searching for cool neutral meduim in UCHVCs

© Bahar Bidaran

The lack of observed dwarf galaxies compared to predictions for numerous low-mass dark matter halos is a well-known cosmological problem, often referred to as the missing satellites problem. Ultra-compact high velocity clouds (UCHVCs) identified in the ALFALFA HI survey have been presented as potential low mass galaxy candidates in the Local Group due to their single-dish HI properties. We observed some of the UCHVCs with the Westerbork Synthesis Radio Telescope (WSRT) to understand them as potential galaxies. By using the HI kinematics from the WSRT observations, we can constrain the underlying mass distribution, addressing the question of whether these systems represent gas in dark matter halos. We can also study the state of the interstellar medium (ISM) on spatially resolved scales, including searching for the presence of a cool neutral medium (CNM) component, addressing the potential of these systems to form stars and be recognized as ' bona-fid ' galaxies.

To accomplish this aim we have written a code to retrieve kinematical information by fitting both single and double component Gaussians to the resolved HI gas. As a test case, we focused on AGC 249525, which is interesting for showing ordered velocity motion, similar to another good galaxy candidate AGC 198606 (see Daily Image from 19-12-2014). We produced WSRT HI data cube at a resolution of 210" (matched to the Arecibo beam) with a velocity resolution of 4 km/s. We fit Gaussians to the data cube, to produce velocity fields (picture on the left). This velocity field reveals ordered velocity motion with a gradient of ~15 km/s. This can be interpreted as rotation with amplitude of ~12 km/s.

In addition, studying the velocity dispersion on spatially resolved scales allows us to understand the star formation (SF) potential of these systems. The velocity dispersion traces the temperature structure of the gas. We are searching for a cool neutral medium (CNM) component that has the potential to collapse and form stars. A single Gaussian fit (or traditional moment 2 map) reveals no evidence for a CNM component (picture on the right). We are involved in an ongoing work to fit 2 Gaussian components to search robustly for a CNM component.