Large-scale galactic turbulence: can self-gravity drive the observed H i velocity dispersions?

The apparent correlation between the specific star formation rate (sSFR) and total stellar mass (M ) of galaxies is a fundamental relationship indicating how they formed their stellar populations. To attempt to understand this relation, we hypothesize that the relation and its evolution is regulated by the increase in the stellar and gas mass surface density in galaxies with redshift, which is itself governed by the angular momentum of the accreted gas, the amount of available gas, and by self-regulation of star formation. With our model, we can reproduce the specific SFR− M relations at z ∼ 1-2 by assuming gas fractions and gas mass surface densities similar to those observed for z = 1-2 galaxies. We further argue that it is the increasing angular momentum with cosmic time that causes a decrease in the surface density of accreted gas. The gas mass surface densities in galaxies are controlled by the centrifugal support (i.e., angular momentum), and the sSFR is predicted to increase as, sSFR(z) = (1 + z) 3 /t H0 , as observed (where t H0 is the Hubble time and no free parameters are necessary). In addition, the simple evolution for the star-formation intensity we propose is in agreement with observations of Milky Way-like galaxies selected through abundance matching. At z > ∼ 2, we argue that star formation is self-regulated by high pressures generated by the intense star formation itself. The star formation intensity must be high enough to either balance the hydrostatic pressure (a rather extreme assumption) or to generate high turbulent pressure in the molecular medium which maintains galaxies near the line of instability (i.e. Toomre Q ∼ 1). We provide simple prescriptions for understanding these self-regulation mechanisms based on solid relationships verified through extensive study. In all cases, the most important factor is the increase in stellar and gas mass surface density with redshift, which allows distant galaxies to maintain high levels of sSFR. Without a strong feedback from massive stars, such galaxies would likely reach very high sSFR levels, have high star formation efficiencies, and because strong feedback drives outflows, ultimately have an excess of stellar baryons.

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“…Turbulence may be driven by the energy injection from young stars at high SF intensities (e.g. Agertz et al 2009). …”

Section: Exponents In the Generalizedmentioning

confidence: 99%

On the cosmic evolution of the specific star formation rate

Lehnert

Driel

Tiran

et al. 2015

A&A

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“…Large-scale turbulence probably plays a role in this. The cause of the Hi velocity dispersions is complicated and not well-understood, but turbulence driven by star formation, supernovae, shearing motions and magnetic instabilities all may play a role (Tamburro et al 2009;Agertz et al 2009). …”

Section: Hi Velocity Dispersionmentioning

confidence: 99%

The dark matter halo shape of edge-on disk galaxies

O’Brien¹,

Freeman²,

Kruit³

2010

A&A

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This is the third paper in a series in which we attempt to put constraints on the flattening of dark halos in disk galaxies. We observed for this purpose the Hi in edge-on galaxies, where it is in principle possible to measure the force field in the halo vertically and radially from gas layer flaring and rotation curve decomposition respectively. For this purpose we need to analyse the observed XV diagrams in such a way as to accurately measure all three functions that describe the planar kinematics and distribution of a galaxy: the radial Hi surface density, the rotation curve and the Hi velocity dispersion. In this paper, we first present the results of the modelling of our Hi observations of 8 Hi rich, late-type, edge-on galaxies. We find that in all of these we find differential rotation. Most systems display Hi velocity dispersions of 6.5 to 7.5 km s −1 and all except one show radial structure in this property. There is an increase in the mean Hi velocity dispersion with maximum rotation velocity, at least up to 120 km s −1 . Next we analyse the Hi observations to derive the radial variation of the thickness of the Hi layer. The combination of these gas flaring measurements with the Hi kinematics measurements allow us to measure the total vertical force of each galaxy assuming hydrostatic equilibrium. We find that with the exception of the asymmetric IC5052, all of the galaxies in our sample are good candidates for 3D mass modelling to measure the dark halo shape. The flaring profiles are symmetric with respect to the galactic centres and have a common shape, increasing linearly inside the stellar disks and exponential outside where the gravitational potential is dominated by the dark halo. In the best example, UGC 7321, we find in the inner regions small deviations from the midplane and accompanying increases in thickness of the Hi layer that are possibly a result of perturbations of the gravitational field by a relatively strong bar.

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“…In our model the gravitational interaction between the gas and the potential of the arms would be responsible for sheared motions that further evolve into turbulence. The main difference between both is that the gravitational collapse drives motions at small scales (∼ λJeans), with subsonic/transonic motions (Agertz et al 2009), while the gas-arm interaction drives supersonic turbulence at large scales. Also, the gravitational collapse drives coherent inward motions that may develop into chaotic motions after the complex interaction between collapsed structures.…”

Section: Discussionmentioning

confidence: 99%

“…In such a scenario multiple and discontinous arms are formedand destroyed -in relatively short timescales. Also, dense and cold regions would form, scarcely distributed in the disk, as a consequence of the gravitational collapse of a cooling ISM (Agertz et al 2009). The relative motion between the clouds and the arms would be reduced compared to the standing shock wave model.…”

Section: Discussionmentioning

confidence: 99%

The onset of large-scale turbulence in the interstellar medium of spiral galaxies

Falceta‐Gonçalves¹,

Bonnell²,

Kowal³

et al. 2014

Monthly Notices of the Royal Astronomical Society

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Turbulence is ubiquitous in the interstellar medium (ISM) of the Milky Way and other spiral galaxies. The energy source for this turbulence has been much debated with many possible origins proposed. The universality of turbulence, its reported largescale driving, and that it occurs also in starless molecular clouds, challenges models invoking any stellar source. A more general process is needed to explain the observations. In this work we study the role of galactic spiral arms. This is accomplished by means of three-dimensional hydrodynamical simulations which follow the dynamical evolution of interstellar diffuse clouds (∼ 100cm −3 ) interacting with the gravitational potential field of the spiral pattern. We find that the tidal effects of the arm's potential on the cloud result in internal vorticity, fragmentation and hydrodynamical instabilities. The triggered turbulence result in large-scale driving, on sizes of the ISM inhomogeneities, i.e. as large as ∼ 100pc, and efficiencies in converting potential energy into turbulence in the range ∼ 10 to 25 % per arm crossing. This efficiency is much higher than those found in previous models. The statistics of the turbulence in our simulations are strikingly similar to the observed power spectrum and Larson scaling relations of molecular clouds and the general ISM. The dependency found from different models indicate that the ISM turbulence is mainly related to local spiral arm properties, such as its mass density and width. This correlation seems in agreement with recent high angular resolution observations of spiral galaxies, e.g. M51 and M33.

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Large-scale galactic turbulence: can self-gravity drive the observed H i velocity dispersions?

Cited by 127 publications

References 67 publications

On the cosmic evolution of the specific star formation rate

On the cosmic evolution of the specific star formation rate

The dark matter halo shape of edge-on disk galaxies

The onset of large-scale turbulence in the interstellar medium of spiral galaxies

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