Accumulating observational evidence for a number of radio galaxies suggests an association between their jets and regions of active star formation. The standard picture is that shocks generated by the jet propagate through an inhomogeneous medium and trigger the collapse of overdense clouds, which then become active star-forming regions. In this contribution, we report on recent hydrodynamic simulations of radiative shock-cloud interactions using two different cooling models: an equilibrium cooling-curve model assuming solar metallicities and a non-equilibrium chemistry model appropriate for primordial gas clouds. We consider a range of initial cloud densities and shock speeds in order to quantify the role of cooling in the evolution. Our results indicate that for moderate cloud densities ( 1 cm −3 ) and shock Mach numbers ( 20), cooling processes can be highly efficient and result in more than 50% of the initial cloud mass cooling to below 100 K. We also use our results to estimate the final H 2 mass fraction for the simulations that use the non-equilibrium chemistry package. This is an important measurement, since H 2 is the dominant coolant for a primordial gas cloud. We find peak H 2 mass fractions of 10 −2 and total H 2 mass fractions of 10 −5 for the cloud gas, consistent with cosmological simulations of first star formation. Finally, we compare our results with the observations of jet-induced star formation in "Minkowski's Object," a small irregular starburst system associated with a radio jet in the nearby cluster of galaxies Abell 194. We conclude that its morphology, star formation rate (∼ 0.3M ⊙ yr −1 ) and stellar mass (∼ 1.2 × 10 7 M ⊙ ) can be explained by the interaction of a ∼ 9 × 10 4 km s −1 jet with an ensemble of moderately dense (∼ 10 cm −3 ), warm (10 4 K) intergalactic clouds in the vicinity of its associated radio galaxy at the center of the galaxy cluster.

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We present results from two-dimensional numerical simulations of the interactions between magnetized shocks and radiative clouds. Our primary goal is to characterize the dynamical evolution of the shocked clouds. We perform runs in both the strong and weak magnetic field limits and consider three different field orientations. For the geometries considered, we generally find that magnetic fields external to, but concentrated near, the surface of the cloud suppress the growth of destructive hydrodynamic instabilities. External fields also increase the compression of the cloud by effectively acting as a confinement mechanism driven by the interstellar flow and local field stretching. This can have a dramatic effect on both the efficiency of radiative cooling, which tends to increase with increasing magnetic field strength, and on the size and distribution of condensed cooled fragments. In contrast, fields acting predominately internally to the cloud tend to resist compression, thereby inhibiting cooling. We observe that, even at modest strengths (β 0 100), internal fields can completely suppress low-temperature (T < 100 K) cooling.

Using three-dimensional, moving-mesh simulations, we investigate the future evolution of the recently discovered gas cloud G2 traveling through the galactic center. We consider the case of a spherical cloud initially in pressure equilibrium with the background. Our suite of simulations explores the following parameters: the equation of state, radial profiles of the background gas, and start times for the evolution. Our primary focus is on how the fate of this cloud will affect the future activity of Sgr A*. From our simulations we expect an average feeding rate in the range of 5 − 19 × 10 −8 M yr −1 beginning in 2013 and lasting for at least 7 years (our simulations stop in year 2020). The accretion varies by less than a factor of three on timescales ≤ 1 month, and shows no more than a factor of 10 difference between the maximum and minimum observed rates within any given model. These rates are comparable to the current estimated accretion rate in the immediate vicinity of Sgr A*, although they represent only a small ( 5%) increase over the current expected feeding rate at the effective inner boundary of our simulations (r = 750R S ≈ 10 15 cm), where R S is the Schwarzschild radius of the black hole. Therefore, the break up of cloud G2 may have only a minimal effect on the brightness and variability of Sgr A* over the next decade. This is because current models of the galactic center predict that most of the gas will be caught up in outflows. However, if the accreted G2 material can remain cold, it may not mix well with the hot, diffuse background gas, and instead accrete efficiently onto Sgr A*. Further observations of G2 will give us an unprecedented opportunity to test this idea. The break up of the cloud itself may also be observable. By tracking the amount of cloud energy that is dissipated during our simulations, we are able to get a rough estimate of the luminosity associated with its tidal disruption; we find values of a few 10 36 erg s −1 .

We have carried out WFPC2 V -and I-band imaging of the young LMC cluster NGC 2157. Construction of a color-magnitude diagram and isochrone fitting yields an age of τ = 10 8 yrs, a reddening E(B − V ) = 0.1 and a distance modulus of 18.4 mag. Our data covers the mass range 0.75 M ⊙ ≤ m ≤ 5.1 M ⊙ . We find that the cluster mass function changes significantly from the inner regions to the outer regions, becoming steeper (larger number of low mass stars relative to high mass stars) at larger radii.The age of NGC 2157 is comparable to its two-body relaxation timescale only in the cluster core. The observed steepening of the mass function at larger radii is therefore most likely an initial condition of the cluster stars. Such initial conditions are predicted in models of cluster star formation in which dissipative processes act more strongly upon more massive stars.

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