3 The results

3.1 The H$\alpha$ + [N II] and [S II] images

The field in H$\alpha +[$N II] is dominated by several bright H II regions, mainly found in the east (Fig. 1). We also find LBN 156 at the south-west edge of our field (Lynds 1965). The two arcs seem to emerge from the northern areas of LBN 156 and extend up to a declination of 34$^\circ$24$^\prime$ (area I in Fig. 1). These arcs seem to define rather well part of a circle's periphery, but not centered in the middle of all the new structures, and with a radius of $\sim$42$^\prime$. Three more new structures are present in this field. The first appears as a $\sim$11$^\prime$ long filament in the south (area II in Fig. 1). Its center is approximately at $\alpha$ $\simeq$ 19$^{\rm h}$59$^{\rm m}$34$^{\rm s}$ and $\delta$ $\simeq$ 33$^\circ$26$^\prime$53 $^{\prime\prime}$, a few arcminutes to the north of Sh 2-99, Sh 2-100 (Sharpless 1959).

The second structure (area III in Fig. 1) is located at $\alpha$ $\simeq$ 20$^{\rm h}$01$^{\rm m}$00$^{\rm s}$ and $\delta$ $\simeq$ 33$^\circ$54$^\prime$46 $^{\prime\prime}$, very close to the bright, variable star HD 189918. It consists of two sub-stuctures oriented in the east-west direction and occupies an angular extent of 4 $.\mkern-4mu^\prime$ $8 \times 2$ $.\mkern-4mu^\prime$5. Optical emission from this area is the brightest among the new structures in this field (Table 2). Finally, the last new structure appears as a partial arc convex to the north with a radius of curvature of 6 $.\mkern-4mu^\prime$8 (area IV in Fig. 1). The center of curvature is estimated at $\alpha$ $\simeq$ 20$^{\rm h}$00$^{\rm m}$01$^{\rm s}$ and $\delta$ $\simeq$ 33$^\circ$42$^\prime$20 $^{\prime\prime}$. The eastern half of this arc is brighter than the western half.

The morphology of the [S II] image is generally similar to, though not as sharp as, that of the H$\alpha +[$N II] image and is not shown here.

We also note that we have examined the published galactic plane survey of Parker et al. (1979). While the scale and quality of the published images may limit the visualization of weak emission, we find no evidence for any of the emission seen here in H$\alpha +[$N II] and [S II] (or in [O III]).

3.2 The [O III] and H$\beta$ line emission

The medium ionization line of [O III]5007 Å (Fig. 2) displays a different morphology than the lower ionization lines of e.g. H$\alpha +[$N II] or [S II]. The two arcs (area I) are present but their emission is very weak. Another filamentary structure is detected in the north, at $\alpha$ = 19$^{\rm h}$59$^{\rm m}$13$^{\rm s}$ and $\delta$ = 34$^\circ$12$^\prime$30 $^{\prime\prime}$ (area Ia), where the arc of area I merges with the background. Emission from area II is much better defined than in its H$\alpha +[$N II] or [S II] counterparts. We now observe a $\sim$23$^\prime$ long filamentary structure characterized by a projected thickness of $\sim$40 $^{\prime\prime}$.

Interestingly, the composite structure in area III looks quite different in this emission line. We observe two thin filaments at the north and south boundaries of this area separated by diffuse emission, while contrary to the low ionization images a $\sim$11$^\prime$ long tail of diffuse emission is detected to the east. A new structure is also detected in [O III] to the south-east of this tail at $\alpha$ $\simeq$ 20$^{\rm h}$02$^{\rm m}$08$^{\rm s}$ and $\delta$ $\simeq$ 33$^\circ$47$^\prime$47 $^{\prime\prime}$ without any obvious counterpart in H$\alpha +[$N II] or [S II]. Diffuse emission is only present in the east half of area IV which would be very difficult to identify if it were not for the H$\alpha +[$N II] image.

The H$\beta$ line emission image resembles the morphology of the H$\alpha +[$N II] image but at a much lower intensity level and is not shown here. The two arcs in area I are barely seen, while the long filament in area II is less well defined. Emission from areas III and IV is traced further to the north where the emission from the latter area is very weak.

   

Table 4: Relative line fluxes from long-slit spectra.

	area I	area II	area IIIs$^{\rm a}$	area IIIn$^{\rm a}$	area IVn$^{\rm b}$	area IVs$^{\rm b}$
Line (Å)
4861 H$\beta$	10$^{\rm c}$ (5)$^{\rm d}$	21 (11)	23 (17)	17 (4)	16 (3)	17 (5)
4959 [O III]	-	23 (22)	8 (11)	33 (8)	-	-
5007 [O III]	17 (7)	68 (45)	24 (22)	120 (30)	17 (3)	38 (2)
6548 [N II]	32 (14)	20 (22)	23 (41)	18 (13)	27 (5)	22 (12)
6563 H$\alpha$	100 (29)	100 (109)	100 (120)	100 (50)	100 (19)	100 (41)
6584 [N II]	102 (31)	66 (73)	73 (110)	67 (38)	87 (17)	71 (33)
6716 [S II]	55 (22)	35 (45)	66 (105)	48 (26)	74 (18)	61 (30)
6731 [S II]	39 (18)	25 (32)	46 (97)	32 (16)	51 (13)	44 (22)
Absolute H$\alpha$$^{\rm e}$	6.7	22.0	63.0	14.7	12.8	15.6
H$\alpha$/H$\beta$	10 (5)	4.8 (11)	4.4 (16)	5.8(4)	6.3 (2.5)	6.1 (5)
[O III]/H$\beta$	1.7 (5)	3.2 (11)	1.0 (22)	7.1(4)	1.1 (3)	2.2 (2)
[S II]/H$\alpha$	0.9 (20)	0.6 (48)	1.1 (92)	0.8(25)	1.2 (16)	1.0 (20)
[S II]/[S II]	1.4 (14)	1.4 (26)	1.4 (71)	1.5(14)	1.5 (10)	1.4 (18)

${\rm ^a}$ Center of aperture IIIs is 55 $^{\prime\prime}$ south of slit center, while IIIn is 22 $^{\prime\prime}$ north.
${\rm ^b}$ Center of aperture IVn is 22 $^{\prime\prime}$ north of slit center, while IVs is 45 $^{\prime\prime}$ south.
${\rm ^c}$ Uncorrected for interstellar extinction.
${\rm ^d}$ The signal to noise of the quoted numbers is given in the parentheses.
${\rm ^e}$ The absolute H$\alpha$ flux is given in units of 10^-17 erg s^-1 cm^-2 arcsec^-2.   All fluxes normalized to F(H$\alpha$) = 100.

3.3 The [O II] line emission

The [O II]3727 Å low ionization line image best describes the newly detected structures (Fig. 3). Emission from the two arcs in area I is detected as well as emission from the filament in area Ia. The long [O III] filament in the south (area II) is also present in [O II] but its extent is shorter $\sim$16$^\prime$, while its projected width is $\sim$34 $^{\prime\prime}$. The south and north boundaries of the structure in area III are very bright with diffuse emission between them. Especially, the north boundary is very sharp. We measure a decrease in flux by a factor of $\sim$3.5 in only 3 pixels, i.e. $\sim$15 $^{\prime\prime}$.

Figure 3: The [O II] image clearly delineates the new structures detected in this field. The shadings run linearly from 0.0 to 50 $\times$ 10-17 erg s-1 cm-2 arcsec-2 and the image has been smoothed to suppress the residuals from the imperfect continuum subtraction. The slit locations are shown as long rectangles.

Figure 4: The radio 4850 MHz contours (Condon et al. 1994) overlaid on the [O II] image shown in Fig. 3. The radio contours scale linearly from 0.002 to 0.06 Jy/beam by 0.00725 Jy/beam. Strong, extended radio emission is seen in the south-west, while faint radio emission seems correlated with the newly detected structures.