Highlights and Conclusions of the Colloquium (.pdf) - Chalonge ...

More documents

Recommendations

Info

FIG. 5: Universality of the average surface density (and gravity) of baryons within the halo core radius (plotted against B-band absolute magnitude; from Gentile et al. 2009). Different symbols indicate different galaxy subsamples. Gentile, G., Burkert, A., Salucci, P., Klein, U., & Walter, F. 2005, ApJ, 634, L145 Gentile, G., Salucci, P., Klein, U., & Granato, G. L. 2007, MNRAS, 375, 199 Gentile, G., Famaey, B., Zhao, H., & Salucci, P. 2009, Nature, 461, 627 Governato, F., et al. 2010, Nature, 463, 203 Kormendy, J., & Freeman, K. C. 2004, Dark Matter in Galaxies, 220, 377 Mashchenko, S., Couchman, H. M. P., & Wadsley, J. 2006, Nature, 442, 539 Milgrom, M. 1983, ApJ, 270, 365 Sellwood, J. A., & McGaugh, S. S. 2005, ApJ, 634, 70 Spano, M., Marcelin, M., Amram, P., Carignan, C., Epinat, B., & Hernandez, O. 2008, MNRAS, 383, 297 van Eymeren, J., Trachternach, C., Koribalski, B. S., & Dettmar, R.-J. 2009, A&A, 505, 1
E. Chanda J. Jog Indian Institute of Science, Bangalore 560012, India Tracing the dark matter halos of galaxies by modeling the observed HI data Abstract We use the observed rotation curves and the HI vertical scaleheight data as simultaneous constraints to obtain the density profile and the shape of the dark matter halos in galaxies, and show these to be varied. A galaxy is modeled as a gravitationally coupled star-gas system in the field of the dark matter halo. This approach is applied to three galaxies: the Milky Way, M31 and UGC 7321, a low surface brightness galaxy. The resulting dark matter profile is not universal. Introduction In a typical spiral galaxy like our Galaxy, stars constitute ∼ 90% of the visible mass while the rest is in the interstellar gas. Due to its lower dispersion, gas is important for the disk dynamics despite its lower mass content. Further, the interstellar atomic hydrogen gas (HI) extends 2-3 times farther out than the stellar disk, hence it is an excellent tracer of dynamics in the outer regions of a galaxy. The observed rotation curve has been routinely used in the literature to study the radial mass distribution within a galaxy, and it is well-known that the dark matter progressively dominates in the outer parts. However, the shape of the dark matter halo is not well-studied. We use the observed vertical thickness of the HI gas distribution as an additional, complementary constraint to model the vertical density distribution and hence the shape of the halo. Calculations and Results A galactic disk is supported vertically by pressure, and the balance of self-gravity and pressure decides its thickness. This was studied for a one-component, gravitating isothermal disk in a classic paper by Spitzer (1942). However, a real galaxy consists of stars and gas, where the gas gravity could play a crucial role in determining the vertical distribution since the gas lies closer to the mid-plane. To study this, we have developed a model where stars and gas are taken to be gravitationally coupled, and are embedded in the field of a rigid dark matter halo (Narayan & Jog 2002). We solve the equations of hydrostatic equilibrium along z for stars and gas and the joint Poisson equation together to obtain a self-consistent solution for the vertical disk density distribution. The coupled, second-order differential equations are solved numerically and iteratively. A general four-parameter dark-matter halo profile as motivated from the dynamical studies of elliptical galaxies (de Zeeuw & Pfenniger 1988) is used where the four parameters are: ρ0, the central density; Rc, the core radius; q, the vertical to planar axis ratio; and p, the power-law density index. The grid of halo parameters is scanned systematically. For different trial halo parameters, we obtain the rotation curve and the vertical scaleheights at different radii using our model. We then compare these with the observations to get the best-fit halo parameters. This approach has been applied to study three galaxies and the results are summarized in Table 1. For the nearby Andromeda galaxy, our best-fit model gives an isothermal, flattened halo with an axis ratio of 0.4 (Banerjee & Jog 2008). This lies at the most oblate end of the distribution obtained from cosmological simulations. For the low surface brightness, superthin galaxy UGC 7321, we find that the best-fit halo-core radius is comparable to the stellar disk scalelength. Thus the dark matter halo dominates the dynamics even at small radii in a LSB galaxy (Banerjee, Matthews & Jog 2010). For the Galaxy, the best-fit to the HI data indicates a spherical halo with a density falling faster than for an isothermal case (Narayan, Saha & Jog 2005). Thus, the dark matter halo profile in spiral galaxies does not appear to be universal. The three crucial parameters that determine the disk vertical HI distribution are the disk-to-halo mass ratio, the gas velocity dispersion, and the shape of the halo. Our work underlines the potential of this approach for further systematic study of the dark matter halo parameters in different galaxy types. For this we need data for outer-galactic HI scaleheights & gas dispersion - this remains a challenging task as was already pointed out by Sancisi & Allen (1979) in their study of NGC 891. The shape of the dark matter halo in the disk plane is also an important parameter in the study of galaxy formation and evolution. However, this has not been studied much so far. Recent, detailed Leiden/Argentine/Bonn HI survey of the Milky Way Galaxy shows the gas thickness to be azimuthally asymmetric with the flaring being much higher in the North. We assume the gas to be in a hydrostatic equilibrium, and model the above asymmetry of thickness
Page 1 and 2: HIGHLIGHTS and CONCLUSIONS of the C
Page 3 and 4: FIG. 2: Photo of the Group
Page 5 and 6: II. PROGRAMME AND LECTURERS • Pet
Page 7 and 8: B. Alfonso Cavaliere & Andrea Lapi
Page 9 and 10: C. Hector J. de Vega LPTHE, CNRS/Un
Page 11: D. Gianfranco Gentile University of
Page 15 and 16: F. Paolo Salucci SISSA, Trieste, It
Page 17 and 18: FIG. 8: Dark matter central surface
Page 19 and 20: 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0
Page 21 and 22: invoking SNRs, pulsars have the big
Page 23 and 24: with mSI/ √ αSI as the energy sc
Page 25 and 26: J. Janine van Eymeren Universität
Page 27 and 28: K. Markus Weber Institut für Exper
Page 29 and 30: ] -1 velocity [km s 400 350 300 250
Page 31 and 32: • None of the predictions of wimp
Page 33 and 34: Penarrubia et al. 2007, 2008; Wolf
Page 35 and 36: man (Kentucky), Clayton Heller (Sta
Page 37 and 38: decreases with increasing mass of t
Page 39 and 40: Moiseenko, B. Nath, G. Pavalas, E.
Page 41: PILO Luigi University of L’Aquila

Highlights and Conclusions of the Colloquium (.pdf) - Chalonge ...

Create successful ePaper yourself

Delete template?

Save as template?