Kinematics of dust outflow

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Nov 13, 2013 (3 years and 8 months ago)

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Spectropolarimetry of the starburst galaxy M82:

Kinematics of dust outflow

Michitoshi YOSHIDA
1),2)
, Koji S. KAWABATA
1)
, and Yoichi OHYAMA
3)

1)

Hiroshima University,
2)

Okayama Astrophysical Observatory, NAOJ,
3)

Academia Sinica, Institute of Astronomy and Astrophysics

Abstract

Spectropolarimetry results for the starburst galaxy M82 are presented. The optical emission lines of the filaments in the ene
rge
tic outflow
(“superwind”) from the nuclear starburst region of M82 are substantially polarized. The polarized emission lines are redshift
ed
with respect to the
emission lines in the total light and systemic motion of the galaxy. The emission line intensity ratios in the polarized ligh
t a
nd the electron density
N
e

derived from the [SII]λ6731/λ6717 line ratio of the polarized light strongly suggest that the emission from the nuclear starb
urs
t of M82 is scattered by
dust grains entrained and transported outward by the superwind. A simple hollow biconical outflow model shows that the veloci
ty
of the outflowing
dust grains,
v
d
, ranges from 100 to 200 km s
-
1

near the nucleus, decreases monotonically with the distance from the nucleus, and reaches ~10 km s
-
1

at around 1 kpc. The motion of the dust is substantially slower than that of both ionized gas (
v


~ 600 km s
-
1
) and molecular gas (
v
CO

~ 200 km s
-
1
) at the same distance from the nucleus of M82. This indicates that dust grains in the superwind are kinematically decoupled
fro
m both gas
components at large radii. Since the dust velocity
v
d
is much less than the escape velocity of M82 (
v
esc

= 170 km s
-
1

at 1.5 kpc from the nucleus),
most of the dust entrained by the superwind cannot escape to intergalactic space, and may fall back into the galaxy disk with
out

any additional
acceleration mechanisms (such as radiation pressure).

Observations

Subaru Telescope + FOCAS

0.”6 slit, VPH650 grism → R
≈ 2100

Wollaston prism + half wavelength plate

(φ=0˚, 45
˚
, 22.5
˚
, 67.5
˚
)



2003/12/21 Slit PA=150
˚

exp.time 600 sec


2003/12/22 Slit PA=134
˚

exp.time 720 sec x 4



Extract eight regions (see Fig.1) from the
obtained spectra

Results & Discussion




























Slit positions of the spectropolarimetry of M82.

Observed spectra of M82. Upper panels show H
α
+[NII] lines and lower panels show [SII] lines. Polarized flux (solid lines) super
-

imposed on total (unpolarized )flux (dotted line), polarization degree, and polarization angle are shown in each panel. The p
ola
rized

emission
-
lines are systematically redshifted with respect to the unpolarized lines.

Velocity field of the polarized and
unpolarized Hα lines

Introduction

M82
D

= 3.6Mpc
M
B
=
-
18.95
L
IR

= 5.4x10
10
L



v
sys
= 214 km/s
v
rot
= 135 km/s

Huge dust in the superwind of M82

optical imaging (e.g Ohyama 2002), imaging polarimetry (e.g
Scarrott 1991), mid
-
infrared imaging (e.g Engelbracht 2006,
Kaneda 2010), sub
-
mm imaging (e.g Leeuw 2009)


Can the dust escape from the galaxy to
intergalactic space ?

→ Spectropolarimetry to study the kinematics of
the scattering material (=dust) in the superwind

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A

(
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α
+

[NII]

[SII]

R1

R2

R3

R4,5,6

R7

R8

Polarized (scattered) light

Total flux

polarization

degree

polarization

angle

polarized

unpolarized

V
sys

of M82

The radial velocities of the polarized emission lines

are higher than the systemic velocity of the galaxy.

Outflow model

Dust flow

velocity field

Electron density
N
e

Excitation diagram

nuclear

region

of M82

The Ne derived from the polarized
[SII] lines (red circle) are significantly
higher

than those derived from the
unpolarized lines (blue circle).

The excitation of the polarized
emission lines (red circles) is
similar to that of the

circum
nuclear gas of M82.

A hollow cone model of the M82 dust wind.

The dust grains in the galaxy disk are
entrained

by the superwind expelled from
a circumnuclear

circular region whose
radius in the disk is
b
. The entrained dust
flows along the walls of a cone whose
opening angle is 2
×
θ. The angle of I

nclination of the axis of the dust flow cone
is
i
.

The dust grains in the wind reflect the
nuclear

emission, acting as mirrors
moving at velocity vd cos
y

with respect
to the nucleus.

The polarized light is the nuclear light

scattered by dust in the superwind.

θ
= 25
°
case

θ
= 12.5
°
case

θ

the opening angle



of the outflow

The dust outflow of M82 is monotonically decreasing with the distance from the disk.

Element

Velocity at 1 kpc

Reference

Ionized gas (Hα)

600 km/s

Shopbell & Bland
-
Hawthorn (1998)

Molecular gas (CO)

220 km/s

Walter et al. (2002)

Dust

50
-
100 km/s

This work

Comparison with other elements

The dust in the superwind of M82 is kinematically

decoupled with both of the ionized gas and the

molecular gas in the wind.

The dust is very slow !!