Cosmic Ray Propagation and Star Formation History of NGC 1961 3

arXiv:astro-ph/9805231v1 18 May 1998 o.Nt .Ato.Soc. Astron. R. Not. Mon. .Lisenfeld U. of history formation 1961 star NGC and propagation ray Cosmic mn(eVuoluse l 91 C)tasae o110 for Mpc to 82.9 translates of RC3) distance 1991, a al. (adopting et kpc Vaucouleurs (de cmin Rmnsi 93;isotcldaee of diameter galaxies is spiral optical known It its largest appearance. 1983); and optical (Romanishin massive most asymmetric the and among peculiar a with 2 1 G 91(r 8)i eymsie(yaia mass (dynamical massive very a is 184) 10 (Arp 1961 NGC INTRODUCTION 1 2021 April 10 eta,102Gaaa pi,emi [email protected] e-mail Spain, Granada, 18012 Central, ⋆ on otaeo egrrmatsc sascn nu- H second the a in as distribution such remnant velocity merger disturbed a or they of since cleus, cloud merger trace gas a no to intergalactic of an found close possibility or sufficiently galaxy the mass another dismissed enough with also large They the a 1961. since with NGC galaxy galaxy by another no 1961 with pos- is NGC interaction the in tidal discarded gas of They (IGM). the sibility medium of intergalactic stripping hot to a disturbed due the is that suggested sharp appearance a SHSVV and south-east. north-west the the to to edge kpc 30 extensive extending SHSVV) gas an of (1982, with “wing” morphology al. asymmetric et unusual an Shostak found by kinematics and structure Mpc eevdrltvl iteatnin ealdsuyo i of study detailed A attention. little relatively received member. massive most and largest the far by c nvria eGaaa autdd ´sc eoiaydlC F´ısica del Teórica de y Facultad Granada, Laborator de Cavendish Universidad Observatory, Astronomy Radio Mullard rsn drs:IA,AeiaDvn atr ,Núcleo 7, Pastora Divina Avenida IRAM, address: Present 12 00RAS 0000 − ept t ieadpcla perne G 91has 1961 NGC appearance, peculiar and size its Despite − 1 .I samme fagopo aaisweei is it where galaxies 6 of group a of member a is It ). 10 13 M ⊙ otsa ta.18)S prlgalaxy spiral Sb 1983) al. et Gottesman , 1 , 2 ⋆ .Alexander P. , 000 0–0 00)Pitd1 pi 01(NL (MN 2021 April 10 Printed (0000) 000–000 , ed omcry ai otnu:galaxies continuum: radio – rays cosmic interg – an galaxy.fields with the collision words: of a features or Key unusual merger various a the likely caused most formatio has past, star cloud, the recent radio in by extended d event The probably violent strong sites. acceleration, a these ray that at cosmic The place indicates recent flatter. taken maxima a much has the indicating is rate at formation spectrum steep, emission m the synchrotron very the the maxima at is of the unusual: spectrum from is Away synchrotron disk electrons. galactic the the spectral emission in nonthermal ther radio distribution the the index determine separate spectral to super to This and the us emission on allow radio frequencies observations 4 thermal These at 1961. data NGC continuum galaxy radio new present We ABSTRACT H 0 D 0k s km 50 = 25 1 ..Pooley G.G. , aais niiul G 91–glxe:IM–glxe:magnetic galaxies: – ISM galaxies: – 1961 NGC individual: galaxies: 4 = i a.In gas. . sH ts ar- 6 − re so,102Gaaa Spain Granada, 18002 osmos, ,MdnlyRa,CmrdeC3OE UK OHE, CB3 Cambridge Road, Madingley y, 1 i n ons )Tedslcmn fteH the of follow- displacement the The noted i) SHSVV points. ing hypothesis stripping the of favour anyfo h eta ein(ec os19) The comes 1997). and Rots H galaxy & asymmetric (Pence spiral the region of a central cause for the typical from fairly mainly is of emission 1961 X-ray the NGC recent that However, shown IGM. have hot observations de- ROSAT from galaxy emission as the interpreted of they south-east the the by curve in tected A feature iv) blue formation. emission south- The star the X-ray and recent iii) wing indicating (SFR). NW ridge the rate of eastern formation emission optical den- star the (CR) high of ray colour a cosmic the to the in due of increase sity enhancement an by an of or by field presence caused magnetic The being which south-east as ii) the galaxy. interpreted in they the emission of continuum south radio increased the in disk optical h at(ntm-clso e 10 few a of i time-scales formation (on star past the the spec about formation, its information contains particular star shape, in day tral and present emission, of synchroton nonther- intensity the and whereas and re- sites thermal emission radio the identify thermal veals to The emission. use (synchrotron) we mal which data uum unclear. eios,adas bu h ceeainadpropagation and acceleration CRs. pro- the of (SN) mechanisms about supernova of also and and electrons genitors), CR the of life-time the nti ae epeetmlifeunyrdocontin- radio multi-frequency present we paper this In 1 n .Wilding T. and A Einstein T E tl l v1.4) file style X aelt Sotke l 92 which 1982) al. et (Shostak satellite i itiuinteeoeremains therefore distribution 1 7 .W ugs hta that suggest We n. rwihi eae to related is which yr msini inof sign a is emission ne distribution. index i a n h non- the and mal ihrsett the to respect with asv,peculiar massive, cieo h star the of ecline e omcray cosmic ged te spectrum steep xm fthe of axima lci gas alactic n d - 2 U. Lisenfeld et al.

2 OBSERVATIONS Table 1. Integrated flux densities NGC 1961 was mapped with the VLA† at 8.41 GHz and with the Cambridge 5-km Ryle Telescope (RT) at 4.92 and Frequency Flux Density Reference 15.4 GHz. Additionally, existing VLA data (B-array) at [MHz] [Jy] 1.46 GHz (Condon 1983) were available. When the obser- 38 3.40 0.85 Howarth (1990) ± vations of NGC 1961 were made (between 1990 and 1993), 57.5 2.0 0.5 Israel & Mahoney (1990) ± the RT had a band-width of 280 MHz split into 28 10-MHz 151 1.17 0.21 Howarth (1990) ± frequency channels; together with the minimum baseline of 1400 0.191 White & Becker (1992) 1465 0.149 0.015 this work 18 m this provides excellent temperature sensitivity at both ± 1490 0.182 0.023 Condon (1987) 4.92 and 15.4 GHz. ± 4850 0.057 0.009 Becker, White & Edwards (1991) For the RT observations a total of five telescope con- ± 4919 0.050 0.006 this work ± figurations at each observing frequency were employed giv- 8414 0.028 0.003 this work ± ing a nearly fully filled aperture out to a baseline of ap- 10550 0.031 0.005 Niklas et al. (1995) ± proximately 1 km. A minimum of two 12-hour runs in 15360 0.019 0.002 this work each configuration were obtained. This gave a resolution of ± approximately 1.0 × 1.0cosec(δ) arcsec2 at 15.4 GHz and 2 such a match, the UV-range of the data in the aperture plane 3.0 × 3.0cosec(δ) arcsec at 4.92 GHz. At 4.92 GHz observa- used in the construction of the images above 1.46 GHz was tions of phase-calibrators were made at the beginning and restricted during the mapping programs so that the min- end of each run, while at 15.4 GHz calibration observations imum and maximum baselines (measured in wavelengths) were interleaved with those of NGC 1961. 3C 286 and 3C 48 used in the imaging were the same; this involved removing were observed regularly as flux calibrators. Calibration and baselines from all of the high-frequency data. While it is not post- data-editing was performed in the MRAO package possible to ensure that the precise sampling of the aperture mortem aips , with subsequent reduction in and the MRAO plane is the same in all cases, for both the VLA and RT the anmap package . dense sampling near the shortest baselines leads to similar The 8.41-GHz VLA observations were in the B, C and overall coverage of the UV plane. Furthermore the use of D arrays and were carried out during 1990/91. Reduction CLEAN does, to some extent, compensate for the resulting of the data followed standard VLA procedures with calibra- marginally different beamshapes. A possible problem results aips tion, editing and imaging performed in . The 1.46 GHz from the fact that the extended, low-brightness structure of- data were from Condon (1983) and no additional reduction ten has a steep radio-frequency spectrum. Missing the short was required. A number of maps were made at each fre- baselines could lead to a negative region (“negative bowl”) quency by tapering the aperture-plane to highlight structure around the source and hence a (frequency-dependent) error on different angular scales. in the zero level of the image. The final images were checked The data in this paper which are most seriously affected for the existence of such an effect and in each case any offset by the lack of short baselines are those at 1.46 GHz, which of the local zero level was found to be less than the thermal are only sensitive to structures on scales smaller than about noise (this is clear from Figs. 1 and 2 where no evidence of a 2 arcmin. Very faint emission from the whole of the optical significant negative bowl exists). We are confident that the disc of NGC 1961 will not be adequately represented in our procedures that we have adopted here are sufficient to reduce maps. The optical diameter of NGC 1961 is 4.6 arcmin; how- any artefacts resulting from the sampling of the aperture to ever the extent of the continuum emission at 1.46 GHz as levels significantly below those of the thermal noise. determined by Shostak et al. (1982) using the WSRT with a All the maps were CLEANed and a set of maps at shortest baseline of only 36 m is approximately 2.5 arcmin, a common resolution of 16 × 16 arcsec2 were used for the with very little low-brightness emission outside that region. multi-frequency comparison (at 1.46 GHz this was achieved A further test of the completeness of our mapping is to com- by convolving the supplied image). All maps have been cor- pare our measured flux densities with single dish data; these rected for the primary beam response of the telescope used comparisons are reported in Table 1. It is apparent that to to make the observations. within the quoted uncertainties our flux densities are consistent with those determined by other workers (including single dish measurements at 1.40 GHz and 4.85 GHz), al- 3 RESULTS though the best estimates represent a shortfall of 18% of the flux density at 1.46 GHz and 12% at 4.92 GHz. These 3.1 Radio images results suggest that we are able to image the extent of the continuum emission satisfactorily at all our observing fre- Fig. 1 shows the radio maps at the four frequencies at a quencies. resolution of 16 arcsec; maps at a resolution of 8 arcsec are So as to make the spectral index comparison as accu- shown in Fig. 2. In Fig. 3 the 1.46 GHz radio map is overlaid rate as possible it is crucial to ensure that the spectral index on an optical image obtained from the Digitized Sky Survey is determined from images all of which are sensitive as far as produced by the Space Telescope Science Institute. The ra- possible to the same range of spatial structure. To achieve dio images at all frequencies show a peculiar morphology which are similar to the optical image. The bright central nucleus is resolved in the higher resolution images and con- sists of two peaks – one which corresponds to the optical † The VLA is operated by the National Radio Astronomy Ob- servatory for Associated Universities Inc., under a cooperative peak and a second peak which coincides with a dust lane. agreement with the National Science Foundation. Furthermore, a bright radio arm is visible in the south-east,

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Cosmic ray propagation and star formation history of NGC 1961 3

1.46 GHz 4.92 GHz 0536+69 IPOL 1464.900 MHZ 1961_1.5.16.IMAP.1 NGC1961 FPOL 4919.167 MHZ 1961_5.16.FPOL.1

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19 45 05 36 50 45 40 35 30 25 20 05 36 50 45 40 35 30 25 20 RIGHT ASCENSION RIGHT ASCENSION Peak flux = 4.0982E-03 JY/BEAM Peak flux = 3.1751E-03 JY/BEAM Figure 1. Maps of NGC 1961 at 1.46, 4.92, 8.41 and 15.4 GHz at a resolution of 16 16 arcsec2. The contour levels start at (-0.85, 1 × 1 -0.60, 0.60, 0.85.....) mJy beam− (1.46 GHz and 4.92 GHz); (-0.28, -0.20, 0.20, 0.28,...) mJy beam− (8.41 GHz) and (-0.48, -0.34, 0.34, 0.48....) mJy beam−1 (15.4 GHz) and increase by factors of √2. The rms noise levels are: 0.30 mJy beam−1 (1.46 and 4.92 GHz), 0.10 mJy beam−1 (8.41 GHz) and 0.17 mJy beam−1 (15.4 GHz).

1.46 GHz 8.41 GHz 0536+69 IPOL 1464.900 MHZ 1961_1.5.8.IMAP.1 NGC1961 IPOL 8414.900 MHZ 1961_8.8.IMAP.1

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05 36 55 50 45 40 35 30 25 20 15 05 36 55 50 45 40 35 30 25 20 15 RIGHT ASCENSION RIGHT ASCENSION Peak flux = 9.4672E-03 JY/BEAM Peak flux = -9.1602E-03 JY/BEAM Figure 2. Maps of NGC 1961 at 1.46 and 8.41 at a resolution of 8 8 arcsec2. The contour levels start at (-0.48, -0.34, 0.34, 0.48 ....) 1 × 1 mJy beam− (1.46 GHz) and at (-0.14, -0.10, 0.10, 0.14.....) mJy beam− (8.41 GHz) and increase by factors of √2. The rms noise levels are: 0.17 mJy beam−1 (1.46 GHz), 0.05 mJy beam−1 (8.41 GHz).

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4 U. Lisenfeld et al.

Figure 3. The radio map at 1.46 GHz from Fig. 2 overlaid on an optical image. The contour lines start at 0.34 mJy and increase by factors of 2. coinciding with the bright optical south-eastern arm. The Figure 4. The radio spectrum of NGC 1961. The data are listed optical spiral arm extending from of the west of the nucleus in Table 1. The line is the best-fit power-law spectrum and has a slope of 0.85. and to the south is also visible in the radio image. At all frequencies, except for 15.4 GHz, there is an extended enve- lope of radio emission visible which surrounds the nucleus and the south-eastern arm and which contains, at 1.46 GHz, about 50% of the total radio flux. effect of the missing short spacings is therefore in the worst case to underestimate very slightly the flux density at 1.46 GHz relative to the higher frequencies and therefore to un- 3.2 Radio spectrum and spectral index derestimate also the spectral index. However, this effect will distribution lead to a systematic error which is considerably smaller than the statistical error due to the thermal noise and in what fol- Integrated flux densities at the four observing frequencies lows it will be ignored. were determined from the low-resolution maps and in Table Fig. 5 shows spectral index maps between 1.46 and 4.92 1 the results are summarized together with flux densities GHz and between 1.46 and 8.41 GHz, each at 16-arcsec reso- taken from the literature. The integrated spectrum is shown lution and gated so that the spectral index is only calculated in Fig. 4; the spectrum is well-fitted by a power law with a where the flux density exceeds 3σ at both frequencies. The spectral index α = 0.85 (S(ν) ∝ ν−α), which is not unusual spectral index between 1.46 and 4.92 GHz (α1.46−4.92GHz)

for a spiral galaxy; the average spectral index for a sample shows a peculiar behaviour in the sense that at the max- 1 of spiral galaxies is 0.74 ±0.03 (Gioia et al. 1982). ima of the radio emission (i.e. at the nucleus and on the As discussed in Section 2 there may be some flux miss- south-eastern arm) the spectral index is steep. Away from ing from our images due to the restricted UV-coverage on these sites the spectral index flattens, except towards the the shortest baselines; our estimate of this is 18% (33 mJy) north and west of the nucleus and towards the north of at 1.46 GHz and 12% (7 mJy) at 4.92 GHz. The spectral the arm. The behaviour of α1.46−8.41GHz is less uniform comparison has been performed using images with matched which is largely due to the contribution of thermal emis- UV-coverage and resolution so that the spectra of structure sion at the higher frequency. The most noticeable flattening of similar spatial scales may be compared. If the emission is of the spectral index in Fig. 5 is between the nucleus and regarded as features superimposed on a background of emis- the south-eastern arm where α1.46−4.92GHz = 0.2 ± 0.3 and sion, then on those scales which are properly represented in α1.46−8.41GHz = 0.55±0.2. The spectral index variation that our imaging the spectral results will be accurate. A prob- would normally be expected due to synchrotron and inverse lem may exist, however, in those regions which do not show Compton aging of the CR electrons is a gradual steepening significant structural variation. In this case we can quantify of the spectrum away from the sites of acceleration. How- the likely errors. If we take a worst case of 33 mJy miss- ever, at the distance of NGC 1961 our 16-arcsec beam cor- ing flux density at 1.46 GHz distributed over an area of 9 2 responds to 6.4 kpc which is larger than the typical scale armin (slightly larger than the observed mappable extent of electron diffusion, and we are effectively averaging over a of the source), then this would correspond to an error of CR electron population of varying ages. 0.26 mJy/beam over the source at 1.46 GHz, which is less than the thermal noise and at a level of approximately 40% of the lowest contour in Fig. 1. Since we would expect the spectrum of the most extended material to be steep, this effect becomes less important at the higher frequencies. The

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Cosmic ray propagation and star formation history of NGC 1961 5

α1.46−4.92GHz α1.46−8.41GHz

Figure 5. Maps of the spectral index between 1.46 and 4.92 GHz (left) and between 1.46 and 8.41 GHz (right) at a resolution of 16 16 2 × arcsec . The greyscales range from 0.1 to 1.2 (α1.46−4.92GHz ) and from 0.6 to 1.2 (α1.46−8.41GHz ). Overlaid is a 1.46 GHz image with contour lines starting at 0.6 mJy and increasing by factors of 2.

4 SEPARATING THE THERMAL AND (ii) Diffusion in the disc of the galaxy is negligible on SYNCHROTRON EMISSION scales larger than our observing beam, which in this case is a good approximation. The maps at 4 frequencies allow us to separate the radio- (iii) The electrons which are radiating within a given res- continuum emission into thermal and nonthermal compo- olution element were all accelerated (injected) a time t ago nents. A spectral-fitting technique is used to estimate the (the electron age). two contributions and determine spatial variations in the (iv) The electron loss time is much greater than the pitch form of the synchrotron spectrum. For a set of maps at a angle randomization time — the so-called JP model (Jaffe common resolution a function of the following form is fitted & Perola 1974). at each pixel (v) The electron injection index is γ = 2.0. −0.1 I(ν)= SC(xB,γ)+ T (ν0)ν , (1) With these assumptions the electron energy distribution −0.1 where T (ν0)ν is the emission from optically thin ther- with an injection index γ is given by: mal Bremsstrahlung and SC(xB,γ) is the synchrotron emis- −γ γ−2 N0E (1 − ǫ(U + Urad)Et) E

2Lbol Lbol/W −35 eV chrotron emission is more extended than the thermal emis- rad × × U = 2 = 2 1.4 10 3 . (2) sion which is mainly concentrated around the maximum of cπR R/kpc cm the radio emission and the south-eastern arm. The electron We approximate Lbol as the sum of the far-infrared and age reﬂects the peculiar behaviour which is seen in the spec- 38 blue emission (yielding Lbol = 1.3 10 W) and hence ﬁnd tral index maps and noted above. The ages are highest at −3 Urad = 1.5 eV cm . the maxima of the radio emission where the spectrum be- c 0000 RAS, MNRAS 000, 000–000

6 U. Lisenfeld et al.

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05 36 50 45 40 35 30 25 20 05 36 50 45 40 35 30 25 20 05 36 50 45 40 35 30 25 20 RIGHT ASCENSION RIGHT ASCENSION RIGHT ASCENSION Grey scale flux range= 0.0 19.0 MilliJY/BEAM Grey scale flux range= 0.000 4.000 MilliJY/BEAM Grey scale flux range= 1.0 20.0 JY/BEAM Figure 6. Maps of the synchrotron emission (left), thermal emission (middle) and break frequency (right) from which the age of the electrons can be derived. For the synchrotron emission the greyscale ranges from 0 (white) to 19 mJy/beam (black), for the thermal emission from 0 (white) to 4 mJy/beam (black) and for the break frequency from 1 (white) to 20 GHz (black) (corresponding to electron ages from 1.4 107 yr (white) to 3 106 yr (black)). tween 1.46 and 4.92 GHz is steepest. The total thermal radio Table 2. Global properties continuum emission found from this fitting procedure is 16 1 mJy at 1 GHz (i.e. ∼ 10% of the total emission). Distance 82.9 Mpc SHSVV D25 4.6 arcmin RC3 Fig. 7 shows selected radio-continuum spectra of ◦ Inclination 50 SHSVV NGC 1961 and the fits associated to these data together 23 −1 P1 49 1.5 10 W Hz Condon (1987) with their positions relative to a 1.46 GHz map. The steep . GHz S100 6.4 Jy IRAS Faint Source Cat. spectrum at the nucleus and at the maximum of the south- S60 21.6 Jy IRAS Faint Source Cat. eastern spiral arm can be seen (positions 3 and 4). At these S60/S100 0.30 2 37 locations the flattening of the spectra towards higher fre- LFIR 3.9 10 W quencies due to a high thermal component is clearly visi- B0 11.01 RC3 T 37 ble and the thermal emission can be determined with good LB 9.5 10 W 0 accuracy. The thermal fraction is relatively high (25% at (B V ) 0.57 RC3 − T 11 1 MH i 1.2 10 SHSVV 1 GHz) and the nonthermal spectrum steep (αnth = 1.7 be- 10 3 MH ii 4 10 Young et al. (1995) tween 1.46 and 4.92 at both positions). The derived break 12 1 Mtotal 2.3 10 SHSVV frequency is low, suggesting that acceleration has not taken 1 −1 −1 adjusted to H0 = 50 km s Mpc place for a considerable period. Away from these positions, 2 32 2 LFIR = 1.5 10 [D/Mpc) (2.58(S60/Jy) + (S100/Jy))] the radio spectra show less flattening towards higher fre- 3 assuming a conversion factor of MH ii /LCO = 4.8 quencies (positions 5 and 6), or none at all (position 1 and 2) and therefore indicate a lower thermal contribution. The −1 radio spectra at the positions 1 and 2 are flat at low fre- ferred H2 mass (LFIR/MH2 = 2L⊙M⊙ ; for our Galaxy −1 quencies and steepen toward higher frequencies. The break LFIR/MH2 = 3.7L⊙M⊙ , Solomon et al. 1992). On the other frequency is high, indicating recent acceleration of CR elec- hand, the radio luminosity is high compared to luminosities trons. in other bands. For example the radio to blue luminosity ra- 11 −1 −1 tio Prad/LB = 6.25 10 WHz L⊙ is a factor of 7.5 higher than found for a sample of Sb galaxies (Hummel et al. 1988). Furthermore, the radio surface brightness is high as reflected 5 DISCUSSION in the magnetic field strength, B = 15µG, estimated using 2/7 5.1 FIR and radio luminosity the minimum energy method in which B ∝ (Prad/area) . Hummel et al. (1988) derived for a sample of Sb galaxies In Table 2 observational properties of NGC 1961 are pre- (with the same assumption for the minimum energy calcu- sented. Because of its size, NGC 1961 is luminous at all lation) an average value of 8 µG. wavelengths. The (B−V )0 colour is typical for an Sb galaxy T Deviations from the FIR/radio correlation have been (Huchra 1977) as is the ratio LFIR/LB = 0.3 (e.g. Lisenfeld found before for cluster (Niklas, Klein & Wielebinski 1995) et al. 1996b). This latter value is a measure of the ratio of and interacting galaxies (NGC 2276, Hummel & Beck 1995). the SFR averaged over the last 108 yr to the SFR averaged Also in these cases the conclusion by the authors was that over the last 109 yr, and indicates that the global SF has the radio emission is enhanced compared to normal galax- proceeded constantly during the past 109 yr. ies. The reasons for this enhancement have been suggested The ratio of the far-infrared (FIR) to radio luminosity, to be either an additional production of CRs by shock accel- log(LFIR/P1 49 ) = 14.44, is a factor of 3-5 lower than . GHz eration at interstellar shocks produced by the interaction of for spiral galaxies. The average values derived for galaxy the galaxy with the intercluster medium (Völk & Xu 1993), samples range from 14.9 (bright galaxies, Condon Ander- or an enhanced magnetic field (Hummel & Beck 1995). son & Helou 1991) to 15.1 (normal spiral galaxies, e.g. Condon et al. 1991, Lisenfeld et al. 1996a) with a typical standard deviation of 0.2 in the logarithm. This result 5.2 The spectral analysis is most likely due to an unusually high radio luminosity; the FIR luminosity does not appear unusually low com- The diffusion scale of the CR electrons in NGC 1961 is pared to its blue luminosity (LFIR/LB = 0.3), or the in- approximately 3.7 kpc at 1.46 GHz (taking B = 15µG,

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Cosmic ray propagation and star formation history of NGC 1961 7

1 2 3

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Figure 7. Selected spectra in NGC 1961 together with the positions where they were extracted relative to a 1.46 GHz image.

−3 Urad = 1.5 eV cm , and a typical diffusion coefficient of ter, indicating recent electron acceleration. The electron age 1029cm2s−1). At the distance of NGC 1961 the 16-arcsec distribution derived from the spectral fitting analysis gives beam corresponds to 6.4 kpc; to a good approximation, a time since acceleration for the CR electron population at therefore, the observations average over the synchrotron the maximum of the radio emission and in the south-eastern emission of both newly accelerated and aged CR electrons. arm (positions 3 and 4) of ∼ 107 yr, whereas the electrons The spectral variations reflect variations in the thermal frac- between these sites are considerably younger (3 × 106 yr). tion, differences in the conditions of the ISM and in the CR This leads to the following picture of the star forma- injection, but not effects due to propagation of the CR elec- 7

trons. tion history. The end of CR injection 10 yr ago indicates

1 1 1 that the SN activity ceased at that time. The life-time of SN progenitor stars is at maximum 8 × 107 years (corresponding to a stellar mass of 5 M⊙), which suggests an epoch of 5.2.1 Star formation history 7 intense SF which ended ∼< 9 × 10 yr ago. The beginning The shape of the non-thermal spectrum reflects temporal of the star formation epoch cannot be constrained by our variations of past CR acceleration and therefore of the star- data. The recent acceleration of CR electrons deduced from formation history if SN remnants are the most important the flat spectral index of the extended radio emission means source for the acceleration of CR electrons. The steep syn- that the CR injection and thus SN activity at these sites has chrotron spectrum at the centre and in the south-eastern recently started. The spectra are too flat to be explained by arm (positions 3 and 4 in Fig. 7) indicate aged CR elec- continous CR acceleration because in this case the contri- trons; the spectra cannot be explained by continous CR bution from aged CR electrons to the radio emission would acceleration since this would require an injection spectral steepen the spectrum, and approximating this situation to index of γ = 3.4. In the region between the centre and the that of a steady-state we would expect a synchrotron spec- south-eastern arm the synchrotron spectrum is much flat- tral index of αnth = γ/2. Taking γ = 2, the expected syn- c 0000 RAS, MNRAS 000, 000–000

8 U. Lisenfeld et al.

σ chrotron spectral index on the basis of a steady state model γ 1 2 (2 + γ) △α = α − = − σ . (6) is much steeper than the observed one, which is, attributing 2 2 2+ 2 as an upper limit all of the observed flux density at 15 GHz For an increasing magnetic field with σ = −1 this yields 1 46−4 92 ± to the thermal emission, αnth, . . GHz = 0.4 0.2. These △α = 0.4 and for σ = −2 △α = 0.66. Thus a signifi- results are most consistent with a recent burst of star for- 7 cant steepening of the spectrum is possible. If however syn- mation 2 − 3 × 10 yr ago followed by a peak SN rate about 2 6 chrotron losses (∝ B ) dominate then, since Urad + UB ≈ 5 × 10 yr ago. 2 UB ∝ B , we set ρ = 2σ and get: The time-scales derived from this analysis imply that 1 a causal connection between the star formation epochs is σ(1 − 4 (2 + γ)) △α = σ (7) possible, in the sense that the earlier star formation period 1 − σ + 4 in the arm may have triggered an episode of star formation In this case only a very weak steepening of the spectrum is in the inter-arm region. The width of the south-eastern arm possible; for γ = 2 △α is zero for all values of ρ and σ. is about 10 kpc and taking a typical sound speed to be 10 8 For NGC 1961 the high radio surface brightness sug- km/s a SN shock would take approximately 10 yr to cross gests that synchrotron losses dominate. In this case, the the arm which is consistent with the time-scale inferred for above analysis shows that under steady-state conditions the star formation history. variations in the field strength do not lead to significant vari- SHSVV suggested that an encounter of NGC 1961 with 8 ation in the spectral index. Steep spectra in the south-east an intergalactic cloud took place less than 5 10 yr ago (the arm could, however, result from a non-steady-state scenario rotation time-scale), and triggered an episode of intense star in which enhancement of the field was followed by a burst formation in the south-eastern arm. They presented an op- of star formation with increased synchrotron loss rate – it tical colour image showing that the south-eastern arm has is not possible to modify the spectrum significantly without indeed a blue colour, indicating recent star-formation. This incorporating into a model variation in the star-formation is consistent with the star-formation history that we infer. rate.

5.3 A dynamical history for NGC 1961 5.2.2 Variations of the magnetic field SHSVV suggest that a collision with an intergalactic cloud In the above analysis we have assumed that the magnetic has resulted in the unusual H i distribution in NGC 1961. field strength is constant over the galactic disk; this is Such an interaction with the IGM can explain in addition very likely not the case. Compression of the ISM and/or to the optical and H i morphology many features of the radio the presence of shocks throughout the disc will lead to en- emission provided the interaction triggered an episode of in- hanced magnetic field strengths and hence increased radio tense SF in the south-eastern arm. The main difficulty with synchrotron luminosity relative to FIR. Such processes were this model is that the massive intergalactic cloud required inferred by SHSVV from the morphology of the H i distribu- has not been observed either as ionized (Pence & Rots 1997) tion. However, an enhanced magnetic field strength in the or in neutral (SHSVV) gas. south-eastern arm alone cannot explain the large radio-to- An alternative view is that the disturbed appearance of FIR ratio since the radio flux density at 1.46 GHz of the NGC 1961 could be the result of a merger. SHSVV discarded south-eastern arm contributes only about 15 % to the total this possibility since they found no evidence for it either in flux density of the galaxy. the optical (i.e. there is no second nucleus) or in the H i If the magnetic field varies on scales shorter than that distribution. However, our high resolution radio images re- of the CR electron propagation it has an effect not only on veal the presence of a second nucleus in the centre of the the local synchrotron emission but also on the integrated galaxy consistent with an advanced merger; the disturbed spectral index. In particular, if the magnetic field strength optical and H i appearance is then naturally explained as a decreases (increases) away from the source of CR electrons, merger remnant. Near-infrared observations will provide an the spectrum can become significantly flatter (steeper). The important indication as to whether a highly obscured optical problem can be solved in a one-dimensional model in which nucleus is present. the magnetic field strength and the energy density of the radiation field change with increasing height over the galactic disk, z. Assuming that the magnetic field strength varies as 6 CONCLUSIONS −σ −ρ B(z) ∝ z , the energy losses as (Urad + U )(z) ∝ z B We have presented radio data at 4 frequencies of the su- and the energy dependence of the diffusion coefficient is permassive spiral galaxy NGC 1961. Using these data we D(E) ∝ Eµ, then the resulting spectral index is, in the fitted a combined aged-synchrotron plus thermal emission steady state, spectrum to each point in the galaxy. The most recent star σ γ 1 − µ ρ − 2 (2 + γ) formation, that is traced by the thermal radio emission, is α = + σ (5) mainly taking place in the centre and the south-eastern arm 2 2 2 − ρ + 2 (1 − µ) of the galaxy, whereas the underlying extended radio emis- (Berezinsky et al. 1990). Taking µ = 0, we see that in the sion is predominantly nonthermal. case ρ = 0 (i.e. spatially constant inverse Compton losses The spectral index distribution between 1.46 and dominate the energy losses) the difference of the spectral 4.92 GHz over the disk of the galaxy shows a peculiar be- index with respect to the case of a constant magnetic field haviour. At the maxima of the radio emission the nonther- is mal spectral index is very steep whereas the extended radio

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Cosmic ray propagation and star formation history of NGC 1961 9 emission has a much flatter spectrum. We discuss various Völk H.J., Xu C., 1993, Infrared Phys.Technol. 35, 527 possibilities to explain this peculiar behaviour. The most White R.L., Becker R.H., 1992, ApJS, 79, 331 likely explanation is that the steep spectrum is due to the Young J., Xie S., Tacconi L., et al., 1995, ApJS, 98, 219 end of the CR injection about 107 years ago. Assuming that CRs are accelerated mainly in SNRs this means that a phase 8 of intense SF has ceased ∼< 10 yr ago. The flat spectrum of the extended emission is a sign of recent CR acceleration in these regions. This could be due to CR acceleration in the shocks of SNRs of a recently formed stellar population. The radio-to-FIR ratio exceeds the average value of spiral galaxies by a factor of 3-4. An overall increase of the magnetic field with respect to the radiation field could account for this high FIR/radio ratio. Spatial variations of the magnetic field, which are likely to be present if the magnetic field is frozen into the gas, are however unable to give the observed distribution of the spectral index. The discovery of a second radio nucleus leads us to suggest that NGC 1961 is most likely the result of a merger rather than collision with a massive intergalactic gas cloud.

ACKNOWLEGDEMENTS This research has made use of the NASA/IPAC extragalac- tic database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aero- nautics and Space Administration. UL gratefully acknowl- edges the receipt of a grant of the Deutsche Forschungsge- meinschaft (DFG) and by the Comisi´on Interministerial de Ciencia y Technolog´ıa (Spain).

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