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Register a new userGalaxy clusters in the Perseus-Pisces region - II. The peculiar velocity field
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M.J. Hudson
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We have measured the mean peculiar motions of 103 early-type galaxies in 7 clusters in the Perseus-Pisces (PP) ridge or PP background, and a further 249 such galaxies in 9 calibrating clusters from the literature, using the inverse Fundamental Plane relation. This relation is found to have a distance error of 20% per galaxy. None of the 6 clusters in the PP ridge has a significant motion with respect to the CMB frame, but the PP background cluster J8 shows marginal evidence of `backside infall into the PP supercluster. The full 16 cluster sample has a mean CMB-frame bulk motion of 420 +/- 280 km/s, towards l=262, b=-25. This result is consistent both with no bulk motion in the CMB frame and with the ~350 km/s bulk motion found by Courteau et al. It is inconsistent at the 98% confidence level with the ~700 km/s bulk flow found by Lauer & Postman (LP). The PP ridge clusters are found to have a small and statistically insignificant mean radial motion with respect to the CMB frame: -60 +/- 220 km/s. A comparison between our cluster peculiar velocities and the predicted peculiar velocities from the IRAS 1.2 Jy density field, smoothed on a 500 km/s scale, yields beta_I = Omega^0.6/b_I = 0.95 +/- 0.48, consistent with previous results. We find agreement between our peculiar motions and published Tully-Fisher results for the same clusters. The disagreement between the 11 clusters common to our sample and that of LP, based on BCGs, is statistically significant at the >~ 99.7% confidence level indicating that the errors of one or both of these data sets are underestimated. When the BCG distances corrected for the X-ray luminosity of the host cluster are used, the disagreement is reduced to the ~94% confidence level. (Abridged)
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We present results of a new study of peculiar motions of 7 clusters in the Perseus-Pisces (PP) region, using the Fundamental Plane as a distance indicator. The sample is calibrated by reference to 9 additional clusters with data from the literature. Careful attention is paid to the matching of spectroscopic and photometric data from several sources. For six clusters in the PP supercluster no significant peculiar motions are detected. For these clusters we derive a bulk motion of 60 +/- 220 km/s, in the CMB frame, directed towards the Local Group. This non-detection is in marginal conflict with previous Tully-Fisher studies. Two clusters in the background of the supercluster exhibit significant negative peculiar velocities, characteristic of backside infall into PP. A bulk-flow fit to all 16 clusters reveals a statistically insignificant motion of 430 +/- 280 km/s towards l=265, b=26 (CMB frame). Comparison with the velocity field predicted from the IRAS 1.2Jy survey yields beta = 1.0 +/- 0.5. We find no evidence for residual bulk motions generated by mass concentrations beyond the limiting depth of the IRAS density field.
We present new spectroscopic and photometric data for 137 early-type galaxies in nine clusters, and for a set of nearby standard galaxies. The clusters studied are Perseus (A0426), Pisces, A0262, A0347, J8, HMS0122+3305, 7S21, A2199 and A2634. Our spectroscopic data comprise radial velocities (cz), central velocity dispersions (sigma) and magnesium line strength indices (Mg). Internal errors (derived from repeat observations) are 7.6% on each measurement of sigma, and 0.010 mag on each Mg measurement. We correct the major published datasets to a standard aperture size, and define a new `standard system by the aperture-corrected Lick data of Davies et al. Through extensive intercomparisons with data from the literature, we present the corrections required to bring the major published datasets onto the standard system. We demonstrate that our new velocity dispersion data can be brought into consistency with the standard system, to an uncertainty of <0.01 dex. From R-band CCD photometry, we derive effective diameter (A_e), mean surface brightness within effective diameter (mu_e) and an R-band diameter equivalent to the D_n parameter of Dressler et al. Internal comparisons indicate an average error of 0.005 in each measurement of log(D_n). The combination log(A_e) - 0.3 mu_e, approximately the quantity used in the Fundamental Plane distance indicator, has an uncertainty of 0.006 per measurement. The photometric data can be brought onto a system consistent with external data at the level of 0.5% in distance. These data will be used in a companion paper to derive distance and peculiar velocity estimates for the nine clusters studied. (Abridged)
We report the discovery of DGSAT I, an ultra-diffuse, quenched galaxy located 10.4 degrees in projection from the Andromeda galaxy (M31). This low-surface brightness galaxy (mu_V = 24.8 mag/arcsec), found with a small amateur telescope, appears unresolved in sub-arcsecond archival Subaru/Suprime-Cam images, and hence has been missed by optical surveys relying on resolved star counts, in spite of its relatively large effective radius (R_e(V) = 12 arcsec) and proximity (15 arcmin) to the well-known dwarf spheroidal galaxy And II. Its red color (V-I = 1.0), shallow Sersic index (n_V=0.68), and the absence of detectable H-alpha emission are typical properties of dwarf spheroidal galaxies and suggest that it is mainly composed of old stars. Initially interpreted as an interesting case of an isolated dwarf spheroidal galaxy in the local universe, our radial velocity measurement obtained with the BTA 6-meter telescope (V_h=5450 +/- 40 km/s) shows that this system is an M31-background galaxy associated with the filament of the Pisces-Perseus supercluster. At the distance of this cluster (~78 Mpc), DGSAT I would have an R_e ~ 4.7 kpc and M_V ~-16.3$. Its properties resemble those of the ultra-diffuse galaxies recently discovered in the Coma cluster. DGSAT I is the first case of these rare ultra-diffuse galaxies found in this galaxy cluster. Unlike the ultra-diffuse galaxies associated with the Coma and Virgo clusters, DGSAT I is found in a much lower density environment, which provides a fresh constraint on the formation mechanisms for this intriguing class of galaxy.
We analyze in detail the peculiar velocity field traced by 56 clusters within 120 h^-1 Mpc in the Streaming Motions of Abell Clusters (SMAC) sample. The bulk flow of the SMAC sample is 687 +- 203 km/s, toward l = 260 +- 13, b = 0 +- 11. We discuss possible systematic errors and show that no systematic effect is larger than half of the random error. The flow does not drop off significantly with depth, which suggests that it is generated by structures on large scales. In particular, a Great Attractor as originally proposed by Lynden-Bell et al. cannot be responsible for the SMAC bulk flow. The SMAC data suggest infall into an attractor at the location of the Shapley Concentration, but the detection is marginal (at the 90% confidence level). We find that distant attractors in addition to the Shapley Concentration are required to explain the SMAC bulk flow. A comparison with peculiar velocities predicted from the IRAS PSCz redshift survey shows good agreement with a best fit value of Beta_I = Omega^0.6/b_I = 0.39 +- 0.17. However, the PSCz density field is not sufficient to acount for all of the SMAC bulk motion. We also detect, at the 98% confidence level, a residual bulk flow of 372 +- 127 km/s toward l = 273, b = 6 which must be generated by sources not included in the PSCz catalogue, that is, either beyond 200 h^-1 Mpc, in the Zone of Avoidance or in superclusters undersampled by IRAS. Finally, we compare the SMAC bulk flow with other recent measurements. We suggest that a bulk flow of 225 km/s toward l = 300, b = 10 at depths greater than 60 h^-1 Mpc is consistent with all peculiar velocity surveys, when allowance is made for sparse sampling effects. (abridged)
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