Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userFragmentation of Expanding Shells in Spiral and Irregular Galaxies
69
0
0.0
(
0
)
Authors
S. Ehlerova
Ask ChatGPT about the research
No Arabic abstract
Conditions for the fragmentation of expanding shells due to gravitational instability are discussed. The self-similar analytical solution is compared with the results of 3-dimensional computer simulations for the expansion into homogeneous media. For realistic galactic disks we show that the formation of fragments is influenced by the amount of energy supply from the final number of young stars in an OB association, the value of the sound speed, the stratification and density of the ambient medium, the galactic differential rotation and the gravitational force perpendicular to the galactic plane. The typical size of gravitationally unstable shells is 1 kpc for an ambient gas density n=1 cm^-3. In thick disk galaxies the fragmentation occurs in nearly the whole shell while in thin disks it is restricted to the galactic equator. Unstable fragments may become molecular and trigger the formation of molecular clouds, and finally new star formation. We conclude that in dwarf galaxies the star formation may propagate in all directions turning the system into a star- burst. Contrary to that, the star formation in spiral galaxies propagates only in some directions in a thin strip near the symmetry plane, basically at the tips of the expanding shell.
rate research
Read More
66 -
S. Ehlerova
, J. Palous (Astronomical Institute
, Academy of Sciences ofn the Czech Republic
2000
We discuss the induced star formation in dense walls of expanding shells. The fragmentation process is studied using the linear perturbation theory. The influence of the energy input, the ISM distribution and the ISM speed of sound is examined analytically and by numerical simulations. We formulate the universal condition for the gravitational fragmentation of expanding shells: if the total surface density of the disk is higher than a certain critical value, shells are unstable. The value of the critical density depends on the energy of the shell and the sound speed in the ISM.
We discuss fragmentation processes which induce star formation in dense walls of expanding shells. The influence of the energy input, the ISM scale-height and speed of sound in the ambient medium is tested. We formulate the condition for the gravitational fragmentation of expanding shells: if the total surface density of the disc is higher than a certain critical value, shells are unstable. The value of the critical density depends on the energy of the shell and the sound speed in the ISM.
The initial cluster mass function (ICMF) is a fundamental property of star formation in galaxies. To gauge its universality, we measure and compare the ICMFs in irregular and spiral galaxies. Our sample of irregular galaxies is based on 13 nearby galaxies selected from a volume-limited sample from the fifth data release of the Sloan Digital Sky Survey (SDSS). The extinctions, ages, and masses were determined by comparing their ugiz magnitudes to those generated from starburst models. Completeness corrections were performed using Monte Carlo simulations in which artificial clusters were inserted into each galaxy. We analyzed three nearby spiral galaxies with SDSS data in exactly the same way to derive their ICMF based on a similar number of young, massive clusters as the irregular galaxy ICMF. We find that the ICMFs of irregular and spiral galaxies for masses >3x10^4 M_sun are statistically indistinguishable. For clusters more massive than 3x10^4 M_sun, the ICMF of the irregular galaxies is reasonably well fit by a power law dN(M)/dM ~ M^-a_M with a_M = 1.88 +/- 0.09. Similar results were obtained for the ICMF of the spiral galaxy sample but with a_M = 1.75 +/- 0.06. We discuss the implications of our result for theories of star cluster formation: the shape of the ICMF appears to be independent of metallicity and galactic shear rate.
We perform numerical hydrodynamic modeling of various physical processes that can form an HI ring as is observed in Holmberg I. Three energetic mechanisms are considered: multiple supernova explosions (SNe), a hypernova explosion associated with a gamma ray burst (GRB), and the vertical impact of a high velocity cloud (HVC). The total released energy has an upper limit of 10^54 ergs. We find that multiple SNe are in general more effective in producing shells that break out of the disk than a hypernova explosion of the same total energy. As a consequence, multiple SNe form rings with a high ring-to-center contrast K<100 in the HI column density, whereas single hypernova explosions form rings with K<10. Only multiple SNe can reproduce both the size (diameter ~1.7 kpc) and the ring-to-center contrast (K ~ 15-20) of the HI ring in Hoolmberg I. High velocity clouds create HI rings that are much smaller in size (< 0.8 kpc) and contrast (K < 4.5) than seen in Holmberg I. We construct model position-velocity (pV) diagrams and find that they can be used to distinguish among different HI ring formation mechanisms. The observed pV-diagrams of Holmberg I are best reproduced by multiple SNe. We conclude that the giant HI ring in Holmberg I is most probably formed by multiple SNe. We also find that the appearance of the SNe-driven shell in the integrated HI image depends on the inclination angle of the galaxy. In nearly face-on galaxies, the integrated HI image shows a ring of roughly constant HI column density surrounding a deep central depression, whereas in considerably inclined galaxies (i > 45 deg) the HI image is characterized by two kidney-shaped density enhancements and a mild central depression.
W49A is a giant molecular cloud which harbors some of the most luminous embedded clusters in the Galaxy. However, the explanation for this starburst-like phenomenon is still under debate. Methods. We investigated large-scale Spitzer mid-infrared images together with a Galatic Ring Survey 13CO J = 1-0 image, complemented with higher resolution (~ 11 arcsec) 13CO J = 2-1 and C18O J = 2-1 images over a ~ 15 x 13 pc^2 field obtained with the IRAM 30m telescope. Two expanding shells have been identified in the mid-infrared images, and confirmed in the position-velocity diagrams made from the 13CO J = 2-1 and C18O J = 2-1 data. The mass of the averaged expanding shell, which has an inner radius of ~ 3.3 pc and a thickness of ~ 0.41 pc, is about 1.9 x 10^4 M*. The total kinetic energy of the expanding shells is estimated to be ~ 10^49 erg which is probably provided by a few massive stars, whose radiation pressure and/or strong stellar winds drive the shells. The expanding shells are likely to have a common origin close to the two ultracompact Hii regions (source O and source N), and their expansion speed is estimated to be ~ 5 km/s, resulting in an age of ~ 3-7 x 10^5 years. In addition, on larger (~ 35 x 50 pc^2) scales, remnants of two gas ejections have been identified in the 13CO J = 1 - 0 data. Both ejections seem to have the same center as the expanding shells with a total energy of a few times 10^50 erg. The main driving mechanism for the gas ejections is unclear, but likely related to the mechanism which triggers the starburst in W49A.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
