arXiv subjects

J. Sommer-Larsen

Publications and source records attributed to J. Sommer-Larsen.

24 records · Page 2Linked to original sources

arXiv2003

On the Mass-to-Light ratio and the Initial Mass Function in disc galaxies

A low mass-to-light ratio for the stellar component of spiral galaxies (M/L<1 in the I-band) is advocated by various dynamical arguments and by recent cosmological simulations of the formation of these systems. We discuss this possibility by means of chemo-photometric models for galactic discs, adopting different Initial Mass Functions. We show that a number of "bottom--light" IMFs (namely, with less mass locked in low-mass stars than the standard Salpeter IMF), suggested independently in recent literature, do imply M/L ratios as low as mentioned above, at least for late type spirals (Sbc/Sc). This conclusion still holds when the bulge contribution to mass and light is included. We also predict the typical stellar M/L ratio, and correspondingly the zero-point of the Tully-Fisher relation, to vary considerably with Hubble type (about 0.5-0.7 mag in the red bands, from Sa to Sc type). For some of the bottom-light IMFs considered, the efficiency of metal production tends to exceed what is typically estimated for spiral galaxies. Suitable tuning of the IMF mass limits, post-supernova fallback of metals onto black holes or metal outflows must then be invoked, to reproduce the observed chemical properties of disc galaxies. In the appendix we provide M/L-colour relations to estimate the stellar M/L ratio of a galaxy on the base of its colours, for several IMFs.

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arXiv2002

The Mass to Light ratio and the Initial Mass Function in galactic discs

A low mass-to-light ratio for the baryonic component of spiral galaxies is advocated by a number of dynamical studies and by cosmological simulations of galaxy formation. We discuss the possibility of obtaining low mass-to-light ratios for the stellar component in discs, by changing the Initial Mass Function and the Star Formation History.

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arXiv1999

No nearby counterparts to the moving objects in the Hubble Deep Field

Ibata et al (1999) have recently discovered faint, moving objects in the Hubble Deep Field. The quantity, magnitudes and proper motions of these objects are consistent with old white dwarfs making up the Galactic dark halo. We review a number of ground-based proper motion surveys in which nearby dark halo white dwarfs might be present, if they have the colours and absolute magnitudes proposed. No such objects have been found, whereas we argue here that several times more would be expected than in the Hubble Deep Field. We conclude it is unlikely that hydrogen atmosphere white dwarfs make up a significant fraction of the halo dark matter. No limits can be placed yet on helium atmosphere dwarfs from optical searches.

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arXiv1999

Formation of disk galaxies: feedback and the angular momentum problem

When only cooling processes are included, smoothed-particle hydrodynamical simulations of galaxy formation in a Cold Dark Matter hierarchical clustering scenario consistently produce collapsed objects that are deficient in angular momentum by a factor of about 25 relative to the disks of observed spiral galaxies. It is widely hoped that proper allowance for star formation feedback effects will resolve this discrepancy. We explore and compare the effects of including two different types of feedback event: uniform reheating of the entire Universe to 5e5 K at a redshift z~6, and gas blow-out from pregalactic gas clouds comparable to present-day dwarf galaxies at a somewhat lower redshift. We find that blow-out is far more successful than early reheating, and that it may even be sufficient to solve the angular momentum problem. We see indications that the remaining angular momentum deficit (a factor of 5 in the blow-out models) is due to current limitations in the numerical method (SPH) used in our and other authors' work. Our most successful models distinguish themselves by the fact that a large fraction of the gas is accreted very gradually, in a cooling flow from a surrounding hot phase, rather than by mergers of massive cold clumps.

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arXiv1996

Microlensing, structure of the galactic halo and determination of dark objects' mass function

We study the accuracy of inference of the massive halo objects' (MHO or `Macho') mass function from microlensing events observed toward LMC. Assuming the spatial distribution and kinematics of the objects are known, the slope and the range of the MHO mass function (modeled here by a simple power law) will be possible to determine from 100-1000 detected events if the slope αis in the range [-2.5, -0.5], with the statistical errors reaching their minima at α=-1.5. Outside this range the errors grow rapidly making the inference difficult even at very large numbers of events (N\approx 10000). On the other hand, the average mass of the MHOs will be determined to better than about 30% accuracy from N\approx 100 events for any slope. We find that the accuracy of inference at fixed N will not be strongly affected by the presently available detection efficiencies if the typical MHO masses are in the range indicated by the events detected so far. We also estimate the effects of uncertainty of the MHOs' spatial distribution and kinematics on the determination of their mass function. The halo models considered are all spherical but we allow for various density profiles and a radius-dependent, anisotropic velocity dispersion. We find that, while the mass function slope and range are weakly affected for -2 < α< 0, the error in the average mass due to the halo uncertainties could be reduced to less than 50% only through the detection of about 1000 or more events. Reliable estimates of the halo structure itself [density profile and velocity dispersion profile] can start only at very large numbers of detections (N ~ 10000).

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arXiv1996

A dynamical and kinematical model of the Galactic stellar halo and possible implications for galaxy formation scenarios

We re-analyse the kinematics of the system of blue horizontal branch field (BHBF) stars in the Galactic halo (in particular the outer halo), fitting the kinematics with the model of radial and tangential velocity dispersions in the halo as a function of galactocentric distance r proposed by Sommer-Larsen, Flynn & Christensen (1994), using a much larger sample (almost 700) of BHBF stars. The basic result is that the character of the stellar halo velocity ellipsoid changes markedly from radial anisotropy at the sun to tangential anisotropy in the outer parts of the Galactic halo (r greater than approx 20 kpc). Specifically, the radial component of the stellar halo's velocity ellipsoid decreases fairly rapidly beyond the solar circle, from approx 140 +/- 10 km/s at the sun, to an asymptotic value of 89 +/- 19 km/s at large r. The rapid decrease in the radial velocity dispersion is matched by an increase in the tangential velocity dispersion, with increasing r. Our results may indicate that the Galaxy formed hierarchically (partly or fully) through merging of smaller subsystems - the 'bottom-up' galaxy formation scenario, which for quite a while has been favoured by most theorists and recently also has been given some observational credibility by HST observations of a potential group of small galaxies, at high redshift, possibly in the process of merging to a larger galaxy (Pascarelle et al 1996).

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