Jarrett et al. 2007, AJ, 133, no. 3 Preprint typeset using LATEX style emulateapj v. 08/13/06 DISCOVERY OF TWO GALAXIES DEEPLY EMBEDDED IN THE GREAT ATTRACTOR WALL T.H. Jarrett Spitzer Science Center, IPAC 100-22, Caltech, Pasadena, CA 91125 B.S. Koribalski Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, AU R.C. Kraan-Korteweg & P.A. Woudt Dept. of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, RSA B.A. Whitney Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301 M.R. Meade, B. Babler & E. Churchwell Astronomy Dept,, 475 N. Charter St., University of Wisconsin-Madison, Madison, WI 53706 R.A. Benjamin Physics Dept,, 800 W. Main St., University of Wisconsin-Whitewater, Whitewater, WI 53190 and R. Indebetouw Astronomy Dept,, PO 318, University of Virginia, Charlottesville, VA 22903-0818 Jarrett et al. 2007, AJ, 133, no. 3 ABSTRACT We report on the discovery of two spiral galaxies located behind the southern Milky Way, within the least explored region of the Great Attractor. They lie at (, b ≈ 317◦, −0.5◦), where obscuration from Milky Way stars and dust exceeds 13 to 15 mag of visual extinction. The galaxies were the most prominent of a set identified using mid-infrared images of the low-latitude (|b| < 1◦) Spitzer Legacy program GLIMPSE. Follow-up H i radio observations reveal that both galaxies have redshifts that place them squarely in the Norma Wall of galaxies, which appears to extend diagonally across the Galactic Plane from Norma in the south to Centaurus/Vela in the north. We report on the near- infrared, mid-infrared and radio properties of these newly discovered galaxies, and discuss their context in the larger view of the Great Attractor. The work presented here demonstrates that mid-infrared surveys open up a new window to study galaxies in the Zone of Avoidance. Subject headings: infrared: galaxies, radio lines: galaxies, superclusters; Great Attractor 1. INTRODUCTION tational influence stretching beyond Virgo and the Lo- Our emerging portrait of the universe is that of an in- cal Supercluster of galaxies (Lynden-Bell et al. 1988; tricate cosmic web of galaxies arrayed in long filaments, Burstein et al. 1990; Kocevski & Ebeling 2006; Er- sheets and bubbles that intersect to form dense mass dogdu et al. 2006a,b). The search for the elusive GA concentrations. The key outstanding problem is the dis- covers a very large expanse of sky that is within the tribution and nature of dark matter and dark energy that Zone of Avoidance (ZoA), where traditional surveys are drives the dynamics of the expanding cosmos. The study limited by the foreground Milky Way. It has been at- of the local Universe, including its peculiar motions and tributed to or associated with visible galaxy overdensi- its clustering at the largest size scales, is essential for ties in Hydra, Centaurus, Pavo and Indus (e.g. Lahav structure formation in the early Universe and its relation 1987; Lynden-Bell, Lahav, & Burstein 1989), and most to the formation and subsequent evolution of galaxies. recently with dedicated surveys of the ZoA, in Norma and Studying superclusters both near and far is paramount Vela (e.g., see Kraan-Korteweg & Lahav 2000, Kraan- in decoding the mass density of the universe. Korteweg 2005, for reviews). We now recognize that the core of one of the most im- To date, the most prominent density peak to be dis- portant mass concentrations in the local universe is lo- covered in the southern ZoA is the Norma cluster (Abell cated behind the southern Milky Way ( ∼ 320◦,b∼ 0◦). 3627; Abell, Corwin & Olowin 1989), whose cumulative The so-called Great Attractor (GA) is pulling on the mass and richness is comparable to the Coma cluster Milky Way and the Local Group of galaxies, its gravi- (Kraan-Korteweg et al. 1996; Woudt, Kraan-Korteweg & Fairall 1999). It is located below the southern Galactic ◦ ◦ −1 Electronic address: [email protected] equator (325 , −7 , 4900 km s ), the limit where opti- 2 Jarrett et al. cal and near-infrared surveys become heavily incomplete. are combined into a single mosaic per band. The com- To the north, emerging on the other side of the dark- posite point source FWHM is ∼2.5. est region of ZoA are the Cen-Crux and CIZAJ1324.7– At longer wavelengths, the MIPSGAL survey (Carey 5736 clusters, both at similar recessional velocities as et al. 2006) covered the same GLIMPSE footprint of Norma (Radburn-Smith et al. 2006). For a velocity the Milky Way using the Spitzer Multiband Imaging range between 4000 and 6000 km s−1, a continuous line Photometer for Spitzer (MIPS). The diffraction limit at of galaxies appear to extend northward from Pavo-Indus 24μm is approximately 6 arcsec, roughly a factor of two up through Norma, bending to lower longitudes and join- larger than the IRAC imaging, adequate for measuring ing with Cen-Crux/CIZAJ1324.7–5736 in the northern the dust-emitting continuum from the GLIMPSE galax- Galactic hemisphere (see Fig. 1 in Jarrett 2004 for a 3D ies. A combined IRAC + MIPS panorama of the GA is view of these large scale structures). This large filamen- shown in Fig. 1, simultaneously illustrating the incredi- tary structure, named the ”Norma Wall”, runs diago- ble beauty of the Milky Way and the major challenge we nally behind the southern Milky Way (Kraan-Korteweg are faced as we look behind the Galactic veil. et al. 1994; Kraan-Korteweg 2005), severely hampering Visually scanning the set of GLIMPSE images that the construction of sufficiently detailed large-scale flow crossed the GA, we identified a number of resolved maps that are needed to identify the full extent, shape sources to be promising extragalactic candidates for and mass of the GA. Despite dedicated multi-wavelength follow-up study. The two most prominent ones (see efforts in mapping the GA in the last decade (Kraan- Fig. 1) lie at α, δ(J2000) = 14h48m36.0s, −60◦0715,(l, b Korteweg 2005), a complete census of the density and = 317.04◦, -0.50◦)and14h47m45.1s, −60◦1704 (l, b = total mass is still unrealized. 316.87◦, -0.60◦), and are referred to as G 1 and G 2, re- From above the Earth’s blocking atmosphere, the mid- spectively. These sources were found in relatively low infrared (MIR) opens a new and promising window for 8μmand24μm background regions compared to the very detecting ZoA galaxies. The MIR can penetrate the thick bright H ii emission encircling the bubble-like clearing. layer of Galactic gas and dust, whilst being sensitive to The stellar confusion noise and the visual extinction, both stellar photospheric emission of early-type galaxies however, were extremely high, rendering both galaxies and interstellar emission from star-forming late-type disk near the detection limit of GLIMPSE. galaxies. The Spitzer Legacy project “Galactic Legacy Photometry was extracted from the four IRAC bands Infrared Mid-Plane Survey Extraordinaire” (GLIMPSE; and the MIPS 24μm image using a set of nested elliptical Benjamin et al. 2003), surveyed a large fraction of the ◦ apertures centered on each galaxy, where the images were Milky Way (within |b| < 1 )usingtheIRACcamera first cleaned of contaminating, foreground stars. The (3.6 − 8μm), passing across the most opaque part of the background was determined using an annulus that was GA. GLIMPSE provides a readily available data set to well outside of the galaxy light, but still local to the test the viability of probing the ZoA with MIR imaging. galaxy environment and considerably smaller than the Inspection of the GLIMPSE MIR images revealed scale of the background ISM gradients. For the IRAC a number of galaxy candidates peaking through the measurements, the flux density of each source was aper- formidable foreground of Galactic dust, gas and stellar ture corrected using the ”extended source” prescription confusion. Intriguingly, a concentration of galaxy can- recommended by the Spitzer Science Center 1.Theex- didates appear to be projected against the Norma Wall pected uncertainty in the IRAC photometry (measure- exactly where the wall supposedly bends from the south- ment errors plus calibration) is ∼10%. For the MIPS ern hemisphere to the northern hemisphere. To confirm 24μm measurements, the appropriate aperture correc- their extragalactic nature, we have followed-up two of tion was determined to be ∼1.1(seeFigure3.2ofthe the most promising galaxy candidates using deep near- MIPS data handbook), while another factor of 1.04 was infrared (NIR) imaging (Sect. 2.2) and radio observa- applied to the integrated fluxes to account for the color tions (Sect. 2.3). Both were found to have radial veloc- correction between normal galaxies and the MIPS ab- ities that are consistent with membership in the Norma solute calibration standard (see Table 3.12 in the MIPS Wall (Sect. 3.2), thus providing compelling evidence that data handbook). The expected uncertainty in the MIPS an extragalactic bridge exists between the southern and photometry is ∼10-15%. northern halves of the Plane. In Section 3 we report on the properties of these ZoA galaxies and in Section 4 2.2. IRSF Near-Infrared Observations discuss their context in the larger view of the GA. Near-infrared JHKs images of the region were ob- 2. OBSERVATIONS & DATA REDUCTIONS tained with the sirius camera on the 1.4m Infrared Sur- 2.1. Spitzer Mid-Infrared Observations vey Telescope (IRSF; Nagata & Glass 2000, Nagayama et al. 2003) in South Africa during April 4/5, 2006. The Active study of the Galactic Plane is now under- exposures were relatively long, reaching the stellar con- way with the Spitzer Space Telescope. The heart of fusion limit at 2μm for much of the region studied.