tokens: focal ratios of these telescopes, along with the use of single, photographic-film size CCDs, allow them to probe large areas of galaxy haloes while reaching surface brightness levels ∼2–3 mag deeper (μ r ∼ 28 mag arcsec −2 ) than the classic photographic plate surveys (e.g. Palomar Observatory Sky Survey) and the available large-scale digital surveys (e.g. the Sloan Digital Sky Survey). Here, we use this amateur telescope discovery approach to report a faint, elongated galaxy projected on to the halo regions of NGC 253 in the Sculptor Group. With a distance of 3.5 Mpc, NGC 253 is one of the nearest large spirals, and has been well studied (e.g. Davidge 2010 ; Bailin et al. 2011 ; Greggio et al. 2014 ; Monachesi et al. 2015 ), but this object, which we call NGC 253-dw2, had been missed. 1 2 OBSERVATIONS We first noticed a candidate satellite galaxy in a visual inspection of several images of NGC 253 available on the internet that were taken by amateur astronomers: Alessandro Maggi using a Takahashi Epsilon 180ED astrograph (18-cm diameter at f /2.8) and Mike Sidonio using a 12-in f /3.8 Newtonian (see Fig. 1 ). In both cases, the pixel scale was coarse and it was not clear if the elongated feature was real or an artefact or reflection as often present in amateur images. Figure 1. Left: amateur images of NGC 253 and its satellite NGC 253-dw2. The grey-scale image is from the 12-in telescope luminance filter, with colour image of NGC 253 from the 0.5 m Black Bird Remote Observatory, and field of view of ∼11 × 10 (67 kpc × 60 kpc). North is up and east is left. The zoom-in on NGC 253-dw2 is from CHART32 and covers ∼8 kpc × 7 kpc. Right: Subaru/Suprime-Cam image, colourized using g and R c bands, and demonstrating that the satellite is composed of red, discrete stars. We investigated the nature of NGC 253-dw2 by follow-up observations using amateur and professional facilities as follows. First, deep imaging of the field centred on the dwarf candidate was collected remotely with the Chilean Advanced Robotic Telescope (CHART32), an 80-cm f /7 corrected Cassegrain telescope located at the Cerro Tololo Inter-American Observatory, Chile. An FLI PL-16803 CCD camera was used, with a pixel scale of 0.331 arcsec over a 22 arcmin × 22 arcmin field-of-view. Two overlapping sets of 20 × 1200 s individual image frames were obtained through a Baader luminance filter over several photometric nights between 2013 September and November. Each individual exposure was reduced following standard image processing procedures for dark subtraction, bias correction, and flat-fielding (Martínez-Delgado et al. 2010 ). The images were combined to create a final co-added image with a total exposure time of 48 000 s. Part of the resulting CHART32 image is shown as an inset in Fig. 1 (left-hand panel). The candidate object looks fairly regular and diffuse, like a nearby dwarf spheroidal (dSph) galaxy rather than a background galaxy or an instrumental artefact. It is also very
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