arccos centre of excellence in evolutionary cosmology local group analogs: where are the hvcs?...
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ARCCOSCentre of Excellence in Evolutionary Cosmology
Local Group Analogs: Where are the HVCs?Local Group Analogs: Where are the HVCs?
Centre for Astrophysics & Supercomputing
Swinburne University
Brad Gibson
D.J. Pisano, Ken Freeman, Lister Staveley-Smith, David Barnes, Virginia KilbornTim Connors, Daisuke Kawata
ARCCOSCentre of Excellence in Evolutionary Cosmology
• Motivation
• Observational Search for Intragroup Gas Clouds
• Computational Search for Anomalous Velocity Gas Clouds
• Conclusions
Local Group Analogs: Where are the HVCs?Local Group Analogs: Where are the HVCs?
ARCCOSCentre of Excellence in Evolutionary Cosmology
Motivation (the “higher” goals)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?
ARCCOSCentre of Excellence in Evolutionary Cosmology
Motivation (a different tack)Motivation (a different tack)
• High-Velocity Clouds• The Satellite/Substructure Problem
ARCCOSCentre of Excellence in Evolutionary Cosmology
Galactic HVCs: What? Where? How?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
ARCCOSCentre of Excellence in Evolutionary Cosmology
Local Group HVCs?Local Group HVCs?
Blitz et al. (1999) suggested that most HVCs were associated with low-mass dark matter halos and had d1 Mpc and MHI107 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 D150 - 200 kpc and MHI ≤ 107 M
All of these models associate HVCs with galaxy formation
The latter models associate HVCs with dark matter halos
ARCCOSCentre of Excellence in Evolutionary Cosmology
The Satellite/Substructure ProblemThe 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)
ARCCOSCentre of Excellence in Evolutionary Cosmology
Testing the HypothesisTesting 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.
ARCCOSCentre of Excellence in Evolutionary Cosmology
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.
Our Local Group Analog SurveyOur Local Group Analog Survey
ARCCOSCentre of Excellence in Evolutionary Cosmology
What is a Loose Group?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.
ARCCOSCentre of Excellence in Evolutionary Cosmology
Groups observed: LGG 93, 106, 180, 293, 478,
HIPASS Group 3
Composition: All spiral + irregular galaxiesNo 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
Group PropertiesGroup Properties
ARCCOSCentre of Excellence in Evolutionary Cosmology
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
Parkes Multibeam ObservationsParkes Multibeam Observations
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The LGG 93 GroupThe LGG 93 Group
IC 1914
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New Detections LGG 93New Detections LGG 93
ARCCOSCentre of Excellence in Evolutionary Cosmology
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
Parkes Survey ResultsParkes Survey Results
ARCCOSCentre of Excellence in Evolutionary Cosmology
ATCA ObservationsATCA 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!)
ARCCOSCentre of Excellence in Evolutionary Cosmology
LGG 93 ATCA FollowupLGG 93 ATCA Followup
LGG 93-2
IC 1914
LGG 93-1
LSBG F200-023
AM 0311-492
No HI detections of any HVCs in ATCA data!
ATCA Observations: LGG 93 FollowupATCA Observations: LGG 93 Followup
Optical CounterpartsOptical Counterparts
All but one new source cataloged in NED.
No HI clouds without stars found yet!
MHI/LB = 6.3 MHI = 5 x 107 M
LSBG F200-023
MHI/LB = 5.6
MHI = 7 x 107 M
AM 0311-492
ARCCOSCentre of Excellence in Evolutionary Cosmology
• 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.
ATCA Survey ResultsATCA Survey Results
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)
A Simple CHVC ModelA Simple CHVC Model
As DHWHM decreases, we expect less detections.As the population decreases, we expect fewer detections.
undetecteddetected
Expected Distances
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.
DHWHM < 160 kpc
Combined Constraints for 3 groups
MHI < 4 x 105 M
Blitz et al. 99
BB99
ARCCOSCentre of Excellence in Evolutionary Cosmology
Summary of ObservationsSummary 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.
ARCCOSCentre of Excellence in Evolutionary Cosmology
“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) -
ARCCOSCentre of Excellence in Evolutionary Cosmology
Multi-resolution Cosmological Disks with GCD+Multi-resolution Cosmological Disks with GCD+
Mvir = 6x109 M Vmax = 65 km/s
J-band
High-z snapshot
High Resolution Region mDM=2x105 M, εDM=0.14 kpc, mgas=3x104 M, εgas=0.08 kpc
Kawata & Gibson (2003a,b)
ARCCOSCentre of Excellence in Evolutionary Cosmology
Dark Halo
Anomalous Velocity Gas in Cosmological “Milky Way” Disk Simulations
Anomalous Velocity Gas in Cosmological “Milky Way” Disk Simulations
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• 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
ARCCOSCentre of Excellence in Evolutionary Cosmology
SummarySummary
• 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
ARCCOSCentre of Excellence in Evolutionary Cosmology
SummarySummary
• 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?)
ARCCOSCentre of Excellence in Evolutionary Cosmology