1 Introduction

Outside our own Galaxy, most of what we know about the structure, evolution and dynamics of stellar populations, and their connection to dark matter, is deduced from high surface brightness features: bars, bulges, and thin disks. Fainter surface brightness components such as stellar halos, thick disks, and globular clusters probe galactic potentials differently, in both time and space owing to their larger age and extent. The formation mechanisms of these faint tracers are still a matter of some controversy; suggestions range from early protogalactic collapse, secular processes such as heating from molecular clouds, black holes and spiral structure, through to later stochastic processes such as accretion (see recent reviews by Buser 2000; Bland-Hawthorn & Freeman 2000; and references therein). These scenarios predict different kinematical, morphological and chemical characteristics, but too few systems have been sufficiently well studied to constrain the models. Due to the difficulty in detecting low surface brightness features reliably in external galaxies, the important complementary information they contain has only begun to be tapped.

In the Milky Way, faint disk and halo components can be separated on the basis of their kinematics and morphology, and - to a certain extent - metallicity, because individual stars can be resolved. The Galactic stellar halo of field stars and the globular cluster systems have volume densities that decrease with galactocentric radius rroughly as $\rho (r) \propto r^{-3.0}$ or r^-3.5 (Harris & Racine 1979; Saha 1985; Zinn 1985), similar to results for halo populations in large spirals like M31 (Racine 1991; Reitzel et al. 1998) and NGC 4565 (Fleming et al. 1995). Giant ellipticals and superluminous CD galaxies, on the other hand, which are thought to be the product of many mergers, have halo luminosities and globular cluster systems that fall less steeply, roughly as $\rho (r) \propto r^{-2.3}$ (Harris 1986; Bridges et al. 1991; Harris et al. 1995; Graham et al. 1996). The total mass, the bulk of which is believed to be contained in dark matter halos, is inferred from kinematical studies to have volume densities that decline as $\rho (r) \sim r^{-2}$ beyond a few disk scale lengths (see Sackett 1996 for a review).

Our Galaxy also has a faint thick disk whose density falls exponentially with increasing height (z) above the plane as ${\rm e}^{-z/h_{z}^{\rm thick}}$. Its scale height $h_{z}^{\rm thick} \simeq 1 \pm 0.3$ kpc (Reid & Majewski 1993; Ojha et al. 1996; Buser et al. 1999) is about three times larger than that of the much brighter thin disk.

The scale length of the thick disk is $h_{R}^{\rm thick} \simeq 3 \pm 1.5$ kpc (Buser et al. 1999), similar to that of the Galactic thin disk. Despite this, the thick disk contibutes only 2-9% of the total local stellar disk light (Reid & Majewski 1993; Ojha et al. 1996; Buser et al. 1999), and perhaps $\sim$13% of the total disk luminosity of the Milky Way (Morrison et al. 1994).

For external galaxies, morphology determined through integrated surface brightness photometry is the only current method to detect and characterize faint galactic components. Detections of extended light that are perhaps indicative of a thick disk component with $h_{z}^{\rm thick} \simeq 1{-}2$ kpc have been reported in a few external edge-on galaxies. Early detections of extra-planar light in excess of that associated with a thin exponential disk were limited to SO (Burstein 1979) and early-type spirals with significant bulges (van der Kruit & Searle 1981a; van der Kruit & Searle 1981b; Wakamatsu & Hamabe 1984; Bahcall & Kylafis 1985; Shaw & Gilmore 1989; de Grijs & van der Kruit 1996; Morrison et al. 1997). Leading to the supposition that thick disks were found in older stellar systems with significant central concentrations (van der Kruit & Searle 1981a; Hamabe & Wakamatsu 1989; de Grijs & Peletier 1997). This hypothesis is consistent with the lack of a thick luminous component around the small, Scd spiral NGC 4244 in deep R-band observations reaching to R = 27.5 mag/sqarcsec (Fry et al. 1999), and in the bulgeless Sd edge-on NGC 7321 (Matthews et al. 1999). On the other hand, observations indicate that there are individual exceptions. Multiband photometry of the later-type Sc spiral NGC 6504 (van Dokkum et al. 1994) revealed extended light interpreted as a weak thick disk with $h_{R}^{\rm thick} \simeq 2$ kpc.

Faint light high above the plane of the well-studied, late-type, edge-on spiral NGC 5907 has further complicated the picture of extra-planar light in small- or no-bulge spirals. First detected at heights of 3 to 6 kpc above the plane in deep R-band observations (Morrison et al. 1994), this extended emission is intriguing because it is unlike any known thick disk or stellar component, having instead a morphology similar to that inferred for the dark matter halo distribution of NGC 5907 (Sackett et al. 1994). Other workers have confirmed the presence of the faint light in other bands (BVRIJK), and showed that the extended light is redder than the thin stellar disk. If the faint light is due to a thick disk, it is unlike any other, with a scale length that is at least twice that of its thin disk (Morrison 1999). The stellar population responsible for this faint light remains highly controversial, ranging from normal or metal-rich populations with steep IMFs (Lequeux et al. 1996; Rudy et al. 1997; James & Casali 1998), old, metal-rich accreted populations with normal IMF (Lequeux et al. 1998), or exceedingly metal-poor or giant-poor populations with few resolvable stars at the tip of the RGB (Zepf et al. 2000). The controversy remains because the full spectral energy distribution is apparently inconsistent with any single explanation (e.g. Zepf et al. 2000; Yost et al. 2000).

The puzzling nature of the extended light in NGC 5907 has motivated new studies to test a possible connection between faint optical and IR light and dark matter in this and other spirals (Gilmore & Unavane 1998; Rauscher et al. 1998; Uemizu et al. 1998; Abe et al. 1999; Beichman et al. 1999; Yost et al. 2000; Zepf et al. 2000). The optical results are mixed, but infrared surface brightness photometry indicates that whatever produces the faint optical light detected to date does not appear to emit strongly at IR wavelengths far from the plane of the galactic disks. Thus, if associated with known stellar populations, the sources of the faint light are unlikely to account for the dark mass of spiral galaxies.

 

 

Table 1: Basic properties of ESO 342-G017.

Parameter	Value	Reference
$\alpha$, $\delta$ (J2000.0)	21 12 10.8, -37 37 38	Karachentsev et al. (1999)
type	Sc+6	Mathewson & Ford (1996)
redshift	$7680\pm 10$ kms-1	Mathewson & Ford (1996)
inclination	88$^\circ$	this paper
PA	$120\hbox{$.\!\!^\circ$ }4 \pm 0\hbox{$.\!\!^\circ$ }5$	this paper
major-axis D$^{\ast}$	86 $^{\prime\prime}$	this paper
mB	$16.67\pm0.09$	Lauberts & Valentijn (1989)
mV	$16.40\pm0.03$	this paper
mR	$15.92\pm0.04$	this paper
mI	$15.47\pm0.06$	Mathewson & Ford (1996)
MR	$-19.1\pm0.3$	this paper
MV	$-18.7\pm0.3$	this paper

$^{\ast}$ Major-axis diameter measured from the R=27.0  mag/sqarcsec contour.
Magnitudes are not corrected for extinction.

In this paper, we report on the collection, reduction and analysis of ultra-deep surface photometry of the isolated, edge-on, low surface brightness, Sd galaxy ESO 342-G017, using some of the first science observations taken with the VLT. The simple optics, good seeing, and extremely well-sampled PSF of our observations ensured a low and well-understood level of scattered light and accurate identification of contaminating sources. Concurrent deep observations of unrelated blank fields with the VLT were used to create dark sky flat fields at the appropriate wavelengths. Considering all sources of uncertainty, including those from light scattered through the wings of the PSF, we conclude that the resulting surface photometry is reliable to a level of R = 28.5 mag/sqarcsec and V = 27.5 mag/sqarcsec. Analysis of these data reveals a faint component that we interpret as a thick disk, to our knowledge the first thick disk discovered in an LSB galaxy.

In Sect. 2 we describe the VLT observations and observing strategy. In Sect. 3 the data reduction process, including the production of dark sky flats and the procedures for masking, mosaicing, calibrating, and determining the sky flux are outlined. The procedure to extract profiles from the deep images is given in Sect. 4, along with a brief description of the error analysis, which is discussed in depth in the appendix. The resulting V and R surface photometry of ESO 342-G017 are presented in Sect. 5, along with a description of the fitting procedure for the thin and thick disk parameters. A thorough analysis of scattered light due to the tightly-constrained PSF is discussed in Sect. 5, and ruled out as the cause of the faint extended light we detect in ESO 342-G017. The thin and thick disks, including their inferred intrinsic properties are described in Sect. 6. We summarize and conclude in Sect. 7. Throughout this paper we assume a distance of 102Mpc to ESO 342-G017 (based on a Hubble constant of H₀=75km/s/Mpc), which yields an image scale of 0.495kpc per arcsecond.