7 Summary and conclusions

Figure 12: The fitted values of $\mu$(0) for our two-component fits to the vertical surface brightness extractions of ESO 342-G017 in both R and V bands. The V-band thick disk fit is not shown due to its poorer S/N. As a rough guide, an exponential of the form $f(R) = f(0) * {\rm exp}(-R/h_R)$has been drawn on the data using a constant scale length of hR = 8.9 kpc and the peak central surface brightness ($\mu (0)$) of each component. The intent is to show that a hR > 8 kpc is consistent with the data, and that the hR is similar for both thin and thick disks.

We have used the VLT test camera on UT1 to obtain deep surface brightness photometry of the edge-on LSB galaxy ESO 342-G017 in the Vand R-bands. Careful masking of foreground and background objects to obtain an accurate value of the sky flux on our science mosaics, and an analysis of flat-fielding uncertainties - both statistical and systematic - on a variety of spatial scales, allow us to estimate confidently the total uncertainty in our deep surface photometry. We conclude that on the size scales important for probing faint, extended structure, we reach V = 28 and R = 29  mag/sqarcsec. A detailed analysis of the PSF of the images, derived from faint isolated stars on the mosaic and standard stars, indicates that scattered light affects the extended vertical luminosity profiles of ESO 342-G017 only for R > 28.5  mag/sqarcsec.

Extended light in excess of that expected for a single-component thin disk is detected at about R > 26.5  mag/sqarcsec in nearly all vertical profiles perpendicular to and up to 17 kpc along the major axis of ESO 342-G017. The same component may have also been detected in the V band frames, but the lower S/N of these frames and the larger PSF in V make this detection less robust. Given the geometric form of the extended light in this apparently bulgeless galaxy, we interpret the faint R-band light as a thick disk.

Two-component exponential disk fits were made to the observed surface brightness profiles and used to determine projected and - after deprojection and deconvolution - intrinsic structure parameters for the thin disk of ESO 342-G017 in the V- and R-bands and for the thick component in R. In particular, we find:

• The thin disk has projected scale heights perpendicular to the major axis of $h_z^{\rm thin} = 380 \pm 35~$pc in the R-band and $h_z^{\rm thin} = 380 \pm 45~$pc in V. The projected scale length of the thin disk is $8.9 \pm 1.5~$kpc in both bands.

• After deprojection and deconvolution with the PSF derived from our observations, we estimate that, within the errors, the intrinsic thin disk scale heights are 80% of the measured values. The intrinsic scale length of the thin disk is 2/3 of the fitted value.

• The face-on central surface brightness of the thin disk is estimated to be $\mu_{0}^{\rm thin} = 23.6~$ mag/sqarcsec in R and $\mu_{0}^{\rm thin} = 24.1~$ mag/sqarcsec in V.

• The thick disk, which is detected robustly in our R surface photometry, has a projected scale height of $h_z^{\rm thick} = 810 \pm 40~$pc in the R-band and $h_z^{\rm thick} = 760 \pm 75~$pc in V. The projected scale length of the thick disk cannot be determined precisely from our observations, but within uncertainties is consistent with that of the thin disk.

• The intrinsic scale parameters of the thick disk are somewhat smaller than these measured projected ones, though not as dramatically different as the projected and intrinsic parameters of the thin disk.

• Simple considerations lead to an estimate for the face-on central surface brightness of the thick disk of $24.1 < \mu_{R,0}^{\rm thick} < 24.9~$ mag/sqarcsec.

• The thick disk is likely to contribute 20-40% of the total (old) disk R-band luminosity of the galaxy.

• The total central surface brightness of the (thin + thick) disk is $\mu_{R,0} > 23.1~$ mag/sqarcsec, which places ESO 342-G017 securely in the category of low surface brightness (LSB) galaxies.

This detection of a thick disk adds to only a few others known in external galaxies (see Sect. 1), and to our knowledge is the first known thick disk in an LSB galaxy. The thick and thin disks of ESO 342-G017 have similar scale heights as their corresponding components in the Milky Way, but larger scale lengths. Importantly, the thick disk of ESO 342-G017 appears to contribute a larger fraction of the overall old disk light than does the Galactic thick disk. (Young HII regions have been masked and so do not enter into the extrapolated estimates we have made.) A prominent thick disk in ESO 342-G017 is particularly interesting since, compared to their high surface brightness cousins, LSB galaxies are thought to be more dark-matter dominated and to have less evolved disks. The VLT observations reported here suggest that, at least in the case of ESO 342-G017, such an unevolved thin disk can coexist with a substantial thick luminous component, perhaps providing a clue to the formation mechanism of thick disks in all spirals.

Acknowledgements

We are grateful to the ESO VLT Science Verification team for their assistance in obtaining the data analyzed here and to Edwin Valentijn for useful discussions. MJN acknowledges support by the European Commission, TMR Programme, Research Network Contract ERBFMRXCT96-0034 "CERES.'' PDS thanks the Anglo-Australian Observatory, Epping and the Institute for Advanced Study, Princeton for hospitality during the completion of some of the work presented here. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.