Sona Ehlerova

HI shells in the outer Milky Way

We present results of a method for an automatic search for Hi shells in 3D data cubes and apply it to the Leiden-Dwingeloo Hi survey of the northern Milky Way. In the 2nd Galactic quadrant, where identifications of structures are not substantially influenced by overlapping, we find nearly 300 structures. The Galactic distribution of shells has an exponential profile in the radial direction with a scale length of σ gsh = 3 kpc. In the z direction, one half of the shells are found at distances smaller than 500 pc. We also calculate the energies necessary to create the shells: there are several structures with energies greater than 10E SN but only one with an energy exceeding 100E SN . Their size distribution, corrected for distance effects, is approximated by a power-law with an index α = 2.1. Our identifications provide a lower limit to the filling factor of shells in the outer Milky Way: f 2D = 0.4 and f 3D = 0.05.

Environmental dependences for star formation triggered by expanding shell collapse

Criteria for gravitational collapse of expanding shells in rotating, shearing galaxy discs were determined using three-dimensional numerical simulations in the thin shell approximation. The simulations were run over a grid of seven independent variables, and the resultant probabilities for triggering and unstable masses were determined as functions of eight dimensionless parameters. When the ratio of the midplane gas density to the midplane total density is small, an expanding shell reaches the disc scaleheight and vents to the halo before it collapses. When the Toomre instability parameter Q, or a similar shear parameter, QA, is large, Coriolis forces and shear stall or reverse the collapse before the shell accumulates enough mass to be unstable. With large values of c 5/( GL ), for rms velocity dispersion csh in the swept-up matter and shell-driving luminosity L, the pressure in the accumulated gas is too large to allow collapse during the expansion time. Considering ∼5000 models covering a wide range of parameter space, the common properties of shell collapse as a mechanism for triggered star formation are: (1) the time-scale is ∼4(csh/2πGρ[ GL ] 0.2 ) 0.5 for ambient midplane density ρ, (2) the total fragment mass is ∼2 × 10 7 M� , of which only a small fraction is likely to be molecular, (3) the triggering radius is ∼2 times the scaleheight, and the triggering probability is ∼0.5 for large OB associations. Star formation triggered by shell collapse should be most common in gas-rich galaxies, such as young galaxies or those with late Hubble types.

Triggered star formation in expanding shells

We discuss fragmentation processes which induce star formation in the dense walls of expanding shells. We test the influence of the energy input, the interstellar medium scaleheight and the speed of sound in the ambient medium, and 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, the shells are unstable. The value of the critical density depends on the energy of the shell and the sound speed in the interstellar medium.

Exploring GLIMPSE bubble N107 - Multiwavelength observations and simulations

Context. Bubble N107 was discovered in the infrared emission of dust in the Galactic Plane observed by the Spitzer Space Telescope (GLIMPSE survey: l ~ 51.0 deg, b ~ 0.1 deg). The bubble represents an example of shell-like structures found all over the Milky Way Galaxy. Aims. We aim to analyse the atomic and molecular components of N107, as well as its radio continuum emission. With the help of numerical simulations, we aim to estimate the bubble age and other parameters which cannot be derived directly from observations. Methods. From the observations of the HI (I-GALFA) and 13CO (GRS) lines we derive the bubbles kinematical distance and masses of the atomic and molecular components. With the algorithm DENDROFIND, we decompose molecular material into individual clumps. From the continuum observations at 1420 MHz (VGPS) and 327 MHz (WSRT), we derive the radio flux density and the spectral index. With the numerical code ring, we simulate the evolution of stellar-blown bubbles similar to N107. Results. The total HI mass associated with N107 is 5.4E3 Msun. The total mass of the molecular component (a mixture of cold gasses of H2, CO, He and heavier elements) is 1.3E5 Msun, from which 4.0E4 Msun is found along the bubble border. We identified 49 molecular clumps distributed along the bubble border, with the slope of the clump mass function of -1.1. The spectral index of -0.30 of a strong radio source located apparently within the bubble indicates nonthermal emission, hence part of the flux likely originates in a supernova remnant, not yet catalogued. The numerical simulations suggest N107 is likely less than 2.25 Myr old. Since first supernovae explode only after 3 Myr or later, no supernova remnant should be present within the bubble. It may be explained if there is a supernova remnant in the direction towards the bubble, however not associated with it.

Correlation of HI shells and CO clumps in the outer Milky Way

HI shells, which may be formed by the activity of young and massive stars, or connected to energy released by interactions of high-velocity clouds with the galactic disk, may be partly responsible both for the destruction of CO clouds and for the creation of others. It is not known which effect prevails. We study the relation between HI shells and CO in the outer parts of the Milky Way, using HI and CO surveys and a catalogue of previously identified HI shells. For each individual location, the distance to the nearest HI shell is calculated and it is specified whether it lies in the interior of an HI shell, in its walls, or outside an HI shell. The method takes into account irregular shapes of HI shells. We find a lack of CO clouds in the interiors of HI shells and their increased occurrence in walls. Properties of clouds differ for different environments: interiors of HI shells, their walls, and unperturbed medium. CO clouds found in the interiors of HI shells are those that survived and were robbed of their more diffuse gas. Walls of HI shells have a high molecular content, indicative of an increased rate of CO formation. Comparing the CO fractions within HI shells and outside in the unperturbed medium, we conclude that HI shells are responsible for approx. 20 % increase in the total amount of CO in the outer Milky Way.

The HI supershell GS061+00+51 and its neighbours

We describe H i observations of a 44 eld in the Milky Way centered on l =6 1, b =0 made by the Eelsberg radiotelescope. The eld contains one previously identied H i supershell, GS061+00+51 (Heiles 1979); apart from it we nd several new structures. We also study the H i distribution in the vicinity of four H ii regions, S86, S87, S88 and S89. We conrm the existence of the shell GS061+00+51, and we nd that it has two smaller neighbours, spherical shells with Rsh 30 pc. We identify at least one more regular shell at vLSR = 18 km s 1 ; and one blown-out shell at vLSR = 54 km s 1 . In two cases we are able to connect H ii regions with features in the H i distribution (S86 and S87), in two other cases no connection is found. Apart from quite regular H i shells we see a number of non-coherent objects, which are probably a result of the turbulence in the interstellar medium.

Triggered Star Formation: From Large to Small Scales

We give examples of star formation triggering. On the large scale, SF is triggered by collisions and interactions of galaxies, and by interactions of a galaxy with the environment in a cluster. On the intermediate scale, SF is triggered in barred galaxies by resonant rings. On the small scale, the star formation is self-regulated by the feedback from young, massive stars.

GS242-03+37: a lucky survivor in the galactic gravitational field

HI shells and supershells, found in discs of many galaxies including our own, are formed by the activity of young and massive stars (supernova explosions and stellar winds), but the formation of these structures may be linked to other energetic events, such as interactions of high-velocity clouds with the galactic disc. The larger structures in particular significantly influence their surroundings; their walls are often places where molecular clouds reside and where star formation happens. We explore the HI supershell GS242-03+37, a large structure in the outer Milky Way. Its size and position make it a good case for studying the effects of large shells on their surrounding. We perform numerical simulations of the structure with the simplified hydrodynamical code RING, which uses the thin-shell approximation. The best fit is found by a comparison with the HI data and then we compare our model with the distribution of star clusters near this supershell. The best model of GS242-03+37 requires, contrary to previous estimates, a relatively low amount of energy, and it has an old age of

Colliding interstellar bubbles in the direction of l = 54

\sim

Expanding shells in low and high density environments

100 Myr. We also find that the distribution of young star clusters (with ages