hell Structure in the Fornax Dwarf Spheroidal Galax

We present initial results from a wide field photometric survey of the Fornax dwarf spheroidal (dSph) galaxy. The focus here is on a small overdensity of stars located approximately $17''$ ($670$ pc in projection) from the centre of Fornax. Based on imaging data in both $V$ and $I$ bands down to $V \approx 21$, the dimensions of the feature are $\sim$1.7$'' \times 3.2''$ ($68 \times 128$ pc) with an average surface brightness of $25.8$ mag/arcsec${}^2$ in $V$. A follow-up analysis using the deep $B$ and $R$ band data obtained by @ste98 indicates that the overdensity is apparently dominated by a relatively young stellar population with an age $\approx 2$ Gyr, though the current data do not rule out the presence of older stars. Our preferred interpretation is that the overdensity represents shell structure in the Fornax dSph galaxy, a phenomenon previously unseen in dwarf galaxies. The shell may be the remnant of a merger with Fornax of a smaller, gas-rich system that occured approximately 2 Gyr ago. The comparitively recent star formation seen in some dSph and dE galaxies may then also be the result of infall of small relatively gas-rich systems.

Introduction
============

Dwarf spheroidal (dSph) galaxies provide important clues to our understanding of the universe. For example, the high mass-to-light ratios found in these systems make them excellent laboratories for the study of dark matter. Further, due to the relatively large number of dwarf galaxies, they are useful tracers of matter (both luminous and dark) in the local universe. At present, the most widely accepted model describing the formation of structure in the universe is the Cold Dark Matter (CDM) paradigm, based on hypothetical non-baryonic particles [@pee93; @pea99]. CDM predicts that relatively small-scale clumps of dark matter formed from fluctuations in the density spectrum of the early universe. Large structures were then formed through a process of hierarchical merging. According to this theory, the initial dark matter haloes had masses of $\sim$10$^7$ M${}_{\odot}$ [@may02; @moo99], somewhat smaller than the masses of Galactic dSphs ($2 \times 
10^7$ M${}_{\odot}$ to $7 \times 10^7$ M${}_{\odot}$, e.g. @mat98). Consequently, the inference from the simulations is that the current Galactic dSphs have also formed from accretion events in the early Universe. However, high resolution simulations of individual dark matter halos (Power 2003, personal communication) indicate that late infall is negligible in typical dwarf halos. Therefore, discounting a fleeting encounter with a dark clump in the Galactic halo, dwarf galaxies are not expected to show any significant substructure. Nevertheless, substructure is observed in at least one Galactic dSph, namely Ursa Minor (e.g. @oa85 [@dmd01; @bel02; @pms03; @kwg03]), though in this particular case the origin of the substructure may lie with the tidal field of the Galaxy.

Although CDM accurately reproduces the large-scale structure of the universe, its predictions do not match observations of small scale regions with a high contrast in density. For example, the mass distribution of low surface brightness (LSB) galaxies inferred from their rotation curves do not have the steep density cusp at the core expected from CDM models (e.g. @wel03). In addition, the number of dwarf galaxies surrounding our Galaxy is an order of magnitude smaller than the mass spectrum predicted by CDM (e.g. @moo99). Further, in the CDM picture, the Galaxy's halo was built up through mergers of low-mass systems such as dwarf galaxies. However, the abundance patterns observed in present day dwarfs appear to be distinct from those exhibited by Galactic halo field stars [@she01; @tol03]. Indeed, age, chemical and dynamical inconsistencies make currently existing dwarfs unlikely to be the major building blocks of large systems such as spiral and elliptical galaxies (e.g. @tos02).

The Fornax dwarf spheroidal galaxy is the brightest object of this type in the Galactic halo ($M_V = -13.2$, from @mat98). It lies at a distance of $138 \pm 8$ kpc, and has a tidal radius of $r_t = 71' \pm 4'$ [@ih95] equivalent to $r_t = 2.85 \pm 0.16$ kpc. Fornax is one of nine known dwarf spheroidal companions of the Milky Way, all of which show little or no indication of HI or HII (with the possible exception of Sculptor, see @car98 [@bou03]). Fornax displays a very complex star formation history, beginning at least 12 Gyr ago and extending intermittently until $\sim 500$ Myr ago (see @ste98 [@sav00] and references therein). There are also indications, in the form of an excess of stars beyond the 'tidal radius' of the best-fit King model, that the gravitational potential of the Galaxy is tidally heating the outer regions of this dSph [@ih95]. However, @wal03 find no such excess, albeit for a larger 'tidal radius' ($r_t = 98'$) than found by @ih95. Possible asymmetries in the distribution of stars in the inner regions of Fornax have been reported by @pwh61, @esk88 and @ste98. The latter authors drew particular attention to the difference in distribution between the youngest stars and that for the bulk of the population. A measurement of the proper motion of Fornax, made using Hubble Space Telescope data, indicates that this dwarf is currently at or near perigalacticon [@pia02].

In this paper we examine an overdensity of stars situated approximately $17'$ from the centre of the Fornax dSph. This overdensity does not correspond to any previously reported asymmetry in the distribution of Fornax stars. By collecting accurate photometry down to the red clump at $V=21$, we have been able to determine the surface brightness and dimensions of this feature. Subsequent analysis of the deep survey data of Fornax provided by @ste98 reveals that the overdensity is apparently dominated by a relatively young main sequence stellar population. We propose that this clump is the first detection of shell structure in a dwarf galaxy: such structure, despite being well observed in large galaxies, was not previously known in small systems such as dwarf galaxies. The recognition of shell structure in elliptical galaxies (e.g. @mal80) provided an important insight into understanding the interactions between a large galaxy and a smaller companion. Phase-wrapped shells in the haloes of large galaxies became obvious through the process of photographic amplification [@mal78] of images of elliptical galaxies (e.g.  @mal83b). Subsequent simulations by @her88 [@her89] indicated that a large galaxy disrupting a small companion will produce the observed structure, with some ($\sim10\%$) of the material of the companion galaxy distributed in shells around the primary. The level of ordered structure in the shells reveals the type of interaction between the two objects: a direct collision will produce highly ordered, phase-wrapped shells around the primary galaxy, while a companion on a non-radial orbit will produce a confused set of shells. Observational evidence supports this process of shell formation. In particular, studies of the colour of the shells shows that they are generally bluer than the colour of the primary galaxy, reflecting the stellar population of the disrupted lower mass companion [@pen86; @mcg90].

In the following section we describe our Fornax observations and the data reduction techniques. Section 3 outlines the analysis of the data and the results are discussed in section 4. The conclusions are presented in section 5.

Observations and Data Reduction
===============================

Initial deep images of the Fornax dwarf galaxy were derived by combining photographically amplified derivatives [@mal78; @jbh93] from two deep plates taken with the UK Schmidt Telescope. The original exposures (J5462 and J8297) were made on hypersensitised Kodak IIIaJ emulsion with a Schott GG395 filter defining a broad passband from 395 to approximately 540nm. The shell-like structure is readily visible on the derivatives made from both plates and is undoubtedly real. In Fig. \[fornax\_malin\], the two photographically-amplified derivatives have been photographically combined to improve the signal-to-noise in the final image. The feature was further confirmed by inspecting the Digitised Sky Survey, and it is also present in the APM catalogue [@mes96]. This prompted further analysis using CCD data.

CCD images were obtained with the Siding Spring Observatory 1 metre telescope using the Wide-Field Imager (WFI) in October/November 2002. WFI comprises eight 4096 $\times$ 2048 CCDs arranged in a $2 \times 4$ mosaic to give a total format of 8192 $\times$ 8192 pixels. This gives a field-of-view of 52 $\times$ 52 arcmin at a scale of $0.38''$ per pixel. The field centre was placed at the coordinates $\alpha=02^h 41^m 34.2^s$, $\delta=-34^{\circ} 48' 06''$ (J2000.0), which lies approximately $31.8'$ southeast from the centre of Fornax. Thus, the field includes the central region of Fornax. We recorded images in both $V$ and $I$ with exposure times of $6 \times 600$s and $6 \times 480$s, respectively, under good conditions (FWHM $\approx 1.5''$). These data form a small part of our large-scale photometric study of the Fornax dSph galaxy.

Image reduction was accomplished using standard IRAF routines. The bias images and overscan region allowed the pedestal current to be subtracted from all object and flat field exposures. Each object image was divided by the appropriate combined flat, accounting for variations in CCD pixel sensitivity. Fringing effects in the $I$-band are negligible. After registering and combining the images for each filter, instrumental magnitudes were measured using the DAOPHOT [@ste87] program within IRAF, which allows the point-spread function to be determined interactively. The full-width half-maximum values for the PSF were 4.8 pixels ($1.8''$) and 3.5 pixels ($1.3''$) in $V$ and $I$, respectively.

The magnitudes and corresponding colours were calibrated using @gra82 and @lan92 standard fields observed across the CCDs of the WFI mosaic throughout the run. The extinction coefficients adopted were $k_V=0.16$ and $k_I=0.086$ from @sung00. As an example, the transformation equations for CCD 7 on the nights the data discussed here were obtained are: $$V-v=24.26+0.006(V-I),$$ $$I-i=24.79-0.014(V-I)$$

where $v$ and $i$ represent extinction corrected instrumental magnitudes using a photometry offset of 23.5. The rms scatter about these relations are 0.025 and 0.027 mag, respectively. The equations for CCD 8 are similar.

We present the results here for CCDs 7 and 8 which occupy the north-west quarter of the WFI mosaic. These CCDs contain the centre of Fornax and the overdense region as shown in Fig. \[F17V\_78\]. The photometric errors used are those returned by the DAOPHOT program.

Results
=======

The first question to be addressed is whether the feature might be the result of differential extinction in the the Fornax field. We approached this question in two ways. First, we chose two regions, each $\sim$10$\arcmin$ in size. One was located near the centre of Fornax and the other centred in the vicinity of the feature. Using the extinction maps of @sch98, the difference in $E(B-V)$ between these two regions was investigated and found to be less than 0.01 mag. Minor changes in the positions of the regions do not alter this result. Second, we inspected the IRAS 100$\mu$m images of this part of the sky and found that there is no 'cirrus' structure in the vicinity of the Fornax feature. We conclude therefore that the feature is not a consequence of differential extinction.

We now consider the region of the feature in more detail. It is centred approximately $17'$ ($670$ pc in projection) from the centre of Fornax at coordinates $\alpha=02^h 40^m 28.5^s$, $\delta=-34^{\circ} 42' 33''$ (J2000.0). The physical characteristics were determined by convolving the $V$-band image with a circular Gaussian kernel using the IRAF subroutine <span>*gauss*</span> and a standard deviation of $\sigma=30$ pixels (11.3$\arcsec$). This $\sigma$ value was chosen to merge small-scale structure within the feature, while allowing us to determine its overall dimensions. Defining a sampling resolution of $x_{res}=y_{res}=64$ pixels (24$\arcsec$), we produced a contour image using the subroutine <span>*contour*</span> in the IRAF package. The feature is clearly detected in this convolved image. It is almost rectangular in shape with approximate dimensions $1.7' \times 3.2'$ ($68 \times 128$ parsecs at the distance of Fornax). The feature major axis has a position angle of $41^{\circ} \pm 2.5^{\circ}$ measured eastwards from north. This agrees closely with the position angle of the major axis of Fornax, which @ih95 give as $41^{\circ} \pm 1.0^{\circ}$. In other words, the major axis of the feature is parallel to the major axis of Fornax. The feature also lies very close to the <span>*minor*</span> axis of Fornax. According to @pia02, the proper motion of Fornax is also approximately in the direction of the galaxy's minor axis.

Using the IRAF routine <span>*polyphot*</span>, we next calculated the average surface brightness of a typical region at the same (ellipticity-corrected) distance from the centre of Fornax as the feature. This is $26.1 \pm 0.1$ mag/arcsec${}^2$ in $V$. The feature, however, has an average surface brightness of $25.8 \pm 0.1$ mag/arcsec${}^2$ in $V$, indicating an increased stellar density of approximately 30%. The average surface brightness and the area of $1.7' \times 3.2'$, together with the background, then correspond to a total magnitude for the excess density of $m_V = 16.6 \pm 0.6$. Assuming the stellar population of the overdensity is at the same distance as Fornax ($(m-M)_V=20.80$ for $E(B-V)=0.03$ [@mat98]), the total absolute visual magnitude is $M_V \approx -4.2 \pm 0.6$ and the visual luminosity is $L_{V} \approx 4 \pm 2 \times 10^3 L_{\odot}$. For comparison, the faintest of the five Fornax globular clusters, Fornax 1, has $M_V = -5.4$ and a limiting radius of $1.25'$ [@mg03]. This cluster is readily visible in the NW of Fig. \[fornax\_malin\] and the difference in size compared to the feature is apparent. Within the Galactic halo, globular clusters of comparable absolute magnitude to the feature include Pal 12 ($-4.5$) and Pal 13 ($-3.7$) [@har96].

In Fig. \[cmd\_region\] we show a colour-magnitude diagram (CMD) for Fornax using the two CCDs in the northwest quadrant of the WFI mosaic. As noted above (see also Fig. \[F17V\_78\]), these two CCDs cover the central parts of Fornax as well as the region of the overdensity. We were able to measure 5195 and 10312 stars, respectively, on these two CCDs. The high galactic latitude of Fornax means that contamination from foreground stars is minimal. The resulting CMD reaches down to the red clump at $V$ $\approx$ 21 with typical magnitude and colour errors less than 0.05 mag. We then compared the CMD for the approximately 100 stars in the overdense region with that for Fornax as a whole. This is also shown in Fig. \[cmd\_region\] where the photometry for the stars in the region are shown overlaid on the Fornax CMD. No obvious differences in the CMD distributions are immediately apparent but this is not surprising given that approximately two in every three stars in the overdense region are members of the Fornax field rather than of the overdense region. A deeper dataset is needed for further analysis.

Fortunately, the deep survey of Fornax conducted by @ste98 is available, providing a basis for additional analysis of the stellar population of the overdensity. The @ste98 data were obtained using the Cerro Tololo 1.5 m telescope, and a full description of their data reduction techniques can be found in their paper. Whereas our data ends at the red clump of Fornax at $V \approx 21$, the Stetson images provide photometry for $119850$ stars down to $V \approx 24$ over a $1/3$ degree${}^2$ field, revealing, inter alia, a relatively young main sequence. The overdense region lies at the edge of the Stetson field.

Figure \[stetsoncmd\] shows the CMD for the entire Stetson dataset with the stars from the overdense region superposed. There are 327 stars in this region and the excess above the expected star numbers at this (ellipticity-corrected) radius is approximately 40%; a value in reasonable agreement with that derived from the WFI data. It is immediately apparent from Fig. \[stetsoncmd\] that there is a high concentration of relatively young main sequence stars in the overdense region. We have confirmed this occurrence in two ways.

First, we selected from the Stetson dataset those stars in the main sequence CMD region outlined in Fig. \[stetsoncmd\]. The spatial distribution of these stars is shown in Fig. \[stetsonxy\_cmdsel\]. The overdense region stands out prominently in the bottom left of the figure. Note that we have excluded the inner regions of Fornax in making this plot as there are potential substantial inter-field completeness variations for stars in this part of the CMD in the central regions of the dwarf [@ste98]. The prominence of the overdensity when using relatively blue stars also explains why the feature is better seen on blue plates. We have not conducted a statistical analysis of this map, but it appears that the overdense region is the largest fluctuation. A more detailed analysis, which allows for variable incompleteness, may reveal additional overdensities. @ste98 did note, however, that the younger main sequence (blue) stars, defined as those with $18.5 \le (B+R)/2 \le 22$ (essentially equivalent limits apply for $B$ for these stars which have $B-R \approx 0$) possess "a clearly flattened distribution on the sky with the long axis in the E–W direction and an axial ratio of 3 or 4 to 1". This distribution is evident in Fig. \[stetsonxy\_cmdsel\] as the excess density of points to the East and West of the excluded region; see also Fig. 12 of @ste98.

Second, in order to make an unbiased assessment of the stellar population belonging to the overdense region, we have endeavoured to generate a background-subtracted CMD for the region. Two complimentary methods were constructed for this CMD subtraction process. For the first, we defined a CMD density function $\Phi$, generated by dividing the Fornax CMD into a grid of $34 \times 34$ cells in colour and magnitude and then counting the stars per cell. The cells were equally spaced, extending through the domain $-0.4 \le (B-R) \le 3.0$ and the range $19.15 \le B \le 23.95$. Each cell covered an area $0.1 \times 0.15$ mag${}^2$. An estimate of the CMD for the overdense region was then determined by subtracting the 'average' Fornax CMD density function $\Phi_{Fornax}$ from that for the overdense region. That is,

$$\Phi_{feature} = \Phi_{region} - \Phi_{Fornax}.$$

To avoid the introduction of errors in the function $\Phi_{Fornax}$ by any possible radial change in the stellar population of Fornax, we chose stars only at the same (ellipticity-corrected) radius from the centre of Fornax as the overdense region. In other words, we used an annulus with the same ellipticity-corrected radius and width as the overdense region to select the Fornax stars for our average CMD for the dSph. The sample for $\Phi_{Fornax}$ was then normalised by reducing the area of the annulus to be the same as that of the overdense region. This gave the number of stars in an average region at the same radius as the overdense region as $\sim$226, while the @ste98 data for the overdense region contains 327 stars. This leads to a further estimate of the feature as a $\sim$45% overdensity, again consistent with the previous estimates above.

We then calculated the difference between $\Phi_{region}$ and $\Phi_{Fornax}$, and the resulting CMD density function for the overdense region is shown in Fig. \[subtractioncmd\]. The greyscale shown ranges from zero (white, indicating no difference between the overdense region and Fornax) to 500 (black, indicating a difference of 500 stars/mag${}^2$ at that location in the CMD) so that the darker regions in Fig. \[subtractioncmd\] indicate higher levels of confidence in the presence of overdensity members at that colour-magnitude value. Once again the CMD of the feature appears dominated by a relatively young main sequence population, consisting of approximately 80 stars. There is also some indication of the presence of red clump and red giant branch stars.

The second subtraction method used the 'average' Fornax field CMD defined above. We constructed a series of field CMDs by using a Monte Carlo alogrithm applied the density function, $\Phi_{Fornax}$. Each field CMD contained 226 stars, the average number of stars in a region with the same area and distance from the Fornax centre as the overdensity. We then subtracted each of these trial field CMDs by 'cancelling out' the nearest star in the CMD of the overdense region. A typical outcome from this process is shown in Fig. \[montecarlo\]. In particular, we find that the occurrence of stars in the region $B \sim 22.5 - 23.5$ and $(B-R) \sim 0.3$ in the subtracted CMDs is a robust result of this process.

To estimate the age of the stars in the overdense region we have used isochrones calculated from the Yonsei-Yale web-based release [@yi01; @kim02]. Shown in Figs. \[subtractioncmd\] and \[montecarlo\] are isochrones for ages of 1, 2 and 3 Gyr and an assumed metallicity of \[Fe/H\] = –1.0. Given that the data are increasingly incomplete below B $\approx$ 23.5 [@ste98][^1], the isochrones in Figs. \[subtractioncmd\] and \[montecarlo\] suggest the overdense region is dominated by a stellar population with an age of $\sim$2 Gyr for this metallicity. However, the limiting magnitude of the @ste98 data in this region makes it difficult to decide whether the overdense region also contains an older population. The metallicity assumed was chosen on the basis of the fit to the CMD, and we note that @tol01 give the mean metallicity of Fornax as \[Fe/H\] $\approx -1.0$. @tol03 argue for an age-metallicity relation in Fornax and assign an age of $\sim$2 Gyr to the Fornax giant M21 whose iron abundance, measured from a UVES high dispersion spectrum, is \[Fe/H\] = –0.7. However, the use of higher metallicity isochrones produces a less satisfactory agreement in colour at the main sequence, and does not substantially alter the age estimate. Deeper imaging specific to the overdense region would aid substantially in constraining the age (or age range) of its stellar population.

Discussion
==========

We turn now to the interpretation of the overdense region. Our current understanding of dSph galaxies indicates they are complex systems, with star formation histories extending back to the earliest times (@mat98 and references therein). These systems may be the darkest in the universe, with mass-to-light ratios ranging from $M/L \sim 5$ for the higher luminosity dwarfs such as Fornax and Sculptor, to $M/L \sim 100$ for systems such as Draco and Ursa Minor [@mat98]. According to CDM theory, the presence of such a dominant dark halo should inhibit the formation of significant substructure, barring strong encounters with objects of a comparable or higher mass (e.g. the Sagittarius and NGC 205 dwarf galaxies are being tidally stripped by their host galaxies). Therefore, we do not expect the feature to be intrinsic structure in the dSph. However, @kwg03 have recently argued that the observed morphological substructure in the Ursa Minor dSph could be a primordial artifact. From their models, the primary requirements for a dynamically cold clump to survive are that the dark matter potential possess a constant density core, and that core size be substantially larger than the orbit: substructure is rapidly destroyed in the cuspy dark matter potential predicted by CDM or for orbits larger than the core radius [@kwg03]. Nevertheless, the Fornax overdense region is unlikely to be a primordial survivor (i.e. formed at early epochs) as it contains stars as young as a few Gyr (cf. Figs. \[subtractioncmd\] and \[montecarlo\]).

An initially plausible interpretation of the overdense region is that it is a consequence of tidal interaction between Fornax and the Milky Way Galaxy. For example, recent studies of the Ursa Minor dSph (UMi) by @dmd01 [@bel02; @pms03] and @kwg03 have revealed a strongly disturbed structure for this dSph. This is frequently interpreted as the result of tidal interactions with the Galaxy. However, this is unlikely to be the origin of the overdense region in Fornax, for two reasons. First, the galactocentric distance and mass of Fornax are considerably larger than those of Ursa Minor and thus Fornax is subject to substantially lower tidal forces. Second, and more compelling, is the fact that the overdense region has a stellar population that appears to be distinct from that for the bulk of Fornax. If the feature had a tidal origin, the expectation would be that the feature population should be the same as that for the dwarf as a whole, and this is not the case.

Another possible interpretation is that the overdense region represents the remnant of a disrupted stellar cluster, which presumably was a few Gyr in age. As noted above, the @kwg03 models illustrate that a clump formed in this way could survive if two main criteria are fulfilled. Firstly, the distribution of the dark matter (DM) in Fornax must have an approximately uniform density core (that is, it differs from the standard CDM prediction). If the core is cusped, then any coherent structure is disrupted within a few orbital periods. We estimate the orbital period of the clump at this radius to be $\sim 10^8$ years using the Fornax central density and core radius values from @mat98, considerably less than the inferred age. Secondly, the core radius of the DM distribution should be at least comparable with the distance of the clump from the centre of Fornax. This latter point is possible; the clump lies in projection at about 1.5 r$_{core, stars}$ and the available velocity dispersion results suggest that the dominant dark mass probably has a more extended distribution than the light [@mow91]. The signature of this interpretation would be that the clump possesses a single age population, and that it is kinematically cold with a velocity dispersion substantially less than that of the dwarf. It would a considerable observational challenge to determine if the overdensity has such as small velocity dispersion. However, with deeper imaging data it should be possible to determine if the overdensity is dominated by a single age population, as would be expected if it is a disrupted cluster, or contains a mix of populations. Such deeper imaging observations are planned. We note, however, that while on-going star formation has certainly occurred in Fornax, there is no evidence for any star clusters in this dwarf other than the five well known globular clusters, which all have ages comparable to the globular clusters of the Galactic halo [@bon98; @bon99] and which all show no signs of disruption [@mg03; @rr94]. Therefore, it seems unlikely that the clump is the remnant of a disrupted $\sim$2 Gyr old cluster. We further note that there are <span>*no*</span> stellar aggregates (bound or unbound) in any other dSph that have ages of a few Gyr. Such aggregates that do exist are either very old (such as the Fornax globular clusters or the Ursa Minor kinematically cold clump) or very young. For example, the Phoenix dSph/dIrr contains an apparent association of young (age $\sim 100$ Myr) stars [@dmd99]. This young association is located towards the centre of the Phoenix dwarf, in contrast to the Fornax feature which is situated significantly beyond the core radius of the dwarf.

Our preferred interpretation is that the overdense region represents a 'shell' analogous to those seen around many E galaxies: the shells arise from orbit phase-wrapping of the debris of a low-mass companion tidally disrupted by a more massive galaxy (e.g.  @her88 [@her89]). Such features are relatively common in more luminous systems, with about $\sim$10% of all E galaxies showing features of this nature [@her88], however such structure has not been previously seen in dE or dSph galaxies. Our shell interpretation is driven by the location and appearance of the Fornax overdense region – the location on the minor axis, the elongation parallel to the major axis, and the relatively 'sharp-edged' nature of the overdense region are all characteristic of shell features. Shells around more luminous E galaxies are also generally bluer than the primary galaxy [@pen86; @mcg90] and that is clearly also the case here. Simulations by @her89 predict approximately $10\%$ of the companion mass is distributed as shells around the primary, depending on the mass ratio and collision parameters of the two objects. If we assume that approximately $1 - 10\%$ of the mass contained in the shells is visible in this clump, we obtain a mass of $\sim 10^6 - 10^7$ M${}_{\odot}$ for the companion object. This is comparable to the smallest dwarf galaxies of the Local Group.

We propose then that a relatively gas-rich low mass dwarf galaxy merged with Fornax approximately 2 Gyr ago. Tidal stirring of gas from the companion produced a new population of stars, which, together with the remnant population of the disrupted system, are distributed as shell structure throughout the Fornax dSph galaxy. Modelling of such a process may well provide insight as to the location and surface density of other possible shells to guide further observations. It may also indicate how retention of gas from such a merger process may have led to the unusual distribution of the youngest Fornax stars as outlined by @ste98.

Conclusion
==========

We have presented photometric data for a stellar clump located $\sim$17$\arcmin$ from the centre of the Fornax dwarf spheriodal galaxy. The clump is approximately 68 $\times$ 128 pc in size and is $\sim$40% more dense than the surrounding regions. It is located on the Fornax minor axis and is elongated in the direction of the dSph's major axis. By removing the Fornax field contamination through a CMD density function analysis, we have shown that the clump appears dominated by a stellar population approximately 2 Gyr in age. Our interpretation of this clump is that it is a shell feature analogous to those frequently seen around more luminous E galaxies. As such, it would represent the first detection of shell structure in a dwarf galaxy. In this interpretation we propose that a smaller gas-rich system merged with Fornax in the relatively recent past ($\sim$2 Gyr ago) and that the shell is a consequence of this merger. Models of such a merger, kinematic studies of the shell stars, and further deep imaging of the region surrounding this dSph to constrain any additional shell structure, are all required to investigate this hypothesis more thoroughly. It may be that in many cases the comparitively recent star formation seen in some dSph and dE galaxies is also the result of the infall of lower mass, relatively gas-rich systems.

The authors are grateful to Peter Stetson for making his Fornax data available for this study. Matthew Coleman acknowledges the financial support provided by an Australian Postgraduate Award. The authors are also grateful to the referee, Dr. Antonio Aparicio, for his helpful comments on the original manuscript.

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[^1]: @ste98 actually state that they regard the data as 'essentially complete' to well below \[$V=(B+R)/2$\] magnitude 23. For main sequence stars with $B-R \approx 0.4$, $V$=23 corresponds to $B$=23.2 from which we assumed that the data are increasingly incomplete below $B \approx 23.5$. It is also worth noting, as Figure 2 of @ste98 illustrates, that the region of the overdensity received less total exposure time than the central regions of the dwarf galaxy.

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