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Astron. Astrophys. 327, 562-568 (1997)

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3. Large-scale morphology and colors of NGC 1427A

The appearance of NGC 1427A resembles that of the LMC with a bar-like main body and several regions of active star formation (Fig. 3). North of the `bar' a large blob of diffuse light and embedded point sources is located, detached from the main body of NGC 1427A (in the following the `nob' [FORMULA] `northern blob'). This region appears similar to Shapley Constellation III north of the bar of the LMC.

[FIGURE] Fig. 1. This picture shows a [FORMULA] color map of NGC 1427A. The greyscale corresponds to color bins of 0.1 mag as indicated in the plot. The sky is arbitrarily set to a color of [FORMULA] mag. Arrows indicate the direction to the central Fornax galaxies NGC 1399 and NGC 1404.

[FIGURE] Fig. 2. The top panel shows a CMD of all objects within a radius of [FORMULA] from the center of NGC 1427A. The dots are unresolved objects as discussed in Sect. 5. Circles indicate slightly extended objects (the FWHM is more than 1.5 times larger than for stellar images). Circles with stars inside are objects with H [FORMULA] emission. Double circles turned out being multiple objects after comparison with the HST image. The lower panel shows the CMD of objects in a 2 times larger control field north of NGC 1427A. The mean errorbars of the color for different magnitudes are indicated.

[FIGURE] Fig. 3. This picture shows a V image of NGC 1427A. The white contours indicate H [FORMULA] detections. The values are aperture [FORMULA] and [FORMULA] colors for the brightest, most prominent OB associations and clusters. In comparison to Fig. 4 the (X,Y) pixel section of this image is (320,190) for the lower left corner and (830,700) for the upper right corner.

The appearance at low surface brightness shows that the main body and the `nob' are embedded in a common envelope of unresolved stellar light (see Fig. 4). The lowest isophote in Fig. 4 corresponds to a surface brightness of [FORMULA] mag/arcsec2, which is 2 [FORMULA] above the background. Inside this envelope, the most luminous part of the main body lies off-centered to the south-west. At the south-west border the regions with the highest surface brightness are aligned in a half-ring. The most prominent of these regions is located at (530,360) in (X,Y) pixel coordinates of Fig. 4. Appearence and size ([FORMULA] kpc) of this star forming region strongly resembles the 30 Dor complex in the LMC. The east side of the `bar' ends in an extended tail of low surface brightness ([FORMULA] mag/arcsec2). At the southern border of the `nob' some low surface brightness spurs point towards the main body of the galaxy (see Fig. 4).

[FIGURE] Fig. 4. The location of the cluster candidates is given in the plot of the isophotes of the main stellar body of NGC 1427A. The distribution of the clusters is more uniform than that of the OB associations (see also Fig 3). The contours are derived from a [FORMULA] pixel median filtered image .The 3 lowest isophotes correspond to [FORMULA] mag/arcsec2, [FORMULA] mag/arcsec2, and [FORMULA] mag/arcsec2, respectively. Frame size and orientation are the same as in Fig. 1.

We constructed a calibrated color map from our V and I image to visualize the colors of the main body and the bright knots (Fig. 1). All pixels with intensities lower than [FORMULA] above the sky were arbitrarily set to [FORMULA] mag. The color map was smoothed by a [FORMULA] pixel median filter and the color values were binned in steps of 0.1 mag. The mean color error is in the order of 0.03 mag in the region of the stellar body. Several features become visible: the bright knots have colors bluer than [FORMULA] mag. The stellar body has a mean color of [FORMULA] mag except a north-south stripe in the middle of the galaxy with colors around [FORMULA] mag. This redder color is probably due to a dust lane at the center of the galaxy. Another explanation would be an older (redder) stellar population that is dominating the color in this region. The `nob' as well as the faint luminosity tail in the east have significantly bluer colors of about [FORMULA] mag, which indicates a different star formation history in these regions than in the main body.

We also measured [FORMULA] colors of the stellar body in small apertures assuming constant sky values. The mean colors are [FORMULA] mag in the main body, which is comparable to the mean color of the LMC, [FORMULA] mag in the eastern tail, and [FORMULA] mag in the `nob'. Compared to other galaxies the `nob' lies in the color range of blue compact dwarf galaxies (hereafter BCDs) or very blue dwarf irregular galaxies (i.e. Thuan 1983, Gallagher & Hunter 1987).

The blue color of the unresolved stellar light can not be explained by an old stellar population alone, even if this population would have a very low metallicity of [FORMULA]. Therefore a young stellar component must have contributed to the light. We estimated ages by comparison with photometric evolutionary synthesis models of dI and BCD galaxies from Krüger & Fritze-v. Alvensleben (1993). In the case of continuous star formation a color of [FORMULA] of the main body is consistent with ages between 2 and 5 Gyr depending on metallicity and a constant or decreasing star formation rate (SFR). The stellar population of the `nob' with [FORMULA] would then be about 1 Gyr old. Assuming a single starburst occuring in a 10 Gyr old galaxy with a previous continuous SFR, colors bluer than [FORMULA] correspond to ages younger than [FORMULA] yr after the burst.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998