The engine of spiral galaxy evolution is star formation, which drives the cycling of matter from ISM to stars and back again. The major sources of feedback into the ISM are massive OB stars, which inject energy via Lyman continuum photons, stellar winds and, finally, supernova explosions. In the MW about half of these explosions are correlated in space and time, generating superbubbles (SBs) typically 100-1000 pc in extent. SB interiors are generally more efficient than individual SNRs in converting thermal into kinetic energy of expansion, because radiative losses are smaller in SBs than for the same number of isolated explosions. Shocks in colliding SNR and SB shells produce high-density layers that fragment into clouds which generate new sites of star formation. Thus, SNRs and SBs are among the prime drivers controlling the morphology and the evolution of the ISM. Observing their properties is crucial to understanding the galactic matter cycle. Both SNRs and SBs radiate copiously at energies of 0.5-2.0 keV, a range that is difficult to study globally in the MW because in the disk an intervening absorbing column density of a few ~1021 cm-2 is reached within less than a kiloparsec, blocking all soft X-ray emission from more distant sources. The multiwavelength image in the Figure below illustrates the rich diversity of diffuse structures present in M33.
This false-color image of M33 suggests the rich structures seen in other wavelength bands:
red = continuum-subtracted Halpha, from warm ionized gas (KPNO Schmidt telescope); green = near-UV, primarily from young stars (Galex); blue = HI, from cool neutral gas (VLA+GBT). ChASeM33 covers approximately the region inside the white circle to give complementary data for the hot gas in M33.
M33 is an ideal laboratory for studying the hot diffuse ISM, not only because of its proximity and orientation, but also because it has low Galactic foreground absorption.
ChASeM33 covers a significant fraction of the disk of M33 to address the following questions:
1) Is the diffuse hot ISM shaped predominantly by SNRs or by SBs? 2) What is the volume filling factor of the hot ISM? 3) Is there a typical pattern in the morphology (i.e., what is the average distance between SBs)? 4) What are the respective size distributions of the SNRs and SBs? Is there a radial gradient in the size distribution? Does size correlate with the gravita
tional potential? What is the fraction of SNR mergers? 5) Are bubbles on average too small for their energy input, as has been claimed for the Large Magellanic Cloud?
In order to measure the properties of the diffuse emission, it is essential to measure and remove point sources to high accuracy in order to avoid contamination. The high angular resolution of Chandra is crucial to our study.
Survey details
ChASeM33 is designed to cover the central region of M33 at a resolution of about 5" in order to resolve the source confusion in the inner galaxy found in earlier surveys, and to increase the number of known associations between X-ray sources and objects identified at other wavelengths. For that we chose seven overlapping ACIS-I fields, arranged such that all regions within a galactocentric radius of 4kpc lie no farther than 8' off-axis in one or more fields. The figure below shows the fields covered by ChASeM33.
Field of views of ChASeM33 pointings are plotted as 8' and 5' circles, respectively, on a three color rgb-image of M33.
Our program was designed to comprise two long (100 ks each) uninterrupted observations per field, separated by about six months to allow us to search for time variability of some of the sources. ChASeM33 observations began in September 2005 and ended in November 2006. The total exposure time of the whole survey amounts to 1.4 million seconds which is equal to 16 days of non-stop observation.
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