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6. MAPS AND MORPHOLOGICAL CLASSIFICATION

In Figure 3 we present greyscale maps corresponding to the R-band and to continuum-subtracted Halpha images obtained for the 114 galaxies in our sample. A blow-up of the nuclear region is also shown in the upper right-hand corner of each image, rescaled in intensity to emphasize details. The size of the compass shown in each of the images is 0.5 kpc at the distance of the corresponding galaxy. The average limiting surface brightness of the main images (3sigma) are 24.6 mag arcsec-2, 24.3 mag arcsec-2, and 7 × 10-17 ergs-1 cm-2 arcsec-2 for B, R, and Halpha, respectively.

Figure 3a

Figure 3. R (left) and continuum-subtracted Halpha (right) images of the galaxies in the sample. The compass is 0.5kpc in size in each image. A blow-up image of the galaxy nuclear region is also shown.

Figure 3b

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3c

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3d

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3e

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3f

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3g

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3h

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3i

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3j

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3k

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Figure 3l

Figure 3. R and continuum-subtracted Halpha images of the galaxies in the sample (cont.)

Within the galaxy sample studied only three galaxies were not detected by us in Halpha emission, namely Mrk 709, UCM 1446+2312, and UGC 4703 NOTES02. In the case of Mrk 709 the galaxy recession velocity provided by NED (1,197 km s-1), which was taken from the 21 cm-line velocity data published by van Zee, Haynes, & Giovanelli (1995), differs significantly from that published by Terlevich et al. (1991) (~ 15,000 km s-1) and recently confirmed by E. Pérez-Montero (private communication). This leads us to suggest that the low-redshift HI emission detected by van Zee et al. (1995) is not related to Mrk 709, but maybe with another object in the beam that, according to our data, does not show significant ionized gas emission. For UCM 1446+2312 the uncertainty in the recession velocity (determined from the position of the Halpha line in objective-prism photographic plates; Alonso et al. 1999) is ~ ± 1,200 km s-1. Therefore, the line-emission arising from this object might well be beyond the wavelength region covered by the 20Å-wide filter used. Finally, UGC 4703 NOTES02 forms with UGC 4703 NOTES01 an interacting pair where only the latter shows Halpha emission. This suggests that UGC 4703 NOTES02 may have either exhausted most of its gas early during its evolution, or it may have lost it as consequence of the interaction with UGC 4703 NOTES01. Is it important to note that, as already pointed out by Cairós et al. (2001a) and we have discussed in Section 3, most of the BCD galaxies were originally identified by their high surface brightness and blue optical colors. Therefore, some of them may be expected to show very low Halpha equivalent widths (EW) (i.e. they are BCDs but they should not be then classified as HII galaxies). Some examples of this may be Mrk 1423 and I Zw 115 (see Section 8).

The maps presented in Figure 3 show that most of the galaxies in our sample have an extended, low-surface-brightness halo visible in the continuum images. Such halos, commonly associated with an evolved stellar population (Papaderos et al. 1996; Cairós et al. 2001a, b and references therein), are usually more extended than the region showing Halpha emission, except perhaps in the case of the most compact objects (I Zw 18, HS 0822+3542, Pox 186). In Paper II we analyze the structural properties of this low-surface brightness component. With regard to the morphology in Halpha the inspection of our images show that 83 galaxies in our sample (73%) show star formation activity distributed over more than one region. In particular, in 45 galaxies (39%) we are able to distinguish 4 regions or more. Only 28 galaxies show their morphology in the light of Halpha to be dominated by only a single, nuclear HII region. It is worth noting that the median distance and seeing for those galaxies showing only one single star-forming region are 23 Mpc and 1.4 arcsec respectively, while the galaxies with multiple regions are typically at 19 Mpc and have 1.5 arcsec seeing. Clearly, the difference between these values is not large enough to explain the different number of regions detected purely in terms of a differing spatial resolution.

Based only on broad-band optical imaging data, Loose & Thuan (1986) undertook a morphological classification of BCD galaxies. They defined four different groups:

The classification of a galaxy as nE or iE BCD is been mainly determined by the shape of the inner isophotes: regular in the case of nE, and irregular for the iE BCDs. However, the greyscale maps presented in Figure 3 show that, in some cases, despite the continuum isophotes in the inner region being regular, the Halpha images may show a complex structure composed of several individual HII regions. An example of this behavior is seen in Mrk 409, where the inner R-band isophotes are regular (which would lead one to classify this galaxy as a nE BCD), but the Halpha emission shows the presence of a ring composed of at least 5 HII regions (see Figure 3). In cases like that of Mrk 409 we have made use of the information provided by the Halpha images, and the galaxies have been reclassified as iE BCDs. The presence of a ring of star-forming regions around the nucleus of the galaxy has been observed in a total of 4 objects in our sample including Mrk 409, namely Mrk 86 (see Gil de Paz et al. 2000b, c; 2002), Mrk 400, Mrk 409, and VCC 0655. The radius of this ring ranges from 0.5 to 1.0 kpc. In these cases we have added a letter `r' to the morphological type of the objects.

The results of the morphological classification are given in Table 5. For the objects with detected Halpha emission 27 out of 111 (24%) were classified as nE BCDs, 37 (33%) as iE, and 39 (35%) as iI. For the 39 iI BCDs, 11 are "cometary" (iI,C) and 10 are "mergers" (iI,M). In only 8 galaxies did we not detect any underlying/extended R-band continuum light (that would be otherwise be associated with an old stellar population) and, consequently, they were classified as i0 BCDs. Within this group are the objects with the lowest metallicity in our sample (I Zw 18, Tol 65, UCM 1612+1308). For comparison, within their sample of 28 BCDs, Cairós et al. (2001a, b) found 21% of nE, 39% of iE, and 32% of iI BCDs (7% and 14% of the total were iI,C and iI,M BCDs, respectively).

Table 5. Integrated Photometry

Object Name B R B-R fHalpha MB log LHalpha T EWHalpha
  (mag) (mag) (mag) (10-14 erg/s/cm2) (mag) (erg/s)   (Å)
(1) (2) (3) (4) (5) (6) (7) (8) (9)

IC 10 5.75±0.03 5.45±0.08 0.30±0.08 51000±3700 -19.25 40.79 iI,C 36
HS 0029+1748 16.74±0.05 15.93±0.14 0.81±0.15 19.1±2.5 -15.87 40.40 nE 228
Haro 14 13.65±0.05 12.91±0.14 0.74±0.15 34±4 -17.08 39.90 iE 22
UM 285 17.00±0.03 17.03±0.08 -0.02±0.08 5.8±0.4 -15.25 39.74 iI,C 187
UCM 0049-0045 14.76±0.04 13.36±0.25 1.39±0.25 20±4 -17.18 40.14 iE 20
UM 323 16.09±0.04 15.24±0.08 0.85±0.09 14.1±1.2 -16.17 40.13 iE 83
Mrk 996 15.01±0.03 14.08±0.14 0.93±0.14 54±7 -16.87 40.56 nE 109
UM 133 15.41±0.03 14.51±0.14 0.90±0.14 20±3 -16.50 40.15 iI,C 60
UM 382 18.20±0.04 17.83±0.22 0.37±0.22 2.6±0.5 -15.33 39.90 iI,C 170
UM 404 18.48±0.05 18.40±0.12 0.08±0.13 3.6±0.4 -15.09 40.06 i0 455
KUG 0207-016A 15.24±0.05 14.77±0.12 0.47±0.13 14.3±1.5 -18.28 40.64 iI 54
UM 408 17.46±0.16 16.70±0.12 0.75±0.20 6.8±0.8 -16.12 40.34 nE 161
UM 417 18.37±0.07 17.73±0.13 0.64±0.15 4.4±0.5 -14.58 39.90 iI,C 279
Mrk 370 13.49±0.03 12.21±0.12 1.28±0.13 70±7 -16.94 40.10 iE 24
Mrk 600 14.85±0.03 14.82±0.15 0.03±0.15 24±3 -15.97 39.78 iE 96
NGC 1522 14.03±0.01 13.22±0.06 0.81±0.06 59.9±0.6 -16.10 39.91 iE 54
NGC 1705 13.09±0.01 12.19±0.06 0.90±0.06 253±3 -15.45 39.90 iE 90
II Zw 33 14.21±0.03 13.34±0.11 0.86±0.11 93±9 -18.77 41.24 iI 95
II Zw 33B 16.87±0.04 15.67±0.11 1.21±0.12 5.1±0.5 -16.12 39.98 iI 43
II Zw 40 11.87±0.04 11.10±0.09 0.77±0.09 1554±120 -18.09 41.25 iI,M 217
Tol 0610-387 15.90±0.05 14.87±0.06 1.03±0.08 16±1 -15.89 40.01 iE 68
Mrk 005 15.13±0.04 14.56±0.13 0.57±0.13 28±3 -15.47 39.77 iI,C 89
Mrk 007 14.29±0.03 13.49±0.18 0.79±0.18 36±6 -19.00 40.95 iI 41
Mrk 86 12.07±0.03 11.49±0.11 0.58±0.11 241±23 -17.13 40.14 iE,r 44
HS 0822+3542 17.85±0.03 17.76±0.14 0.09±0.14 7.4±0.9 -12.18 38.95 iI,C 546
UGC 4483 15.14±0.03 14.56±0.13 0.58±0.13 36±4 -12.38 38.64 iI,C 115
UGC 4703N1 15.86±0.04 15.01±0.03 0.85±0.05 24.9±0.5 -17.58 40.85 iI,M 120
UGC 4703N2 17.04±0.04 16.10±0.03 0.94±0.05 ... -16.39 ... ... ...
Mrk 1416 16.32±0.03 15.75±0.02 0.58±0.04 19.4±0.8 -16.32 40.42 iI 191
Mrk 108 15.15±0.03 14.66±0.08 0.49±0.08 73±5 -16.54 40.62 iI,M 275
Mrk 400 14.30±0.04 13.63±0.11 0.67±0.12 39±4 -18.41 40.76 iE,r 51
NGC 2915 11.93±0.01 10.96±0.06 0.97±0.06 209±3 -15.85 39.51 iE 23
I Zw 18 16.05±0.04 16.24±0.07 -0.18±0.08 35.2±2.4 -14.45 39.83 i0 679
Mrk 1418 13.86±0.03 12.61±0.19 1.25±0.20 95±16 -16.42 40.17 iE 49
Mrk 1423 14.90±0.03 13.52±0.10 1.38±0.11 6.8±0.4 -16.65 39.53 nE 8
SBS 0940+544C 17.18±0.04 17.05±0.10 0.13±0.11 5.1±0.5 -14.73 39.55 iI,M 165
Mrk 709 16.32±0.03 15.65±0.02 0.67±0.04 ... -14.66 ... ... ...
Mrk 1426 15.88±0.04 15.44±0.12 0.44±0.12 7.8±0.8 -16.24 39.82 iE 55
UGCA 184 15.99±0.04 15.80±0.10 0.19±0.11 14.0±1.4 -15.82 39.95 iI 142
Mrk 409 14.37±0.03 13.33±0.05 1.04±0.06 29.2±1.7 -17.27 40.20 iE,r 29
Tol 001 15.74±0.05 15.48±0.07 0.26±0.09 12.0±0.9 -16.09 39.89 iE 88
Tol 002 14.06±0.01 13.34±0.06 0.72±0.06 121.3±1.8 -15.19 39.86 iE 127
NGC 3125 13.05±0.01 12.25±0.06 0.80±0.06 342±4 -16.79 40.55 iE 131
SBS 1006+578 16.42±0.04 15.78±0.11 0.64±0.12 5.6±0.6 -15.34 39.53 iE 53
Haro 2 13.39±0.04 12.87±0.11 0.52±0.11 157±16 -18.28 40.94 nE 105
Mrk 1434 16.77±0.04 16.43±0.13 0.33±0.14 16.4±2.0 -15.85 40.34 i0 324
Haro 3 13.22±0.04 12.61±0.13 0.61±0.14 275±30 -17.57 40.83 iE 148
SBS 1054+504 16.08±0.04 15.46±0.15 0.62±0.15 4.3±0.5 -15.44 39.32 nE 30
Haro 4 15.59±0.04 15.44±0.17 0.15±0.17 66±12 -14.14 39.79 iI 589
VII Zw 403 14.11±0.04 13.58±0.11 0.53±0.12 59±6 -14.30 39.21 iE 75
Mrk 178 14.15±0.04 13.60±0.09 0.55±0.10 109±9 -13.96 39.36 iI,M 146
UM 439 14.77±0.03 14.09±0.06 0.67±0.07 42.8±2.5 -15.96 40.00 iE 88
Mrk 1450 15.75±0.05 15.09±0.09 0.67±0.10 42±4 -15.08 40.04 nE 234
UM 452 15.25±0.03 14.07±0.11 1.19±0.12 4.7±0.5 -16.20 39.33 nE 9
SBS 1147+520 16.95±0.05 15.98±0.09 0.97±0.10 1.4±0.2 -14.44 38.78 nE 16
Tol 17 15.99±0.05 15.05±0.06 0.94±0.08 17.5±1.0 -16.20 40.20 iE 86
UM 455 17.02±0.03 16.26±0.01 0.77±0.03 6.9±0.3 -16.62 40.38 iI 105
UM 456 15.37±0.03 14.66±0.01 0.71±0.03 24.5±0.9 -16.48 40.21 iI 85
UM 456A 16.71±0.03 16.05±0.01 0.66±0.03 4.20±0.16 -15.24 39.48 iI 51
Pox 4 15.27±0.04 14.88±0.10 0.38±0.11 94±9 -18.18 41.43 iI,M 478
ESO 572-G025 15.02±0.04 14.07±0.03 0.94±0.05 60.6±1.6 -16.80 40.59 nE 125
VCC 0001 15.80±0.03 14.42±0.01 1.37±0.04 1.90±0.06 -15.22 38.77 nE 5
Mrk 1313 16.02±0.03 15.50±0.06 0.52±0.06 8.6±0.5 -16.49 40.02 i0 64
VCC 0130 17.05±0.05 16.27±0.01 0.78±0.05 1.75±0.06 -13.97 38.73 iE 26
Haro 8 14.27±0.03 13.20±0.11 1.07±0.11 43±4 -16.75 40.12 nE 38
UM 491 15.54±0.03 14.95±0.07 0.59±0.08 8.8±0.6 -16.66 39.90 nE 39
ISZ 399 14.18±0.01 12.99±0.06 1.19±0.06 41.8±2.4 -15.99 39.77 nE 30
VCC 0459 14.95±0.05 14.13±0.01 0.83±0.05 20.4±0.5 -16.07 39.80 iI 42
VCC 0655 13.32±0.04 12.12±0.03 1.20±0.05 75.1±2.2 -17.70 40.36 iE,r 24
Tol 65 17.26±0.04 16.84±0.03 0.42±0.05 24.9±1.0 -15.52 40.59 i0 916
VCC 0848 15.03±0.05 14.10±0.01 0.93±0.05 14.2±0.4 -15.99 39.64 iI,M 28
Mrk 209 14.15±0.03 13.94±0.13 0.21±0.13 173±20 -14.67 39.84 iE 349
Mrk 1329 14.08±0.03 13.38±0.09 0.70±0.10 67±6 -16.94 40.31 iI,C 70
UGCA 290 15.67±0.03 15.04±0.13 0.63±0.13 6.2±0.9 -13.46 38.52 iI,M 29
VCC 1750 16.73±0.03 15.93±0.05 0.80±0.06 3.8±0.2 -14.29 39.06 iI 41
Haro 9 13.02±0.05 12.24±0.10 0.78±0.11 221±21 -17.91 40.79 nE 82
NGC 4861 12.68±0.03 11.91±0.09 0.76±0.09 560±5 -17.82 41.03 iI,C 160
UM 533 14.63±0.04 13.64±0.11 0.99±0.11 31±3 -15.71 39.71 iE 41
Mrk 450 14.44±0.05 13.65±0.10 0.79±0.11 11.8±1.1 -16.13 39.38 iE 15
NGC 5058 14.05±0.03 13.37±0.07 0.68±0.08 45±3 -16.58 39.99 iI 47
PGC 046448 15.47±0.03 14.51±0.01 0.96±0.03 8.9±0.4 -17.48 40.21 iE 26
Pox 186 17.73±0.03 17.39±0.01 0.35±0.04 14.8±0.5 -13.20 39.62 i0 907
Tol 35 14.17±0.03 13.22±0.05 0.94±0.06 83±4 -17.99 40.86 iE 76
SBS 1331+493 14.87±0.03 14.16±0.17 0.71±0.17 25±4 -15.11 39.47 iE 54
Tol 85 16.51±0.03 15.99±0.01 0.52±0.03 20.5±0.9 -16.96 40.78 iI 261
Mrk 67 16.10±0.03 15.34±0.08 0.76±0.09 25.1±2.0 -14.67 39.79 nE 170
Mrk 1480 16.17±0.03 15.56±0.08 0.61±0.08 13.9±1.0 -16.01 40.09 nE 111
Mrk 1481 16.19±0.03 15.57±0.06 0.62±0.07 4.4±0.3 -15.99 39.60 iI,M 34
Tol 1345-420 15.87±0.03 15.00±0.03 0.87±0.05 33.5±1.1 -16.67 40.62 nE 165
HS 1400+3927 17.04±0.05 15.94±0.10 1.10±0.12 8.6±0.8 -14.58 39.66 nE 97
SBS 1415+437 15.43±0.03 14.77±0.12 0.66±0.13 50±6 -14.59 39.79 iI,C 203
SBS 1428+457 15.42±0.05 14.67±0.09 0.75±0.10 48±4 -17.32 40.85 iI 173
Tol 1434+032 15.86±0.03 15.42±0.06 0.44±0.07 11.3±0.7 -15.80 39.79 iI 78
Mrk 475 16.20±0.03 15.67±0.07 0.53±0.08 35.6±2.4 -13.73 39.60 nE 355
HS 1440+4302 17.58±0.05 16.54±0.11 1.04±0.12 4.1±0.4 -15.32 39.85 nE 80
HS 1442+4250 15.55±0.05 15.01±0.07 0.54±0.09 14.7±1.1 -14.90 39.42 iE 69
UCM 1446+2312 15.77±0.03 14.52±0.09 1.25±0.10 ... -16.96 ... ... ...
Tol 1448+116 16.15±0.05 15.95±0.10 0.21±0.12 8.9±0.9 -16.02 39.90 iE 101
II Zw 70 14.84±0.12 14.29±0.13 0.55±0.18 83±11 -16.52 40.54 iI 217
II Zw 71 14.45±0.15 13.54±0.12 0.91±0.19 40±4 -16.91 40.23 iI 49
I Zw 115 15.04±0.09 14.14±0.08 0.91±0.12 1.88±0.16 -15.24 38.47 iE 4
SBS 1533+574 16.02±0.10 15.30±0.10 0.72±0.14 30±3 -17.46 40.95 iE 198
I Zw 123 15.42±0.10 14.85±0.08 0.57±0.13 35±3 -14.92 39.76 nE 151
HS 1609+4827 15.11±0.11 14.55±0.09 0.56±0.14 20.9±1.8 -18.04 40.66 nE 64
UCM 1612+1308 17.21±0.14 17.31±0.07 -0.10±0.15 7.1±0.5 -16.42 40.38 i0 313
UGCA 412 15.55±0.05 14.62±0.09 0.92±0.11 35±3 -17.52 40.85 nE 120
HS 1704+4332 18.41±0.03 17.87±0.08 0.54±0.09 3.3±0.3 -14.13 39.61 i0 236
NGC 6789 13.74±0.03 12.69±0.07 1.05±0.08 32.7±2.0 -14.04 38.70 iE 18
Tol 1924-416 13.97±0.04 13.50±0.03 0.47±0.05 297±5 -19.09 41.78 iE 413
Tol 1937-423 15.14±0.04 14.25±0.03 0.89±0.05 21.7±0.4 -17.96 40.66 iE 50
Mrk 900 14.17±0.03 13.56±0.12 0.61±0.13 55±6 -17.20 40.37 nE 68
Mrk 314 13.55±0.03 13.12±0.24 0.43±0.24 64±13 -18.99 40.90 iI,M 53
Mrk 324 15.17±0.03 14.60±0.13 0.57±0.14 16.0±1.9 -16.84 40.09 nE 51
Mrk 328 14.93±0.05 14.18±0.09 0.74±0.10 17.4±1.4 -16.82 40.02 nE 38

Note. - Columns stand for: (1) Galaxy name (2) B-band integrated magnitude and its 1-sigma error. (3) R-band integrated magnitude and its 1-sigma error. (4) (B - R) integrated color its 1-sigma error. (5) Integrated Halpha flux and error in units of 10-14 erg s-1 cm-2 (corrected for underlying stellar absorption). (6) B-band absolute magnitude. (7) Halpha luminosity in erg s-1. (8) Morphological type according to the classification of Loose & Thuan (1986). (9) Equivalent width of Halpha in Å. Magnitudes, colors, fluxes, and luminosities are corrected for Galactic extinction but not for internal extinction.

It is worth mentioning that, although the fraction of BCDs with elliptical envelopes is < 60 per cent, the four galaxies where a ring of star-forming regions was found are all iE BCDs. This fact suggests that the shaping of these rings requires a highly symmetric distribution of the galaxy stellar and total mass.

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