9.9.2005 Kashi 1

Radio continuum observations Radio continuum observations of the Sombrero galaxy NGC4594 (M104) of the Sombrero galaxy NGC4594 (M104)

and other edge-on spiralsand other edge-on spirals

Marita Krause MPIfR, Bonn

Michael Dumke ESO, Chile

Richard Wielebinski MPIfR, Bonn

9.9.2005 Kashi 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

9.9.2005 Kashi 3

M104: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☼

HST image

i = 84 º

p.a. = 90 º

3kpc=70“

9.9.2005 Kashi 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

HST image

i = 84 º

p.a. = 90 º

9.9.2005 Kashi 5

PI is uneffected by reversals of B

RM is sensitive to direction of B

9.9.2005 Kashi 6

Sombrero galaxy M104: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?Is M104 a `normal´ spiral galaxy?

Where does the huge bulge come from?Where does the huge bulge come from?

Comparison with other edge-on galaxies like

NGC891, NGC4631, NGC3628, NGC5907, NGC4565NGC891, NGC4631, NGC3628, NGC5907, NGC4565

9.9.2005 Kashi 7

Magnetic field strength from synchrotron intensitiesMagnetic field strength from synchrotron intensitiesAssumtions:

•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)

9.9.2005 Kashi 8

Dumke, Krause et al., 1995

9.9.2005 Kashi 9

Dumke, Krauseet 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

9.9.2005 Kashi 10

Edge-on GalaxiesEdge-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.

9.9.2005 Kashi 11

M104 HST image

9.9.2005 Kashi 12

• 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)

M104 VLA 6cm 23“HPBW

9.9.2005 Kashi 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

9.9.2005 Kashi 14

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☼)

HHT observations at λ 870μm

9.9.2005 Kashi 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.

9.9.2005 Kashi 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.

9.9.2005 Kashi 17

Thank you