussion to those cosmic rays produced in the disk of NGC 3310, together with the relativistic electrons. We make no attempt to discuss the nature of cosmic rays originating outside the disk.
DATA ANALYSIS The problem of cosmic ray production in this galaxy is addressed by comparing the distribution of synchrotron radiation with that of optical radiation in the form of starlight and line emission. The following is a progress report on the ongoing data analysis being done in collaboration with L.E. Davis (KPNO, USA), P.C. Crane (NRAO, USA), R.C. Bignell (NRAO, USA) and E.R. Seaquist (University of Toronto, Canada). Throughout this discussion it will be assumed that variations in synchrotron emission are correlated with variations in the density of relativistic electrons. This is not a bad assumption in light of the study by Haslam et al (1981) of our own galaxy. Our optical data consist of CCD images taken through U, B, V, R, I and Ha filters with the KPNO #i 0.9 meter telescope. The radio data consist of aperture synthesis maps obtained with the VLA at 5 and 1.4 GHz. The data are presented in the form of contour maps in figure I -
91
92
N. Duric
which shows the U, V, I and Ha optical images and the 6 and 20 cm radio maps. The resolution varies from map to map but is generally between 2 and 3" in the optical and ~1.5" in the radio.
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Contour map representations of the optical and radio data.
It is apparent that the two dimensional distribution of the radio continuum emission is most similar to the Ha distribution in the optical and least similar to the distribution of starlight in the I band. Since the radio emission is dominated by synchrotron radiation we infer that the relativistic electrons are associated with the ionized gas and extreme population I stars and not with the disk population. Our radio data also indicated that the total disk emission under the arms is only about 2% of the total arm emission• Thus, it appears that population II stars play little or no role in cosmic ray production in this galaxy. By determining positions of spiral arm ridges it was found that the synchrotron and Ha radiation are displaced from the starlight in the manner predicted by the density wave theory. This suggests that the synchrotron radiation is, at least in part, associated with the shock front of the density wave. Much of the synchrotron emission appears to be coincident with the Ha arms but with a resolution of 150 pc it is difficult to say whether this is inconsistent with a density wave picture.
Cosmic Rays in the Spiral Galaxy NGC 3310
93
DISCUSSION It is tempting to attribute the synchrotron emission along the arms to an enhancement of cosmic rays in the disk as in M51 (Mathewson et al., 1972). However, in NGC 3310 this enhancement is an order of magnitude greater (arm to disk ~50) which suggests that cosmic rays are produced in the spiral arms. Generally, these enhancements are modelled only with adiabatic shocks but we dismiss the possibility of isothermal shocks on the grounds that gas travel times through the shock fronts are much less than their cooling times (106 vs l0 T -108 years). The mechanism most often invoked for accelerating particles to relativistic speeds in spiral arms is the supernova. Certainly, our maps of pure synchrotron emission (extracted from the data in the manner of Turner and Ho, 1983) show that locally ~cale lengths <~300 pc) it originates in star forming regions. However, the bulk of the arm emission occurs on global scale lengths (up to a few kpc) and appears to be relatively smooth and coherent. Propogation of cosmic rays, at the Alfven velocity, amounts to ~25 pc/Myr so that kpc scale lengths would require tens of millions of years of travel time from star forming regions. Since this time is greater than the average lifetimes of supernova producing stars it seems unlikely that supernovae can account for the global, non-clumped synchrotron emission. In order to satisfactorily explain the global distribution of cosmic rays it may be necessary to invoke non-stellar acceleration mechanisms. The Fermi (e.g. Bell, 1977) and betatron (e.g. Cowsik and Mitteldorf, 1974) mechanisms are 2 such processes. Both involve the presence of shocked gas and are capable of enhancing the synchrotron emission much more than adiabatic compression alone. They would be consistent with the observation that the global distribution of cosmic rays mimics the distribution of gas more than the stars and that these cosmic rays are associated with the global shock fronts along the density wave. In addition, we have examined the radio data for linear polarization. We report no detectable polarization and place upper limits of 5 - 10% on the degree of polarization in the spiral arms. This implies that the magnetic fields along the arms are tangled which supports the non-stellar acceleration mechanisms since they rely on gas turbulence.
SUMMARY Our preliminary study of the data suggests that, in NGC 3310, the source of cosmic rays can be constrained to the extreme population I for local production and to regions of shocked gas for global production. Population II stars seem to play only a minor role, if any, in cosmic ray production. This may prove to be an important result since it would show that global shocks are capable of generating cosmic rays directly.
REFERENCES Balick, B. and Heckman, T.M., 1981, Astr. Ap., 96, 271 Bell, A.R., 1977, M.N.R.A.S., 179, 573 Cowsik, R. and Mitteldorf, J., 1974, Ap.J. 189, 51 Haslam, C.G.T., Kearsey, S., Osborne, J.L., Phillips, S., Stoffel, H., 1981, Nature, 289, 470 Kormendy, J. and Norman, C.A., 1979, Ap.J., 233, 539 Kruit, P.C. van der, 1976, Astr. Ap., 49, 161 Mathewson, D.S., Kruit, P.C. van der, Brouw, W.N., 1972, Astr. Ap., 17, 468 Schweizer, F., 1982, in IAU Symposium i00, Internal Kinematics and Dynamics of Galaxies, ed. E. Athanassoula (Dordrecht:Holland) Turner, J.L. and Ho, P.T.P., Ap.J. (Letters), 268, L79
0273-1177/84 $0.00 + .50 Copyright © COSPAR
O N T H E O R I G I N OF C O S M I C R A Y S IN T H E S P I R A L G A L A X Y N G C 3310 N. Duric University of Toronto, Toronto, Canada
ABSTRACT The problem of cosmic ray production in the spiral galaxy NGC 3310 is addressed by analysing and comparing optical and radio continuum data. Tentative results indicate that on global scales relativistic electrons may be produced in the shock front associated with the density wave while on local scales extreme population I objects may be producing them. It is inferred that the same conclusions apply to all cosmic rays produced in the disk. KEYWORDS Cosmic rays; relativistic electrons; spiral galaxies; radio continuum emission; synchrotron emission; density waves; shock fronts.
INTRODUCTION NGC 3310 is a high surface brightness galaxy whose spiral structure may be of recent origin. The inner region consists of 2 very open and symmetric arms which become tightly wound within 1 kpc of the nucleus. The fainter outer regions are very assymetric and appear to be highly disturbed. Kinematical studies (van der Kruit, 1976) point to the presence of a spiral density wave and indicate that the optical nucleus is offset from the dynamical center of the galaxy. Balick and Heckman (1981) verify the offset and in addition observe that the inner region has an anomolously low metal abundance. They propose that all these observations are indicative of a merger with another galaxy which they suggest was a metal poor, gas rich dwarf and that the merger occurred less than l0 s years ago. Schweizer (1982) also favours a merger. The merger idea is consistent with the suggestion by Kormendy and Norman (1979) that the spiral arms are young and probably tansient. Studying a young, recently established, spiral density wave would allow us to address many interesting problems relating to the dynamical and chemical evolution of a galaxy. For the purposes of the symposium, we restrict our discussion to the role such a density wave would play in the production of relativistic electrons. Although it is tempting to associate all cosmic ray particles with the electrons recent work by Hermsen (1983, private communication) suggests that this is not the case. He shows that not all cosmic ray nuclei in our galaxy follow the distribution of relativistic electrons and that some may in fact be extragalactic. Thus, throughout our talk we will restrict our discussion to those cosmic rays produced in the disk of NGC 3310, together with the relativistic electrons. We make no attempt to discuss the nature of cosmic rays originating outside the disk.
DATA ANALYSIS The problem of cosmic ray production in this galaxy is addressed by comparing the distribution of synchrotron radiation with that of optical radiation in the form of starlight and line emission. The following is a progress report on the ongoing data analysis being done in collaboration with L.E. Davis (KPNO, USA), P.C. Crane (NRAO, USA), R.C. Bignell (NRAO, USA) and E.R. Seaquist (University of Toronto, Canada). Throughout this discussion it will be assumed that variations in synchrotron emission are correlated with variations in the density of relativistic electrons. This is not a bad assumption in light of the study by Haslam et al (1981) of our own galaxy. Our optical data consist of CCD images taken through U, B, V, R, I and Ha filters with the KPNO #i 0.9 meter telescope. The radio data consist of aperture synthesis maps obtained with the VLA at 5 and 1.4 GHz. The data are presented in the form of contour maps in figure I -
91
92
N. Duric
which shows the U, V, I and Ha optical images and the 6 and 20 cm radio maps. The resolution varies from map to map but is generally between 2 and 3" in the optical and ~1.5" in the radio.
V
I
O
Q
©
O
<>
HC~
8om o
.
•
O o-
O
•
4
D o ¢= Q "
°.~o
io.°Oo.
o~ O
°~ O : :
• o
Fig. i.
_~o
°
"
O
Contour map representations of the optical and radio data.
It is apparent that the two dimensional distribution of the radio continuum emission is most similar to the Ha distribution in the optical and least similar to the distribution of starlight in the I band. Since the radio emission is dominated by synchrotron radiation we infer that the relativistic electrons are associated with the ionized gas and extreme population I stars and not with the disk population. Our radio data also indicated that the total disk emission under the arms is only about 2% of the total arm emission• Thus, it appears that population II stars play little or no role in cosmic ray production in this galaxy. By determining positions of spiral arm ridges it was found that the synchrotron and Ha radiation are displaced from the starlight in the manner predicted by the density wave theory. This suggests that the synchrotron radiation is, at least in part, associated with the shock front of the density wave. Much of the synchrotron emission appears to be coincident with the Ha arms but with a resolution of 150 pc it is difficult to say whether this is inconsistent with a density wave picture.
Cosmic Rays in the Spiral Galaxy NGC 3310
93
DISCUSSION It is tempting to attribute the synchrotron emission along the arms to an enhancement of cosmic rays in the disk as in M51 (Mathewson et al., 1972). However, in NGC 3310 this enhancement is an order of magnitude greater (arm to disk ~50) which suggests that cosmic rays are produced in the spiral arms. Generally, these enhancements are modelled only with adiabatic shocks but we dismiss the possibility of isothermal shocks on the grounds that gas travel times through the shock fronts are much less than their cooling times (106 vs l0 T -108 years). The mechanism most often invoked for accelerating particles to relativistic speeds in spiral arms is the supernova. Certainly, our maps of pure synchrotron emission (extracted from the data in the manner of Turner and Ho, 1983) show that locally ~cale lengths <~300 pc) it originates in star forming regions. However, the bulk of the arm emission occurs on global scale lengths (up to a few kpc) and appears to be relatively smooth and coherent. Propogation of cosmic rays, at the Alfven velocity, amounts to ~25 pc/Myr so that kpc scale lengths would require tens of millions of years of travel time from star forming regions. Since this time is greater than the average lifetimes of supernova producing stars it seems unlikely that supernovae can account for the global, non-clumped synchrotron emission. In order to satisfactorily explain the global distribution of cosmic rays it may be necessary to invoke non-stellar acceleration mechanisms. The Fermi (e.g. Bell, 1977) and betatron (e.g. Cowsik and Mitteldorf, 1974) mechanisms are 2 such processes. Both involve the presence of shocked gas and are capable of enhancing the synchrotron emission much more than adiabatic compression alone. They would be consistent with the observation that the global distribution of cosmic rays mimics the distribution of gas more than the stars and that these cosmic rays are associated with the global shock fronts along the density wave. In addition, we have examined the radio data for linear polarization. We report no detectable polarization and place upper limits of 5 - 10% on the degree of polarization in the spiral arms. This implies that the magnetic fields along the arms are tangled which supports the non-stellar acceleration mechanisms since they rely on gas turbulence.
SUMMARY Our preliminary study of the data suggests that, in NGC 3310, the source of cosmic rays can be constrained to the extreme population I for local production and to regions of shocked gas for global production. Population II stars seem to play only a minor role, if any, in cosmic ray production. This may prove to be an important result since it would show that global shocks are capable of generating cosmic rays directly.
REFERENCES Balick, B. and Heckman, T.M., 1981, Astr. Ap., 96, 271 Bell, A.R., 1977, M.N.R.A.S., 179, 573 Cowsik, R. and Mitteldorf, J., 1974, Ap.J. 189, 51 Haslam, C.G.T., Kearsey, S., Osborne, J.L., Phillips, S., Stoffel, H., 1981, Nature, 289, 470 Kormendy, J. and Norman, C.A., 1979, Ap.J., 233, 539 Kruit, P.C. van der, 1976, Astr. Ap., 49, 161 Mathewson, D.S., Kruit, P.C. van der, Brouw, W.N., 1972, Astr. Ap., 17, 468 Schweizer, F., 1982, in IAU Symposium i00, Internal Kinematics and Dynamics of Galaxies, ed. E. Athanassoula (Dordrecht:Holland) Turner, J.L. and Ho, P.T.P., Ap.J. (Letters), 268, L79