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Deep Imaging of Galaxy Outskirts Using Telescopes Large and Small Future Prospects: Deep Imaging of Galaxy Outskirts using Telescopes Large and Small Roberto Abraham1;2, Pieter van Dokkum3, Charlie Conroy4, Allison Merritt3, Jielai Zhang1;2;5, Deborah Lokhorst1;2, Shany Danieli3;4;6;7, Lamiya Mowla3 Abstract The Universe is almost totally unexplored at low surface bright- ness levels. In spite of great progress in the construction of large telescopes and improvements in the sensitivity of detectors, the limiting surface bright- ness of imaging observations has remained static for about forty years. Re- cent technical advances have at last begun to erode the barriers preventing progress. In this Chapter we describe the technical challenges to low sur- face brightness imaging, describe some solutions, and highlight some relevant observations that have been undertaken recently with both large and small telescopes. Our main focus will be on discoveries made with the Dragon- fly Telephoto Array (Dragonfly), which is a new telescope concept designed to probe the Universe down to hitherto unprecedented low surface bright- ness levels. We conclude by arguing that these discoveries are probably only scratching the surface of interesting phenomena that are observable when the Universe is explored at low surface brightness levels. 1Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada. Primary contact: [email protected] 2Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada 3Department of Astronomy, Yale University, Steinbach Hall, 52 Hillhouse Avenue, New Haven, CT 06511, USA 4Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA 5Canadian Institute for Theoretical Astrophysics, 60 St. George Street, Toronto, ON M5S 3H8, Canada arXiv:1612.06415v1 [astro-ph.GA] 19 Dec 2016 6Department of Physics, Yale University, 266 Whitney Avenue New Haven, CT 06511, USA 7Yale Center for Astronomy and Astrophysics, 52 Hillhouse Avenue, New Haven, CT 06511, USA 1 2 Team Dragonfly 1 Motivation A fundamental prediction of hierarchical galaxy formation models in a dark energy-dominated Cold Dark Matter cosmology (ΛCDM) is that all galaxies are surrounded by a vast and complex network of ultra-low surface brightness filaments and streams, the relics of past merger events (Purcell et al 2007; Johnston et al 2008; Cooper et al 2013; Pillepich et al 2014). Resolved (star count-based) analyses have revealed the existence of streams and filaments around the Milky Way (e.g., Majewski et al 2003; Belokurov et al 2007; Car- ollo et al 2007; Bell et al 2008) and M31 (e.g., Ibata et al 2001; Ferguson et al 2002; Ibata et al 2007; Richardson et al 2008; McConnachie et al 2009; Gilbert et al 2012). These impressive results provide strong evidence that some massive spiral galaxies formed, at least in part, hierarchically. In the ΛCDM picture many thousands of such streams and filaments combine over time to define galactic extended stellar haloes, with the bulk of the material distributed at surface brightnesses well below 30 mag/arcsec2 (Johnston et al 2008). In contrast to the star count-based results on stellar streams, the de- tection of these haloes has proven elusive. The Hubble Space Telescope (HST) has undertaken some very successful deep pencil beam star count surveys of a number of galaxies outside the Local Group (e.g., the GHOSTS survey; c.f. Radburn-Smith et al 2011 and references therein) but since these stel- lar haloes are very extended (tens of arcminutes for nearby objects), nearby haloes may not be well sampled by pencil beams, and the seemingly much simpler strategy of trying to image them directly using ground-based tele- scopes is quite attractive. Many such studies have claimed detections of the extended low surface brightness stellar haloes of galaxies on the basis of direct imaging, but these claims are now controversial, with recent investigations dismissing these \haloes" as simply being scattered light. This point was first made by de Jong in 2008, and the putative detection of low surface brightness stellar haloes from unresolved imaging has recently been the subject of two exhaustive investigations by Sandin (2014, 2015), who concludes that most claimed detections are spurious. 2 Why is Low Surface Brightness Imaging Hard? The faintest galaxies detected at present are about seven magnitudes (over a factor of 600) fainter than the faintest galaxies studied during the \pho- tographic era" of astronomy prior to the mid-1980s. It is therefore quite remarkable that over this same period of time there has been essentially no improvement in the limiting surface brightness of deep imaging observations of galaxies. (For example, the low surface brightness limits presented in Ko- rmendy and Bahcall 1974 are quite impressive even by modern standards). As we will describe below, this is because low surface brightness imaging is 7 reproduced in Fig. 1).for As will example, be the described\cleaner" very below, than this striking those clean examples of focalre-imaging in large plane telescopes, is Duc optics with an et fewerthe al internal imaging reflections. 2015, systems (See, tend one to of be which relatively simple. is The absence of complicated permission from Duc etwhich al. confuse (2015) thenote CFHT the data absence atsmall of low telescope large surface than stellar brightness on haloes levels. the in Figure CFHT, the reproduced but image with the obtained limiting by surface the brightness amateur is telescope the same, and telescopes (Mart´ınez-Delgadoet al 2009, 2010,appeared 2012, recently, based 2016). onof amateur-professional interesting collaborations investigations using ofderable small tidal work can structures be around donelimited by nearby by relatively imperfect galaxies small control of telescopes, have systematic andnot errors. indeed usually The a limited implication number is by that photon cons- statistics or byDeep spatial Imaging resolution. of Instead, Galaxy it Outskirts is using Telescopes Large and Small 3 pixel sizes. designs have intrinsically well-correctedcorrectors, flat or focal even planesscales image that on straight relatively are onto small areasons, their sensors. e.g., good sensors, Most to match reduce small since to their telescopes some long1 CCD utilize native small small focal telescope and lengths in optical simple order flat to field provide reasonable image ( West) obtained with twoFig. different cameras 1 and telescopes: MegaCam on the 3.6 m CFHT left Re-imaging optics are an essential component of cameras on large telescopes for many A major benefit of the work done to date using small telescopes is that ) and a 12 inch amateur telescope ( Images of the same field around NGC 474 (to the East) and NGC 467 (to the Images of the same field around NGC 474 (to the East) and Figure 7. NGC 467 (to the West) obtainedMegaCam with two on different the cameras and 3.6m telescopes: band CFHT – (top) and – total antelescope, exposure ATIK located 4000m of on the CCD 0.7 site camera of hour the mountedvatory in Bulgarian Rohzen National on the – Astronomical a total Obser- g- exposure 12” timefilter RC of 21.5 (L); amateur hour Image with the credit: ClearNote Irida Luminance telescope, the Velimir absence Popov of andtelescope. large Emil stellar Ivanov halos – in the image obtained by the Irida lowing to reach a similarobserving time. surface brightness limit in 30 times less 3.1.3 Nuclear halos A potentially more devastating effectuntil of internal recently reflections, has which been oftenthe minimized, brightest is parts the of radialradius. galaxies, spreading This i.e. of PSF their far nucleus, wingtaken at for generates large real structures stellar effective which halos (Michard may 2002;low be Sandin imaging 2014). mis- surveys, With the shal- PSF signatureages may (de be seen Jong in 2008; stacked La im- With Barbera our et deep al. imaging 2012; survey, D’Souza it et is al. directly 2014). visible on individual 1 tends to produce images from small telescopes that are Early-type galaxies as seen by deep optical images Top- Bottom-left: , is used. 2 right − ). The integration time is 30 resulting image at the same intensity mag arcsec , 000–000 composite g+r–band image of the field around 000 Bottom-right: Top-left: Imprints of the internal reflections of bright stars on deep images Extended low surface brightness nuclear and stellar halos as seen empirical model of the reflections in the r–band. A surface brightness Some surveys have circumvented this difficulty by minimizing 0000 RAS, MNRAS c Figure 5. with MegaCam. NGC 2764. The position ofright: the ETG is indicated byscale, the ranging white square. between 24original and r–band image. 30 scale after subtraction of the stellar halos. Figure 6. on composite g+r+i images of the(right). galaxies The NGC nucleus 5473 of (left) each anda galaxy NGC size 3489 generates and its colour own roughly ghost similar halo to which that has of the surrounding bright stars. should be removed in the 7tiplying individual frames by before the stacking, same mul- To amount avoid this, the the time stellar halos necessaryand were models to computed were remove from slightly the them. blurred. final image light scattering and reflectionsantireflective coatings with inside light-absorbing the material telescopeet and al. and 2005; the Slater camera et (Mihos al.radical 2009), different or technologies developing (see concept the camerasvan with Dragonfly Dokkum Telephoto et Array, al.with 2014). simple Images optics generated alsoand by suffer amateur produce much cameras less images fromcleaner. that internal An at reflections instructive first comparisonthe sight field between around may the the galaxies be CFHT NGCCFHT/MegaCam 474 and image considered and with of NGC a to 467 small 12 obtained be of inch with (0.3 a m) collaboration telescope with as amateur part Most astronomers is of presented the in LSB Fig.visible 7.
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