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Observing Galaxy Mergers at the Epoch of Reionization

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Observing Galaxy Mergers at the Epoch of Reionization DRAFT VERSION AUGUST 11, 2018 Typeset using LATEX twocolumn style in AASTeX61 OBSERVING GALAXY MERGERS AT THE EPOCH OF REIONIZATION EVGENII A. CHAIKIN,1 NADEZDA V. TYULNEVA,1 AND ALEXANDER A. KAUROV2 1Peter the Great St. Petersburg Polytechnic University, Politechnicheskaya 29, 194021, St. Petersburg, Russia 2Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA ABSTRACT The galaxies with photometric redshifts observed in a close angular proximity might be either projection coincidences, strongly lensed images of the same galaxy, or separate galaxies that are in a stage of merging. We search for the groups of galaxies in the Hubble Ultra Deep Field (HUDF09) in z ∼ 7 and z ∼ 8 drop-out samples. We find no close pairs among 50 galaxies in the z ∼ 7 sample, while in the z ∼ 8 sample we find that 6 out of 22 galaxies have a companion within ∼ 100 (3 pairs). Adopting a numerical simulation and performing forward modeling we show that even though mergers are unlikely to have such a high fraction, the projection coincidences and the strong lensing are even less likely mechanisms to account for all of three pairs. Alternatively, there is a possibility of the contamination in the drop-out catalog from lower redshifts, which potentially can account for all of the groups. Finally, we make projection on the sensitivity to mergers of the James Webb Space Telescope, and discuss the possible applications of the high-redshift merging galaxies for decreasing cosmic variance effect on the luminosity function and for improving the accuracy of photometric redshifts in general. Keywords: galaxies: high-redshift — galaxies: photometry arXiv:1712.04110v1 [astro-ph.GA] 12 Dec 2017 Corresponding author: Alexander A. Kaurov [email protected] 2 CHAIKIN ET AL. 1. INTRODUCTION Lyman-break Galaxies at z ∼ 7 and z ∼ 8 from B15. We The next decade will be rich for new space observatories show that even without spectroscopic redshifts there is a sub- sensitive to the infrared, such as the James Webb Space Tele- stantial statistical signal in the data (x2), and using one group scope (Gardner et al. 2006) and the Wide Field Infrared Sur- as an example we argue that, indeed, it can be identified as vey Telescope (Spergel et al. 2015), that would allow us to one system (x2.1). greatly extend the observational horizon well into the epoch The theoretical considerations (Lacey & Cole 1993) also of reionization, and the ground-based instruments like the predict a significant fraction of dark matter halos and, there- Giant Magellan Telescope (Johns et al. 2012) would provide fore, galaxies to be in some stage of merging. In order to greater angular resolution in infrared. The main statistics that compare theoretical predictions with observations, we per- will be immediately extracted from the deep galaxy surveys form forward modeling using numerical simulations of cos- is the luminosity function and the average star formation rate mological size boxes with galaxy formation (x3.1). We (e.g. Bouwens et al. 2015, hereafter B15). In this paper we mimic the HST’s Wide Field Camera 3 (WFC3), including study another piece of information that can be potentially noise and point spread functions, and generate mock obser- contained in the existing and upcoming imaging data – the vations with a depth similar to the XDF (x3.2). The mock statistics of galaxy mergers at the epoch of cosmic reioniza- images are then processed with common pipelines and source tion. catalogs are created (x3.3). In x4 we summarize the results of The fraction of mergers is well studied at the redshifts be- our modeling and compare it with other possible mechanisms low ∼ 4 (i.e. Lotz et al. 2011; Man et al. 2016; Conselice that can create observed grouped galaxies. Finally, in x5, we et al. 2008;L opez-Sanjuan´ et al. 2009), and was compared discuss the possible application of the grouped galaxies. with simulations (i.e. recent comparison with the Illustris simulation Snyder et al. 2017). See Rodriguez-Puebla et al. 2. OBSERVATIONS OF MERGERS (2017) for a combined review of available data on merger ob- In this study we consider the HUDF09 and the XDF project servations. Also, the fraction of clumpy galaxies was stud- data release, since it is likely to be the deepest imaged region ied up to redshifts 8 (see compilation of various studies in until the JWST will start taking measurements. Also, this Shibuya et al. 2016). The large number of both spectro- field is well imaged by ground based telescopes and the pho- scopic and photometric galaxies at low redshifts allows one tometry is conveniently available (Brammer et al. 2012). The to make magnitude cuts and study major/minor mergers sep- Frontiers fields (with images of high-redshift galaxies magni- arately. At the redshifts of reionization, on contrary, there fied by foreground clusters) effectively provide even deeper are much less observed galaxies, especially spectroscopically imaging; however, for this study we avoid the complications confirmed. Nevertheless, in this paper we attempt to see what caused by lensing effects. is observed at z ∼ 7 − 8 and how consistent is it with the nu- We adopt the source catalog used to determine the lumi- merical simulations. nosity function at 4 < z < 10 in B15 which is available The galaxies at redshifts beyond 6 are expected to be com- online1. Another catalog with open access – the HST 3D pact and their detailed morphology can not be presently re- survey (Skelton et al. 2014) – also covers this field, but it solved (e.g. Liu et al. 2017). However, individual blobs (that differs by the way sources are detected, as well as the pho- appear to be gravitationally bound) can be observed. There tometry measurements and final photometric redshifts. We is already a handful of Lyα emitters (LAEs) observed at do not use the 3D HST redshift catalog since by construction high redshifts that exhibit either multiple clumps, merging, it assumes the upper limit for the photometric redshifts to be or have another galaxy nearby: “Himiko” object has three 6, i.e. the objects that are determined as z > 6 in the B15 clumps that may correspond to a triple merger at z ≈ 7 catalog are z < 6 in the 3D HST catalog. Also, there is a (Ouchi et al. 2009, 2013); “CR7” (COSMOS Redshift 7) at catalog by Mclure et al.(2011a) that covers this region and z = 6:6 that exhibits three distinct clumps with significantly redshift range. We discuss it in x4.4. different photometric SEDs (Sobral et al. 2015); A1689-zD1 The galaxies in B15 catalog have two redshifts. First one is is a potential dusty merger at z ≈ 7:5 (Watson et al. 2015; the photometric redshift evaluated with template fitting code Knudsen et al. 2016); the lensed object at z = 6:3 that has EAZY (Brammer et al. 2008) using the photometric fluxes two distinct clumps and three images (Rydberg et al. 2016). in different filter bands. Second is the redshift bin based on However, Lyα line is detected only for a fraction of ob- drop-out technique. We adopted z ∼ 7 and z ∼ 8 samples jects. In the absence of spectroscopic redshifts, we have only that correspond to the drop-outs from F850LP and F105W photometric estimates that have much lower precision and HST filters (Bouwens et al. 2011). By its nature, the pho- accuracy especially at high redshifts. Therefore, the projec- tometric and drop-out redshifts are much less precise than tion uncertainty fundamentally limits the search of grouped spectroscopic redshifts and have big uncertainties; therefore, of objects. Nevertheless, we show that there is still some sig- it is not possible to accurately measure the spatial distance nal that can be extracted. In this paper we focus on the Hub- between two galaxies with photometric redshifts. ble Ultra Deep Field 2009 (HUDF09, Beckwith et al. 2006) and adopt the Hubble eXtreme Deep Field legacy data (XDF, Illingworth et al.(2013)) and consider drop-out samples of 1 http://cdsarc.u-strasbg.fr/viz-bin/Cat?J/ApJ/ 803/34 OBSERVING GALAXY MERGERS AT THE EPOCH OF REIONIZATION 3 F775W F850LP F105W F140W F160W 5.82 7.49 7.49 3.87 7.33 7.49 5.96 6.46 7.84 7.75 7.58 4.43 Figure 1. Cutouts of galaxies from z ∼ 8 (red squares) drop-out sample with separation less than 300. Only galaxies with z > 4 are labeled with cyan circles and corresponding photometric redshift. All panels are 6 arcseconds across, and the colormap in all panels spans from −1σ to +2σ. The galaxy in the last cutout with photometric redshift z = 7:75 belongs to z ∼ 7 drop-out sample. Coordinates of groups from up to bottom (03:32:44.7; -27◦ 460 44:800), (03:32:40; -27◦ 470 17:500), (03:32:43; -27◦ 460 27:300). The effect of photometric redshifts on the identifying 50 objects with redshifts z ∼ 7, and 22 with z ∼ 8 (B15). mergers is studied at lower redshifts (e.g.L opez-Sanjuan´ As we show in x4, the former sample agrees well with the et al. 2015). In case of redshifts beyond 6 the main problem simulations and does not include any groups (defined by 100 is likely to be not the uncertainty of photometric redshifts (if threshold), while the latter sample does include groups and the probability density function of the redshift is known, one therefore it is of greater interest. can marginalize over it), but the so-called catastrophic out- Among 22 galaxies at z ∼ 8, there are 3 groups (with 6 liers, i.e. when a low-redshift dusty galaxy is misidentified galaxies in total), where the distance between galaxies is less with a high-redshift galaxy (see further discussion in x4.4).
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