6 The thin and thick disks of ESO 342-G017

The extended light in ESO 342-G017 is reasonably well fit by a thick exponential disk with nearly constant projected scale height (h_z) as a function of galactocentric radius (R) along the major axis of the galaxy. This is illustrated in Figs. 7 and 8, and in Fig. 11, where the fitted values of h_zfor each averaged extraction are shown in both the R and Vbands for the thick and thin exponential disk components. The error-weighted mean of the projected scale heights are: $h_z^{\rm thin} = 380 \pm 35~$pc and $h_z^{\rm thick} = 810 \pm 40~$pc in the R-band and $h_z^{\rm thin} = 380 \pm 45~$pc and $h_z^{\rm thick} = 760 \pm 75~$pc in the V-band. The projected scale length, h_R, of the thin disk is more difficult to assess, but is estimated from the fitted values of $\mu (0)$ as a function of position along the major axis to be about $8.9 \pm 1.5~$kpc in both bands. The projected scale length of the fitted thin disk is indeterminate from the V-band frames, but is consistent, within the uncertainties, with the projected scale length of the thin disk in the R-band.

When deprojected and deconvolved (that is, taking into account line-of-sight effects due to the inclination of the galaxy and seeing), the true face-on surface brightness of the thin disk in the R-band is $\mu_{R,0}^{\rm thin} = 23.6~$ mag/sqarcsec, with a true scale height and scale length of ${h_{\rm z,0}^{\rm thin}} = 310 \,$pc and ${h_{\rm R,0}^{\rm thin}} = 5.9\,$kpc, respectively. These estimates were made by convolving model thin exponential disks inclined at 88$^\circ$ with the measured PSF, and requiring that the resulting vertical and radial profiles matched those fitted to the observed profiles (see Fig. 10). The intrinsic thin disk scale heights and lengths in the V-band are the same as those in the R-band, within uncertainties. The projected scale height, h_z, is larger than the intrinsic value h_z,0primarily due to convolution with the comparably sized PSF. On the other hand, the projected scale length, h_R, is larger than h_R,0 because of line-of-sight effects due to the extreme inclination of the galaxy. The thin disk has an inferred face-on surface brightness in Vof $\mu_{V,0}^{\rm thin} = 24.1~$ mag/sqarcsec, implying an intrinsic color of $V - R \approx 0.5$ for the thin disk. Since the color is found by extrapolating the fitted parameters into the plane of the disk, it is relatively, though not completely, insensitive to dust and clumpy luminosity from HII regions. We estimate that the uncertainty in our inferred intrinsic parameters is about 10-15%, primarily coming from uncertainties in inclination and the fit parameters.

Figure 10: The effect of inclination and PSF convolution on the observed radial and vertical surface brightness profiles along the major and minor axes of a thin exponential disk similar to that of ESO 342-G017. The inferred intrinsic vertical and face-on radial profiles are shown as thin dashed lines. The thick solid line indicates the result after inclination by 88 degrees and convolution with the high signal-to-noise PSF determined from isolated faint stars on the science mosaic and a bright standard star. The vertical (minor axis) and radial (major axis) profiles of an exponential thin disk with typical fitted parameters for the projected scale height, hz = 380 pc, and scale length, hR = 8.9 kpc (see Sect. 6) are shown as thin solid lines.

The structural parameters of the extended light are more uncertain, but also much less affected by inclination and seeing effects. We have not attempted, therefore, to deproject the thick disk scale parameters, but expect that in the R band the intrinsic scale height is close to the projected value of $h_z^{\rm thick} = 810 \pm 40~$pc, while the true scale length of the thick disk is between $\sim$6 and 9 kpc, (the intrinsic thin disk and projected thick disk values, respectively). The value of the central surface brightness of the thick disk is uncertain, but can be constrained. For a pure exponential disk, the edge-on central surface brightness $\Sigma_{\rm EDGE}(0)$ (in linear units) can be shown to be given by $\Sigma_{\rm EDGE}(0) = ({h_{R,0} / h_{z,0}}) \Sigma_0(0)$, where $\Sigma_0(0)$ is the face-on central surface brightness. If we assume that the fitted value $\mu_{R}^{\rm thick} = 22.1~$ mag/sqarcsec of the thick disk in the R-band is a good approximation to the actual edge-on value for the thick disk, then, based on our estimates of these quantities and their uncertainties, we can deduce that $24.1 < \mu_{R,0}^{\rm thick} < 24.9~$ mag/sqarcsec. The PSF may have a small effect that would cause the fitted value to be higher than the actual value, in which case these constraints would be pushed to slightly fainter magnitudes. The detection of the thick disk in the V-band is less secure, both because the S/N of our relative surface brightness photometry is lower in V and because the PSF (and thus scattered light problems) is larger in V. Furthermore, beyond galactocentric radii of 5 kpc, there is only a small statistical difference in the inferred scale heights of the fitted thin and thick disks (Fig. 11), and the extrapolated in-plane surface brightness of the thick component in the V-band shows no clear trend with major axis radius.

Figure 11: The fitted values of hz for two-component (thin + thick exponential disks) model of the vertical surface brightness extractions of ESO 342-G017 in both R (left) and V (right) bands. The error bars indicate the formal errors of the fit, and are clearly larger in V-band and at larger galactocentric radius R where the S/N is poorest. Horizontal dashed lines indicate the error-weighted mean of hz for the two components in each photometric band.

The intrinsic R-band scale heights of the thin and thick disk components of ESO 342-G017 are similar to those of the Milky Way, but because the intrinsic scale length of its thin disk is larger than the commonly accepted Galactic value of h_R,0 = 3-3.5 kpc (see references in Sackett 1997), the ratio h_R,0 / h_z,0 is $\sim$50% larger for ESO 342-G017 than for the Galaxy. Since the total luminosity of any pure exponential disk is given by $L = 2 \pi \Sigma_0(0) h^2_{R,0}$, if the intrinsic scale lengths of the thick and thin components are equal, the ratio of total light in each is given by the ratio of their intrinsic central surface brightness. Together with the constraints on $\mu_{R,0}$ for the two components derived above, this assumption implies that the thick disk contributes $\sim$20-40% of the total R-band light of ESO 342-G017, excluding the light in individual masked HII regions.

Finally, we note that these constraints on the luminosity contribution of the thick disk imply a combined (thin+thick disk) face-on central surface brightness for ESO 342-G017 of $\mu_{R,0} > 23.1$. Since the B - R color of the galaxy is certainly greater than zero, and probably $\ge$0.5, this places ESO 342-G017 firmly in the class of low surface brightness (LSB) galaxies, which are generally defined as those disks with B-band face-on central surface brightnesses $\mu_{B,0} > 23$ mag/sqarcsec (cf., de Blok et al. 1995).