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A Local Hubble Bubble from Type Ia Supernovae? A Local Hubble Bubble from Type Ia Supernovae? The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Zehavi, Idit, Adam G. Riess, Robert P. Kirshner, and Avishai Dekel. 1998. “A Local Hubble Bubble from Type Ia Supernovae?” The Astrophysical Journal 503 (2): 483–91. https:// doi.org/10.1086/306015. Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:41399824 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Other Posted Material, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#LAA THE ASTROPHYSICAL JOURNAL, 503:483È491, 1998 August 20 ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A. A LOCAL HUBBLE BUBBLE FROM TYPE Ia SUPERNOVAE? IDIT ZEHAVI,1 ADAM G.RIESS,2 ROBERT P. KIRSHNER,3 AND AVISHAI DEKEL1,4 Received 1997 September 16; accepted 1998 April 1 ABSTRACT We analyze the monopole in the peculiar velocities of 44 Type Ia supernovae (SNe Ia) to test for a local void. The sample extends from 20 to 300 h~1 Mpc, with distances, deduced from light-curve shapes, accurate to D6%. Assuming) \ 1 and) \ 0, the most signiÐcant deviation we Ðnd from the m " Hubble law is an outward Ñow of 6.5% ^ 2.2% inside a sphere of radius 70 h~1 Mpc, as would be produced by a void of D20% underdensity surrounded by a dense shell. This shell roughly coincides with the local great walls. Monte Carlo analyses, using Gaussian errors or bootstrap resampling, show the probability for chance occurrence of this result out of a pure Hubble Ñow to be D2%. The mono- pole could be contaminated by higher moments of the velocity Ðeld, especially a quadrupole, which are not properly probed by the current limited sky coverage. The void would be less signiÐcant if)m is low and) is high. It would be more signiÐcant if one outlier is removed from the sample, or if the size of the void" is constrained a priori. This putative void is not in signiÐcant conÑict with any of the standard cosmological scenarios. It suggests that the Hubble constant as determined within 70 h~1 Mpc could be overestimated by D6%, and the local value of ) may be underestimated by D20%. While the present evidence for a local void is marginal in this data set, the analysis shows that the accumulation of SN Ia distances will soon provide useful constraints on elusive and important aspects of regional cosmic dynamics. Subject headings: cosmology: observations È cosmology: theory È galaxies: distances and redshifts È large-scale structure of universe È supernovae: general 1. INTRODUCTION tively high values of the baryon density(Hu & White 1996). A local void has been proposed as one way to reconcile Large-scale redshift surveys of galaxies show underdense the age of the universe based on the Hubble expansion with regions of typical extent D50 h~1 Mpc. These ““ voids ÏÏ the ages of globular clusters within the framework of the appear to be bordered by dense ““ walls ÏÏ(Kirshner et al. EinsteinÈde Sitter cosmology (e.g.,Turner, Cen, & Ostriker 1981; Huchraet al. 1983 [CfA]; Broadhurst et al. 1990; 1992; Bartlettet al. 1995) Measurements of the Hubble con- Shectmanet al. 1996 [Las Campanas Redshift Survey stant within the void would overestimate the universal (LCRS)]). In particular, maps of our cosmological neigh- value by do/oB[3dH/H. Indeed, the values obtained for borhood display the Great Wall of Coma and the Southern the Hubble constant from the longest-range distance indica- Wall, which appear to connect into a shell-like structure of tors, the SNe Ia(Jacoby et al. 1992; Sandage & Tammann radius 70È80 h~1 Mpc about the Local Group(Geller & 1993; Tammann& Sandage 1995; Hamuy et al. 1995, Huchra1989 [CfA2]; da Costa et al. 1994 [Southern Sky 1996b;Riess, Press, & Kirshner 1995a, 1996; Branch, Redshift Survey 2 (SSRS2)]). The volume encompassed by Nugent, & Fisher1997) and the gravitational lenses (Falco this structure appears to be of lower density. et al.1997; Keeton & Kochanek 1997) are typically smaller Despite these impressive maps, it is difficult to quantify than values obtained more locally using Tully-Fisher (TF) the large-scale radial density proÐle of this region. First, the distance indicators(Kennicutt, Freedman, & Mould 1995; true galaxy density is hard to distinguish from the sample Mouldet al. 1995; Freedman et al. 1994; Freedman 1997, selection function when the structure of interest approaches Giovanelliet al. 1997). A local void would also imply that the sample size. Second, we do not know how well galaxies local estimates of ) underestimate the global value of ). trace mass. And third, portions of the galaxy distribution Finally, a local outÑow would reduce the distances derived are obscured from our viewpoint within the Milky Way. from TF peculiar velocities for features such as the Great The imprint of wall and void structure on the power Attractor, bringing them into better agreement with the spectrum may possibly be associated with excess power positions derived from redshift surveys(Sigad et al. 1998). observed at a wavelength of D100È150 h~1 Mpc (e.g., It is important to separate impressions and theoretical Broadhurstet al. 1990; Landy et al. 1996; Einasto et al. wishes from Ðrmly established observational facts. Attempts 1997). This scale might be naturally attributed to the scale to establish monopole deviations from pure Hubble Ñow of the cosmological horizon after the universe became have not yet produced conclusive results. For example, Shi matter dominated and before the plasma recombined, (1997) claimed Ðnding a monopole gradient in subsamples though this peak is only expected to be signiÐcant for rela- of the Mark III catalog of Tully-Fisher peculiar velocities (Willicket al. 1997a). But he also found a marginal mono- 1 Racah Institute of Physics, Hebrew University, Jerusalem 91904, pole gradient of the opposite sign in the peculiar motions of Israel. rich clusters based on their brightest members as standard 2 Astronomy Department, University of California, Berkeley, candles (Lauer & Postman1992, 1994). Finally, he found no CA 94720. signiÐcant deviation in an early subsample of 20 supernovae 3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138. (Riesset al. 1996). None of these data sets was ideal for 4 UCO/Lick Observatory, University of California, Santa Cruz, testing for a void of radius D70 h~1 Mpc: the Mark III CA 95064. data include only a small number of galaxies beyond 70 h~1 483 484 ZEHAVI ET AL. Vol. 503 Mpc, the earlier supernova sample is too sparse, and the ““ Formal ÏÏ estimates of the distance errors for the individ- rich clusters, while of comparable abundance to the current ual supernovae, including the intrinsic scatter in the dis- supernova sample within 70 h~1 Mpc, have a much larger tance indicator and the measurement errors, are provided error per cluster.Kim et al. (1997), based on comparisons of by the covariance matrix of the MLCS Ðt (RPK), * . supernova distances at high redshifts (z D 0.4) and low red- These internal errors are typically 5%È8% of the distance.MLCS shifts (z \ 0.1), set a 2 p upper limit of 10% for an outÑow Additional systematic errors in the distances are uncertain, within the local 300 h~1 Mpc, but their sample has only a so we adopt a conservative approach by adding in quadra- few supernovae within 70 h~1 Mpc, which is the suggested ture* \ 0.05 mag in our ““ standard ÏÏ study case (in ° 3), domain of the putative void. and lateradd discuss the implications of varying* in the We now have a sample of 44 SNe Ia from the Calan/ range 0È0.1 mag. add Tololo Survey (Hamuy et al.1993a, 1996a) and from the CfA supernova program(Riess 1996; Riess et al. 1998), 3. PECULIAR MONOPOLE which reaches the threshold for an interesting assessment of 3.1. Fitting a Monopole ProÐle the dynamical signature of a void contained within the Figure 1 shows the radial peculiar velocities of the 44 Great Walls. The nearest supernovae in this sample are at supernovae, u, translated to the corresponding relative devi- distances of D20 h Mpc, the farthest are beyond 300 h ~1 ~1 ations from a pure Hubble Ñow,d 4 dH/H u/d, at the Mpc, the median is at 95 h Mpc, and 17 supernovae lie H \ ^ ~1 inferred distances d, assuming (in this section) ) 1 and inside 70 h Mpc. This distribution in distance is well \ ~1 " 0. The plot suggests positived , an outÑow, inside 70 È \ H suited for detecting a monopole perturbation within 50 100 h Mpc, and possibly a marginal indication for an inÑow h Mpc while determining the universal Hubble Ñow ~1 ~1 in the surrounding shell 70È110 h~1 Mpc, before the Ñow outside this sphere. These are the scales which have been converges to a global Hubble expansion at larger distances. suggested and where the standard cosmological theories The 26th supernova from the Local Group at d 107 predict perturbations on the order of several percent (e.g., ^ h~1 Mpc, named SN 1992bl, is the most discrepant point in Wang,Spergel, & Turner 1998). Because supernova light this plot. Its inferred peculiar velocity of D2400 km s is a curves provide distances with a precision of 5%È8%, we ~1 D4 p deviation from the global Hubble Ñow. As described believe it is worth carrying out a preliminary analysis with below, the exclusion of this outlier from the analysis makes only 44 tracers.
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