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Radio continuum observations of the Sombrero galaxy NGC4594 (M104) and other edge-on spirals

This paper discusses the radio polarization and sub-mm observations of the Sombrero galaxy (M104) and other edge-on spiral galaxies, focusing on the magnetic field, dust content, and evolutionary phase of M104.

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Radio continuum observations of the Sombrero galaxy NGC4594 (M104) and other edge-on spirals

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  1. Radio continuum observations of the Sombrero galaxy NGC4594 (M104) and other edge-on spirals Marita Krause MPIfR, Bonn Michael Dumke ESO, Chile Richard Wielebinski MPIfR, Bonn

  2. Outline • Introduction of M104 and the new observations in the radio polarization and • sub-mm observations • How can we observe the magnetic field by radio polarization? • Some examples of the large-scale magnetic field in some face-on spiral galaxies • What are the results of radio polarization observations for edge-on galaxies? • Presentation and discussion of the results for M104

  3. M104: Sa galaxy HI ring/spiral arm at ± 140 ″,170″ d = 8.9 Mpc CO at ± 140 ″ nucleus is LINER, inner disk < r=15’’, central BH with 109 M☼ i = 84 º p.a. = 90 º HST image 3kpc=70“

  4. Emsellem (1995): spatial photometric model → not only extinction, but also light scattering by dust → galaxy would appear dust free if viewed face-on, significant cold dust expected in M104 which should be detectable in mm / sub-mm wavelength range i = 84 º p.a. = 90 º HST image

  5. PIis uneffected by reversals of B RM is sensitive to direction of B

  6. Sombrero galaxy M104: Linear polarization at λ = 6.2 cm (VLA) λ = 3.6 cm (100-m Effelsberg) Sub-mm continuum at λ =870 μm (HHT, Arizona) Is M104 a `normal´ spiral galaxy? Where does the huge bulge come from? Comparison with other edge-on galaxies like NGC891, NGC4631, NGC3628, NGC5907, NGC4565

  7. Magnetic field strength from synchrotron intensities • Assumtions: • Equipartitionbetween the average energy densities of cosmic rays and magnetic field • Results: • Average strength of total field (74 spirals): 9 ± 3 µG • Average strength of regular field: 1-5 µG • Maximum strength of total field in spiral arms: • 25 µG (NGC6946), 35 µG (M51) • Strength of regular field in interarm regions: • 10 µG (NGC6946), 15 µG (M51)

  8. Dumke, Krause et al., 1995

  9. Plane-parallel B-field is expected by the dynamo theory for differential rotation. • Thin disk (300 pc) and thick disk (1.8 kpc), except for NGC 4631 Dumke, Krauseet al.1995

  10. Edge-on Galaxies • Most galaxies have a magnetic field configuration parallelto the disk. • NGC891, NGC3628, NGC4565 and NGC5907 have all similar exponential scale heights for the thinandthickdisk/halo (0.3 / 1.8 kpc) resp., despite their different star forming activties. NGC4631 has only about 50% larger scale heights. • NGC4631 has a vertical magnetic field in the central ~7 kpc, M82, NGC4666 and NGC5775 have also vertical fields.

  11. M104 HST image

  12. M104 VLA 6cm 23“HPBW • First detection of large-scale • magnetic field in an Sa galaxy • Increasing vrot up to r = 8 kpc (180 “) • Gaussian z-distribution with scale heights of about 1.4 kpc in TP and PI (thin disk inside • huge mass distribution, the huge bulge)

  13. RM < ± 100 rad/m² for most points → M104 B-vectors • B is parallel to disk in midplane • significant vertical components at higher z • Bt = 6 ± 1 μG, Breg = 3 ± 1 μG • → magnetic field in M104 is similar to that of other edge-on galaxies

  14. HHT observations at λ870μm Where does the huge bulge comes from? Is Emsellem‘s dust model correct? • Detection in the nuclear region with • S 870μm = 230 ± 35 mJy • Upper limit fo extended emission of • (r.m.s. = 40 mJy/beam at 40”HPBW) • S 870μm≤ 200 mJy → with T = 22 K: M cold dust ≤ 1.2 107 M☼ (Emsellem: M cold dust ≥ 8 106 M☼)

  15. Where does the huge bulge comes from? • Simulations of galaxy evolution revealed that bars form and dissolve. • A spherical bulge can evolve while the bar starts to decrease. • →The bulge in M104 may be due to a dissolving bar(first proposed by • Emsellem, 1995) • A dissolving of a bar is a relatively short-living period compared to a galaxy‘s lifetime → This evolutionary phase should be observable only in a few number of galaxies. • The large-scale magnetic field seems to have persisted during the galaxy‘s evolution.

  16. Conclusions: • Our dust observations support Emsellem‘s dust model (i.e. M104 would look dust free if viewed face-on). • We have detected a large scale magnetic field in M104 with a typical configuration for spiral galaxies seen edge-on. • The idea that the bulge is due to a dissolving bar (in a short-living evolutionary phase) also explains the outstanding appearance of M104 (as observable with present telescopes) → M104 seems to be a ‘normal‘ early type galaxy in a special (short-living) evolutionary phase.

  17. Thank you

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