Cosmic Voids Introductory Comments PJE Peebles, Amsterdam, December 2006 Present conditions are sensitive to initial conditions
Catalog of Neighboring Galaxies Karachentsev et al. 2004 The nature of large-scale structure, as we under- stand the situation, is a product of: (1) the initial conditions for the Friedmann-Lemaˆıtre cosmology; (2) the interactions of galaxies and the intergalactic medium with their surroundings; (3) the evolution of galaxies as island universes; and (4) our belief system. Factor (1) means we are misled if we don’t have excellent understanding of conditions in the very early universe, including the physics. Factor (4) is a real and proper part of every physi- cal science. Extragalactic astronomy and cosmol- ogy is special only in that the bones of our belief system tend to stick out under the thin flesh of the observations. In this table, I mean by Nature, initial conditions, or else conditions within about 30 kpc of a • protogalaxy when the accumulation of baryon mass within that radius has grown close to the present value; Nurture, the effect of interactions on scales larger than about 30 kpc, as • in the effect of the environment on the evolution of protogalaxies once assembled, and the evolution of the intergalactic medium; Theory, ideas about what has happened that are more strongly sup- • ported by theory than observation. The assignments in this table are debatable, to be sure, but then we’re here to debate the issues.
Phenomenon, real or virtual Nature Nurture Theory morphology-density relation √ near universal red-sequence √ color-magnitude relation the most luminous galaxies prefer the densest environments, consistent √ with their strong clustering the faintest galaxies & L galaxies are similarly distributed,∗and they √ √ √ avoid common voids the ΛCDM Cosmic Web defines the voids √ inhabitants of voids & low density regions: grand-design spirals √ close to normal ellipticals √ √ low surface density Lyα absorbers √ the warm-hot IGM √ baryon-free dwarf DM halos √ The morphology-density relation This has a long history. For example, Hubble (1936): in clusters all “types of nebulae are represented, but in contrast to the general field, the earlier types, and especially the ellipticals, predominate”; Spitzer and Baade (1950): “Dense clusters of galaxies, such as the Coma and Corona clusters, contain large numbers of S0 galaxies” which “presum- ably contain stars only of population type II. It is suggested that collisions between galaxies sweep any interstellar matter out of the galaxies in such clusters, and thereby prevent the appearance of any type I systems”; Gunn and Gott (1972): intracluster gas, perhaps indicated by the UHURU X-ray detection of the Coma cluster, might sweep gas out of cluster mem- bers; and “the cD galaxies might grow once again at the expense of the intracluster medium;” van den Bergh (1976): many Virgo cluster spirals “have a rather anemic appearance which is, no doubt, due to the fact that they are forming stars less vigorously than are typical galaxies of the same type in the field”, which “might possibly be understood in terms of the sweeping out of interstellar gas in Virgo spirals by ram pressure.” Cosmological initial condi- tions surely played a di- rect and important role in establishing the density- morphology relation. For example, cD’s likely mark the positions of exception- ally large primeval density fluctuations. But the role of environment is clear too, as in van den Bergh’s (1976) anemic Virgo spirals and van Dokkum’s (2005) dry mergers in el- lipticals. So I am inclined to assign the dominant role in de- termining the density- morphology relation to Nurture. Pieter van Dokkum (2005) Phenomenon, real or virtual Nature Nurture Theory → morphology-density relation √ near universal red-sequence √ color-magnitude relation the most luminous galaxies prefer the densest environments, consistent √ with their strong clustering the faintest galaxies & L galaxies are similarly distributed,∗and they √ √ √ avoid common voids the ΛCDM Cosmic Web defines the voids √ inhabitants of voids & low density regions: grand-design spirals √ close to normal ellipticals √ √ low surface density Lyα absorbers √ the warm-hot IGM √ baryon-free dwarf DM halos √ The red edge of the color-magnitude-density diagram The red edge of the color-magnitude-density diagram
M M David W. Hogg, Michael R. Blanton, Jarle Brinchmann, Daniel J. Eisenstein, David J. Schlegel, James E. Gunn, Timothy A. McKay, Hans-Walter Rix, Neta A. Bahcall, J. Brinkmann, and Avery Meiksin Received 2003 July 11; accepted 2003 December 2; published 2004 January 16 (bowdlerized)
The local number density contrast is the average within a cylinder of radius 1h−1 Mpc and half-length 8h−1 Mpc in redshift space. The SDSS magnitudes and colors are measured at ∼ 80% of the nom- inal Petrosian magnitude, that is, well outside the half-light radius. The red edge of the color-magnitude-density diagram Park and the SDSS Collaboration Reda, Forbes and Hau 2005 Circles with error bars are Reda et al. isolated ellipticals. Solid line is the Coma cluster CMR.
The local density, ρ, is the number density within a shaped smooth window in redshift space that contains 20 L > L galaxies. ∗ The curves are modes; the dashed curves in the lower panel, ρ/ ρ < 0.5, are the high density cases shown in th! e" upper panels.
Bernardi et al. 2006
Fig. 8.— Location of C4 BCGs (symbols) with respect to the color-magnitude relation defined by the bulk of the early-type galaxy population (solid line). Dashed line shows a linear fit of color as a function of absolute magni- tude, and jagged line with error bars shows the mean color in a few bins in luminosity. Inset shows that BCGs are not offset from the relation defined by the bulk of the popula- tion, though the relation they define is slightly tighter. T
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Phenomenon, real or virtual Nature Nurture Theory morphology-density relation √ near universal red-sequence √ → color-magnitude relation the most luminous galaxies prefer the densest environments, consistent √ with their strong clustering the faintest galaxies & L galaxies are similarly distributed,∗and they √ √ √ avoid common voids the ΛCDM Cosmic Web defines the voids √ inhabitants of voids & low density regions: grand-design spirals √ close to normal ellipticals √ √ low surface density Lyα absorbers √ the warm-hot IGM √ baryon-free dwarf DM halos √ The luminosity-density relation The luminosity-density relation The most luminous galaxies, at L ! 10 times that of the Milky Way, favor the densest environments. Consistent with that, the spatial autocorrelation function is largest for the largest galaxies, and at L ! 10L∗ it is comparable to that of rich clusters of galaxies. Cannibalism aids the density-luminosity relation, and argues for Nurture, but the conven- tional and sensibile wisdom is that the most luminous galaxies mark the largest primeval density fluctuations, an example of Nurture (in my terminology).
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e Phenomenon, real or virtual Nature Nurture Theory morphology-density relation √ near universal red-sequence √ color-magnitude relation the most luminous galaxies prefer the densest environments, consistent √ with their strong clustering the faintest galaxies & L galaxies → are similarly distributed,∗and they √ √ √ avoid common voids the ΛCDM Cosmic Web defines the voids √ inhabitants of voids & low density regions: grand-design spirals √ close to normal ellipticals √ √ low surface density Lyα absorbers √ the warm-hot IGM √ baryon-free dwarf DM halos √ The ΛCDM Cosmic Web The Cosmic Web
50 × 50 × 15h−1 Mpc
H. Mathis & S. D. M. White, MN 337, 1193, 2002 The Cosmic WEB paradigm is well motivated by the ΛCDM cosmology, which has passed demanding tests. And the Web has proved to be a powerful organizing concept: the galaxy distribution does resemble a sponge.
Catalog of Neighboring Galaxies Karachentsev et al. 2004 But do you see tendrils — streams of dwarfs — running into the Local Tullly Void?
Here the scales of depth and width are about the same. These people found a web of another sort. Marshall L. McCall, York University Preliminary Map of the Local Sheet of Nearby Galaxies, 1999 The de Vaucouleurs Local Supercluster −1 IRAS PSCz galaxies, czLG < 1500 km s Will Saunders et al. (2000) The Large-Scale Supercluster P. A. Shaver, Australian J. Phys. 44, 759 (1991) Another look at the Shaver effect:
Saunders et al. (2000) PSCz galaxies at −1 1500 < vLG < 6000 km s
−1 Abell-Corwin-Olwin clusters with Struble-Rood (1999) redshifts 1000 < czlg < 6000 km s Galaxies at distances < 3 Mpc are aligned with the de Vaucouleurs Local Supercluster at ∼distance 20 Mpc. ∼ At distances 20 < r < 100 Mpc clusters and AGNs are aligned with the Local Superc∼luste∼ r, but the general galaxy population shows little evidence of it. If these observations are not just curious accidents, but rather phys- ically significant, then we may ask how did this Web of AGNs avoid entangling the general galaxy • population? how does this Web of AGNs relate to the Cosmic Web of the ΛCDM • cosmology? The observers’ Cosmic Web is operational and well established. The theorists’ ΛCDM Cosmic Web is an established part of the community belief system, with good reason. But I have not yet seen the evidence that would promote it to an established part of physical reality.
Phenomenon, real or virtual Nature Nurture Theory morphology-density relation √ near universal red-sequence √ color-magnitude relation the most luminous galaxies prefer the densest environments, consistent √ with their strong clustering the faintest galaxies & L galaxies are similarly distributed,∗and they √ √ √ avoid common voids → the ΛCDM Cosmic Web defines the voids √ inhabitants of voids & low density regions: grand-design spirals √ close to normal ellipticals √ √ low surface density Lyα absorbers √ the warm-hot IGM √ baryon-free dwarf DM halos √ Inhabitants of the Voids Void inhabitants: a grand design spiral NGC 6946
Adam Block/NOAO/AURA/NSF
Hα and continuum-free images of the eight dwarf irregular companions of NGC 6946 (Karachentsev et al. 2005)
Catalog of Neighboring Galaxies Karachentsev et al. 2004 Void inhabitants: a grand design spiral NGC 6946
Might the low density around NGC 6946 have suppressed growth of an extended DM halo? Carignan et al. (1990) find a close to flat 21-cm rotation curve with
1 vc = 160 km s− at R < 20 kpc.
The Hα curve extends to about the same radius. 10 At the luminosity of NGC 6946, LB = 3 10 L , I think its circular velocity is below the×template". The rms line-of-sight velocity dispersion of the eight dwarf satellites relative to NGC 6946 is σ = 1 70 km s− , which indicates circular velocity
1 v 100 km s− at R 100 kpc, c " " which again seems low. What is the best probe for more clues to the mass structure: deep 21-cm or Hα? imaging searches for more satellites? weak lensing? Void inhabitants: a grand design spiral NGC 6946
What produced the angular momentum of the isolated spiral NGC 6946? Under the tidal torque picture, ideas are the dwarf companions? but they seem very small; • massive dark companions? but how could a massive neighbor • have remained dark? massive protogalaxies that were nearby at high redshift, before • the void opened up? but in the standard model voids don’t open up so much as fail to produce visible galaxies. Again, more clues to dynamics at larger radii could be very interesting. Center
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Evidence for evolving spheroidals in the Hubble Deep Fields North and South
w F. Menanteau,1 ² R. G. Abraham1,2 and R. S. Ellis1,3 Some ellipticals outside rich clusters have 1Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA 2Astronomy Department, University of Toronto, 60 St George Street, Toronto, ON, M5S 3H8, Canada blue cores. I gather that it’s generally agreed 3Astronomy Department, Caltech, 105-24, Pasadena, CA 91125, USA that this is largely due to young stars in the core. The star formation surely is fueled in part by accretion. But I gather the young stars have high heavy element abundances, which I suppose indicates recycling of gas shed by stars in the galaxy.
Figure 1. The methodology of internal colour variations as applied to two HDF-North spheroidals. The upper panel shows an I 20:48 mag 814 example with low internal scatter at z 0:77 whereas the bottom panel illustrates an I 21:66 example with a bluer core at z 0:48: The 814 pixel-by-pixel colour distributions are shown alongside each case. Coloured dots refer to pixels where the S/N ratio (SNR) is above the adopted threshold. Void inhabitants: early-type galaxies
These elliptical galaxies likely aren’t as isolated as NGC 6946, but they do sample lower density regions.
They show effects of environment — some Reda et al. early-type galaxies are over-luminous for their masses, and some ellipticals have blue cores — but am I wrong to be impressed by the modest extent of these effects?
Phenomenon, real or virtual Nature Nurture Theory morphology-density relation √ near universal red-sequence √ color-magnitude relation the most luminous galaxies prefer the densest environments, consistent √ with their strong clustering the faintest galaxies & L galaxies are similarly distributed,∗and they √ √ √ avoid common voids the ΛCDM Cosmic Web defines the voids √ inhabitants of voids & low density regions: grand-design spirals √ → close to normal ellipticals √ √ low surface density Lyα absorbers √ the warm-hot IGM √ baryon-free dwarf DM halos √ The void inhabitants: Lyα absorbers, but not detectable 21-cm emitters
I understand that 21-cm sources not arguably associated with a galaxy or a system of galaxies are exceedingly rare. McLin, Stocke et al. (2002) show there are void Lyα absorbers, at
12.5 14.5 2 10 < NHI < 10 cm− , that show no conspicuous association with galaxies. So how did the process of formation of these absorbers avoid pro- ducing appreciable numbers of clouds at the higher surface densities and areas that would be detectable as isolated 21-cm sources? And how are these absorbers related to the Lyα forest at z 3, which (I think I remember) shows strikingly few voids? ∼ Void Inhabitants: the warm-hot intergalactic medium
6 1 Plasma at T = 10 ± K is detected in and around groups of galaxies. I suppose the Cen & Ostriker (1999) shock-heating of the IGM tends not to happen in voids, but that plasma heated elsewhere could flow into voids, depending on how strongly the plasma is gravitationally bound to the DM. Might this flow produce void DM-free Lyα absorbers? Might that fit the observations? t O t T s A h h u h r a e g s e t n g m c a e v o
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