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Local Group Analogs: Where are the HVCs?

Local Group Analogs: Where are the HVCs?. Brad Gibson. Centre for Astrophysics & Supercomputing Swinburne University. D.J. Pisano, Ken Freeman, Lister Staveley-Smith, David Barnes, Virginia Kilborn Tim Connors, Daisuke Kawata. Local Group Analogs: Where are the HVCs?. Motivation

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Local Group Analogs: Where are the HVCs?

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  1. Local Group Analogs: Where are the HVCs? Brad Gibson Centre for Astrophysics & SupercomputingSwinburne University D.J. Pisano, Ken Freeman, Lister Staveley-Smith, David Barnes, Virginia KilbornTim Connors, Daisuke Kawata

  2. Local Group Analogs: Where are the HVCs? • Motivation • Observational Search for Intragroup Gas Clouds • Computational Search for Anomalous Velocity Gas Clouds • Conclusions

  3. Motivation (the “higher” goals) • Is galaxy assembly/formation still occurring today? • Specifically, are HI clouds around galaxies and in galaxy groups associated with galaxy formation? • What do these data say about models of galaxy formation in groups of galaxies? • What do these data say about the evolution of galaxies in different environments?

  4. Motivation (a different tack) • High-Velocity Clouds • The Satellite/Substructure Problem

  5. Galactic HVCs: What? Where? How? • kinematics inconsistent with Galactic rotation • no stellar component • unknown distances (Chris Thom) • large sky covering factor • origin scenarios: fountains, tidal debris, building blocks

  6. Local Group HVCs? Blitz et al. (1999) suggested that most HVCs were associated with low-mass dark matter halos and had d1 Mpc and MHI107 M Braun & Burton (1999) suggested that just the compact HVCs had these properties de Heij et al. (2002) proposed the CHVCs are concentrated around the Milky Way and M31 with a Gaussian distance distribution with D150 - 200 kpc and MHI ≤ 107 M All of these models associate HVCs with galaxy formation The latter models associate HVCs with dark matter halos

  7. The Satellite/Substructure Problem Cold Dark Matter models of galaxy formation predict 300 dark matter halos in the Local Group, most with velocity widths less than 20 km/s, but only 20 satellites known. Is the Local Group typical? Could the remaining DM halos be associated with HVCs or are they devoid of baryons? Gill, Knebe & Gibson (2003)

  8. Testing the Hypothesis If CHVCs are associated with the “missing satellite” dark matter halos and group/galaxy formation, then we should see them in other groups. If CHVCs are tidal debris, galactic ejecta, or something else, then they will not be so ubiquitous.

  9. Our Local Group Analog Survey Since HVCs could be a group phenomenon, perhaps we should look for them in groups. We observed the entire area of 6 spiral-rich, loose groups with the Parkes Multibeam and ATCA.

  10. What is a Loose Group? • A working definition is: • A grouping of a few large galaxies and tens of smaller ones, with the large galaxies typically separated by 100-200 kpc. They are of order 1 Mpc in total extent. • The Local Group of galaxies is a loose group. • Of particular interest are analogs to the Local Group which are dominated by spiral galaxies. • We observed 6 such groups with Parkes and ATCA.

  11. Group Properties Groups observed: LGG 93, 106, 180, 293, 478, HIPASS Group 3 Composition: All spiral + irregular galaxies No X-ray emission. Group distances: 10.6 - 13.4 Mpc Number of galaxies in optically defined group: 3 - 9 Group velocity dispersions: 30 - 300 km/s Average diameter of groups: 1.6 Mpc Average galaxy-galaxy separation: 550 kpc

  12. Parkes Multibeam Observations Groups observed: LGG 93, 106, 180, 293, 478, HIPASS Group 3 Area observed: 1-2 Mpc2 ≈ 25 - 35 sq. deg. Velocity Resolution: 1.65 or 3.3 km/s Velocity Coverage: 1700 - 3400 km/s Spatial Resolution: 14´ = 50 kpc Mass sensitivity (1MHI over 3.3 km/s): 5 - 8 x 105 M

  13. The LGG 93 Group IC 1914

  14. New Detections LGG 93

  15. Parkes Survey Results 110 HI-rich objects detected towards the 6 groups 67 HICAT sources in the same fields 64 HI-rich objects in the 6 groups 33 optically cataloged group galaxies

  16. ATCA Observations Purpose: Confirm detections, ID optical counterparts, find confused sources Number of sources: 110 potential galaxies Area observed: 30´ FOV = 100 kpc Velocity Resolution: 1.65 or 3.3 km/s (same as Parkes) Velocity Coverage: 1700 km/s centered on galaxy Spatial Resolution: 2´ ≈ 7 kpc Mass sensitivity (1MHI over 3.3 km/s): 5 - 8 x 105 M (Same as Parkes Observations!)

  17. LGG 93 ATCA Followup

  18. ATCA Observations: LGG 93 Followup LGG 93-1 AM 0311-492 LGG 93-2 LSBG F200-023 IC 1914 No HI detections of any HVCs in ATCA data!

  19. LSBG F200-023 AM 0311-492 MHI = 5 x 107 M MHI = 7 x 107 M MHI/LB = 6.3 MHI/LB = 5.6 Optical Counterparts All but one new source cataloged in NED. No HI clouds without stars found yet!

  20. ATCA Survey Results • Detected all known optically-cataloged group galaxies (33 galaxies). • 64 HI detected galaxies in the 6 groups, 40% of which were not detected in HIPASS. • All but 2 of 110 galaxies detected with Parkes are confirmed with ATCA. All detections have optical counterparts. HI masses are 107-109 M. • No HI clouds without stars found!!! • Two Parkes sources found to be pairs of galaxies.

  21. undetected detected A Simple CHVC Model If CHVCs are associated with the formation of the Local Group or the galaxies therein, then they should be in other groups. (Pisano et al. 2004, ApJ, 610, L17) As DHWHM decreases, we expect less detections. As the population decreases, we expect fewer detections.

  22. DHWHM = 50 - 500 kpc For DHWHM = 50 kpc, average CHVC MHI = 1.5 x 105 M. For DHWHM = 200 kpc, average CHVC MHI = 6 x 105 M. For DHWHM = 500 kpc, average CHVC MHI = 1 x 107 M. We do NOT detect average CHVC analogs, but only the most massive ones. If population of CHVC analogs is different, this model may not be valid. Expected Distances

  23. Blitz et al. 99 BB99 Combined Constraints for 3 groups MHI < 4 x 105 M DHWHM < 160 kpc

  24. Summary of Observations Non-detections imply HVCs must be within DHWHM < 160 kpc at 95.45%confidence level. Average CHVC MHI < 4 x 105 M Total HI Mass in CHVCs < 1 x 108 M Therefore, There is very little NEUTRAL gas around the Milky Way to participate in galaxy formation. Yet, it can still provide fuel for star formation. May be plenty of ionised gas! Furthermore, Unless MDM/MHI = 100, the total mass in CHVCs is much less than the total mass in the Milky Way or M31, and they are dynamically unimportant.

  25. “This is a great piece of science. I pray that after this paper, the number of papers you get claiming to "disprove" the Blitz et al. model of HVCs will drop to zero. It's the best argument against it yet, and it distills all the other, weaker arguments.”- Bob Benjamin (referee) -

  26. Multi-resolution Cosmological Disks with GCD+ Kawata & Gibson (2003a,b) High-z snapshot High Resolution Region mDM=2x105 M, εDM=0.14 kpc, mgas=3x104 M, εgas=0.08 kpc J-band Mvir = 6x109 MVmax = 65 km/s

  27. Anomalous Velocity Gas in Cosmological “Milky Way” Disk Simulations Dark Halo

  28. fraction of sky covered by anomalous high-velocity gas comparable to Milky Way • distribution of velocities also comparable to Milky Way • by analogy with simulation, conclude that HVC gas is at d<150 kpc

  29. Summary • Observational • There are many detections of HI clouds around individual galaxies innearby Local Group analogs, but none of these can be unambiguouslyassociated with galaxy formation no HI clouds without stars down to MHI < 5x106 M • Non-detections imply HVCs must be within 150 kpc at the 95% c.l.Very little neutral gas remaining for galaxy assembly, however theremay be a significant reservoir of ionised gas. original Blitz et al. (1999) and Braun & Burton (1999) models are ruled out

  30. Summary • Computational • Low density environment cosmological N-body + Hydrodynamical simulations of L* disk galaxies naturally yield anomalously high-velocity neutral hydrogen gas clouds with spatial and kinematical properties comparable to those of the Milky Way’s HVC population (Tim Connors’s thesis) • The galactocentric distances to these simulated HVCs is 10-50 kpc, with a tail out to <150 kpc (consistent with above observational program and direct distance constraints from Chris Thom’s thesis) • Under the assumption of collisional ionisation equilibrium (no doubt flawed), an associated population of ionised OVI anomalous velocity clouds are distributed over the range 30-400 kpc, but the sky covering fraction as a function of limiting OVI column density is 2 orders of magnitude lower than observed (resolution-related issues? Conduction fronts? Photoionisation?)

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