3.2. Results and notes on selected galaxies

In this subsection a brief description of the observed eDIG morphology of selected galaxies is presented in addition to some background information, relevant in the DIG context for those targets. The galaxies are listed according to their increasing Right Ascension. The H images of all survey galaxies are shown together with the accompanying broad band images (R-band, and unsharp-masked R-band image) in Figs. 22-54 (available only in electronic form at EDP Sciences). However, some enlargements of selected galaxies with characteristic and spectacular eDIG morphology are included in logarithmic scale as separate figures in this section, in order to highlight some finer details of eDIG emission. A comparison with observations by other researchers from various wavelength regimes in the context of the disk-halo interaction is made, whenever such observations were available in the literature.

NGC24

The H morphology of NGC24 comprises of planar DIG which is visible between several bright HII regions in the disk. No extraplanar DIG is detected which is basically due to the fact that this galaxy is not perfectly edge-on. Guthrie (1992) lists an inclination of 78°. Although the L_FIR/D²₂₅ ratio (for a definition of this expression please cf. Paper I) is quite low, the S₆₀ / S₁₀₀ ratio is moderate, and as the H distribution implies there is considerable star formation activity all over the disk. Additional evidence comes from a UV-study of nearby galaxies, where the morphology of the nuclear region in NGC24 was studied (Maoz et al., 1996). They classify NGC24 as a galaxy with star-forming morphology, and several knots or compact sources can be identified on the UV (~ 2300Å) image, obtained with the Faint Object Camera (FOC) on-board HST. These regions are either compact star clusters or individual OB stars.

UGC260

UGC260 is shown with another smaller edge-on galaxy, CGCG434-012, located 2'.4 west of UGC260 (see Fig. 21), which has a similar redshift (v = 47 km s^-1). Reshetnikov & Combes (1996) included them in their list of tidally-triggered disk thickening galaxies. Indeed, the morphology in the continuum-subtracted H image looks quite distorted. The HII regions are not aligned and just below the disk in the very northern part seems to be a small additional galaxy possibly in the process of merging, or this represents debris tidal tails. Extraplanar DIG is detected, representing a faint layer, with a few individual emission patches. The NED lists another galaxy pair between UGC260 and CGCG434-012, which is barely visible in our R-band image. We cannot rule out in this particular case, that the eDIG emission is triggered by interaction of one of the nearby galaxies.

MCG-2-3-16

This edge-on galaxy is paired with another edge-on spiral, MCG-2-3-15. However, these two galaxies are not physically associated, as MCG-2-3-15 has a much higher radial velocity of v_rad = 5765 km s^-1. Hence its H emission is shifted outside the passband of the used H filter. MCG-2-3-16 on the other side is not very prominent in H. It is one of our 12 survey galaxies, which has no detectable FIR flux at 60µm or 100µm at the sensitivity of IRAS. A small arc is seen in the western portion, extending about 430 pc north of the disk, which is not extraplanar. The disk shows a slight asymmetry in thickness from east to west which might be a projection effect due to a slight deviation from the edge-on character.

UGC1281

UGC1281 was included in the Effelsberg/VLA radio continuum survey by Hummel et al. (1991). However, no radio halo was detected. The H image did not reveal any extraplanar DIG emission. UGC1281 has also not been detected by IRAS, so this altogether hints for a low star formation activity. However, the H images show several bright HII regions in the disk which are not aligned. Two bright emission knots are slightly offset from the plane to the south.

UGC2082

UGC2082 is a northern edge-on spiral galaxy, which is located in the direction of the NGC1023 group (Tully, 1980). It has been investigated on the basis of a HST survey of large and bright nearby galaxies, studied with the FOC in the UV-regime at ~ 2300Å (Maoz et al., 1996). However, UGC2082 has not been detected, which is an indication that the SF activity within this galaxy is very low. That is reflected in our H images as well. We do not detect significant emission except some HII regions in the disk. The diagnostic FIR ratios are very low, too.

ESO362-11

ESO362-11 has a L_FIR / D²₂₅ ratio of ~ 2.6, which indicates a moderate SF activity. In our H images a weak layer of eDIG is found, but no filaments or plumes are detected. ESO362-11 is listed in the catalog of Southern Peculiar Galaxies and Associations (Arp & Madore, 1987) as AM0514-370, possibly due to a chain of 4 galaxies which are located in the vicinity of ESO362-11. These are on the outskirts of our covered field, and therefore are not shown in our appendix image. Telesco et al. (1988) report that galaxy interactions enhance the efficiency of SF activity, which is very likely. However, in the case of ESO362-11 it is not clear, whether or not the chain of galaxies, which are not in the close vicinity of ESO362-11, are able to enhance the SF activity. Coziol et al. (1998) even have classified ESO362-11 on the basis of their Pico Dos Dias Survey as a starburst galaxy, based on FIR spectral indices.

ESO209-9

This little studied nearby, southern edge-on galaxy has both a moderate ratio of L_FIR / D²₂₅, and S₆₀ / S₁₀₀. Its H morphology reveals an extended layer, not as prominent as in NGC891 or NGC3044, but still quite intense. The distribution of the H emission is asymmetric which could be a projection effect, if we were looking at the very end of the spiral arm to the south along the line of sight, whereas the spiral arm to the north could be winded more tightly. Quite extraordinary is an additional emission component, which shows up in the whole field around ESO209-9. We speculate that this emission, which is shown in its fully covered extent in the H+continuum image (see Fig. 3), is of Galactic origin. Quite remarkably, there is no hint of any filamentary emission in the broad R-band image. Our first impression was that this could be straylight from a bright star just outside the covered CCD field. However, as there was neither a shift (after alignment) nor a change in the emission pattern noticed in the two individual H images, which were offset by 20" from one another, and furthermore such structures were never recorded prior or after these exposures in other object frames, we suspect that these structures indeed have a Galactic origin. A visual inspection of both the blue and red DSS images did not reveal anything conclusive. Unfortunately, the IR DSS plate of the region around ESO209-9 is not yet obtained/digitized. The IRAS maps show some emission in that area, however, the resolution is not sufficient enough to resolve these structures clearly. A possible explanation could be, that this emission is Galactic cirrus. This would not be unreasonable, since ESO209-9 has a galactic latitude of only b -11°. Unfortunately, this position is just outside the regions studied by the AAO/UKST H survey ⁴ of the southern galactic plane (e.g., Parker et al., 1999), which would have been a good test for comparison. It could also be extended red emission (ERE) in the diffuse interstellar medium. This ERE was found for high-galactic cirrus clouds by Szomoru & Guhathakurta (1998). They found a peak of cirrus ERE at ~ 6000Å. However, in order to unravel the true nature of this filamentary emission, this should be re-investigated by independent deep H images using a different instrumental setup, and supplemental spectroscopy. Fortunately, very recently, we downloaded an available H image from the Southern H Sky Survey Atlas (SHASSA) ⁵ (see Fig. 2), which became available very recently. For details on SHASSA we refer to Gaustad et al. (2001). Indeed, the field around the galaxy ESO209-9 shows detectable H emission, and the H morphology is clearly recovered, despite the spatial resolution of the SHASSA is much lower than in our images.

Figure 2. Field No. 51 of the Southern H Sky Survey Atlas (SHASSA), which includes the Galactic region in the direction of ESO209-9. The field measures 13°. The field around ESO209-9 is indicated by a circle.

Figure 3. Field around ESO209-9, showing very structured, cirrus-like galactic emission in this H image. The corresponding R-band image does not show anything filamentary in the field of view. North is to the top and East is to the left. The field of view measures ~ 13'. Orientation: N is to the top and E to the left.

NGC3003

NGC3003 has been investigated spectroscopically by (Ho et al., 1995) in search of dwarf Seyfert nuclei, who concluded, based on a detected broad emission complex centered at 4650Å, that this galaxy is a Wolf-Rayet galaxy. NGC3003 has a modest L_FIR / D²₂₅ ratio (1.65), however, the S₆₀ / S₁₀₀ ratio of ~ 0.34 hints that there is some SF activity due to enhanced dust temperatures. Furthermore, due to the slight deviation from its edge-on character, no extraplanar emission can be identified reliably. The H distribution, however, reveals strong planar DIG, consistent with the observations by Hoopes et al. (1999). Several bright emission knots can be discerned. About four decades ago a SN has been detected 34"E, and 17"N of the nucleus (SN1961F).

NGC3221

This is one of the galaxies with the highest L_FIR / D²₂₅ ratio in our survey (13.8). The S₆₀ / S₁₀₀ ratio is also quite high (0.37), suggesting enhanced SF activity. Indeed, there are many bright HII regions visible in the disk, preferentially located in the spiral arms, which are slightly recognizable, due to the small deviation from its edge-on inclination. A faint eDIG layer is visible. There is a slight asymmetry between the northern part and the southern part of the galaxy (i.e. the two spiral arms) discernable. NGC3221 has also been investigated in the radio regime (radio continuum), where Irwin et al. (2000) detected extended disk emission. It is noteworthy to mention that a SN has been detected (SN1961L) in NGC3221.

NGC3501

NGC3501 was observed with the Effelsberg 100m radio dish at 5GHz in a survey to detect radio halos in edge-on galaxies, related to SF driven outflows (Hummel et al., 1991). No extended radio emission has been detected. The optical appearance, based on our H imaging reveals also a quiescent galaxy, with no extraplanar emission. Besides some modest HII regions in the disk, its morphology looks rather inconspicuous. In fact, it is also one of the 12 galaxies in our survey which has no FIR detections.

NGC3600

This edge-on spiral galaxy has been included in the catalog of Markarian galaxies as < ahref="/cgi-bin/objsearch?objname=Mrk+1443&extend=no&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES" target="ads_dw">Mrk1443 (Mazarella & Balzano, 1986). The broadband R image shows a prominent bulge and indicates a slight warp which was also noticed by (Sánchez-Saavedra et al., 1990). In our H image it becomes even more obvious that the disk is warped. There is some extraplanar DIG detected, basically around the central regions, where extended emission seems to be ejected from the nuclear region. There is, besides the nucleus, one bright emission region, presumably consisting of several smaller components, and several fainter knots visible in the disk. Although the L_FIR / D²₂₅ is quite moderate (1.4), NGC3600 has a relatively high S₆₀ / S₁₀₀ ratio of ~ 0.44. Spectroscopy of the nuclear region reveals strong emission from the Balmer lines (H, H), as well as from several forbidden low ionization lines, such as [NII], and [SII] (Ho et al., 1995). We show a detailed view of NGC3600 in Fig. 4.

Figure 4. Extraplanar DIG layer in NGC3600. The eDIG morphology is reminiscent of a starburst galaxy. The field of view measures ~ 11.3 kpc × 11.3 kpc. Orientation: N is to the top and E to the left.

NGC3628

NGC3628 is a member of the Leo triplet, which also includes NGC3623 and NGC3627. It is a starburst galaxy with a very prominent dust lane, which obscures most parts of the emission in the galactic midplane. It has been studied extensively in almost all wavelength regimes, and has also been the target for a multi-wavelength study in the context of the disk-halo connection. Radio continuum observations (Schlickeiser et al., 1984) reveal extended emission, and in the X-ray regime a T ~ 2 × 10⁶K, extended halo has been detected by sensitive PSPC observations with ROSAT (Dahlem et al., 1996). Prominent X-ray emission, tracing the collimated outflow from the nuclear starburst, was found to be spatially correlated with the H plume (Fabbiano et al., 1990). Extraplanar dust has already been detected (Howk & Savage, 1999; Rossa, 2001). We also observed extended emission from the nuclear outflow, localized extended emission (filamentary), and several extraplanar HII regions.

NGC3877

NGC3877 is a northern edge-on spiral galaxy, which ranks among the top 15 galaxies of our survey, according to the SFR per unit area. A SFR from observations in the UV has been derived, which yielded 0.55 M yr-1 (Donas et al., 1987). A nuclear spectrum reveals quite strong emission of H (Ho et al., 1995), whereas H, and the [NII] lines are also clearly detected. Just recently a SN of type II has been detected in NGC3877, namely SN1998S (Filippenko & Moran, 1998). In Fig. 6 we show our broadband R image, which was obtained almost a year after its discovery. The SN (marked by a circle), although considerably fainter, is still visible in our image. Niklas et al. (1995) report a 2.8cm flux for NGC3877 of S_tot = 23 ± 8 mJy. However, no radio map was shown, and they did not comment further on this galaxy. The H morphology, as seen in our images, reveal extended emission with some small filaments and plumes. Quite remarkable is the distribution of very strong HII regions and knots which are seen all over the disk (cf. Fig. 5). The nuclear region is very compact in H.

Figure 5. Extraplanar DIG layer in NGC3877 seen in this continuum subtracted H image. There is a pervasive eDIG layer clearly visible. The field of view measures 12 kpc × 12 kpc. Orientation: N is to the top and E to the left.

Figure 6. SN1998S (marked by a circle) in NGC3877. Our R-band image was obtained 354 days after the discovery of SN1998S. Image size and orientation is same as above.

NGC4216

Another Virgo cluster spiral, NGC4216 has been classified as a WR-galaxy, based on detected HeII4686 emission from a few regions within NGC4216 (Schaerer et al., 1999). Pogge (1989b) investigated the morphology of the nuclear emission, and concluded it as diffuse, while the circumnuclear region was characterized as faint and patchy. There is nuclear emission detected from X-rays (Fabbiano et al., 1992). Maoz et al. (1996) did not detect UV emission in NGC4216 in their HST survey. Our H image did not reveal extraplanar emission. The nucleus is the strongest source, whereas several smaller HII regions in the outer spiral arm contribute also to the H emission.

NGC4235

NGC4235 has been classified as a Seyfert galaxy of type 1 (Weedman, 1978), and has no physical companion (Dahari, 1984). Pogge (1989a) has studied the nuclear environment using narrowband imaging of H and [OIII]. He found a bright nucleus with an extended region towards the NE direction at P.A. ~ 48°, which extends ~ 4.4". Neither disk HII regions were detected, nor ionized gas above the plane. Radio continuum observations (Hummel et al., 1991) also did not find evidence for extended emission. In the X-ray regime the strong nuclear region is detected with a luminosity of 1.55 × 10⁴² erg s^-1 (Fabbiano et al., 1992). In a study of large-scale outflows in edge-on Seyfert galaxies Colbert et al. (1996b) found no double peaked line profiles, or any evidence for extended line regions, and no minor axis emission, too. However, on radio continuum images obtained with the VLA at 4.9 GHz there was a diffuse, bubble-like, extended structure (~ 9 kpc) found in addition to the unresolved nucleus (Colbert et al., 1996a). Our H image reveals a bright nucleus with a faint extended layer, which is restricted to the circumnuclear part. NE of the nucleus a depression is visible, possibly due to absorbing dust, as already noted by Pogge (1989a), and easily visible in the broad band HST image by Malkan et al. (1998). This is one of the very few Seyfert galaxies that appear in our survey. The role of minor axis outflows in Seyfert galaxies, and thus the contribution to the IGM enrichment and heating still has to be explored.

NGC4388

NGC4388 has been identified as the first Seyfert2 galaxy in the Virgo cluster (Phillips & Malin, 1982), and has been studied extensively in various wavelength regimes, including optical line imaging and spectroscopy (e.g., Keel, 1983; Corbin et al., 1988; Pogge, 1988), radio continuum (e.g., Stone et al., 1988; Hummel & Saikia, 1991), and in the high energy waveband extended soft X-ray emission out to a radius of 4.5 kpc has been reported (Matt et al., 1994). From the optical morphology and kinematics it was derived that NGC4388 possesses complex gas kinematics and is composed of several nucleated emission line regions. A prominent feature reaches out to ~ 18" at P.A. ~ 10° (Heckman et al., 1983). The radio continuum maps revealed a double peaked radio source close to the optical nucleus plus a cloud of radio emitting material, apparently ejected from the nucleus (e.g., Stone et al., 1988; Irwin et al., 2000).

Some speculation on the true membership to the Virgo cluster exists, although NGC4388 is located near the core of the Virgo cluster. This is because of its relatively high systemic velocity. However, most authors assume it is a member of the Virgo cluster. Therefore, ram pressure stripping seems to play a role as NGC4388 interacts with the ambient intracluster medium (ICM). Optical narrowband imaging in H and [OIII] has revealed that the morphology of the extended ionized gas is composed of two opposed radiation cones (Pogge, 1988), which give rise to a hidden Seyfert1 nucleus, as favored in the unification scheme of AGN. Recent investigations of NGC4388 using Fabry-Perot imaging techniques have revealed a complex of highly ionized gas ~ 4 kpc above the disk (Veilleux et al., 1999). They found blueshifted velocities of 50 - 250 km s^-1 NE of the nucleus. Furthermore they assume the velocity of the extraplanar gas to be unaffected by the inferred supersonic motion of NGC4388 through the ICM of the Virgo cluster, and suggest that the galaxy and high-| z| gas lies behind the Mach cone.

Our narrowband imaging of NGC4388 (see Fig. 7) reveals also extended emission which is pointing away from the galactic disk to the halo in the NE direction. Furthermore a faint eDIG layer is visible. Very recently deep H images obtained with the Subaru telescope revealed very extended emission-line region in H and [OIII] at distances of up to ~ 35 kpc (Yoshida et al., 2002).

Figure 7. Continuum subtracted H image of NGC4388, revealing an eDIG layer and an extended plume. The displayed area equals ~ 36 kpc × 32 kpc at the distance of NGC4388. Orientation: N is to the top and E to the left.

NGC4700

NGC4700 has been listed as a HII region galaxy in the list of Rodriguez Espinosa et al. (1987), while Hewitt & Burbidge (1991) list it as a Seyfert2 galaxy. It has been included in the HST imaging survey of nearby AGN (Malkan et al., 1998). NGC4700 has a relatively large ratio of S₆₀ / S₁₀₀ of ~ 0.51, making it a promising candidate with a potential DIG halo according to the diagnostic DIG diagram. Indeed, a relatively bright, and extended gaseous halo with a maximum extent of 2.0 kpc above/below the galactic plane is discovered. One of the filaments can even be traced farther out to ~ 3 kpc. The morphology of DIG in NGC4700 is asymmetrical, with the eastern and middle part being most prominent. Five distinct bright HII region complexes, which are composed of several individual smaller regions across the disk, can be discerned. Some prominent filaments above active regions protrude from the disk into the halo. A small number of bright HII regions is seen in this part of the disk, and the maximum extent of the eDIG is visible above those regions. The western part of the galaxy is lacking in bright HII regions, hence the suppressed extend of the halo in this western part. NGC4700 also bears an extended radio halo (radio thick disk), with the maximum extent correspondingly on the same position above the disk (Dahlem et al., 2001), although slightly more extended in the radio continuum image as in our H image (see Figs. 8 + 9).

Figure 8. A detailed H view of NGC4700 showing a bright and extended gaseous halo, superposed by individual filaments.

Figure 9. The extended radio halo of NGC4700 at = 1.4 GHz (depicted in contours), overlaid onto the DSS image (Dahlem et al., 2001), courtesy M. Dahlem.

NGC4945

NGC4945 is a well studied southern edge-on spiral galaxy, which belongs to the Centaurus group. It has been studied at various wavelengths, including the visual, IR, and radio regime. Heckman et al. (1990) found evidence for a starburst driven superwind in NGC4945, and Moorwood & Olivia (1994) derive a ~ 400 pc size starburst in addition to the presence of a visually absorbed Seyfert nucleus. They conclude that NGC4945 is in an advanced stage of evolution from a starburst to a Seyfert galaxy. Extended emission from the disk was detected by radio continuum observations (Harnett et al., 1989; Colbert et al., 1996a), reaching a diameter of ~ 23 kpc. X-ray emission was detected from the nuclear region, however, no extended emission was found in ROSAT PSPC observations (Colbert et al., 1998), probably due to quite large absorbing columns (N_H = 1.5 × 10²¹ cm^-2), as NGC4945 is situated near the galactic plane (b ~ 13°). This decreases the sensitivity for the soft X-ray regime.

NGC4945 is one of the few starburst galaxies that we have included in our H survey, and it is no surprise that it has the second highest L_FIR / D²₂₅ ratio of ~ 14.8 of our studied galaxies. Already investigated by Lehnert & Heckman (1995), NGC4945 shows strong extraplanar DIG with many filaments protruding from the disk into the halo. There are at least two bright filaments on either side of the disk visible on our narrowband images. Several prominent dust patches, which obscure some parts of the emission south of the galactic plane is a further characteristic pattern for this galaxy. In Fig. 10 we show an enlargement of the middle part, which nicely shows the outflow cone from the nuclear starburst.

Figure 10. A zoom onto the outflowing cone in this H image of NGC4945. The displayed portion of this galaxy measures ~ 5.6 kpc × 5.6 kpc. Orientation: N is to the top and E to the left.

NGC5290

This northern edge-on galaxy is part of the LGG361 group (Garcia, 1993), which teams up with NGC5289, which is located ~ 13' to the south, and has a velocity difference to NGC5290 of ~ 67 km s^-1 (Huchra et al., 1983). In the optical NGC5290's most striking character is a box-shaped bulge (de Souza & Dos Anjos, 1987). The L_FIR / D²₂₅ ratio is moderate (~ 2.6), and NGC5290 shows some extended emission, where a faint layer is detected, and some filaments, basically coming from the nuclear region and reaching into the halo, can be discerned (see Fig. 11). The morphology that is visible on the H image resembles those of a starburst galaxy, although NGC5290 has no starburst-like FIR parameters, as the S₆₀ / S₁₀₀ ratio is considerably lower (0.31).

Figure 11. Continuum subtracted H image of NGC5290, showing a pervasive eDIG layer and its morphology resembles those of a nuclear starburst galaxy. The displayed area measures ~ 28.6 kpc × 28.6 kpc at the distance of NGC5290. Orientation: N is to the top and E to the left.

NGC5297

Another northern edge-on spiral, NGC5297 forms a binary galaxy with NGC5296 (Turner, 1976), which is separated by about 1.5' from NGC5297, and has a velocity difference of v 360 km s^-1. Another interesting source is also located in the direct vicinity, a quasar of V = 19.3 mag (Arp, 1976). This quasar ([HB89]1342+440), which has a redshift of z = 0.963, is located 2.5' to the SW. Arp reports on a luminous extension from NGC5236 pointing at the QSO, which Sharp (1990) did not confirm. However, Sharp (1990) did comment on the unusually bright off-center secondary nucleus of NGC5296. The QSO is marked by a circle in our R-band image in Fig. 40. The outer spiral arms of NGC5297 show evidence of perturbation by the S0 companion, as reported by (Rampazzo et al., 1995).

Radio continuum observations of NGC5297 have been performed (Irwin et al., 2000; Hummel et al., 1985; Irwin et al., 1999). While Irwin et al. (1999) claims extended radio continuum emission from NGC5297, still, higher resolution observations show only very weak emission (Irwin et al., 2000). Our H image does not reveal any extraplanar emission. Thus, we do not see enhanced SF activity due to the interaction with the companion galaxy in this case. The L_FIR / D²₂₅ ratio is moderate. We also note, that this galaxy actually is not perfectly edge-on.

NGC5775

From HI observations it was found that NGC5775 is an interacting galaxy with its neighbor face-on spiral NGC5774. Emission along a bridge has been detected, although no tidal arms are discovered (Irwin, 1994). NGC5775 is a good studied northern edge-on galaxy in the disk-halo context, which has been imaged in H (Tüllmann et al., 2000; Collins et al., 2000; Lehnert & Heckman, 1995), where extraplanar emission was detected out to over 5 kpc above the galactic plane. It is the galaxy with the highest L_FIR / D²₂₅ ratio in our sample and is a starburst-type galaxy, although no clear indications exist on a nuclear starburst. A bright halo with individual filaments superposed in a prominent X-shape are discovered on our narrowband image, consistent with earlier observations, mentioned above. Extraplanar DIG has been detected spectroscopically out to 9 kpc (Tüllmann et al., 2000; Rand, 2000), and extended radio continuum emission has been detected as well (Duric et al., 1998; Hummel et al., 1991). Extended X-ray emission coexistent with a radio continuum spur and optical filament was discovered on ROSAT PSPC archival images (Rossa, 2001).

ESO274-1

ESO274-1 belongs to the Cen A group of galaxies, and Banks et al. (1999) derive a HI mass of 430 × 10⁶ M, ranking on position five of the Cen A group. This little studied southern edge-on galaxy was included in the recent list of dwarf galaxies by Cote et al. (1997), who studied new discovered dwarf galaxies in the nearest groups of galaxies. From optical inspection of the DSS images, it is obvious that this galaxy has a low surface brightness. It is the galaxy with the lowest L_FIR / D²₂₅ ratio among the IRAS detected galaxies in our sample (0.19)! Quite remarkably however, the S₆₀ / S₁₀₀ ratio is among the highest in our sample, making ESO274-1 a promising candidate in search for eDIG. We show in Fig. 12 an enlargement of the most active SF regions, which show strong local extended emission. The SW and middle part is most prominent, whereas there are regions in the NE part, where this galaxy (superposed by a crowded field of foreground stars) is almost invisible in H.

Figure 12. Prominent extraplanar emission regions in this continuum subtracted H image of ESO274-1. The displayed area equals ~ 4.4 kpc × 4.4 kpc at the distance of ESO274-1. Orientation: N is to the top and E to the left.

ESO142-19

This is a perfect edge-on galaxy, and it is the only one of type S0-a in our sample. It was only included because of its classification as type Sa in the NED. It shows a strong dust lane, and a strikingly bright, and extended bulge in the R-band image. An upper radio continuum flux limit of 20 mJy at 11cm has been given by Sadler (1984), and a upper flux limit of F_{[N II]} < 1.6 × 10^-14 erg s^-1 cm^-2 has been derived by Phillips et al. (1986). We detect no extended emission in ESO142-19. Even the disk is almost invisible on our H image.

IC5052

IC5052 is a late-type spiral with classifications Scd or Sd. Radio emission detected by 35cm observations revealed an unusually double peaked radio emission complex associated with the disk, which was not coming from the nucleus (Harnett & Reynolds, 1991).

Our H image reveals a bright layer of extraplanar DIG superposed by individual filaments and shells (see Fig. 13). There is a depression in the H distribution noticed in the southern part of the galaxy, due to significant dust absorption, which almost separates the galaxy in two parts. An asymmetrical distribution is also visible in the R-band image. The southern edge is dominated by H emission, whereas the broadband image shows almost no intensity. Several arcs and shells make this galaxy a good target for further high resolution studies.

Figure 13. Continuum subtracted H image of IC5052, showing an eDIG layer, filaments and shells. The displayed area is ~ 11.8 kpc × 11.8 kpc. Orientation: N is to the top and E to the left.

Pa observations with NICMOS 2 HST (Böker et al., 1999) showed excess emission of an isolated region in the disk-halo interface, slightly offset from the disk, which is possibly associated with one of the brighter emission regions in the southern part of our H image. The diagnostic ratios are both moderate, so it is quite remarkable that extended DIG emission is detected in that abundance in IC5052.

NGC7064

NGC7064 is a HII region like galaxy (Kirhakos & Steiner, 1990). Radio emission has been detected at 35 cm (Harnett & Reynolds, 1991), which extends over the inner disk with a maximum intensity at the nucleus. The disk emission is more pronounced to the east, which corresponds to the optical emission distribution. They further show in their map a weak extension to the south, which apparently has no optical counterpart. NGC7064 has also been detected in the ROSAT All Sky Survey (RASS), which was included in a study of IRAS galaxies (Boller et al., 1998). The X-ray intensity maximum is offset from the IRAS position (nucleus). Furthermore, weaker emission is detected to the south of NGC7064 at larger distances far out in the halo region. Presently it is not clear if it is related to NGC7064. In Fig. 14 we show our H image, scaled logarithmically, to show the faint halo. A few emission patches and plumes are visible to the south of the disk, and also in the northern part. The disk emission is asymmetrically distributed, with the eastern part being most prominent. There is a prominent hole in the southern disk, bisecting the disk almost entirely. In our logarithmically scaled image the hole is almost filled with diffuse emission, but its intensity is weaker than in the eastern and western part of the disk-halo interface. Although the L_FIR / D²₂₅ ratio is quite low, it should be noted that the S₆₀ / S₁₀₀ ratio is the highest in our sample.

Figure 14. Continuum subtracted H image of NGC7064, revealing a faint extraplanar DIG layer. The displayed area measures ~ 10.0 kpc × 10.0 kpc. Orientation: N is to the top and E to the left.

NGC7090

This southern edge-on spiral has radio continuum detections at 35cm (Harnett & Reynolds, 1985). The radio emission follows the plane of the galaxy, and extends above it in two distinct spurs out to a distance of ~ 1.5 kpc. The radio peak coincide with the optical nucleus, however, they note no obvious nuclear radio source. Our R-band image shows that a prominent dust lane with patchy structures runs across most parts of the disk offset slightly to the north. It is very irregular, unlike most other prominent dust lanes, such as in NGC891 or IC2531. The H image (Fig. 15) reveals a faint halo, in addition to some filaments and extended emission in the form of knots. The northeastern part of the galaxy has the highest intensity in emission, whereas the southern half is almost absent, except very few emission patches (HII regions) in the disk. The filaments protrude basically from the nuclear region into the halo. It should be noted that the extended radio continuum emission is coincident with the H extended emission.

Figure 15. Continuum subtracted H image of NGC7090, showing an eDIG layer, with filaments and plumes. The displayed area of the galaxy measures ~ 7.8 kpc × 7.8 kpc. Orientation: N is to the top and E to the left.

NGC7184

NGC7184 was detected in the radio regime at 20cm continuum emission (Condon, 1987), and bears a double peaked brightness distribution. This was confirmed by Harnett & Reynolds (1991), who detected two maxima at 35cm. NGC7184 is located at a projected distance of 163 kpc (20.9') to NGC7185, as quoted by Kollatschny & Fricke (1989), who studied the group environment of Seyfert galaxies. A peculiar supernova was detected in NGC7184, named SN1984N (Barbon et al., 1999). Our H image did not show any extraplanar emission, which might be due to the fact, that NGC7184 is not perfectly edge-on. The outstanding feature in NGC7184 is the inner ring, which is also prominently seen on the R-band image. There is a well pronounced sub-structure visible, and several HII regions can be discerned in the ring and in the outer spiral arms.

NGC7339

The late type spiral NGC7339 is accompanied by the galaxy NGC7332 at a projected distance of 5.2', the latter one being a peculiar S0 galaxy, which has a velocity difference of v = - 141 km s^-1 in comparison to NGC7339. A supernova (SN1989L) has been discovered in NGC7339 (Barbon et al., 1999). Several bright, and compact knots are seen in our H image, but no extraplanar emission is detected. NGC7332, which is also visible on our frame, is completely absent in H.

UGC12281

UGC12281 has already been studied in the DIG context a few years ago by Pildis et al. (1994). They claim to have detected two extraplanar emission line features (discrete clouds), and a clumpy plume. We can confirm the presence of these two clouds, and the plume is also marginally detected in our image, which suffers from low S/N.

NGC7462

This southern edge-on spiral was detected with the VLA at = 1.49 GHz (Condon et al., 1987), and Maoz et al. (1996) have conducted UV observations at (2300Å) of the inner 22" × 22" region with the HST, where they detected a star-forming morphology in several distinct regions (knots). Our H image (Fig. 16) reveals an extended DIG layer with individual plumes and faint filaments superposed. The disk emission is composed of several distinct emission complexes, clustered basically in three regions (nucleus + two regions within the disk), and some fainter regions in the outskirts of the disk.

Figure 16. Continuum subtracted H image of NGC7462, showing extraplanar emission. The displayed area equals ~ 10 kpc × 10 kpc at the distance of NGC7462. Orientation: N is to the top and E to the left.

UGC12423

UGC12423 belongs to the Pegasus I cluster of galaxies, and was once reported being one of the most massive and luminous spirals, with a derived HI mass of ~ 9 × 10⁹ M, and logM_H / L_B = + 0.14 (Schommer & Bothun, 1983; Bothun et al., 1982). Hummel et al. (1991) did not detect extended emission from UGC12423 in their radio continuum survey. Our H image also does not show any extraplanar emission. The galaxy is almost invisible in H. This can partly be attributed to insufficient S/N, as we unfortunately only acquired one exposure for H, and also the R-band image was not long enough integrated. This needs to be re-investigated with more sensitive observations.

⁴ http://www.roe.ac.uk/wfau/halpha/halpha.html Back.

⁵ http://amundsen.swarthmore.edu/SHASSA/ Back.