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The Search for HVCs in Other Galaxies

Next: HVC Analogs Up: Minutes of the 1st Previous: Ionization

Minutes of the talk on :
The Search for HVCs in Other Galaxies

Mary Putman (U. Colorado)

A more detailed version of this talk by the speaker is available in HTML, PDF or PPT format.

The principal questions I want to ask are:

Are HVCs a common feature of star forming galaxies?
Are there intergalactic HI clouds in the Local Group?

HVCs as Local Group building blocks, at distances up to a Mpc

Consider the typical properties of the smallest HVCs (compact HVCs) found in HIPASS:

Distance HI Mass HI diameter

At 5 kpc meager objects ~116 solar masses 60 pc

At 50 kpc 1.15 x 10⁴ solar masses 600 pc

At 1 Mpc 5 x 10⁶ solar masses 6 kpc

Distance	HI Mass	HI diameter
At 5 kpc	meager objects ~116 solar masses	60 pc
At 50 kpc	1.15 x 10⁴ solar masses	600 pc
At 1 Mpc	5 x 10⁶ solar masses	6 kpc

That makes them vastly different objects at the different distances. Establishing reliable distances is critical to the understanding of HVCs.

What do we know about HVC distances?

Direct distances (background stars) only in the big complexes in range 5-10 kpc.
H-alpha emission measurements
- Ionizing radiation from MW could be detected if HVC within 100 kpc of MW.
- Detection proves closer distances.
Searches in nearby groups:
- Sculptor
  - Putman et al. (2003) found none to 10⁷ solar masses.
  - 2% of Sculptor to 3 x 10⁶ solar masses by de Blok et al. (2001) found none.
- Centaurus: Zwaan et al. (2001) found none.
Constraints by Lya absorbers
Ionization constraints

HVC's do not fit into the likely Local Group model, but they do fit into a paradigm where they are located in the halo of the Milky Way. They could be infalling fuel for star formation.

Bryan & Norman suggest that the HVC's represent the smooth accretion of intergalactic gas
Harding et al. suggest that they trace minor mergers and satellite accretion.
Some of the clouds may be galactic fountain material.

Known examples

High velocity gas around other galaxies can be identified from maps and dynamics:

Certainly seen around MW (Putman et al. 2003)
Accretion in M81 group: Yun et al
Leo ring
M101 (van der Hulst & Sancisi)
M83 (Park et al.)
FCC35 (Putman et al. 1998)
NGC 2442 (Ryder et al. 2001)
NGC 2403 (Fraternali et al. 2002); anomolous velocities
Look at Halpha around galaxies
NGC 1533: clearly HVC's around it (Ryan-Weber et al. 2003)

Questions to be answered:

Is there a relation between the presence of HVCs and the star formation rate in the galaxy?
What type of environments are galaxies with HVCs found in?

Do the galaxies with HVCs always have nearby companions?

How are HVCs related to absorbers which are also commonly found in galaxy halos

How are HVCs related to the OVII and lower column density Lyalpha absorbers which are often 100s of kpc from a galaxy?

How do HVCs fit into the CDM galaxy formation paradigm?

What would an ALFA survey find?

We can use the northern part of HIPASS extensions (decl +2 to +25^o). Based on the number of sources in the southern HIPASS zone, there should be about 900 sources. If we stick to the inner zone, say cz < 2000 km/s, there should be about 300 sources. Note that the ALFA resolution gives 10 kpc at 10 Mpc. For a survey of high sensitivity mapping of the sample assuming an expected 30 km/s linewidth, the resulting sensitivity would be N_H ~ 10¹⁹ cm^-2. Such a survey would provide information of the location and frequency of HVCs.

Questions/Discussion:

Riccardo	Have you calculated the required integration times?
Mary	I haven't worked it all out yet.