The last few years have seen an explosion in the number of works exploring the outermost regions of nearby galaxies using very deep imaging. These works can be grouped into three different flavours: deep (∼ 1 h) multipurpose surveys with medium sized (2−4 m) telescopes, extremely long integrations (≳ 20 h) of particular galaxies with small (≲ 1 m) telescopes or long integrations (≳ 5 h) with large (≳ 8 m) telescopes. In what follows we will summarize some of these efforts.
3.1. Survey Data
The 3.6 m Canada-France-Hawaii Telescope (CFHT), with its MegaCam wide-field camera comprising 36 CCDs and covering a 1 square degree FOV, has played a significant role in the new generation of deep imaging surveys. This telescope has been used for general purpose surveys like the Wide Synoptic CFHT Legacy Survey (155 square degrees; Cuillandre et al 2012), or more specific projects like the Next Generation Virgo Cluster Survey (NGVCS; Ferrarese et al 2012) or the deep imaging follow-up (352015Duc et al) of the ATLAS3D project (202011Cappellari et al). These surveys are characterized by having similar depths (r ∼ 25 − 25.5 mag; S / N = 10 for point sources) using integration times of 40−60 min, reaching a limiting surface brightness of 28.5−29 mag arcsec−2 (3σ in 10 × 10 arcsec2).
Among the most important results of the NGVCS is the connection between the distribution of the metal-poor globular clusters and the intra-cluster light (Durrell et al 2014), suggesting a common origin for these two structures. The deep imaging by Duc et al (2015) revealed a large number of features surrounding their galaxies. Both these projects targeted mostly early-type galaxies. Unfortunately, the pipeline used for the reduction of the Wide Synoptic CFHT Legacy Survey removed the low surface brightness features around the brightest extended galaxies in the images, producing obvious “holes” that prevent the use of this reduced dataset for the exploration of the outermost parts of the spiral galaxies. In the later MegaCam surveys this problem does not occur.
Another telescope that has revolutionized our understanding of galaxies is the Sloan 2.5 m telescope. The Sloan telescope is well known for producing the SDSS (York et al 2000). Among the different projects that Sloan has covered, of particular interest here is its deep imaging survey in the Southern Galactic cap, popularly known as the “Stripe 82” survey (Jiang et al 2008, Abazajian et al 2009). The Stripe 82 survey covers an area of 275 square degrees along the celestial equator (−50 < R.A. < 60, −1.25 < Dec. < 1.25) and has been observed in all the five SDSS filters: ugriz. The typical amount of time on source was ∼ 1.2 h. Being located at the equator, the Stripe 82 area is accessible from most ground-based facilities. A third of all the available SDSS data in the Stripe 82 area were combined by Annis et al (2014). They reached a depth (50% completeness for point sources) of r ∼ 24.2 mag. Later on, Jiang et al (2014) used the entire dataset and reported a gain of 0.3−0.5 mag in depth compared to the previous reduction. None of these reductions were done with the aim of exploring the lowest surface brightness features of the objects. This task was performed by Fliri and Trujillo (2016) and is known as the IAC Stripe 82 Legacy Project. Fliri and Trujillo (2016) reached a depth of r ∼ 24.7 mag (50% completeness for point sources) and a limiting surface brightness of µr ∼ 28.5 mag arcsec−2 (3σ in 10 × 10 arcsec2). The reduced images of Fliri and Trujillo (2016) have been made publicly available through a dedicated webpage (http://www.iac.es/proyecto/stripe82).
3.2. Small Telescopes
As surface brightness is independent of telescope aperture, in principle one can use small telescopes to reach ultra-faint surface brightness levels. Direct advantages of using small telescopes over larger ones include the larger field of view, and the reduced competition for observing time, in particular when private telescopes are used. Using modern CCD technology, this has been done by several workers in the field, aiming to uncover light sources as diverse as intra-cluster light, diffuse galaxies, or the outer regions of bright galaxies. The Chapter by Abraham et al (this volume) gives significantly more detail on this; we review the basics in this short Section.
Mihos and collaborators used the Case Western Reserve University's Burrell Schmidt 0.6 m telescope to obtain ultra-deep imaging of the core region of the Virgo cluster, down to limits of µV = 28.5 mag arcsec−2 (see Mihos et al 2016 for a recent update). From these images, they were able to reveal a number of interesting features of the intra-cluster light, including several tidal streamers of more than 100 kpc in length, and many smaller tidal tails and bridges between galaxies, and to conclude that cluster assembly appears to be hierarchical in nature rather than the product of smooth accretion around a central galaxy (Mihos et al 2005; the colours of features in the intra-cluster light were measured by Rudick et al 2010). Mihos et al (2015) used the same data set to find three large and extremely low surface brightness galaxies, only one of which shows the signs of tidal damage that might be expected for such galaxies in a dense cluster environment. The same telescope was used by Mihos et al (2013) to image the galaxy M101 over an area of 6 deg2 and to a depth of µB = 29.5 mag arcsec−2, and by Watkins et al (2014) to push down to µB = 30.1 mag arcsec−2 in the M96 galaxy group. The former authors found a number of plumes and spurs but no very extended tidal tails, suggestive of ongoing evolution of the outer disk of the galaxy due to encounters in its group environment, whereas the latter found no optical counterpart to the extended Hi surrounding the central elliptical M105, and in general only very subtle interaction signatures in the M96 group.
In a fruitful collaboration with amateur astronomers, Martínez-Delgado et al (2008, 2010, 2015) have used small private telescopes to obtain deep images of tidal streams around a number of galaxies, most with pre-existing evidence for the presence of some kind of outer structure, and more recently of low surface brightness galaxies in the fields of large nearby galaxies (e.g., Javanmardi et al 2016). This group uses very long exposures taken at dark sites, imaging through wide filters. Among the most beautiful and well-known results obtained by Martínez-Delgado's group is the discovery of the optical analogues to the morphologies predicted from N-body models of stellar haloes constructed from satellite accretion (e.g., Bullock and Johnston 2005, Johnston et al 2008). While the resemblance between models and observations is indeed striking and important, it must be kept in mind that most of the structure seen in the models is at lower or much lower surface brightness levels than even the deepest currently available imaging, and that most galaxies observed by Martínez-Delgado et al were targeted specifically to have some previous evidence for tidal structure and are thus not representative of the general galaxy population. In fact, many nearby galaxies show no evidence at all for any tidal or other disturbances in deep images (e.g., Duc et al 2015, see also Merritt et al 2016).
A third strand, besides using small existing research telescopes or amateur installations, is to use simple, small, custom-built telescopes optimized for deep imaging through simple optics and a very careful treatment of systematics. The best-known example of this is the Dragonfly Telephoto Array (Abraham and van Dokkum 2014), a set of up to 48 commercial telephoto lenses with excellent coatings coupled to CCD cameras, which minimizes the amount of scattered light produced inside the telescope. In the current configuration, the array is equivalent to a 1 m aperture telescope with a field of view of 6 square degrees (Abraham and van Dokkum 2014, Abraham 2016; see also Abraham et al, this volume).
Among the results obtained with Dragonfly images and profiles (which go down to µg ∼ 28 mag arcsec−2; 3σ in 12 × 12 arcsec boxes; Merritt et al 2016) are the finding that there is a significant spread in the stellar mass fraction surrounding galaxies (van Dokkum et al 2014, Merritt et al 2016, see also Sect. 4.4), and a study of a so-called ultra-diffuse galaxies in the Coma cluster (van Dokkum et al 2016 and references therein).
3.3. Large Telescopes
Large (8-10 m class) telescopes have barely been used so far to obtain very deep imaging of nearby galaxies. To the best of our knowledge the first successful attempt of going ultra-deep (i.e., surpassing the 30 mag arcsec−2 barrier) was conducted by Jablonka et al (2010), who used the imaging mode of the VIsible MultiObject Spectrograph (VIMOS) on ESO's Very Large Telescope (VLT) to target the edge-on S0 galaxy NGC 3957 reaching surface brightness limits (Vega system) of µr = 30.6 mag arcsec−2 (1σ; 6 h) and µV = 31.4 mag arcsec−2 (1σ; 7 h). These authors found that the stellar halo of this galaxy, calculated between 5 and 8 kpc above the disk plane, is consistent with an old and preferentially metal-poor normal stellar population, like that revealed in nearby galaxy haloes from studies of their resolved stellar content. Also worth mentioning is the work by Galaz et al (2015), who used the more “modest” 6.5 m Magellan telescope to observe the extremely large galaxy Malin 1. Galaz et al (2015) used the Megacam camera to image the galaxy for about 4.5 h in g and r, reaching µB ∼ 28 mag arcsec−2. Using these images, they obtain an impressive result for the diameter of Malin 1 of 160 kpc, ∼ 50 kpc larger than previous estimates. Their analysis shows that the observed spiral arms reach very low luminosity and mass surface densities, to levels much lower than the corresponding values for the Milky Way.
The currently deepest ever image of a nearby galaxy was recently obtained by Trujillo and Fliri (2016). These authors pointed the Gran Telescopio Canarias (GTC, a 10.4 m telescope) at the galaxy UGC 00180, an object similar to M31 but located at a distance of ∼ 150 Mpc (see Fig. 3). Their r-band image reached a limiting surface brightness of 31.5 mag arcsec−2 (3σ; 10 × 10 arcsec2) after 8.1 h on-source integration. This image revealed a stellar halo with significant structure surrounding the galaxy. The stellar halo has a mass fraction of ∼ 3% of the total stellar mass of the galaxy. This value is close to the one found in the Milky Way and M31 using star counting techniques. This is the first time that integrated-light observations of galaxies reach a surface brightness depth close to that achieved using star counting techniques in nearby galaxies. It is a major step forward as it allows the exploration of the stellar haloes in hundreds of galaxies beyond the Local Group, in particular when viewed in the context of future imaging possibilities with the Large Synoptic Survey Telescope (LSST; see Sect. 5).
Figure 3. The galaxy UGC 00180 and its surrounding field observed down to a surface brightness limit of 31.5 mag arcsec−2 in the r-band (around 10 times deeper than most of the previous deep images obtained from the ground). The image is a combination of SDSS imaging (colour part) and 8.1 h of imaging with the GTC; grey part). In addition to the stellar halo of the galaxy, the image shows a plethora of details. Among the most remarkable is the filamentary emission from dust of our own Galaxy located in the bottom-left part of the image. There are also distant galaxies which are seen to be merging with other objects, and a high-redshift cluster towards the bottom-left corner of the galaxy where the intra cluster light is visible. Credit: GTC, Gabriel Pérez and Ignacio Trujillo (IAC).