University of Groningen
A Westerbork blind HI imaging survey of the Perseus-Pisces filament in the Zone of
Avoidance
Ramatsoku, Mpati Analicia
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Publication date: 2017
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Ramatsoku, M. A. (2017). A Westerbork blind HI imaging survey of the Perseus-Pisces filament in the Zone of Avoidance. University of Groningen.
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A Westerbork blind Hi imaging survey
of the Perseus-Pisces filament
in the Zone of Avoidance
PhD thesisto obtain the degree of PhD of the University of Groningen
on the authority of the Rector Magnificus Prof. E. Sterken
and in accordance with the decision by the College of Deans.
and
to obtain the degree of PhD of the University of Cape Town
on the authority of the Vice-Chancellor Dr. M. Price
and in accordance with
the decision by the Doctoral Degrees Board. Double PhD Degree
This thesis will be defended in public on Tuesday 17 October 2017 at 11:00 hours
by
Mpati Analicia Ramatsoku born on 16 August 1987 in Bethlehem, South Africa
Promotores
Prof. M.A.W. Verheijen Prof. R.C. Kraan-Korteweg Copromotor Dr. W.J.G de Blok Beoordelingscommissie Prof. C. Carignan Prof. M.J. Drinkwater Prof. J.H. van Gorkom Prof. J.M. van der Hulst
Plagiarism Declaration
I, Mpati Ramatsoku, know the meaning of plagiarism and declare that all of the work in the document, save for that which is properly acknowledged, is my own.
Cover: A picture of the Westerbork telescope looking at the Milky Way. ISBN: 978-94-6233-760-2
v
Abstract
In this thesis we study a structure associated with the Perseus-Pisces Super-cluster (PPS; cz ∼ 6000 km s−1), where it crosses the Zone of Avoidance (ZoA) at the Galactic longitude of ` ≈ 160◦. This largely unexplored region contains the X-ray emitting 3C 129 cluster which hosts two strong radio sources with bent morphologies, thus indicative of a rich intra-cluster medium (ICM). Due to its low Galactic latitude of b = 0.27, where Galactic extinction is severe at optical wavelengths (AB = 1.8 − 8.0), the details of its galaxy population and dynamics had not been investigated in detail before. Additionally, the diverse and dynamic cosmic environments of the PPS with the embedded galaxy clus-ter make this region an ideal laboratory to study the effects on these environ-ments on the galaxy properties and galaxy transformation processes therein. We used the Westerbork Synthesis Radio Telescope (WSRT) to blindly map this ZoA crossing of the PPS in the 21 cm HI-wavelength. This was conducted through 35 individual pointings observed for a total of 420 hours, covering a large area of about 9.6 sq.deg and a velocity range of approximately cz ∼ 2000− 16000 km s−1, thus mapping the immediate surrounding regions to enable investigations of the various cosmic environmental effects on the HIproperties of galaxies. With the resulting spectral line data cubes of an angular resolution of 2300× 1600 and a velocity resolution of 16.5 km s−1 with a rms sensitivity of
∼ 0.4 mJy/beam, we detect 211 galaxies within the entire surveyed volume. These new HIdetections reveal a total of four distinct overdensities. Of these overdensities, the two major ones are located at the distance of the PPS at cz ∼ 4000 − 8000 km s−1 and behind it at cz ∼ 8000 − 12000 km s−1. The galaxy overdensity associated with the PPS is used to demonstrate how this
supercluster connects across the ZoA and confirms earlier indications of a fil-amentary connection between Perseus, Pisces and the A569 clusters through this region of the sky. Galaxies in the background of the PPS seem to be part of the CID15 structure that had been earlier predicted in the reconstructed density and velocity maps from the 2MASS Redshift Survey.
We carry out a detailed census of the galaxy population in the 3C 129 clus-ter by combining the HI-data of the gas-rich galaxies with the near-infrared (NIR) images of the gas-poor galaxies. The NIR galaxies are identified from the high resolution (0.200/pix, seeing ∼0.800) images of the UKIDSS Galac-tic Plane Survey. We obtain photometry in the J, H and K bands for about 9700 galaxies identified in the NIR images within the WSRT HIsurveyed area. These measurements are used to derive the red-sequence of this cluster through the (J − K) vs K colour-magnitude diagram to identify the gas-poor cluster member candidates. Within the spatial extent of the cluster of radius ∼ 1.7 Mpc, a total of 261 galaxies are identified as cluster members, with 23 detected in HI. An assessment of the morphologies of these galaxy members reveals a clear morphological segregation, with E and E/S0 galaxies dominating the in-ner regions of the 3C 129 cluster and the late-type spirals found in the cluster outskirts.
We also examine the richness of the cluster by comparing it to two well-known clusters at similar redshifts. One being a massive cluster in the Great Attractor region, namely the Norma cluster, also located in the ZoA but in the South, and the other being the Coma cluster. The comparison shows that the 3C 129 cluster is quite rich as was suspected. The galaxy density in the core of the 3C 129 cluster is similar to that in the Norma cluster and slightly less than in the Coma cluster. Furthermore, an assessment of the spatial distribution of galaxies in the core shows a slight asymmetry aligned with the irregular distribution of the X-ray emission, thus consistent with the results from the X-ray analysis which surmised that the 3C 129 cluster seems to have undergone a merger and has not yet reached a dynamically relaxed state. In support of this we find a large substructure dominated by gas-rich galaxies North of the main cluster at a slightly higher recession velocity which could be falling into the cluster. All these findings seem to support a scenario where the 3C 129 cluster is still growing through accretion of galaxies from the PPS filament. The wealth of HI data allows an analysis of the environmental effects on the galaxy properties within the WSRT surveyed volume. We first characterise the cosmic environments found within the two major overdensities as outlined by the HI-detected galaxies. This is conducted by perfoming tests to search for substructures. A total of four distinct substructures are found within the PPS ZoA overdensity (cz ∼ 4000 − 8000 km s−1 ) and three in the
back-ground galaxy overdensity (cz ∼ 8000 − 12000 km s−1). Our analysis of the HI properties of galaxies in these varying cosmic environments shows that highly disturbed HI-disks are found in larger and tightly bound groups, due to the higher incidence of tidal interactions between the galaxies. Furthermore, an indication of HI-gas deficiency is found in the core of the 3C 129 cluster. We assess the cause of this deficiency through models of the ICM of the 3C 129 cluster and find ram-pressure stripping to be the dominant gas removal process in this region. Additionally, highly HI-gas deficient galaxies are also found in the cluster outskirts. Most of these galaxies are located within galaxy groups where galaxy-galaxy interactions and mergers are prevalent, thus highlighting the importance of these processes in removing gas from galaxies in groups.
CONTENTS ix
Contents
Table of Contents ix
1 Introduction 1
1.1 The Zone of Avoidance . . . 1
1.1.1 Observations behind the Galaxy . . . 2
1.2 Why Study the ZoA? . . . 6
1.2.1 The large-scale structures in the Local Universe . . . 6
1.2.2 The mass distribution in the Local Universe . . . 7
1.3 Galaxy clusters . . . 8
1.4 This thesis . . . 10
1.4.1 Thesis outline . . . 12
2 The WSRT PP ZoA I. HI catalogue & atlas. 13 2.1 Introduction . . . 14
2.2 Observations and Data Reduction . . . 18
2.2.1 Data Processing . . . 19
2.3 Source Finding . . . 24
2.4 Data Products of Detected HI Sources . . . 26
2.4.1 The Global HI Profiles . . . 26
2.4.2 Total HI maps . . . 27
2.4.3 Radial Column Density Profiles . . . 28
2.4.4 HI Velocity Fields . . . 28
2.4.5 Position-Velocity Diagrams . . . 29
2.4.6 Counterparts . . . 29
2.5 The HI catalogue and atlas . . . 31
2.5.1 The HI catalogue . . . 31
2.5.2 The Hi atlas . . . . 37
2.5.3 Previous Hi Detections . . . . 39
2.6 The Distribution of Hi Properties of Detected Galaxies . . . . . 46
2.7 Large Scale Structures Crossing the ZoA . . . 48
2.8 Summary . . . 53
x CONTENTS
3 The WSRT PP ZoA II. The 3C 129 cluster 189
3.1 Introduction . . . 190
3.2 Data Samples . . . 194
3.2.1 The HI data . . . 194
3.2.2 The Near-Infrared Data . . . 194
3.2.3 Star-Galaxy Separation . . . 196
3.2.4 Star-Subtraction . . . 199
3.2.5 The NIR Photometry . . . 200
3.2.6 Photometric checks . . . 201
3.2.7 Extinction . . . 203
3.3 The Colour-Magnitude Relation . . . 203
3.3.1 The Red Sequence and Cluster Membership . . . 205
3.3.2 Morphologies of the Cluster Members . . . 206
3.4 The Spatial Distribution of the 3C 129 Cluster Galaxies . . . . 216
3.4.1 Substructure in the 3C 129 Cluster . . . 218
3.5 Summary and Discussions . . . 222
Appendix B: The Near-Infrared Catalogue and Images . . . 224
4 The WSRT PP ZoA III. Environmental Effects on HI 243 4.1 Introduction . . . 245
4.2 The WSRT PPZoA project . . . 247
4.3 Identifying Substructures . . . 248
4.3.1 The Dressler-Shectman test . . . 249
4.3.2 The 2D-projection density . . . 251
4.4 Characterising environments . . . 254
4.4.1 Substructures in Aur 2 - The Perseus-Pisces ZoA Filament254 4.4.2 Substructures in Aur 3 - Behind the PPS ZoA Filament 255 4.5 The Hi Gas Content in Various Environments . . . 257
4.5.1 The HI-Content of the 3C129 Cluster Galaxies . . . 258
4.5.2 Comparison with other environments . . . 261
4.6 HI Morphologies . . . 263
4.6.1 HI Morphologies in Aur 2 . . . 264
4.6.2 HI Morphologies in Aur 3 . . . 269
4.7 A phase-space view of the 3C 129 cluster . . . 274
4.7.1 HI Stripping . . . 277
4.7.2 Orbital Trajectories of the Galaxies . . . 278
4.8 Discussion and Summary . . . 281
Appendix C: The HI and near-infrared in various environments . . . 284
5 Summary and Future Work 293 5.1 Introduction . . . 293
CONTENTS xi 5.1.1 The WSRT HI-imaging of the Perseus-Pisces
Superclus-ter in the Zone of Avoidance . . . 294
5.1.2 Linking large-scale structures across the ZoA . . . 295
5.1.3 The galaxy composition of the 3C 129 cluster. . . 296
5.1.4 Effects of the environment on HI-properties . . . 297
5.2 Future work . . . 298
5.2.1 Perseus-Pisces Supercluster ZoA flow-fields . . . 298
5.2.2 Spectroscopic follow-up . . . 299
5.2.3 The Cluster Dynamics . . . 300
5.2.4 Radio Continuum and Star-Formation . . . 300
Nederlandse Samenvatting 302
Kakaretso ka Sesotho 311
Bibliography 314