aa r X i v : . [ a s t r o - ph . GA ] A ug IAU Astronomy in FocusXXIXth IAU General Assembly, August 2015Edith Falgarone & Bruce Elmegreen, ed. c (cid:13) Galaxy Goup Scaling Relations
R. Brent Tully Institute for Astronomy, University of Hawaii2680 Woodlawn Drive, Honolulu, HI 96822, USAemail: [email protected]
Abstract.
What is a galaxy group?
Keywords.
Galaxies: groups; mass function
This brief account summarizes a lecture on group scaling relations that itself was asummary of two recent publications (Tully 2015 a, b ). The overarching motivation was tobuild a group catalog. Such catalogs are subject to biases created by selection criteria. Itis an obvious concern that we access fewer group member with increasing distance. Therelationship between luminosity and mass changes with group properties. Most funda-mentally, the dimensions and velocity dispersions of groups scale with mass, so it is quiteinappropriate to fix these parameters at single values in attempting to identify groups.In Tully(2015 a ) the goal was to establish the size and velocity dispersion properties ofgroups over 3 decades in mass, from 10 to 10 M ⊙ . It was demonstrated that the secondturnaround radius, R t , is an observable proxy for the virial radius. Velocity dispersions, σ p , are determined from spectroscopy of galaxies within this radius and masses, M v , aredetermined from the virial theorem.The identification of group members is often messy. However it is possible to isolateclean cases, for example, by choosing groups that project against voids. Candidate groupswere chosen to span a wide range in galaxy numbers and types. Wide fields encompassingtarget groups were observed with the Canada-France-Hawaii and Subaru telescopes. Thesmallest groups to be studied, those with the most limited memberships, lie sufficientlynearby that their constituents can be resolved with Hubble Space Telescope imaging.These observations provided accurate distances, hence unambiguous membership identi-fications.The observations were designed to test theoretical expectations. In the approximationof spherical collapse, any specified phase of collapse depends on the inverse square root oflocal density. Focusing on the specific phase of second turnaround, the relation t today ∼ ρ − / t implies scaling relations between the two observables R t and σ p and the virialradius M v .The main result of the first paper was the unambiguous demonstration that the antic-ipated scaling relations are, in fact, seen. The products of interest are the coefficients ofthe fits. There are two independent relations and one derivative relation: R t = 0 . M ) / h − / Mpc σ p /R t = 368 h km / s M v = 2 . × σ p h − M ⊙ where M = M v / M ⊙ . In this study, distances were directly measured but the scaleis compatible with H = 75 km/s/Mpc. The observed correlations are seen in Figure 1.1 R. Brent Tully ComaVirgoN5846N1407N5353/4N1023M81M31
Figure 1.
Scaling relations between R t , σ p and M v . In each case, the slope is given by thetheoretical expectation and the only free parameter is the scale zero point. The second paper (Tully 2015 b ) used these scaling relations to build a group catalog.The input sample was the 2MASS Redshift Survey complete to K s = 11 .
75 mag (Huchraet al. 2012) with 43,038 galaxies. An assumptions was needed regarding the relationshipbetween the mass of a group and the luminosity of constituent galaxies. A formulation wasmotivated that ran linearly with log mass from M v /L K = 40 at 10 M ⊙ to M v /L K = 120at 10 M ⊙ .A group catalog could be built with these pieces. Using redshifts for distance, themost intrinsically luminous galaxy in the catalog was identified, its inferred mass wascalculated, whence its expectation second turnaround radius and associated velocity dis-persion. Galaxies within this radius and twice the velocity dispersion were linked asgroup members. The added candidates increased the joint luminosity, hence the asso-ciated parameters so the search was renewed for additional members. In this way, theentire catalog was processed.The range of optimal validity of the group catalog is 3,000 − − × to 5 × M ⊙ .Support for this program has been provided by the US National Science Foundation, theNASA Astrophysics Data Analysis Program, and awards from the Space Telescope Sci-ence Institute in connection with observations with Hubble Space Telescope. Additionalobservations were made with the Canada-France Hawaii and Subaru telescopes. References
Huchra, J.P, Macri, L.M., Masters, K.L., et al. 2012,