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Deep Optical Imaging of the Dark Galaxy Candidate AGESVC1 282 Michal Bílek, Oliver Müller, Ana Vudragović, Rhys Taylor

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Deep Optical Imaging of the Dark Galaxy Candidate AGESVC1 282 Michal Bílek, Oliver Müller, Ana Vudragović, Rhys Taylor Deep optical imaging of the dark galaxy candidate AGESVC1 282 Michal Bílek, Oliver Müller, Ana Vudragović, Rhys Taylor To cite this version: Michal Bílek, Oliver Müller, Ana Vudragović, Rhys Taylor. Deep optical imaging of the dark galaxy candidate AGESVC1 282. Astronomy and Astrophysics - A&A, EDP Sciences, 2020, 642, pp.L10. 10.1051/0004-6361/202039174. hal-02962092 HAL Id: hal-02962092 https://hal.archives-ouvertes.fr/hal-02962092 Submitted on 8 Oct 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License A&A 642, L10 (2020) Astronomy https://doi.org/10.1051/0004-6361/202039174 & c M. Bílek et al. 2020 Astrophysics LETTER TO THE EDITOR Deep optical imaging of the dark galaxy candidate AGESVC1 282? Michal Bílek1, Oliver Müller1, Ana Vudragovic´2, and Rhys Taylor3 1 Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg (ObAS), UMR 7550, 67000 Strasbourg, France e-mail: [email protected] 2 Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia 3 Astronomical Institute of the Czech Academy of Sciences, Bocníˇ II 1401/1a, 141 00 Praha 4, Czech Republic Received 13 August 2020 / Accepted 21 September 2020 ABSTRACT The blind Hi survey Arecibo Galaxy Environment Survey (AGES) detected several unresolved sources in the Virgo cluster, which do not have optical counterparts in the Sloan Digital Sky Survey. The origin of these dark clouds is unknown. They might be crucial objects since they could be the so-called dark galaxies, that is, the dark matter halos without stellar content that are expected from cosmological simulations. In order to reveal the nature of the dark clouds, we took a deep optical image of one them, AGESVC1 282, with the newly-commissioned 1.4 m Milankovi´c Telescope. After observing it for 10.4 h in the L-filter, the image reached a surface- brightness limit of about 29.1 mag arcsec−2 in V. No optical counterpart was detected. We placed an upper limit on the V-band 7 7 luminosity of the object of 1:1 × 10 L , giving a stellar mass below 1:4 × 10 M and a Hi-to-stellar mass ratio above 3.1. By inspecting archival Hi observations of the surrounding region, we found that none of the standard explanations for optically dark Hi clouds fits the available constraints on this object. Key words. galaxies: individual: AGESVC1 282 – galaxies: formation – techniques: image processing – galaxies: interactions – galaxies: structure – galaxies: clusters: intracluster medium 1. Introduction dark matter content, and simulations have shown that such dark galaxies would be able to withstand a disruption by other cluster Recently, there has been a rise of interest in the low surface members (Taylor et al. 2016). brightness (LSB) universe. While the existence of LSB galax- These objects were first reported in Taylor et al.(2012, 2013) ies has been long established (e.g., Sandage & Binggeli 1984; as part of the Arecibo Galaxy Environment Survey (AGES). Bothun et al. 1987); in the last few years, it has become pos- Their radial velocities (1000−2000 km s−1) and distribution on sible to detect them in large numbers (e.g., van Dokkum et al. the sky suggest that they are members of the Virgo cluster (in the 2015; Koda et al. 2015; Venhola et al. 2017; Müller et al. 2017; case of AGESVC1 282 in the B cloud, at a distance of 23 Mpc, Román & Trujillo 2017; Greco et al. 2018; Prole et al. 2019; Gavazzi et al. 1999). Their Hi masses are ∼1−3 × 107 M , which Habas et al. 2020). Given the difficulties in detecting these faint is consistent with other tidal debris; however, their combination objects, it is natural to ask whether there might be a signifi- of isolation (> 100 kpc from the nearest galaxy) and high line cant population of even fainter galaxies with perhaps no stellar widths (∼150 km s−1) make them more unusual. Tidal debris is content at all (Collins et al. 2020). The existence of dark mat- known to be found at large distances from its parent galaxy ter halos without stellar content is the standard way to explain (e.g., Hess et al. 2017; Leisman et al. 2016; Serra et al. 2015), the missing satellites problem of cosmological dark-matter- but with line widths typically <50 km s−1. In contrast, high line only simulations (Simon & Geha 2007; Sawala et al. 2016; width features are known to exist within large Hi streams (e.g., Simpson et al. 2018). Koopmann et al. 2008; Kent et al. 2009), but they are rarely One proposed way to detect such “dark galaxies” is through found without a clear association to a likely parent galaxy. In this Hi surveys. Indeed, there appears to be a gas density threshold particular respect, the AGES Virgo clouds are highly unusual. for star formation (see Davies et al. 2006 for a detailed discus- In addition, their low mass and high velocity widths make them sion). While numerous dark clouds, that is to say Hi sources strongly deviant from the baryonic Tully–Fisher relation (BTFR, without optical counterparts, have been detected, establishing McGaugh 2005) if at the distance of the Virgo cluster. In order to their nature is often difficult: Hi removed through tidal encoun- reconcile them with the usual BTFR, they would need distances ters or ram pressure stripping might have a similar morphology of ∼2 Mpc, but there are no other known foreground galaxies in and kinematics as dark galaxies (Duc & Bournaud 2008). How- the direction of the Virgo cluster. Analogous objects have not ever, there is a set of objects in the Virgo cluster that does not been detected in other galaxy clusters. appear to be explicable by tidal encounters (Taylor et al. 2017) or In this Letter, we describe extremely deep optical imaging to through interactions with the intracluster medium (Taylor et al. try to detect the optical counterpart of one of these dark clouds, 2018). Their velocity widths are consistent with a significant AGESVC1 282. This can potentially bring several possible bene- ? Data underlying to Fig. 1 are only available at the CDS via anony- fits (see Sect.3 and also Minchin et al. 2007): we might discover mous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http:// that the object has a faint galaxy as an optical counterpart (as in cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/642/L10 Impey et al. 1990 or Mihos et al. 2018), that the gas has been L10, page 1 of5 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A&A 642, L10 (2020) displaced from a faint parent galaxy that is much closer to the cloud than to the known brighter objects (as in the case of the Hi cloud near VCC 1249, Arrigoni Battaia et al. 2012), or, alternatively, we might find stellar tidal debris that would indi- 8°25' cate a tidal origin of the clouds. No optical counterpart was actually detected. The luminosity of the object must be below 7 −1 −1 1:1 × 10 L . In this Letter, we assumed H0 = 71 km s Mpc for consistency with earlier AGES papers, and we assume a dis- 20' tance to the object of 23 Mpc. 2. Observations and data reduction Declination 15' The Hi observations and data reduction of AGES are exten- sively described in Auld et al.(2006), Taylor et al.(2012, 2013). In brief, AGES is a drift scan survey with a sensitivity of 10' 0.7 mJy, a spatial resolution of 3.50, and a spectral resolution of 10 km s−1 after Hanning smoothing. This gives an approxi- 6 mate mass sensitivity of 8 × 10 M at the Virgo distance with a physical resolution of 23 kpc in the B cloud of the cluster. 05' 5 arcmin = 33 kpc Objects in AGES were detected through a combination of visual 12h26m00s 25m40s 20s 00s 24m40s and automated source extraction procedures, and they were Right Ascension (J2000) confirmed with follow-up observations using Arecibo’s L-wide Fig. 1. Fully stacked and calibrated image. The image is centered on receiver. Eight optically dark clouds were discovered in Virgo the dark cloud AGESVC1 282. The field-of-view is 21:09 × 22:07. The by AGES. We selected AGESVC1 282 as our target (coordinates box indicates the stamps presented in Fig.2. The black line shows the RA = 12h 25m 24.10s, Dec = +08◦ 160 5400:0) because it has one scale at an assumed distance of 23 Mpc. of the highest Hi signal-to-noise ratios (11.4) and velocity widths (W20 = 164 km s−1), though these features are similar to those of the other clouds. The width of the Arecibo beam puts an upper (Bertin 2010), where we used a median for stack. The final, limit on the Hi radius of AGESVC1 282 of 10500. The lower limit fully calibrated, and dithering image reached a field-of-view of 0 0 on the Hi radius is harder to establish, but it is likely higher than 21:9 × 22:7, see Fig.1.
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