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Evidence for Ultra-Diffuse Galaxyformation'through Galaxy Draft version September 6, 2018 Typeset using LATEX twocolumn style in AASTeX62 Evidence for Ultra-Diffuse Galaxy `Formation' Through Galaxy Interactions P. Bennet,1 D. J. Sand,2 D. Zaritsky,2 D. Crnojevic,´ 1, 3 K. Spekkens,4, 5 and A. Karunakaran5 1Physics & Astronomy Department, Texas Tech University, Box 41051, Lubbock, TX 79409-1051, USA 2Steward Observatory, University of Arizona, 933 North Cherry Avenue, Rm. N204, Tucson, AZ 85721-0065, USA 3University of Tampa, 401 West Kennedy Boulevard, Tampa, FL 33606, USA 4Department of Physics and Space Science, Royal Military College of Canada P.O. Box 17000, Station Forces Kingston, ON K7K 7B4, Canada 5Department of Physics, Engineering Physics and Astronomy, Queens University, Kingston, ON K7L 3N6, Canada Submitted to ApJL ABSTRACT We report the discovery of two ultra-diffuse galaxies (UDGs) which show clear evidence for association with tidal material and interaction with a larger galaxy halo, found during a search of the Wide portion of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). The two new UDGs, NGC2708-Dw1 and NGC5631-Dw1, are faint (Mg=−13.7 and −11.8 mag), extended (rh=2.60 and 2.15 kpc) and have low central surface brightness (µ(g; 0)=24.9 and 27.3 mag arcsec−2), while the stellar stream associated −2 with each has a surface brightness µ(g)&28.2 mag arcsec . These observations provide evidence that the origin of some UDGs may connect to galaxy interactions, either by transforming normal dwarf galaxies by expanding them, or because UDGs can collapse out of tidal material (i.e. they are tidal dwarf galaxies). Further work is needed to understand the fraction of the UDG population `formed' through galaxy interactions, and wide field searches for diffuse dwarf galaxies will provide further clues to the origin of these enigmatic stellar systems. Keywords: galaxies: dwarf | galaxies: evolution | galaxies: formation 1. INTRODUCTION 2016; Rom´an& Trujillo 2017; Spekkens & Karunakaran The last several years have seen a resurgence of inter- 2018; Cohen et al. 2018) and field examples (Bellazzini est in the low surface brightness universe, and in par- et al. 2017; Leisman et al. 2017; Kadowaki et al. 2017). ticular the population of so-called ultra-diffuse galaxies There is considerable debate as to the origin of UDGs, (UDGs; van Dokkum et al. 2015), a term that refers to and it is likely that they are a `mixed bag' of popula- the largest, lowest surface brightness objects, with half tions with multiple origins (e.g. Zaritsky 2017; Lim et al. light radii >1.5 kpc and central surface brightnesses >24 2018). For instance, some UDGs may be `failed galaxies' mag arcsec−2. Although UDGs have been discussed in with Milky Way-like total masses, but with dwarf galaxy the literature for some time (e.g. Sandage & Binggeli stellar masses (e.g. van Dokkum et al. 2015, 2016; Tru- 1984; Caldwell & Bothun 1987; Impey et al. 1988; Dal- jillo et al. 2017) while others appear to simply be the arXiv:1809.01145v1 [astro-ph.GA] 4 Sep 2018 canton et al. 1997; Conselice et al. 2003, among others), low surface brightness extension of the standard dwarf recent work has found hundreds of examples in cluster galaxy population (Beasley & Trujillo 2016; Sif´onet al. environments (van Dokkum et al. 2015; Koda et al. 2015; 2018; Amorisco et al. 2018). Most UDGs with metal- Mihos et al. 2015; Mu~nozet al. 2015; Yagi et al. 2016; licity measurements point to a dwarf galaxy origin con- van der Burg et al. 2016), along with lower density group sistent with their metal poor stellar populations (e.g. (Crnojevi´cet al. 2016; Toloba et al. 2016; Merritt et al. Kadowaki et al. 2017; Ferr´e-Mateuet al. 2018; Pandya et al. 2018). Different formation scenarios posit that UDGs have been subject to extreme feedback, which Corresponding author: Paul Bennet inhibited early star formation (Di Cintio et al. 2017; [email protected] Chan et al. 2018), or that they are the `high-spin' tail of the dwarf galaxy population (Amorisco & Loeb 2016). 2 Bennet et al. A more prosaic explanation would be that UDGs are suggesting that they are being shaped by ongoing galaxy the product of tidal and/or ram pressure stripping (e.g. interactions. This further, direct observational evidence Conselice 2018), which can remove stars and expand the that UDGs can be the product of interactions suggests galaxy's size (e.g. Errani et al. 2015); semi-analytic cal- that this is a viable formation channel for this enigmatic culations show that this scenario is viable for cluster galaxy population. UDGs (Carleton et al. 2018). Similarly, although this has rarely been discussed in the literature (although see 2. THE DATA AND UDG DETECTION Trujillo et al. 2018, and their discussion of NGC1052- We are searching for diffuse galaxies in the Wide DF2; van Dokkum et al. 2018a), some UDGs could plau- portion of the CFHTLS, concentrating on fields W1, sibly be large, low surface brightness tidal dwarf galaxies W2 and W3, using an updated version of the semi- (TDGs). Born during gas-rich galaxy collisions, TDGs automated detection algorithm presented in Bennet should generally be lacking in dark matter and be metal et al.(2017). The total area being searched is ∼150 rich in comparison to normal dwarfs of the same lumi- deg2. The CFHTLS data was taken with the ∼1×1 deg2 nosity (e.g. Hunsberger et al. 1996; Duc 2012, among MegaPrime imager (Boulade et al. 2003), with typical many others). This could be a way to produce a dark exposure times for each field of ∼2750 and 2500s in the matter free UDG, such as is claimed for NGC1052-DF1 g and r bands, respectively. The fields were downloaded (van Dokkum et al. 2018a), however in that case in- directly from the Canadian Astronomy Data Centre, as terpretation is still under extensive discussion and the were the Point Spread Functions (PSFs) for those im- presence of a GC population (van Dokkum et al. 2018b) age stacks, which were used for measuring dwarf struc- is a significant problem for a TDG interpretation. Ob- tural parameters and simulating injected dwarfs. The servationally, some TDGs can survive for ∼4 Gyr, and construction and calibration of these stacks used the have size and surface brightness properties similar to MegaPipe data pipeline (Gwyn 2008), and is described the recently identified UDG class of galaxies (Duc et al. in detail by Gwyn(2012). 2014). Here we briefly outline our diffuse dwarf detection al- There is some observational evidence for a UDG gorithm, which has been updated slightly from that pre- `galaxy interaction' formation scenario in the radial sented in Bennet et al.(2017); the algorithm borrows el- alignment of Coma UDGs (Yagi et al. 2016), the kine- ements from previous work (e.g. Dalcanton et al. 1997; matics of the globular clusters in at least one Virgo van der Burg et al. 2016; Davies et al. 2016). All diffuse UDG (Toloba et al. 2018), and in the very elongated dwarf detection is done on the g-band stacked images UDG associated with NGC 253 (Scl-MM-Dw2; Toloba from the CFHTLS. First, bright stars and galaxies are et al. 2016). Other UDG-like systems also have sug- directly masked by matching source positions with the gestive features pointing to a recent galaxy interaction Guide Star Catalog 2.3.2 (Lasker et al. 2008), and then (e.g. Rich et al. 2012; Koch et al. 2012; Merritt et al. fainter objects are identified and masked by a call to 2016; Greco et al. 2018a), or even spatial/kinematic SExtractor (Bertin & Arnouts 1996). This step leaves substructure that could result from such interactions only very faint objects (<3 σ above the background), ex- (e.g. And XIX; Collins et al. 2013). To our knowledge, tended galaxy halos and low surface brightness features the only direct observational evidence that UDG-like remaining in the image. After masking, each image is objects can form from galaxy interactions comes from binned by 150×150 pixels (28×28 arcsec), a spatial scale a) the disrupting dwarf, CenA-MM-Dw3, which has a chosen to maximize the detection of large, diffuse ob- r =2.5 kpc and µ =26.0 mag arcsec−2, with clear half 0 jects while also remaining sensitive to smaller features. tidal streams extending over ∼60 kpc in the outskirts Another round of SExtractor is run to identify objects of the nearby elliptical Centaurus A (Crnojevi´cet al. on the binned images, and all candidates are forwarded 2016) and b) VLSB-A a nucleated Virgo UDG that has for visual inspection via a web interface, where our final clear tidal features, and is possibly associated with M86 diffuse dwarf candidates are selected. (Mihos et al. 2015) We implant simulated dwarf galaxies directly into our Here we present two additional UDGs discovered dur- images before performing the search in order to better ing a semi-automated, ongoing search for diffuse dwarf characterize our detection efficiency. Simulated dwarfs galaxies in the Wide portion of the Canada-France- are injected in batches of ten, randomly placed through- Hawaii-Telescope Legacy Survey (CFHTLS) { see Ben- out each image. Each simulated dwarf has a S´ersic net et al.(2017) for initial results around M101, and a profile (Sersic 1968) with index n=0.5{2.0, and ellip- description of our algorithm. Both UDGs show associ- ticity 0.0{0.7, randomly chosen, which is representative ated stellar streams connected to a parent galaxy halo, of past UDG measurements. Each dwarf is given a g- UDG formation mechanism 3 band magnitude between g=16{23 mag, with half light in the GALFIT process, and there may be an additional radii in the range ≈2{370 arcsec.
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