Push the redshift limits to larger distances, to investigate the possibility to detect further overdensities

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1 Introduction Despite nearly 20 years of concerted effort, the dynamics in the local Universe remain poorly understood. Galaxy and redshift surveys, as well as peculiar velocity surveys, result in controversial values for the apex and foremost the convergence radius that are supposed to explain the dipole observed in the Cosmic Microwave Background (e.g. Hudson 2004; Erdogdu et al 2006a,b; Basilikos and Plionis 2006; Kocevski & Ebeling 2006). Early discussions focused on the prominence of the Great Attractor (GA) and the Perseus-Pisces Supercluster (PPS) at about 50 Mpc (in units of h 1 ), while other papers claimed significant contributions to the dipole from depths of up to 200 Mpc (such as the Shapley Supercluster Concentration, possibly in combination with Horologium-Reticulum (Lavaux et al 2010; Lavaux & Hudson 2011). More recent bulk flow studies suggest that a major fraction of the local bulk flow ( 400km/s) is generated at distances beyond 100 Mpc (Watkins et al 2009, 2010; Feldman et al 2010; Macaulay et al 2011; Bilicki et al 2011; Nusser & Davis 2011). Moreover, the debate about the convergence radius has intensified recently with numerous claims of a significant increase in the dipole amplitude up to some even beyond 300 Mpc, based on X-ray cluster and also SDSS LRG samples (Kashlinsky et al 2010; Thomas et al 2011; Abate & Feldman 2011). The persistent discrepancies are thought to originate partly from the incomplete mapping of the Zone of Avoidance (e.g. Kraan-Korteweg & Lahav 2000; Loeb & Narayan 2008), and partly from to the loss of sensitivity at the relevant higher distance range ( Mpc) in the probed whole-sky galaxy surveys (e.g. optical, 2MASS, IRAS). This leaves us with several unanswered burning questions. What is the convergence radius? What is the amplitude of bulk flows on largest scales? Are these compatible with the standard Λ CDM model? Who are the major contributors? It is here that the future systematic whole-sky-surveys H I surveys envisioned for the SKA Pathfinders Apertif and ASKAP (WNSHS led by Józsa, and Wallaby led by Staveley-Smith and Korbalski) can shed light. These two H I-surveys combined will provide (a) true 4Π coverage, (b) unprecedented depth in volume, angular resolution and sensitivity, and (c) cover the Zone of Avoidance (ZOA) without any of the major selection effects and biases as in deep optical (extinction), near- and far-infrared (confusion), X-ray (H I-absorption) surveys for b < 5 (see Kraan-Korteweg & Lahav 2000). Status of H I ZOA-surveys and recent discoveries in the northern ZOA However, these deep H I-surveys and subsequent results are still a few years in the future. In the mean time we do not even yet have a complete census of the most important structures (clusters, walls, filaments and voids) in the local Universe (v < 10000km/s) along the great circle of the ZOA. For the southern ZoA ( b 5 ) a deep (rms 6mJy) systematic H I survey exists up to declinations of +15 (Kraan-Korteweg 2005; Donley et al 2005; Henning et al in prep). But no similar systematic H I-survey exists for the northern ZOA, apart from selected ZOA regions accessible from Arecibo (e.g. Henning et al 2008; 2010), leaving the major part of the northern ZOA (80 < l < 180 ) unexplored in H I. For this reason a subgroup (led by R. Kraan-Korteweg) of the current proposer team started collecting 21cm H I line data of the intrinsically largest and brightest 2MASX galaxies (K o < 11 ṃ 25) with unknown redshifts in the highly obscured ZOA (mostly b 5 ) with the Nançay Radio Telescope (NRT). These NRT observations complement the whole-sky 2MASS Redshift Survey (2MRS; Huchra et al. 2011) as well as whole-sky 2MASS Tully Fisher Relation Survey

2 (2MTF; e.g. Masters et al 2008). While both these major surveys claim to be whole-sky, they actually are not they exclude the inner part of the ZoA ( b > 5 ). Since 2010, 85% of the approximately 1000 bright 2MASX galaxy candidates without redshifts in the ZOA accessible to NRT have been observed to an rms 3mJy and v < km/s). This resulted in over 250 new H I galaxy detections with good coverage out to 8000 km/s. It is quite remarkable how well these detections fill in the previously mostly empty northern ZoA. Their resulting redshift distribution allow a tantalizing glimpse into hitherto uncharted filaments surrounding voids, wall-like features and clusters in and across the ZOA that are closely linked to the general Perseus-Pisces SCL; see Fig. 1. The Perseus-Pisces Wall crossing and the 3C129.1 cluster One of the most striking new features outlined by the new H I-detections, ZOA penetrations of the large scale structure, notably a well-defined Galactic Plane crossing at l 161 centered around v 6500km/s (see Figs 1 & 2). While the filamentary connection between the Perseus- Pisces SCL and A569 has been surmised before (e.g. Focardi et al 1984, Chamaraux et al 1990) it has never been substantiated so clearly with actual redshift measurements because most of its galaxies are too heavily obscured to have been identified optically. It is, however, quite prominent in the 2MASX whole-sky map of galaxies (Fig. 1) as derived from photometric redshifts (Jarrett 2004) and also in the 2MRS reconstructed density fields (Erdogdu et al 2006). The PPS may extend and connect to the Auriga and Cassiopeia filamentary structures, forming one of the largest (>>100 Mpc) structures known. Interestingly, this wall seems to embed a major, so far mostly ignored, rich X-ray galaxy cluster about half a degree above the Galactic Plane (see Fig. 2b). While optically inconspicuous, it shines like a beacon in the 2MASX-photz all-sky distribution (Fig. 1). The near-infrared (eg, 2MASS) is sensitive to older, evolved galaxies that tend to trace the baryonic mass. Inspection in the mid-infrared (MIR) using WISE imaging, which is sensitive to gas-rich star-forming galaxies, the wall crossing and the cluster also clearly abound in galaxies. As such this ZOA area seems to host a major potentially massive and evolving component of the Perseus-Pisces supercluster that has escaped attention to date. The galaxy cluster hosts two associated bright radio galaxies (confirmed by Spinrad 1975), namely 3C129 with a steeply rising radio emission at lower frequencies, and the slightly fainter Wide-Angle-Tail (WAT) radio source 3C This alone implies a rich cluster environment. Although optically very faint (B 19 m ) the radio galaxies are seen through an extinction layer of A B 4 ṃ 5). The less affected K s -band magnitudes confirm their brightness (K s = 9 ṃ 8, 9 ṃ 4) indicating extinction-corrected absolute magnitudes of the order of M o K < 25ṃ 0). This ZOA cluster in also a strong X-ray source. As early as 1994, Fabian pointed out that four of the (then) known 39 brightest X-ray clusters do lie at low Galactic latitudes ( b < 10 ), namely the Ophiuchus cluster, Cygnus A, 3C129.1, and PKS He also commented the that 3C129.1 was the least studied of the lot. This has not changed much since then, despite the cluster being identified as CIZAJ in the Clusters in the ZOA survey ( b < 20 ; Ebeling et al 2002). While it is not amongst the brightest X-ray clusters for the ROSAT energy range, it has only a 20% lower X-ray luminosity compared to the Norma cluster A 3627, which defined the core of the Great Attractor (Kraan-Korteweg et al 1996) and which also hosts two bright radio sources. But its luminosity might be an underestimate given the high H I-column density (N HI = cm 2 ) through which it has been measured. The structures outlined by the new NRT H I-detections, the wall-like ZOA crossing seems to indeed merge seamlessly in the on-sky distribution with the previously mapped PPS as observed on either side of the ZOA. However, when displaying the galaxies in a redshift slice, it

3 appears that the majority of the detections seem to define a little semi-circle around the radio galaxies, respectively the cluster core, although it is not devoid of H I-detections. This might be the effect of either the radio galaxies or H I-deficiencies cause by the intracluster X-ray gas - although the latter only extends over a radius of about R < 20 (Ebeling et al 2002). As such it is clear that this largely unexplored ZOA area requires much deeper investigation. This major nearby rich cluster on its own merits detailed observation and analysis, its role as possible major component of the PPS, as well as its relevance to the observed flow fields in this and the wider PPS area. This cannot be achieved in the optical or through spectroscopy. Neither will the NRT, with its limited sensitivity and extended beam allow us to take this exploration further. It is for this reason, that we propose a deep H I-survey with the WSRT of a mosaic covering a region of that will contain both the cluster as well as the most prominent part of the PPS Wall crossing as outlined by the extinction-corrected brightest 2MASX galaxies which so far has only allowed us to sample the tip-of-the-iceberg of this massive structure. It should be emphasized again that while this project is of high scientific interest in its own right it also designed to serve as a pilot-project that will result in a interferometric data cube that will prove invaluable in the planning and preparation for the forthcoming whole-sky H I surveys to be proposed/done with the SKA Pathinders Apertif and ASKAP. Immediate Objectives Our immediate aim is obtain WSRT H I observations to construct a spectral data cube from which we will derive H I maps of the large-scale structure around the local overdensity in the Perseus-Pisces wall. Given the high extinction at other wavelengths, H I is the sole available kinematical tracer, and only an H I study with the sensitivity and the resolution as provided by the WSRT will allow us to reach the sensitivity to achieve a good statistics. Given the spatial resolution of the WSRT, we will at the same time be able to scrutinise the physical status of the 4th brightest Xray cluster on the sky (reference), performing a detailed analysis of the environmental interaction of the neutral gas in the cluster galaxies with the ICM. We hence aim at a measurement that allows us to, Detect galaxies down to a limit of 10 8 M, hence allowing us to dip deep into the H I mass function, ensuring suitable statistics Determine rotation velocities and the kinematical status of the galaxies down to a limit of M, hence probing the cluster environment, as well as providing Tully-Fisher distances for a significant sub-sample of the targeted galaxies Investigate the neutral gas distribution and the individual galaxy gas kinematics within the cluster as well as in the transition towards the normal overdensity within the Perseus- Pisces wall Push the redshift limits to larger distances, to investigate the possibility to detect further overdensities With the current single-pixel receiver of the WSRT, these requirements are hard to meet without requiring a substantial amount of observing time. But again, we emphasise that while Apertif will provide a suitable future solution for the science outlined above, it is now that its surveys need to be designed and planned. Here, we propose to observe an H I mosaic of 35 pointings with the WSRT, each with an integration time of 12h. The area covered by this mosaic spans approximately 6 square degrees.

4 Proposed mosaic and instrumental setup For our study, it is crucial that any detection limit is well beyond the knee of the H I mass function. A good assumption is that a lower mass detection limit should lie below a limit of M. In Fig. 3a, we show the 5-σ rms (point-source) mass detection limit in dependence of the recessional velocity, substituting distance via H 0 = 72 km s 1 Mpc 1, for average onsource integration times of 12h, 6h, and 4h, respectively, assuming a line width of our target galaxies of 150 km s 1 and optimal filtering in the velocity regime. Our main target lies at a velocity of cz = 7000 kms 1, corresponding to a distance of 100 Mpc. It can be seen, that while with an integration time of 6h per pointing this limit is touched, however not quite reached (and an optimistic 5σ rms limit), an integration time of 12h is required to securely match our requirements. This detection limit is calculated under the assumption that we will not make use of the kinematical information, since optimal filtering implies that the velocity information is lost at the low mass end. We hence consider the total S/N for an H I mass of M in Fig. 3b, bottom panel, since this is indicative of the possibility to derive kinematical information from our detections. Only using an integration time of 12h per pointing, the S/N lies above a value of 30, meaning that we can derive velocity information making use of 6 independent velocity bins above a S/N of 5. We hence strongly tend to make use of a full integration per pointing. The choice of the mosaic grid is lead by the aim of covering the largest possible FOV with an acceptable noise variability across the field. With a goal of 20% noise variation, we adopt a similar layout as suggested for Apertif with a spacing of 0.9 primary beam HPBWs on a hexagonal grid (see Fig. 4). We aim at a full integration per pointing to ensure the best possible uv coverage for each pointing, as opposed to a uv coverage achieved fast mosaicing on a narrower grid, and prefer to accept the noise variation to reach that goal. Our targets are not extended enough to make Nyquist sampling of the pointings necessary. In Fig. 4 we show the layout of the mosaic, overlaid with detections and non-detections in the NRT observations. In total, we cover an area of 6.2 square degrees, large enough to map the interface between the cluster and the Perseus-Pisces sheet. The aim of our project is to map out large overdensities, possibly extending a distance of 100 Mpc by a significant factor. Unfortunately, the WSRT correlator does not provide the bandwidth to scan a large velocity range at the highest velocity resolution. Hence, a tradeoff has to be found that enables us to map our prime target with acceptable velocity resolution, while still being able to attempt a shallow dip into the more distant universe. We deem a velocity resolution of 33km s 1, sufficient for our kinematical analyses, such that we can make use of 4 bands of 20 MHz each, distributing the available correlator capacity evenly over all channels. This provides us with a total bandwidth (with 11% overlap of the bands) corresponding to 15,000 km s 1. Data reduction and analysis Our team is well suited for a project of the size as proposed. Our WSRT specialists will ensure the best possible data reduction towards a full mosaic, while we have number of experts in source finding on the team. We will hence provide a maximum possible exploit of the data in terms of source identification. Furthermore, our kinematical analysis is ensured by including members with firm expertise in the kinematical analysis of H I data cubes. Our team has also firm expertise in exploiting ancillary data (NIR) to accomplish our survey. Finally, specialists in bulk flow analyses and ZOA science in general ensure the best possible scientific output. It is foreseen that aided by the team the observations will partly be analysed in a common student program in preparation of the SKA pathfinders.

5 Figures Fig. 1: The Local Universe as viewed from the northern hemisphere (from Jarrett 2004). The image shows a Galactic (equal-area) projection of the 2MASS Galaxy Catalog (XSCz), highlighting some of the large filamentary structures in this region of the sky, including the Perseus-Pisces Supercluster (PPS). The pink-shaded region shows how the massive PPS appears to cross the Galactic Plane at glon = 161 o and 132 o, extending into the northern half of the sky, connecting Perseus to the Auriga and Cassiopeia regions. The wall-like crossing at 161 o is the focus of this H I-survey. Fig. 2: (LEFT) The space distribution of galaxies toward the northern Zone of Avoidance for the velocities between 4500 and 7500 km/s. The colors denote the galaxy redshift: cyan is nearest, blue is intermediate and red is furthest. The PPS dominates the large scale structure in the region of the sky. (RIGHT) Zooming down into the Galactic Plane and including more information, the over-density at glon = 161 o, glat = +1 o is very apparent (dashed box) and is the subject of this proposal. The symbols denote: green = 2MRS galaxies; blue = LEDA galaxies (dots have H I redshifts); red dots = NRT detections; red crosses = NRT non-detections; red open boxes = NRT-non-observations. The symbol sizes indicate distance.

6 Fig. 3: (LEFT): 5-σ rms detection limits in dependence of recession velocity for an assumed line width of 150 km s 1. (RIGHT) S/N for a galaxy with a mass of 10 9 M. Blue line: 12h integration per pointing. Red line: 6h integration per pointing. Green line: 4h integration per pointing. Fig. 4: Layout of the H I observations. 35 pointings are arranged in a hexagonal grid with a separation of 0.9 primary beam HPBWs. The minimum rms noise is 0.94 times the minimum rms noise of a single pointing. Magenta contours indicate a level of 1.2 times the minimum rms noise, blue contours indicate a level of 1.1 times the minimum rms noise. Red circles show the position of detected galaxies, green crosses nondetections. Blue dots indicate the position of galaxies yet to be observed with the NRT. The white stars show the positions of the radio sources 3C129 and 3C References Abate A. & Feldman H. 2012, MN 419, 3482 Basilakos S. & Plionis M. 2006, MN 373, 1112 Bilicki M. et al. 2011, ApJ 741, 31 Chamaraux, P.et al. 1990, A&A, 229, 340 Donley, J.L. et al. 2005, AJ, 129, 220. Ebeling, H. et al. 2002, ApJ, 580, 774. Erdogdu P. et al. 2006, MN 368, 1515 Erdogdu P. et al. 2006, MN, 373, 45 Fabian, A. C. 1994, ASP Conf. Ser. 67, p. 76 Feldman H. et al. 2010, MN 407, 2328 Focardi, P., Marano, B., & Vettolani, G. 1984, A&A, 136, 178 Henning P.A. et al in AIP Conf. Proc. Volume 1035, p. 246 Henning P.A. et al. 2010, AJ, 139, 2130 Huchra, J. et al. 2011, arxiv/ Hudson M. et al. 2004, MN 352, 61 Jarrett T. 2004, PASA 21, 396 Kashlinsky A. 2010, ApJ, 712, L81. Kocevski D.D. & Ebeling H ApJ, 645, 1043 Kraan-Korteweg, R.C. et al. 1996, Nature, 379, 519 Kraan- Korteweg, R.C., Lahav O. 2000, A&ARv 10, 211 Kraan- Korteweg, R.C. 2005, Reviews in Modern Astronomy 18, p. 48. Lavaux G. & Hudson, M.J. 2011, MN 416, 2840 Lavaux G. et al. 2010, ApJ, 709, 483 Loeb A. & Narayan R. 2008, MN 386, 2221 Macaulay, E. et al. 2011, MNRAS, 414, 621. Masters, K.L., et al. 2008, AJ, 135, Nusser A. & Davis M. 2011, ApJ, 736, 93 Thomas S. et al. 2011, PhRvL 106, Watkins R. et al. 2009, MN 392, 743