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Extragalactic Large-Scale Structures Behind the Southern Milky Way

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Extragalactic Large-Scale Structures Behind the Southern Milky Way A&A manuscript no. (will be inserted by hand later) ASTRONOMY AND Your thesaurus codes are: ASTROPHYSICS 03(03.04.1; 07.09.1; 07.19.1: 09.13.1; 20.01.2) 18.7.2018 Extragalactic Large-Scale Structures behind the Southern Milky Way. – I. Redshifts Obtained at the SAAO in the Hydra/Antlia Extension⋆ R.C. Kraan-Korteweg1, A.P. Fairall2, and C. Balkowski3 1 Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands 2 Department of Astronomy, University of Cape Town, Rondebosch, 7700 South Africa 3 Observatoire de Paris, DAEC, Unit´eassoci´ee au CNRS, D0173, et `al’Universit´eParis 7, 92195 Meudon Cedex, France Received date;accepted date Abstract. Spectroscopic observations have been carried sions can be made. However, a distinct overabundance out for galaxies in the Milky Way with the 1.9 m telescope of redshifts around v ≈ 16000 km sec−1 is found, cor- of the South African Astronomical Observatory (SAAO). responding to that of the adjacent dense Shapley (Alpha) The galaxies were selected from a deep optical galaxy region in the northern galactic hemisphere and to that ◦ ◦ ◦ search covering 266 ∼< ℓ ∼< 296 , |b| ∼< 10 (Kraan- of the Horologium superclusters in the southern galactic Korteweg 1994). This is in the extension of the Hydra and hemisphere. This might indicate that these two overdensi- Antlia clusters and in the approximate direction of the ties are part of a single massive structure, bisected by the dipole anisotropy in the Cosmic Microwave Background Milky Way. radiation. The galaxies in the SAAO observing program – one of the three complementary approaches in mapping the 3D galaxy distribution in the ZOA – were selected for high central surface brightness and for even distribution over Key words: redshifts of galaxies – clustering of galaxies the whole search area. The majority of the galaxies have – zone of avoidance – large-scale structure of the Universe m m (absorbed) magnitudes in the range 14 .5 < BJ < 17 . 5. Good S/N redshifts were determined for 115 galaxies. One 1. Introduction spectrum confirmed a planetary nebula, whereas another object with v = 57 km sec−1 most likely is galactic as 1.1. Indications of interesting extragalactic structures in well. The spectra of the other 25 galaxies either have low the Southern Milky Way S/N or were dominated by superimposed foreground stars. Some of the nearest and apparently most influential con- A preliminary description of the distribution in ve- arXiv:astro-ph/9411089v1 22 Nov 1994 centrations of galaxies in the sky are largely obscured by locity space is given. We do find evidence for a contin- the Southern Milky Way (220◦ <l< 360◦). Their impor- uation of the Hydra/Antlia supercluster across the ZOA ◦ tance follows from various lines of evidence. to b ≈ −10 , making it one of the larger structures (su- The dipole of the Cosmic Microwave Background percluster?) in the nearby Universe. However, the promi- (CMB) at ℓ = 264◦,b = +48◦ (Kogut et al. 1993) im- nent overdensity unveiled in the galaxy search in Vela −1 ◦ ◦ plies a motion of our galaxy of ∼ 600 km sec towards (l ≈ 280 ,b ≈ +6 ) does not, contrary to what may ℓ = 268◦,b = +27◦, after the Sun’s galactocentric veloc- be expected from the 2D distribution, blend with the −1 ity is subtracted and corrections for the motion of our Hydra and Antlia supercluster (v ≈ 3000 km sec ). −1 galaxy within the Local Group (LG) have been applied. The latter seems concentrated at v ≈ 6000 km sec . If an infall of about v = 220 km sec−1 towards the Whether it is related to the Great Attractor cannot yet vc Virgo cluster (Tammann and Sandage, 1985) is taken into be assessed. Another weaker concentration is found at −1 account the direction shifts closer to the Galactic plane v ≈ 9700 km sec . Further out, only tentative conclu- (ℓ = 274◦,b = +11◦), in the region of the Hydra and ◦ ◦ ◦ ◦ Send offprint requests to: Ren´ee C. Kraan-Korteweg Antlia clusters (270 , +26 and 273 , +18 , respectively). ⋆ Table 1 is also available in electronic form. See the Editorial This leads immediately to the question as to whether un- in A&AS 1994, Vol. 103, No.1 know extragalactic features – which might prove relevant 2 R.C. Kraan-Korteweg et al.: Large-Scale Structures behind the Southern Milky Way. - I. in explaining the dipole motion – are hidden behind the an optical galaxy search started by one of us. It concerns ◦ ◦ ◦ obscuration layer of our Milky Way. a region of the ZOA, with 266 <ℓ< 296 , |b| ∼< 10 Such a previously unsuspected overdensity has, for in- (cf. Kraan-Korteweg 1989,1992a,b,1994), i.e., in the close stance, been predicted in the zone of avoidance (ZOA) in proximity of the CMB dipole. At the time of writing, this Puppis (l = 245◦), based on an analysis of IRAS galax- survey is being continued in collaboration with Woudt be- ◦ ◦ ies using spherical harmonics by Scharf et al. (1992). This yond ℓ = 296 to ℓ ∼< 330 to cover the ”Great Attractor” overdensity seems to match successfully a nearby cluster region (cf. Kraan-Korteweg and Woudt 1994b, for a status identified by one of us through HI-observations of obscured report on these surveys). galaxies (Kraan-Korteweg and Huchtmeier 1992). From a In order to map the filamentary features and overden- subsequent analysis in this area, we concluded that this sities discovered in 3-dimensional space we have started previously unknown cluster is second only to Virgo in the to measure the redshifts of a representable part of these nearby Universe and adds at least 30 km sec−1 to the galaxies. This paper reports on redshift observations made motion of the LG perpendicular to the SGP (Lahav et with the 1.9m telescope of the SAAO in the ZOA in the al. 1993). extension of the Hydra and Antlia clusters. The peculiar motions of galaxies can be established by The optical search procedure and the detected galaxy comparing redshifts and distances estimated by the Tully- distribution in the Hydra/Antlia region is reviewed in sec- Fisher method for spirals (Aaronson et al. 1982) and the tion 1.2. In Section 1.3, our observing strategy in deter- modified Faber-Jackson relation for ellipticals (Lynden- mining the 3-dimensional distribution of these galaxies is Bell et al. 1988). Such peculiar motions have revealed outlined. Section 2 then concentrates on the optical spec- large-scale streaming motions towards a ”Great Attrac- troscopy of the brighter galaxies in our search area that tor” in the southern sky, close to the galactic plane. An up- was done at the SAAO: the observing and reduction pro- date by Dressler (1991) predicts its center at ℓ = 307◦,b = cedures are discussed in 2.1, and the resulting recession +9◦,cz = 4350 km sec−1 , while the most recent density velocities together with the optical parameters of the ob- reconstructions from the analysis of the assumed irrota- served galaxies are presented in 2.2. This is followed in tional peculiar velocity field (POTENT) put it even closer section 2.3 with a discussion of the properties of the ob- to the galactic equator at ℓ ≈ 320◦,b ≈ 0◦ (Kolatt et served galaxy sample, including velocity data for 31 galax- al. 1994). ies in our sample region taken from other sources. In the The existence of this massive overdensity is still quite third section we describe the resulting galaxy distribution controversial. Dressler and Faber (1990a,b) find weak ev- in redshift space, and discuss the indicated clustering and idence for backside infall into the putative Great Attrac- filamentary structures, in context to known large-scale fea- tor, but Mathewson et al. (1992) and Hudson (1993,1994) tures adjacent to the zone of avoidance. dispute this and claim that no major mass density can be hidden in the ZOA. They, and others, using deep 1.2. The galaxy search behind the Milky Way in the exten- IRAS-redshift surveys and cluster samples (e.g., Rowan- sion of the Hydra/Antlia clusters Robinson et al. 1990, Scaramella 1991, Lauer and Post- man, 1993, Plionis et al. 1993), suggest a residual bulk The obscuration of the Milky Way reduces the apparent motion caused by much more distant overdensities (v > magnitudes and apparent angular diameters of galaxies 10000 km sec−1 ). But this is contradicted with inves- (cf. Cameron 1990). For this reason, galaxies which lie tigations by e.g., Lynden-Bell et al. (1989), Strauss et close to the galactic plane fail to meet the criteria for in- al. (1992), and Jerjen and Tammann (1993), who find con- clusion in catalogues (such as e.g., Lauberts 1982) and vergence of the dipole motion between 3500 km sec−1 to only few galaxies are known below galactic latitudes of −1 ◦ 6500 km sec . Still, it should be noted that this large- |b| ∼< 10 . Added to this are the enormous numbers of scale bulk motion – if real – is also located in the general foreground stars that frequently fall on the galaxy images vicinity of the CMB dipole and the southern Milky Way. and crowd the field of view. A separation of galaxy and Whatever the case, the mass distribution hidden by star images cannot as yet be done by automated measur- the Galactic zone of avoidance appears crucial. This is ing machines such as COSMOS or APM on a viable basis ◦ manifested also with the recent POTENT analysis by Ko- below |b| ∼< 10 − 15 . Examination by eye is still the best latt, Dekel and Lahav (1994), in which they show that the technique, though surveys by eye are clearly both very gravitational acceleration at the Local Group changes by trying and time consuming and maybe not as objective.
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