5 Results

5.1 Analysis of the surface brightness profiles

In Figs. 7 and 8, we show the vertical extractions, averaged above and below the plane of ESO 342-G017, derived from our deep VLT imaging and discussed in Sect. 4. Individual extractions consist of the horizontal average of 21 pixel wide rectangles, with foreground stars and background galaxies masked. Extractions above and below the galaxy disk were averaged about their axis of symmetry and averaged in groups to produce the profiles shown in the figures. In order to display meaningfully our data at the faintest levels, insets in Figs. 7 and 8 show fluxes on a linear scale for distances greater than 6kpc from the major axis of ESO 342-G017. The scatter in each inset about zero indicates clearly that the sky flux has been well-subtracted in our final mosaic within our calculated uncertainties.

The deviation from pure exponential behaviour in nearly all of the profiles indicates the presence of extended light beyond that expected for a purely exponential stellar disk. This motivated our choice of one- (thin disk only) and two-component (thin+thick disks) least-squares fits to the profiles, which are overplotted in Figs. 7 and 8 on the data. Both components were modeled as exponential disks (see Eq. (1)) with the scale height h_z and central surface brightness $f_{\rm0}$ as free parameters. The scale heights and central surface brightnesses (expressed in  mag/sqarcsec) derived from the simultaneous thin plus thick disk fits are summarized in Table 5.

 

 

Table 5: Fitted disk structure parameters for ESO 342-G017.

	Thin disk					Thick disk
	$\mu (0)$ ( mag/sqarcsec)		hz (pc)			$\mu (0)$ ( mag/sqarcsec)		hz (pc)
R (kpc)	R	V	R	V		R	V	R	V
-17.5	24.2 +1.3-1.2	22.7 +1.5-1.3	$340\pm240$	$350\pm210$		22.49 +0.90-0.85	22.9 +1.8-1.5	$600\pm120$	$600\pm230$
-11.8	22.3 +1.3-1.2	22.1 +1.4-1.2	$520\pm190$	$320\pm210$		23.0 +1.9-1.8	22.08 +1.0-0.96	$720\pm180$	$600\pm160$
-7.1	20.83 +0.03-0.03	21.36 +0.13-0.12	$421\pm18$	$410\pm140$		23.05 +0.34-0.26	23.0 +1.1-1.1	$870\pm55$	$760\pm270$
-3.3	21.25 +0.12-0.10	21.37 +0.09-0.09	$494\pm35$	$428\pm91$		22.43 +0.59-0.38	22.73 +1.6-0.62	$807\pm59$	$820\pm140$
0	20.01 +0.04-0.04	20.49 +0.17-0.17	$346\pm9$	$360\pm46$		22.11 +0.09-0.08	22.53 +0.57-0.43	$808\pm16$	$806\pm84$
3.3	20.19 +0.02-0.02	20.68 +0.12-0.10	$396\pm13$	$384\pm75$		22.33 +0.22-0.18	22.29 +1.7-0.63	$806\pm33$	$740\pm120$
7.1	20.61 +0.04-0.03	21.03 +0.14-0.12	$416\pm22$	$422\pm87$		22.58 +0.33-0.25	23.21 +0.90-0.88	$856\pm52$	$880\pm260$
11.8	20.90 +0.06-0.06	21.05 +0.28-0.22	$431\pm33$	$390\pm110$		23.62 +1.1-0.55	23.0 +1.5-1.4	$950\pm160$	$760\pm320$
17.5	21.44 +0.51-0.34		$324\pm51$			24.85 +0.90-0.49		$1100\pm220$

Note: These structure parameters are the results of the model fits to the observed profiles and have not been corrected for projection effects.

   
5.2 Could the faint extended emission be scattered light?

The first-order effect of turbulence in the atmosphere causes the radial point spread functions (PSFs) of point-like objects measured by an astronomical detector to have a roughly Gaussian shape, but many effects, including scattering in the telescope optics, can lead to broader wings. Although the simple optics of the VLT test camera (Giacconi et al. 1999) should minimize such a scattering, faint wings in the PSF are still present. To quantify the effect of these wings on our faint surface brightness photometry, we measured the PSFs of isolated fainter stars in the field of ESO 342-G017 and bright standard stars observed on the same nights as our science frames.

In order to be meaningful, such PSFs must be constructed with high signal-to-noise data. A conservative estimate of the precision required can be made by assuming that all of the light of ESO 342-G017 is confined to a point at a distance equal to the angular separation between the center of the galaxy and the most distant point above the plane we consider. Such an estimate shows that, at 6 kpc above the plane, the amount of R-band light brighter than 30  mag/sqarcsec scattered from ESO 342-G017 can be quantified easily if the PSF is known to a precision of $\sim$ $2.5 \times 10^{-6}$ at that distance.

Only three relatively isolated stars near the center of the ESO 342-G017 mosaic are available; the R- and V-band images of these were masked, added, and azimuthally averaged. The result is shown in Fig. 9. Note that although the extended emission around ESO 342-G017 is seen more clearly in the R-band, median seeing in R is better than in V. At faint light levels, both the R and V faint-star PSFs have broader wings than a pure Gaussian. Unfortunately, the statistical noise in these faint-star PSFs, and the relative size of the systematic photometric uncertainties over the relevant radii, precludes measurement in the wings to the accuracy we require. In principle, saturated stars on the mosaic could be used to study the wings of the PSF, but our small field contained only two; one has a near bright neighbor and the other does not fall on the V-band mosaic.

Figure 9: Measured radial point spread functions (PSF) for standard stars and fainter stars in the ESO 342-G017 field shown on magnitude (top) and linear (bottom) scales chosen to display the full dynamic range in a meaningful way. The data shown in the two plots are otherwise identical, and the apparent change in PSF shape in the linear plot is solely due to the different scales of the logarithmic and linear representation. The R and V PSFs taken from isolated stars in the ESO 342-G017 field are shown as solid and open dots, respectively. The higher signal-to-noise PSF derived from a brighter standard star is shown as the dashed line. The model PSF formed by matching the faint star PSF to the wings of the bright star PSF is indicated by the solid line. For comparison, a Gaussian with $FWHM = 1\hbox {$^{\prime \prime }$ }$ (dotted line) is also shown. Error bars shown in the lower panel represent photon noise only.

We therefore study the PSF wings using much brighter standard stars imaged during the same observing run. We build a model PSF directly from the data, using the isolated three stars on the mosaic of ESO 342-G017 to derive the PSF out to 2.5 $^{\prime\prime}$, and a bright reference star observed with similar seeing to derive the PSF from 2.5 to 16 $^{\prime\prime}$ (i.e., out to 8 kpc above the galaxy plane). Since the seeing was slightly better during the imaging of the reference star, the standard star profile was horizontally displaced to create a smooth match to the inner PSF derived from the mosaic. The result is shown as the thin solid line in Fig. 9. The R-band PSF of the standard star is consistent with zero at the level of $2.5 \times 10^{-6}$ from 7 to 12 arcsec ($\sim$3.5 to 6 kpc), satisfying the conservative requirement that we derived above. We conclude, therefore, that the PSF is well enough understood to determine its effect on the observed shape of the vertical surface brightness profiles of ESO 342-G017.

In order to examine whether the extended light apparent in Figs. 7 and 8 might be due to thin disk light scattered through the broad wings of the PSF to other positions on the detector, we convolved a model exponential disk with intrinsic structural characteristics similar to those of ESO 342-G017 with our model R-band PSF. The intrinsic thin disk model parameters reported in Sect. 6 were determined by requiring that, after inclination and convolution with the observed PSF, the projected thin-disk fitted parameters were retrieved. The degree to which the thin disk fits are reproduced is illustrated in Fig. 10.

Due to its high inclination, ESO 342-G017 has an observed surface brightness along its length that is much larger than the intrinsic (input) face-on value. Except for the central regions, which suffer a net loss of light from scattering, the primary effect of inclination - and to a lesser extent scattering - is to increase the amount of light observed at a given angular distance from the plane of ESO 342-G017. The result (output) is an observed profile that is approximately exponential, but with a projected scale height larger than the intrinsic value. More importantly, however, Fig. 10 clearly illustrates that for surface brightnesses brighter than $R \approx 28.5~$ mag/sqarcsec, no substantial light in excess of the projected thin disk profile is generated by inclination and scattered light effects. The extended light R > 26.5  mag/sqarcsec in many of the profiles of Fig. 7, therefore, must have another cause; we conclude that it is intrinsic to the galaxy itself. This conclusion is supported by the constant color (or possible slight reddening) of the extended light with increasing distance from the galaxy plane, despite the fact that the scattering in the V-band images is larger than that in Ras measured from the stellar PSF on the science mosaic.