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Cosmic Voids and Void Galaxies

Cosmic Voids and Void Galaxies. Michael S. Vogeley Department of Physics Drexel University vogeley@drexel.edu. KIAS Workshop , October 27-28, 2008. SDSS galaxy map. Voids as Cosmic Laboratories. Voids form by gravity through expansion of initially underdense regions

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Cosmic Voids and Void Galaxies

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  1. Cosmic Voids and Void Galaxies Michael S. Vogeley Department of Physics Drexel University vogeley@drexel.edu KIAS Workshop , October 27-28, 2008

  2. SDSS galaxy map

  3. Voids as Cosmic Laboratories • Voids form by gravity through expansion of initially underdense regions • Voids grow more spherical with time • Matter falls outward onto dense filaments and walls • Velocity field is radial and regular (smooth) • CDM theory predicts many low mass dark matter clumps (halos) in voids. Are these found? • Galaxy formation connects mass to light • Dark matter halos collapse • Baryons cool and fall onto halos • Stars form and cause energy feedback (supernovae) • Black holes form, feed on gas, and cause feedback • Galaxy interactions both stimulate and disrupt SF

  4. Finding Voids in Galaxy Redshift Surveys Hoyle & Vogeley 2002, ApJ 566, 641 (PSCz, CfA) Hoyle & Vogeley 2004, ApJ 607, 751 (2dfGRS) Pan, Hoyle, & Vogeley 2008, in prep (SDSS)

  5. The Void Finder Algorithm Goal: identify large voids that are dynamically distinct elements of large-scale structure = “bucket-shaped” voids with flat density profiles and sharp edges, dr/r<-0.8 Procedure: • Initial classification of galaxies as wall/void galaxies • Detection of empty cells • Growth of maximal spheres • Unification of overlapping voids • Calculation of void underdensity, profile Hoyle & Vogeley 2002, ApJ, 566, 641

  6. Initial Classification of Void vs. Wall Galaxies If d3>7h-1 Mpc, then dr/r<-0.6 7h-1 Mpc 7h-1 Mpc WallGalaxy Void Galaxy Then grow maximal spheres bounded by wall galaxies

  7. VoidFinder Works in Simulations • VoidFinder applied to “galaxies” in simulations • Radii of voids match sharp boundaries of voids seen in both galaxies and dark matter • Density within voids nearly constant, reaches mean at nearly twice void radius Benson, Hoyle, Torres, & Vogeley 2003, MNRAS 340, 160

  8. Voids in the PSCz Survey Yellow – voids Red – void centers Blue – wall galaxies Largest void = 17.85 Mpc/h Average (of rmin = 10 Mpc/h) = 12.4 +/- 1.7 Nearly identical results for UZC. Same voids found in IR-selected PSCz and optically-selected UZC. Hoyle & Vogeley 2002, ApJ, 566, 641

  9. Voids in the 2dFGRS Red – void centers Black – wall galaxies 289 voids in total (zmax=0.1) Largest void radius =19.85 Mpc Average(r > 10) =12.4 +/- 1.9 Mpc Similar to PSCz and UZC results Hoyle & Vogeley 2004, ApJ, 607, 751

  10. Voids in SDSS: intersection with 10 h-1 Mpc Slices Pan, Hoyle, Vogeley 2008

  11. Density Profiles Observed voids show “bucket shape” profile with sharp edges, as predicted by linear gravitational theory. Linear Theory (Sheth & van de Weygaert) SDSS Voids

  12. Voids in SDSS DR5 Parent galaxy sample: r<17.77, z<0.107, area = 5000 sq deg, 394,984 galaxies (after boundary cuts) Volume-limited sample: density field from 61,084 galaxies, M<-20.0 Results of voidfinder: 617 voids R>10 Mpc/h 40,635 void galaxies r<17.77 (10% of galaxies are in voids) Pan, Hoyle, Vogeley 2008

  13. Filling Factor of Different Void Sizes Voids with effective radii ~14-22 Mpc/h fill most of void volume. Total filling factor is 50% for voids with average underdensity =-0.8 (20% of cosmic mean). Void volume per bin of void radius

  14. Very Prolate Very Oblate Shapes of Voids: Oblate or Prolate? Fit triaxial ellipsoid to each void, with axes a, b, c (largest to smallest) More very prolate voids (c/b>0.75) than very oblate. Very prolate voids are found to be caused by overlap of pairs of nearly spherical sub-voids. Same behavior in mock surveys from Park & Kim and no significant variation from real to redshift space.

  15. Alignment of Voids We find no statistically significant alignment of the principal axis of voids with the line of sight in observations or simulations (neither real nor redshift space). SDSS voids Simulations: Real Space Redshift Space Mock surveys by Park & Kim

  16. Summary of Void Properties • Void distributions of 2dFGRS, PSCz, UZC, and SDSS agree; void properties are robust • No detected evolution of void size with redshift in nearby universe • Voids are on average ~12 h-1 Mpc in radius, but largest void larger in larger surveys • Density profiles plateau in center to dr/r = -0.95 • Density profiles reveal sharp void edges, as predicted from gravitational instability • Filling factor 40% at density contrast dr/r = -0.9for dr/r = -0.8 • Voids with effective radii 14-22 h-1 Mpc occupy most of void volume • Oblate or prolate: very slight tendency towards prolate • Alignment: not statistically significant • Evidence of redshift space effects? Not so far… • Voidfinder appears to detect dynamically distinct elements of large-scale structure (peaks in initial gravitational potential, outflows in velocity, sharp boundaries in density)

  17. Properties of Void Galaxies Earlier papers on photometric and spectroscopic properties of SDSS void galaxies found that void galaxies are fainter, bluer, more disk-like, and have higher specific star formation rates. See Rojas, Vogeley, & Hoyle 2004, ApJ, 617, 50 (Photometric Properties) Rojas, Vogeley, & Hoyle 2005, ApJ, 624, 571 (Spectroscopic Properties) Hoyle, Rojas, & Vogeley 2005, ApJ, 620, 618 (Luminosity Function) Newer evidence places these trends in the context of a Morphology-Luminosity-Local Density Relation that extends from clusters deep into voids and reveals a few residual environmental effects peculiar to voids.Park, Choi, Vogeley, Gott, & Blanton 2007, ApJ, 658, 898

  18. Morphology-Luminosity-Local Density Relation At fixed L, morphology is a strong function of density At fixed density, morphology is a strong function of L

  19. Color-magnitude relations • Early type “red sequence” shifts blueward by 0.025 mag from high to low density • Late type “blue sequence” shifts blueward by 0.14 mag at low density

  20. Size and environment • Small monotonic dependence of size on local density for all galaxies except the brightest E’s. • Galaxies in voids are slightly smaller: M=-19.7 galaxies are 8% smaller at the lowest density, for both early and late types (but possible sky subtraction error?).

  21. Star formation rates (Ha) Early Late At fixed morphology (early vs. late) and luminosity, very small dependence of SFR on local density: log[W(Ha)]=1.466-0.046 log(1+d) for M=-18.9 galaxies, weaker for brighter galaxies

  22. Results on Environmental Dependence • Strong local density-morphology-luminosity relation. Environment matters down to very low density! • At fixed luminosity and morphology, properties of M<-19 galaxies show only weak dependence on density • Residual effects at low density • Color-magnitude shifts blueward, particularly for late types • Sizes of galaxies are smaller • Star formation in late types is higher • Morphology-density relation steepens for faint galaxies (for smoothing ~12 h-1 Mpc)

  23. Mergers and Interactions in Voids: rare events in voids are clean test of galaxy formation/interaction models • Close Encounters of the First Kind (possible tidal interaction) • Close Encounters of the Second Kind (close tidal interaction) • Close Encounters of the Third Kind (merged) White, Pan, Hoyle, & Vogeley (2008)

  24. Interacting Void Galaxies Tidal interaction Overlap Merged

  25. Close Encounters of the First Kind

  26. Close Encounters of the Second Kind Strong star formation in tidally-disrupted void galaxies

  27. Close Encounters of the Third Kind Recently merged void galaxies show evidence of nuclear activity (LINER, Transition spectral types?)

  28. Yes, there are Active Galactic Nuclei in voids! Constantin & Vogeley 2006, ApJ 650, 727 Constantin, Hoyle, & Vogeley 2008, ApJ 673, 715

  29. Seyferts LINERs Transition objects H II nuclei emission-line Finding AGN among SDSS galaxies 20% of all galaxies are strong line-emitters 52% H II 20% LINERs 7% Transition obj’s 5% Seyferts (Constantin & Vogeley 2006)

  30. AGN of all types in voids and walls Constantin, Hoyle, & Vogeley 2008 “AGN in Void Regions”

  31. fractions Compare at fixed brightness in voids in walls AGN in Voids: Populations (Constantin, Hoyle, & Vogeley 2007) AGN of all types exist in voids: • No AGN in bright (L>>L*) void galaxies • Seyferts more frequent among Mr ~ -20 mag void galaxies • Otherwise very similar AGN occurrence rate for S’s, L’s, and T’s (but not HII’s!)

  32. Log L[O I]/4 Log L[O III]/4 in walls in voids AGN in voids: accretion activity Seyferts LINERs • Accretion rates may be lower in void AGN…lower fueling rate? • Void galaxies have higher SFR per unit mass(Rojas, Vogeley, Hoyle 2005) • Gas available for forming stars but not driven efficiently to nucleus? (Constantin, Hoyle, & Vogeley 2007)

  33. AGN in voids: local environment* *as measured by distance to nearest bright (M<-19.5) galaxy in volume-limited sample In voids: HII’s have the closest nn, LINERs have the farthest nn LINERs most isolated In walls: LINERs have closest nn HI’s have the farthest nn HII’s most isolated HII’s in poor groups? LINERs not driven by close interactions (Constantin, Hoyle, & Vogeley 2007)

  34. higher peaks in the density field are more clustered Seyferts&H II’s: - less massive Dark Matter halos LINERs: - more massive Dark Matter halos MDM halo ~ MBH (Ferraresse 2002, Baes et al. 2003) MBHSeyferts < MBHLINERs AGN Clustering (Constantin & Vogeley 2006) H IIs:s0= 5.7  0.2 h-1 Mpc Seyferts:s0= 6.0  0.6 LINERs:s0= 7.3  0.6 All galaxies: s0= 7.8  0.5 Absolute Magnitude limited samples, -21.6 <M < -20.2

  35. Results on Void AGN • All types of AGN are found in voids, though 50% fewer LINERs and 50% more HII’s • No AGN in the brightest void galaxies • Excess of Seyferts in ~L* void galaxies • Possible lower accretion rate in void AGN • Local environments (nearest neighbor) of AGN types are opposite in voids/walls • LINERs more clustered than Seyferts. HII’s least clustered of all.

  36. ALFALFA HI Survey Optical HI-only Both Haynes (2008) Does the Void Phenomenon indicate an intrinsic failing of the CDM model? • Puzzle: Suppression of star formation in voids is required to match CDM theory to observation, but void galaxies actually have higher star formation rates than elsewhere. • Hypothesis: Higher star formation rate in void galaxies occurs because void galaxies retain a reservoir of baryons that continues to feed star formation long after it shuts down in denser regions. • No evidence for “failed” galaxies in voids. Extensive searches for HI emission find gas-rich optically-detected void galaxies but no evidence for dark objects at the masses predicted by CDM.

  37. Publications about nothing Methods for void finding VoidFinder (Hoyle & Vogeley 2002, 2004) Comparison of void finders (Colberg et al. 2008) Statistics and Properties of Voids Void Probability Function (Hoyle & Vogeley 2004) Void shapes and alignments (Pan, Hoyle, Vogeley 2008) Properties of observed void galaxies Photometry (Rojas, Vogeley, Hoyle 2004) Spectroscopy (Rojas, Vogeley, Hoyle 2005) Luminosity Function (Hoyle, Rojas, Vogeley 2005) Mass function (Goldberg et al. 2005) AGN in voids (Constantin, Hoyle, & Vogeley 2008) Environmental dependence (Park, Choi, Vogeley, Gott, & Blanton 2007) Mergers in voids (White, Vogeley, Pan, Hoyle 2008) Simulations of void galaxies N-body + Semi-Analytic Models (Benson et al. 2003) Specialized void simulations (Goldberg & Vogeley 2004)

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