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    {\small\rm\zihao{6}\songti\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
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    {\small\rm \zihao{6}\songti suppl.\hfill R.-J. Dettmar: Gaseous Halos of Spiral Galaxies  %%（作者：题目名称）
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\zihao{5}\songti

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    \zihao{6}\songti
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    Vol.44~ Suppl.
    \hfill\hspace{8mm} %%% if ``October/January'' then \hspace{8/10 mm} July
    \small ACTA ASTRONOMICA SINICA
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      Feb.,~2003 \\[-1.5 true mm] %%January
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\vs\vs\vs \bc {\Large\bf
Gaseous Halos and the Interstellar Disk-halo \\[2mm] Connection in Spiral Galaxies    %%（文题）
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize  Ralf-J\"urgen Dettmar}  %%（本文作者）

\vs
({\footnotesize \it Astronomical Institute, Ruhr-University Bochum, Germany})\\
 %%（本文作者单位）
 {\footnotesize dettmar@astro.ruhr-uni-bochum.de}
\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
The presence of gaseous halos in star forming disk galaxies is
reviewed in the context of a proposed disk-halo connection of the
interstellar medium (ISM). New results from a quantitative survey
for H$^+$ halos of edge-on galaxies (Rossa \& Dettmar 2002) are
presented. The data confirm that the presence of diffuse ionized
gas (DIG) in the disk-halo interface of spiral galaxies is related
to star formation processes in the underlying disk and allow us to
establish a minimum energy release per unit area that is required
to start the disk-halo mass exchange. By comparing  some recent
observational results for diagnostic emission lines with model
predictions from photoionization we demonstrate that the  origin
and excitation  of the DIG is, however,  still not completely
understood and that a discussion gives important constraints for
models of the ISM. In comparison with similar findings for the
Milky Way the need for an additional heating source is
established. Special emphasis is given to some recent
developments. In particular, new kinematical information for the
DIG layer in NGC\,5775 from ESO/VLT long-slit spectra is discussed
in connection with the magnetic field structure
 in the halo of this  object as deduced from VLA radio-continuum polarization
 data (T\"ullmann et al. 2001). Finally, the r\^ole of dust for the physical processes
 in the disk-halo interface is briefly addressed.

 \vn
 \no{\bf Key words}~~  galaxies: evolution---galaxies: ISM---galaxies:
halos---ISM: general
\end{quotation}

\para{\large\bf{1}~~ Introduction}

\vs\vs
Ever since the first evidence for radio-continuum halos in
the Milky Way and
 other galaxies -- such as in the prototypical case of NGC\,4631
-- the physical
processes for the origin of gaseous galactic halos were mainly sought in the
feedback of energy into the interstellar medium (ISM) by young stars through
winds, the interstellar radiation field, and (multiple)
supernova explosions. From this general idea the concept of the disk-halo
interaction emerged in which the disk medium can connect to the halo.
This is described in different model approaches in terms of galactic fountains,
chimneys, or super-bubble-outbreak by theory. Models for the gas transport from
the disk into the halo are proposed in
several theoretical approaches such as the chimney scenario
(Norman \& Ikeuchi 1989), galactic  fountains  or
superbubble blow-outs (e.g., McLow \& Ferrara 1999).

Observational support for the idea of such a large scale matter exchange
between disk and halo comes from observations of gaseous halos in
external galaxies
(e.g., Dahlem 1997, Dettmar 1998).
While several phases of the ISM -- from cold HI to  X-ray coronae and even molecular
gas --
have meanwhile been found in the halos of
spiral galaxies, the H$^+$ with a scale height of typically 1\,kpc in the disk-halo
interface of spiral galaxies (corresponding to the Reynolds layer
in our Milky Way)  represents a particularly
important constituent, since it is relatively easily observed in the optical
(Dettmar 1992, 1998). Therefore,  imaging in and spectroscopy of optical
emission lines allow us to study the distribution and excitation of this
Diffuse
Ionized Gas (DIG, or WIM for Warm Ionized Medium) with a spatial resolution not achievable for other phases of the ISM in external
galaxies. To demonstrate the distribution of DIG in halos of galaxies we
reproduce in Fig. 1 a H$\alpha$-image of one of our target galaxies (Rossa
\& Dettmar 2000).

\begin{figure}[h]
\center {\includegraphics [scale=0.75]{xian_fig0.ps}}

   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \vs\footnotesize

      Fig.\,1~~ The galaxy NGC\,4634 exhibits a bright and thick
layer of extraplanar H$^+$ (from Rossa \& Dettmar 2000). The
encircled object is a probable dwarf companion}

    \end{figure}

In the following we present results of an ongoing investigation of
extraplanar DIG (eDIG) in edge-on galaxies. Based on a
quantitative H$\alpha$ survey and emission line spectroscopy we
are trying to answer questions such as: How common are galactic
gaseous halos? What is the minimal SFR per unit area which
triggers gaseous galactic outflows? Is there a correlation between
gaseous halos and other halo components such as radio continuum
``thick disks'' that are seen in several edge--on galaxies? Is the
magnetic field structure in galaxies related to outflows? What are
the ionization and excitation processes for DIG at high galactic
latitudes?

\para{\large\bf{2}~~DIG Halo Gas and the Star Formation Rate}

\begin{figure}[h]
\center {\includegraphics [scale=0.8]{xian_fig1.eps}}

   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \vs\footnotesize

      Fig.\,2~~ Diagnostic DIG diagram for starburst and non--starburst
edge--on galaxies. The ratio of the FIR fluxes at 60$\mu$m and
100$\mu$m ($S_{60}/S_{100}$) is plotted versus the ratio of the
FIR luminosity to the optical galaxy diameter squared of the
25$^{th}$\,mag in units of $10^{40} \rm{\,erg\,s^{-1}\,kpc^{-2}}$.
The filled lozenges denote the starburst galaxy sample studied by
Lehnert \& Heckman (1995). Squares indicate our new observations
with filled symbols for detections of H$^+$ gas in the halo and
open squares for non-detections (from Rossa \& Dettmar 2003).}

    \end{figure}

A halo component of DIG for external galaxies was first discovered
in NGC\,891 a decade ago (Dettmar 1990, Rand et al. 1990). A few
small samples have been studied so far (e.g.,\,Pildis et al. 1994,
Rand 1996; Rossa \& Dettmar 2000), however, more complete  and
larger samples are required to clearly demonstrate the suggested
correlations of halo gas properties with the star formation rate
(SFR) in the disk. In particular, a broader range in SFR has to be
covered since up to now emphasis was given to starburst galaxies.
An attempt to relate  the presence of DIG in halos of spiral
galaxies to their far infrared (FIR) properties as a measure of
the SFR is shown in Fig. 2. Here a sample of galaxies was chosen
such that it supplements the previously studied ``starburst''
objects at the low activity end. If the starformation rate per
unit area is low as determined by the FIR luminosity normalized to
the disk surface the presence of halo DIG indeed diminishes. The
FIR luminosity per unit area therefore seems to be a promising
indicator for the presence of halo  H$^+$ gas and a minimum SFR
per area seems to be required to drive the disk-halo interaction
(Rossa \& Dettmar 2002).
\begin{figure}[tbph]
\center {\includegraphics [scale=1]{xian_fig2.eps}}
\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize Fig.\,3~~ Emission line rations and temperature profile (uppermost
panel) of the DIG in NGC\,5775 along a slit cutting the SE disk
perpendicular at $\sim$4\,kpc from the nucleus (from T\"ullmann et
al. 2001).}
\end{figure}


 \para{\large\bf{3}~~Ionization of Halo DIG}

\vs\vs It has been shown in the past (Domg\"orgen\,\&\,Mathis
1994, Golla et al. 1996, T\"ullmann \& Dettmar 2000) that the
emission line characteristics of DIG in galactic halos is not
explained by photoionization in a straight forward manner. The
detailed ionization structure rather remains  a major problem for
pure photoionization models and several  recent studies for
extraplanar DIG in the Milky Way and NGC\,891 (Reynolds et al.
1999, Rand 1997, 1998) therefore claimed a need for secondary
heating and/or ionizing sources. Ideas for such supplemental
mechanisms include shock ionization, turbulent dissipation, and
magnetic reconnection. In order to examine possible ionization
and/or excitation mechanisms of the eDIG we compared
spectroscopically observed emission line ratios and diagnostic
diagrams with model predictions (photoionization codes of Mathis
1986 and Domg\"orgen\,\&\,Mathis 1994) for a small sample of
edge-on galaxies (T\"ullmann \& Dettmar 2000). The relation
between the
 [SII]/H$\alpha$ and [NII]/H$\alpha$ ratios
which are both raising with
increasing distance from the galactic mid-plane is also observed in the
Milky Way. Here it is explained as an effect of  an increase of
temperature at higher latitudes (Reynolds et al. 1999). This behavior of emission
lines for halo DIG is also
observed in NGC\,5775 as shown in Fig. 3.
The very deep spectrum of this galaxy, tracing diagnostic emission lines
out to 10\,kpc from the midplane, was observed with the FORS1 instrument at
the ESO VLT-UT1. From the observed emission line ratios the temperature
profile in the upper panel of the
figure was deduced showing the same general behaviour of an increasing temperature
as it is discussed for the  Milky Way (see above).

\begin{figure}[t]
\center {\includegraphics [scale=0.80]{xian_fig3.eps}}

   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize Fig.\,4~~ DIG
kinematics along slit perpendicular to major axis. The velocity
drops from rotational velocity at the midplane to systemic in the
halo (from Rossa \& Dettmar et al. 2003).}

    \end{figure}

\para{\large\bf{4}~~Kinematics of Halo Gas}

\begin{figure}
\center {\includegraphics [scale=0.8]{xian_fig4.ps}}

   \parbox[tb]{13cm}
   %{\hspace{3.5cm}
   {%\center
   \vs\footnotesize Fig.\,5~~ VLA radiocontinuum map of NGC\,5775 at 6cm; the left panel gives
total power while the right panel represents polarized intensity.
The bars represent the magnetic field direction. For details see
T\"ullmann et al. (2000).}

    \end{figure}

\vs\vs A particularly striking result is related to the
kinematical information regarding the DIG layer which is
extracted from the same deep VLT spectrum discussed in
 the previous section. As shown in Fig. 4 the velocity of the DIG at a
galactocentric distance of $\approx$4\,kpc is dropping from the rotational
velocity in the disk plane to systemic at a distance of $\sim$10\,kpc
above the
plane. This is most likely not just a local perturbance in the velocity field
since similar spectra
of NGC\,5775 by Rand (2000) show the same phenomenon for two different slit
positions. A model to explain such a  velocity field reaching systemic
velocity at large distances is currently not available. Observations of UV
absorption line systems by C\^ot\`e et al. (2000), however, show a similar
phenomenon and therefore ask for a more general solution. Theoretical attempts
may require a reconsideration of all relevant processes such as the
discussion of forces by magnetic tension as introduced by, e.g.,
Benjamin (2000) in response to the
observed slight drop in rotational velocity for halo gas in NGC\,891 by Rand
(1998) and Swaters et al. (1997).

\begin{figure}[h]
\center {\includegraphics [scale=0.85]{xian_fig5.eps}}
\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
  \vs \footnotesize Fig.\,6~~ Unsharp masked R-band continuum image of NGC\,891
   showing the
highly structured dust distribution in the halo. (from Rossa et
al. 2002).}
\end{figure}

\para{\large\bf{5}~~The Magnetic Field Structure in the Halo}

\vs\vs The presence of a disk-halo interaction in a galaxy traced
by DIG in the halo is also supported by the frequently found
simultaneous presence  of other ISM constituents such as X-ray
emitting plasmas or cosmic ray electrons  observed in the radio
continuum. The break-out condition for cosmic rays as a function
of star formation rate per unit area is discussed by Dahlem et al.
(1995) and a compilation of observations of edge-on galaxies in
H$\alpha$, radio continuum, and X-rays is given in Dettmar (1998).

The previously discussed case of NGC\,5775 also shows a prominent radio
continuum halo (as well as a X-ray halo and extraplanar CO emission; Lee et al. 2001)
and a  study of its polarization characteristics with the
VLA reveals even an
highly structured large scale magnetic field with field lines opening
up into the halo (Fig. 5). It is well possible that the velocity structure
of the halo gas described above in Sect. 4 is related to the origin of the
magnetic field since the theory of galactic dynamos relates the z-component of the magnetic field and a shear caused by the gradient dv/dz to the generation of
an azimuthal magnetic component. This is, e.g., discussed by
Men and Han (2003) in these proceedings.


\para{\large\bf{6}~~Dust in the Halo}

\vs\vs Since the star formation process in the disks will also
locally enhance the radiation field it was suggested more than a
decade ago that dust could be elevated by radiation pressure from
the disk into the halo (Franco et al. 1991). Ferrara (1997)
modeled the evolution in time of the dust distribution under the
influence of the radiation field caused by an
  OB association
and showed that the dust would be dispersed into a large volume. This,
however, is in contrast to the observed dust distribution in several galaxies.
Sofue et al. (1994) demonstrate that the dust in the disk of NGC\,253 is highly structured in large scale
arches much resembling solar prominences in shape.
Also the dust distribution in the
edge-on galaxy NGC\,891 studied, e.g., by Howk and Savage (1997) is organized in long filaments reaching from the disk into the halo. These filaments
 end in a level
of dust sheets high above the disk plane. The case of NGC\,891 is also studied
by Rossa et al. (2002) with the high resolution imaging capability of HST
and in Fig. 6 we present a first image of the dust distribution from these
WFPC observations. The possible influence of magnetic
fields on the charged dust particles as an additional process to
structure  the dust distribution  in galactic halos was put forward
by Shchekinov et al. (1999). In the presence of large scale organized
magnetic fields in galactic halos as demonstrated above for
the case of NGC\,5775 such processes have to be discussed in more detail
in the future. It should be mentioned here that in such an environment also
the dipole radiation of spinning dust (Ferrara \& Dettmar 1994, Draine \& Lazarian 1999) could contribute to the radioemission from galaxies at GHz frequencies.

\para{\large\bf{7}~~Summary}

\vs\vs Gaseous DIG halos are found in galaxies with sufficient SFR
per unit area. Typically these layers of extraplanar DIG can be
traced out to  distances of z $\leq$1--2\,kpc --  sometimes even
up to 5\,kpc or more -- from the mid-plane of the disk. The
emission line ratios indicate an increase in the electron
temperature of the DIG with increasing hight above the midplane.
This behaviour is similar to the Reynolds layer in the Milky Way
and a heating mechanism in  addition to photoionization is
currently being discussed. These H$^+$ halos seem to be associated
with halos of cosmic rays and X-ray plasma as expected if caused
by the disk-halo interaction of the ISM. The polarized
radiocontinuum emissions indicates an ordered magnetic field in
the halo. The peculiar velocity structure of the halo gas in the
case of NGC\,5775 is possibly related to this ordered magnetic
field and also the highly structure dust distribution in the halos
of some galaxies could be influenced by magnetic fields due the
coupling of charged dust to both the radiation and magnetic
fields.

\vs\vn{\bf ACKNOWLEDGEMENTS}~~ The author wishes to thank his
students T. Elwert, J. Rossa, and R. T\"ullmann as well as
numerous  colleagues involved in the presented work for a
 fruitful collaboration. This work is supported by Deutsche
For\-schungs\-ge\-mein\-schaft (DFG) and Deutsches
Zentrum f\"ur Luft- und Raumfahrt
(DLR). The discussed observational material was obtained at ESO
(Chile), the NASA/ESA HST, and the VLA/NRAO (USA). The presentation
would not have been possible without travel support by the Max-Planck-Society.

%\newpage\vspace*{1mm}
\bc {\bf References} \ec

\vspace{-4mm} \no \fns

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