Kashi1 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

Kashi2 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

Kashi3 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 10 9 M ☼ HST image i = 84 º p.a. = 90 º 3kpc=70“

Kashi4 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 HST image i = 84 º p.a. = 90 º

Kashi5 PI is uneffected by reversals of B RM is sensitive to direction of B

Kashi6 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

Kashi7 Magnetic field strength from synchrotron intensities Assumtions: Equipartition between 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)

Kashi8 Dumke, Krause et al., 1995

Kashi9 Dumke, Krause et al.1995 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

Kashi10 Edge-on Galaxies Most galaxies have a magnetic field configuration parallel to the disk. NGC891, NGC3628, NGC4565 and NGC5907 have all similar exponential scale heights for the thin and thick disk/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.

Kashi11 M104 HST image

Kashi12 First detection of large-scale magnetic field in an Sa galaxy Increasing v rot 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) M104 VLA 6cm 23“HPBW

Kashi13 RM < ± 100 rad/m² for most points → M104 B-vectors B is parallel to disk in midplane significant vertical components at higher z B t = 6 ± 1 μG, B reg = 3 ± 1 μG → magnetic field in M104 is similar to that of other edge-on galaxies

Kashi14 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 ≤ M ☼ (Emsellem: M cold dust ≥ M ☼ ) HHT observations at λ 870μm

Kashi15 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.

Kashi16 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.

Kashi17 Thank you