197,007 research outputs found
Estimating gas accretion in disc galaxies using the Kennicutt-Schmidt law
No full textWe show how the existence of a relation between the star formation rate (SFR) and the gas density, i.e. the KennicuttSchmidt law, implies a continuous accretion of fresh gas from the environment into the discs of spiral galaxies. We present a method to derive the gas infall rate in a galaxy disc as a function of time and radius, and we apply it to the disc of the Milky Way and 21 galaxies from the THINGS sample. For the Milky Way, we found that the ratio between the past and current star formation rates is about 23, averaged over the disc, but it varies substantially with radius. In the other disc galaxies, there is a clear dependence of this ratio on galaxy stellar mass and Hubble type, with more constant star formation histories for small galaxies of later type. The gas accretion rate follows very closely the SFR for every galaxy and it dominates the evolution of these systems. The Milky Way has formed two-thirds of its stars after z = 1, whilst the mass of cold gas in the disc has remained fairly constant with time. In general, all discs have accreted a significant fraction of their gas after z = 1. Accretion moves from the inner regions of the disc to the outer parts, and as a consequence star formation moves inside out as well. At z = 0, the peak of gas accretion in the Galaxy is at about 67?kpc from the centre
Modelling the gas kinematics in disk galaxies
No full textOver the last few years, evidence that nearby spiral galaxies are surrounded by massive halos of cold gas has been accumulating. This extra-planar cold gas, rotating more slowly than the disk gas, is observed in galaxies with a range of different properties (such as mass and star formation rate, SFR) and it appears analogous to the Intermediate and High Velocity Clouds of the Milky Way. Models for the origin of extra-planar gas have been proposed taking into account the effects of supernova feedback (galactic fountain), cooling flow accretion and hydrostatic equilibrium. Several techniques have been used from analytical treatments and ballistic orbit integration to hydrodynamical simulations. I present a new model where a galactic fountain sweeps up ambient medium as it travels through the halo. This seems to give the best results in reproducing the kinematics of the extra-planar gas and it implies a gas accretion rate of the order of the SFR of the host galaxy
Kinematics of the ionised gas in the spiral galaxy NGC 2403
No full textWe present a study of the kinematics of the ionised gas in the nearby spiral galaxy NGC 2403 using deep long-slit spectra obtained with the 4.2-m William Herschel Telescope. The data show the presence of a halo component of ionised gas that is rotating more slowly than the gas in the disk. The kinematics of this ionised halo gas is similar to that of the neutral halo gas. On small scales. broad line profiles (up to 300 km s(-1) wide) indicate regions of fast Outflows of ionised gas. We discuss these new results in the context of galactic fountain models
Efficiency of gas cooling and accretion at the disc-corona interface
No full textIn star-forming galaxies, stellar feedback can have a dual effect on the circumgalactic medium both suppressing and stimulating gas accretion. The trigger of gas accretion can be caused by disc material ejected into the halo in the form of fountain clouds and by its interaction with the surrounding hot corona. Indeed, at the disc-corona interface, the mixing between the cold/metal-rich disc gas (T ≲ 104 K) and the hot coronal gas (T ≳ 106 K) can dramatically reduce the cooling time of a portion of the corona and produce its condensation and accretion. We studied the interaction between fountain clouds and corona in different galactic environments through parsec-scale hydrodynamical simulations, including the presence of thermal conduction, a key mechanism that influences gas condensation. Our simulations showed that the coronal gas condensation strongly depends on the galactic environment, in particular it is less efficient for increasing virial temperature/mass of the haloes where galaxies reside and it is fully ineffective for objects with virial masses larger than 1013 M⊙. This result implies that the coronal gas cools down quickly in haloes with low-intermediate virial mass (Mvir ≲ 3 × 1012 M⊙) but the ability to cool the corona decreases going from late-type to early-type disc galaxies, potentially leading to the switching off of accretion and the quenching of star formation in massive systems
How axi-symmetric is the inner HI disc of the Milky Way?
No full textWe modelled the distribution and the kinematics of HI in the inner Milky Way (R < Ro) at latitude b = 0∘ assuming axi-symmetry. We fitted the line profiles of the LAB 21-cm survey using an iterative approach based on the tangent-point method. The resulting model reproduces the H I data remarkably well, with significant differences arising only for R < 2 kpc. This suggests that, despite the presence of a barred potential, the neutral gas in the inner Milky Way is distributed in a fairly axi-symmetric disc
On the convergence of 3D free discontinuity models in variational fracture
No full textFree discontinuity problems arising in the variational theory for fracture mechanics are considered. A Gamma-convergence proof for a r-adaptive 3D finite element discretization is given in the case of a brittle material. The optimal displacement field, crack pattern and mesh geometry are obtained through a variational
procedure that encompasses both mechanical and configurational forces. Possible extensions to cohesive fracture and quasi-static evolutions are discussed
Gas accretion from minor mergers in local spiral galaxies
No full textWe quantify the gas accretion rate from minor mergers onto star-forming galaxies in the local Universe using Hi observations of 148 nearby spiral galaxies (WHISP sample). We developed a dedicated code that iteratively analyses Hi data-cubes, finds dwarf gas-rich satellites around larger galaxies, and estimates an upper limit to the gas accretion rate. We found that 22% of the galaxies have at least one detected dwarf companion. We made the very stringent assumption that all satellites are going to merge in the shortest possible time, transferring all their gas to the main galaxies. This leads to an estimate of the maximum gas accretion rate of 0.28 M⊙ yr-1, about five times lower than the average star formation rate of the sample. Given the assumptions, our accretion rate is clearly an overestimate. Our result strongly suggests that minor mergers do not play a significant role in the total gas accretion budget in local galaxies. Appendix A is available in electronic form at http://www.aanda.or
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