Star forming dwarf galaxies

Star forming dwarf galaxies (SFDGs) have a high gas content and low metallicities, reminiscent of the basic entities in hierarchical galaxy formation scenarios. In the young universe they probably also played a major role in the cosmic reionization. Their abundant presence in the local volume and their youthful character make them ideal objects for detailed studies of the initial stellar mass function (IMF), fundamental star formation processes and its feedback to the interstellar medium. Occasionally we witness SFDGs involved in extreme starbursts, giving rise to strongly elevated production of super star clusters and global superwinds, mechanisms yet to be explored in more detail. SFDGs is the initial state of all dwarf galaxies and the relation to the environment provides us with a key to how different types of dwarf galaxies are emerging. In this review we will put the emphasis on the exotic starburst phase, as it seems less important for present day galaxy evolution but perhaps fundamental in the initial phase of galaxy formation.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon, September 2010, Springer Verlag, in pres

COLOR GRADIENTS IN COOLING FLOWS IN CLUSTERS OF GALAXIES

We have obtained U-, b-, V-, and 1-band CCD images of 19 centrally dominant galaxies in cooling-flow clusters (CFDs) and four centrally dominant galaxies in clusters without cooling flows. Using photometry through synthetic elliptical apertures, we find that many CFDs exhibit color-profile anomalies (bluer central colors or positive color gradients) with respect to the control sample and to gE photometry from the recent literature. The anomalies are stronger and extend to larger galactic radii for higher mass accretion rates (m(CF)) as estimated from X-ray observations. At least eight of eleven CFDs with m(CF) greater than or similar to 70 M. yr-1 have blue color-profile anomalies that extend radially 5-20 kpc. Some CFDs with smaller m(CF)s have milder anomalies extending only less than or similar 5 kpc. The color anomalies are probably due to recent star formation induced by the cooling flows. Only one object (NGC 1275) has nuclear color profiles that unambiguously indicate a nonthermal contribution from an active nucleus. Processes related to nuclear activity are apparently not primarily responsible for the extended color anomalies. The correlation with X-ray properties also argues against an origin fueled by gas-rich galaxies in tidally induced starbursts. The prevalance of color anomalies suggests that star formation which includes massive blue stars occurs continuously for a substantial fraction of the Hubble time, or in frequent episodes. However, in no case are the blue color excesses consistent with star formation with the local initial mass function at the rates inferred from the X-ray observations. In addition, the radial extent of the color anomalies are only approximately 5%-10% of the cooling radii. If the X-ray inflow rates are correct, then most of the accreting gas must reside in very low mass stars and/or opaque gas. Despite the harsh cluster environment, dust appears to be common in CFDs. At least approximately 40%-50% of the sample objects show evidence for dust patches or lanes. Seven of the nine objects with dust features also have evidence for star formation. Significant negative color gradients are often observed in the inner approximately 5 kpc of nonaccreting templates and the CFDs without strong anomalies. These are likely due to metallicity gradients originating from dissipative processes during the early stages of galaxy assembly. Beyond approximately 5 kpc the color gradients usually flatten to roughly constant color, suggesting that some combinations of dissipationless infall and collisional processes (stripping, mergers) involving other cluster members is important here. High-frequency structure (e.g., ripples) is often found in the radial color profiles. This may be due to dust or to local age/metallicity inhomogeneities. However, we cannot at present rule out the possibility that incomplete removal of interfering images plays a role. Red upturns in the color profiles are present at large radii (r approximately 7-30 kpc) in approximately 40% of the objects in our sample. These occur at surface brightnesses that are only 10%-30% of the sky background, so they are difficult to establish with certainty. We have been unable to find an instrumental cause for the upturns, and similar effects are present in earlier CCD photometry in the literature. The upturns are often as red as or redder than the metal-rich nuclear stellar populations, so they are unlikely to be the result of stripping or mergers. Their presence in the nonaccreting galaxies suggests that they are not related to the current epoch cooling flows. The flat or redward-rising color gradients in the halos are consistent with high stellar metallicities that extend to large galactic radii. The most intriguing possibility is that the red upturns are related to the dark matter known to be present in clusters from kinematic data

X-RAY EMITTING FILAMENTS IN THE COOLING FLOW CLUSTER-A2029

We present high-resolution X-ray observations of the cluster A2029 with the ROSAT HRI which confirm the presence of a cooling flow, despite the lack of optical line emission or evidence for recent star formation. The cooling rate and radius are M(c) almost-equal-to 370 M. yr-1 and r(c) almost-equal-to 230 kpc, respectively. Emission from the inner cooling flow is dominated by a number of X-ray-emitting filaments. This may be the first case where such inhomogeneities have been clearly resolved. The filaments may be supported in part by magnetic fields and may be connected with the filaments of very strong Faraday rotation seen in several nearly cooling flows

X-RAY AND OPTICAL EMISSION-LINE FILAMENTS IN THE COOLING FLOW CLUSTER 2A 0335+096

We present a new high-resolution X-ray image of the 2A 0335 + 096 cluster of galaxies obtained with the High Resolution Imager (HRI) aboard the ROSAT satellite. The presence of dense gas having a very short cooling time in the central regions confirms its earlier identification as a cooling flow. The X-ray emission from the central regions of the cooling flow shows a great deal of filamentary structure. Using the crude spectral resolution of the HRI, we show that these filaments are the result of excess emission, rather than foreground X-ray absorption. Although there are uncertainties in the pointing, many of the X-ray features in the cooling flow region correspond to features in H-alpha optical line emission. This suggests that the optical emission line gas has resulted directly from the cooling of X-ray-emitting gas. The filament material cannot be in hydrostatic equilibrium, and it is likely that other forces such as rotation, turbulence, and magnetic fields influence the dynamical state of the gas