Tobias Westmeier

Astrophysicist & Radio Astronomer

The high-velocity clouds of M31

A major problem in HVC research is the unknown distance towards most of the HVCs. So far, accurate distance estimates have only been determined for parts of the HVC complexes C, M, and WB based on stellar absorption measurements. Distance, however, is the key parameter to recover important physical properties of HVCs such as their mass, density and size. Moreover, accurate distance measurements for HVCs would provide us with a detailed picture of their radial distribution around the Galaxy, allowing us to constrain the origin of HVCs and to compare their spatial distribution with the results of ΛCDM structure formation scenarios.

One solution to the distance problem is to study the HVC populations of external galaxies. The nearest galaxy with a mass and size comparable to that of the Milky Way is the Andromeda Galaxy (M31). Thilker et al. (2004) used the Green Bank Telescope (GBT) to map an area of 7° × 7° around M31 in the H Ⅰ emission. They discovered a population of HVCs located within a projected distance of about 50 kpc from M31. Models by de Heij, Braun & Burton (2002), however, suggest a spatial distribution of HVCs with a Gaussian dispersion of about 150 to 200 kpc around the Milky Way. Hydrostatic simulations by Sternberg, McKee & Wolfire (2002) also agree with a characteristic distance of 150 kpc. If these models were correct, we would expect to find more HVCs far beyond the boundaries of the area mapped by Thilker et al. (2004).

Effelsberg blind survey of M31

To search for additional HVCs around M31, we conducted an H Ⅰ blind survey of a large region around the Andromeda Galaxy using the 100-m radio telescope at Effelsberg. The survey was performed between July 2003 and July 2004 and covers a total area of 15° × 5° around M31, corresponding to a projected area of about 215 × 65 kpc. Our observations confirmed most of the HVCs discovered near the disk of M31 by Thilker et al. (2004) with the Green Bank Telescope. However, we failed to detect any additional HVCs beyond a projected distance of about 50 kpc from M31 with a 3σ H Ⅰ mass detection limit of 6 × 104 M☉.

Effelsberg H Ⅰ blind survey of M31
Figure 1: Effelsberg H Ⅰ blind survey of M31.

One explanation for our non-detection could be that the proportion of ionised gas in HVCs is increasing with radial distance from M31 so that HVCs outside a 50 kpc radius might not be observable in the 21-cm H Ⅰ line any longer. Another possibility is that HVCs are concentrated in the vicinity of their host galaxies. If we assume that the Milky Way and M31 have similar populations of HVCs, we can use our non-detection to estimate an upper limit for the distance of HVCs from their central galaxies of about 60 kpc. This result places the HVCs much closer to their host galaxies than previously thought and much closer than the 150 kpc suggested by various models and simulations. Our results demonstrate that HVCs, including CHVCs, are forming a circumgalactic population instead of being scattered all over the Local Group.

Interferometric follow-up observations

H Ⅰ column density map of M31 with extra-planar HVCs
Figure 2: H Ⅰ column density map of M31 with extra-planar HVCs.

To further investigate the properties of the HVCs around M31, we performed high-re­so­lu­tion synthesis observations with the Wester­bork Synthesis Radio Telescope (WSRT). The WSRT was pointed at 9 positions showing HVC emission in the GBT data of Thilker et al. (2004). As part of the 2004 ASTRON / JIVE summer school I had the opportunity to reduce and analyse the WSRT data. At an angular resolution of 2 arcmin we detected 16 individual HVCs with typical H Ⅰ masses of a few times 105 M☉ and physical sizes of about 1 kpc. Twelve HVCs are clustered in an area of 1° × 1° close to the disk of M31. We consider these HVCs to form a coherent HVC complex which might represent tidally disrupted gas associated with the giant stellar stream of M31 discovered by Ibata et al (2001). Tidal interaction might also have created the HVC located close to NGC 205. The remaining HVCs around M31 are isolated in position and velocity and may represent primordial, dark-matter-dominated clouds.

Publications

  • Relics of structure formation: extra-planar gas and high-velocity clouds around the Andromeda Galaxy
    Westmeier, T., Brüns, C. & Kerp, J., 2008, MNRAS, 390, 1691
    (ADS | arXiv)
  • Westerbork H Ⅰ observations of high-velocity clouds near M31 and M33
    Westmeier, T., Braun, R. & Thilker, D., 2005, A&A, 436, 101
    (ADS | arXiv)