Multi-method-modeling of interacting galaxies

Abstract

Abstract. NGC 4449 is an active star-forming dwarf galaxy of Magellanic type. From radio observations, van Woerden et al. (1975) found an extended HI-halo around NGC 4449 which is at least a factor of 10 larger than the optical diameter D25 ≈ 5.6 kpc. Recently, Hunter et al. (1998) discerned details in the HI-halo: a disc-like feature around the center of NGC 4449 and a lopsided arm structure. We combined several N-body methods in order to investigate the interaction scenario between NGC 4449 and DDO 125, a close companion in projected space. In a first step fast restricted N-body models are used to confine a region in parameter space reproducing the main observational features. In a second step a genetic algorithm is applied for a uniqueness test of our preferred parameter set. We show that our genetic algorithm reliably recovers orbital parameters, provided that the data are sufficiently accurate, i.e. all the key features are included. In the third step the results of the restricted N-body models are compared with self-consistent N-body simulations. In the case of NGC 4449, the applicability of the simple restricted N-body calculations is demonstrated. Additionally, it is shown that the HI gas can be modeled here by a purely stellar dynamical approach. In a series of simulations, we demonstrate that the observed features of the extended HI disc can be explained by a gravitational interaction between NGC 4449 and DDO 125. According to these calculations the closest approach between both galaxies happened ∼4 − 6 10 8 yr ago at a minimum distance of ∼25 kpc on a parabolic or slightly elliptic orbit. In the case of an encounter scenario, the dynamical mass of DDO 125 should not be smaller than 10% of NGC 4449's mass. Before the encounter, the observed HI gas was arranged in a disc with a radius of 35-40 kpc around the center of NGC 4449. It had the same orientation as the central ellipsoidal HI structure. The origin of this disc is still unclear, but it might have been caused by a previous interaction.

Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

“…Through the Westerbork HI Survey of Spiral and Irregular Galaxies (WHISP), rotation curves have been measured for a sample of 62 galaxies, finding typical rotational velocities of 20 − 80 km/s on scales of a few disk scale lengths (Swaters et al 2009). In general, a strong variation is found from dwarf to dwarf, and in some cases like NGC4449 (Theis & Kohle 2001) or IC 10 (Ashley et al 2014, the rotation curves have been explained by tidal interactions of disks with other dwarfs. The implications of the large diversity of dwarf galaxy rotation curves has therefore been discussed also in recent studies (Oman et al 2015).…”

FitSKIRT: genetic algorithms to automatically fit dusty galaxies with a Monte Carlo radiative transfer code

“…Genetic algorithms are often applied to optimize noisy objective functions (Metcalfe et al 2000;Larsen & Humphreys 2003). During the past decade, genetic algorithms have become increasingly more popular in numerous applications in astronomy and astrophysics ranging from cosmology and gravitational lens modeling to stellar structure and spectral fitting (Charbonneau 1995;Metcalfe et al 2000;Theis & Kohle 2001;Larsen & Humphreys 2003;Fletcher et al 2003;Liesenborgs et al 2006;Baier et al 2010;Schechtman-Rook et al 2012;Rajpaul 2012a). For a recent overview of the use of genetic algorithms in astronomy and astrophysics, see Rajpaul (2012b).…”

Fitting of dust spectra with genetic algorithms

“…Noyes et al (1997) used the code for optimisation of the derived orbital parameters in their work on a possible planet orbiting ρ CrB. In a much larger context genetic algorithms are also used for example to model the interaction of galaxies (Theis 1998;Theis & Kohle 2001). Metcalfe et al (2000) as well as Mokiem et al (2005) used the optimisation code PIKAIA as an improvement and automatisation of the fitting routines of the respective studied objects.…”