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we have selected nearby edge - on oriented starburst galaxies to perform a multi - wavelength ( x - ray , radio - continuum , h and optical ) study of all phases of extraplanar gas to investigate the dependence of galactic halo properties on the energy input rate due to star formation ( sf ) in the underlying galactic disk .
special care must be taken to separate halos created by a sufficiently high energy input and extra - planar emission which is caused by galactic interactions or nuclear activity .
the galaxy s size , i.e. the depth of the gravitational potential , is another parameter affecting the evolution of galactic halos .
one of the main goals of this project is the determination of the energy budget in the halos ( magnetic field , thermal and radiation energy densities ) .
( @xcite ) x - ray observations presented here are used to detect previously unknown extended halo emission due to hot gas and/or to re - visit galaxies with known ( from earlier x - ray missions ) extra - planar x - ray emission features but now with a hitherto unreached sensitivity that can only be provided by deep   observations .
such observations also allow us to investigate with unprecedented signal - to - noise the characteristics ( temperature , metallicity , energy density ) of the hot gas via x - ray spectroscopy .
an earlier report on the status of our ongoing work was given by @xcite and @xcite .
a more detailed motivation and general introduction of our multi - wavelength project can be found in the paper by m. dahlem ( this volume ) .
this paper is based on  observations of the starburst galaxies ngc  1511 , ngc  4666 and ngc  3628 carried out as part of the guaranteed time proposal 011098 in july 2000 , june 2002 and november 2000 , respectively .
the x - ray data have been cleaned for periods of high radiation background and images and spectra were generated with the science analysis system ( sas ) software package ( in its version 5.4.1 , except for ngc  1511 that was processed with 5.3.3 ) .
we also made use of the epic background blank field files and tools provided by a. read @xcite .
the starburst saa pec : h type galaxy ngc  1511 was studied as part of our project and x - ray results presented at this meeting have been published in @xcite .
epic ( @xcite ) revealed for the first time the presence of a diffuse hot gaseous phase in ngc  1511 partly extending out of the disk plane .
extra - planar emission due to cosmic rays and magnetic fields was earlier seen in radio continuum emission @xcite and is suggestive of a common origin for the outflow of these components of the ism .
the x - ray spectral analysis of the integrated 0.2 - 12 kev emission ( excluding a strong point source about 30 north of the centre , which - if associated with ngc  1511 - might be an ultra - luminous @xmath0  erg s@xmath1 x - ray source ) showed a complex emission composition : one ( although not the only possible ) best - fitting model was found consisting of two thermal components and a power law contributing 12% ( 0.19 kev ) , 11% ( 0.59 kev ) and 77% ( powerl ) to the total flux , respectively .
the finding that a spectral model with a single temperature gas component is not sufficient to describe the emission , points toward the fact that the x - ray emitting gas contains several phases .
the best - fit model corresponding total x - ray luminosity ( @xmath2  erg s@xmath1 ) leads to a far - infrared - to - x - ray luminosity ratio for ngc  1511 which is typical for starburst galaxies ( @xcite ) .
the optical monitor ( om , @xcite ) on board  was used to observe ngc  1511 in the uv during the x - ray observations and obtained images showing that this galaxy is heavily disturbed ( as it also can be seen in h@xmath3 ( @xcite ) and in the near infra - red ) .
strong evidence for tidal interactions has only recently been obtained through h observations @xcite , unfortunately rendering ngc  1511 unsuitable as target galaxy to study the dependence of its gaseous halo s properties on the distribution and level of sf in the underlying disk .
based on multi - wavelength ( optical , radio continuum and  x - ray observations ) , we ` classified ' ngc  4666 as a ` superwind ' sc galaxy @xcite harboring an extra - planar outflow cone emanating from a central sf region of @xmath4  kpc in radius , and having an opening angle of @xmath5 .
the outflow could be traced up to @xmath6  kpc above the disk plane by optical emission line filaments .
the  x - ray observations ( @xcite , see fig .
[ fig_n4666_xray ] , left panel ) indicated the presence of soft extended x - ray emission outside of the disk on the north - western ( closer to us ) side of ngc  4666 .
our  epic observation , in contrast , show for the first time that such soft x - ray emission exists on both sides of the galactic disk , originating from a huge , structured hot gas halo ( fig .
[ fig_n4666_xray ] , right panel , and fig .
[ fig_n4666_multi ] ) .
the diffuse emission detected by epic - pn is strong enough to allow us to perform a spectral analysis in several areas in the galactic disk and halo : the complex spectrum of the diffuse disk emission ( fig .
[ fig_n4666_pnspectrum ] , top ) can be fitted by a combination of an internally absorbed mekal ( 0.54  kev ) and power law component plus another mekal model ( 0.18  kev ) all affected by galactic foreground absorption . the lower halo emission ( fig .
[ fig_n4666_pnspectrum ] , middle ) shows a similar spectral behavior as the disk spectrum .
the spectrum of the upper halo emission ( fig .
[ fig_n4666_pnspectrum ] , bottom ) , however , does not need a second mekal component but is fitted reasonably well also with a single 0.23  kev thermal plasma .
most of the total flux above 0.9  kev originates from the power - law type emission ( presumably due to unresolved point - like sources ) , whereas the thermal plasma clearly dominates in the soft 0.3 - 0.9  kev band ( contributing 72% , 69% and 92% to the ` diffuse ' disk , lower and upper halo emission , respectively ) .
if the diffuse x - ray emission in the soft energy band is assumed to be due to hot gas , it is possible to calculate the gas density @xmath7 and its mass @xmath8 . to this end
we used the model of thermal cooling and ionization equilibrium of @xcite where @xmath9 .
the unknown filling factor @xmath10 allows for some clumpiness of the gas .
if we fit all the spectra extracted from the three areas with a single - temperature mekal model , the unabsorbed flux in the 0.3 - 12  kev and corresponding luminosity ( adopting @xmath11  mpc ) are given in tab .
[ tab_n4666 ] . for the gas temperatures of the hot gas , @xcite give a cooling coefficient @xmath12 of @xmath13 erg @xmath14 s@xmath1 .
assuming an ellipsoid for the disk and the outflow cone geometry ( see above ) for the emitting halo volume @xmath15 , and typical volume filling factors of 0.1 - 0.8 , we derive gas densities and masses listed in tab .
[ tab_n4666 ] .
lcccccc & t & @xmath16 & @xmath17 & @xmath18 & @xmath19 & @xmath20 + & & & & & & + disk & @xmath21 & 7.21 & 5.97 & 1.0 - 2.9 & 2.5 - 7 & 0.1 - 0.2 + lower halo & @xmath22 & 2.77 & 2.29 & 2.5 - 7.1 & 0.5 - 1 & 0.7 - 2.0 + upper halo & @xmath23 & 4.40 & 3.64 & 1.2 - 3.3 & 1 - 4 & 0.2 - 0.6 + total halo & @xmath24 & 7.38 & 6.11 & 1.4 - 4.0 & 2 - 5 & 0.3 - 0.9 +    such estimated gas densities together with the fitted temperature allow us to compare the energy density of the hot gas with e.g. that of the magnetic field ( magnetic field strengths were derived from radio continuum observations as 14.4  @xmath25 g in the disk and 7.1  @xmath25 g in the halo ( see @xcite ) . whereas in general the ratio between the thermal and magnetic energy densities is @xmath26 ( see tab .  [ tab_n4666 ] ) , and hence the magnetic field important for the dynamics of the hot gas ( ` channeling ' the outflow ) , in the lower halo the relatively high thermal energy density is supportive of the idea that in this region the gas outflow might easily take place , even against a disk - parallel magnetic field configuration .    a detailed analysis of the halo emission and a possible correlation with h@xmath3 and radio polarization filaments (
as started in @xcite , see also fig .
[ fig_n4666_multi ] ) will be addressed in an upcoming paper by ehle et al . ; there we also plan to present a self - consistent dynamical and thermal spectral model @xcite which does no longer depend on the usual assumption of collisional ionization equilibrium ( cie ) .
we note that the same  observations have been discussed by @xcite comparing the disk - emission with spatially unresolving _ bepposax _ data .
the authors discuss their findings with respect to the discovery of starburst plus agn activity both contributing to the x - ray emission of ngc  4666 : the sf activity was found to be extended over most of the disk and associated with diffuse thermal emission , whereas the quite small low - luminosity type-2 agn contribution ( revealed by prominent k@xmath3 line emission from `` cold '' iron at 6.40  kev ) was detected from the nuclear region .    in ngc  4666 , @xcite failed to detect conclusive evidence for the existence of h gas in its halo .
however , seen in their h maps are prominent tidal arms probably generated by interactions with the galaxy ngc  4668 and previously undetected dwarf companions .
the high sf rate responsible for the superwind of ngc  4666 hence might be triggered by gravitational interaction .
ngc  3628 is a peculiar sbc galaxy known to be an interacting member in the leo triplet ( arp 317 , assumed distance @xmath27  mpc ) .
earlier x - ray observations with _ einstein _ @xcite and  ( @xcite , see fig .
[ fig_n3628_rosat+xmm ] , left panel ) showed evidence for a collimated outflow along the minor axis from a starburst nucleus in ngc  3628 and led to the detection of an extended soft x - ray halo .
our xmm - newton observations ( fig .
[ fig_n3628_rosat+xmm ] , right panel ) are able to detect the extraplanar diffuse emission with much higher significance : the epic image clearly separates the southern collimated spur - like halo emission from nearby ( most possibly background ) point sources ( see fig .
[ fig_n3628_xmm+opt ] , left panel ) and calls the proposed link between this x - ray filament and qsos @xcite to question .    a detailed comparison of the diffuse extraplanar x - ray emission with for example the h@xmath3 filaments ( a plume extending about 130 to the sw in position angle @xmath28 and faint more widespread filamentary extraplanar structures to the north - see h@xmath3 map from @xcite or @xcite and fig .  [ fig_n3628_xmm+opt ] , right panel ) is ongoing .
in the framework of our multi - wavelength project to investigate gaseous halos around late - type spiral galaxies and their dependence on the level of star - formation activity in the underlying disks ( cf .
m. dahlem ( this volume ) ) we showed in this paper x - ray results from three of our sample galaxies .
the presented observations demonstrate that only with the advent of high sensitivity observatories ( like ) , it is becoming possible to detect and study in detail the hot halo gas component of the interstellar medium .
our original sample selection criterion ( based on far infra - red colors , see @xcite and references therein ) , that showed to be a good tool to select candidates for the search for radio halos , is likely to also work in the x - ray regime . in all three  targets
we find extra - planar x - ray emission which is bright enough to allow us detailed spectral investigations and also to test model assumptions ( i.e. collisional ionization equilibrium versus radiative cooling with the dynamics in full non - equilibrium ) .
we note the co - existence of extra - planar soft x - ray emission above the most actively star forming regions in the galactic disk and the co - existence of such features with h@xmath3 filaments as well as vertical magnetic field structures ( in case of ngc  4666 ) .
ehle , m. , dahlem , m. , jimnez bailn , e. , santos - lle , m. , & read , a. m. 2004 , in iau symp .
217 , recycling intergalactic and interstellar matter , eds .
duc , j. braine , e. brinks , san francisco : astronomical society of the pacific , 154
we report on first results of  observations of nearby starburst galaxies that form part of a multi - wavelength study of gaseous halos around late - type spiral galaxies and their dependence on the level of star formation activity in the underlying disks .    
 , with its extraordinary sensitivity for faint extended x - ray emission , is used to derive spatial and spectral properties of the very hot extraplanar / halo gas . 
 for example , spectral models can be tested and hot gas properties like density , mass and energy can be estimated . comparing the distribution of the halo x - ray emission with optical filaments and/or observed magnetic field structures uncovers interesting correlations on which work just has started .    
 our study aims - in general - at assessing the importance of galactic halos as repositories of a metal - enriched hot medium and their significance in terms of galactic chemical evolution and possible metal enrichment of the intergalactic medium .