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Circumgalactic Medium (CGM) and the Large-scale Distribution of Cool Gas


Circumgalactic medium (CGM) refers to the gas halo surrounding galaxies, contains signatures of key processes in galaxy formation and evolution, such as accretion and outflows, and is believed to account for the majority of the baryons in the Universe. Originally proposed to explain the absorption lines detected in bright quasars by John Bahcall and Lyman Spitzer (1969), the idea has gone through a revolution in the last few years.


The progress has been made on two fronts. On the one hand, the new Cosmic Origins Spectrograph (COS) onboard the Hubble telescope drastically increases the throughput and sensitivity of HST in the UV wavelength region and allows UV observations of large samples of pre-selected quasars behind known galaxies (Tumlinson et al. 2011). On the other hand, the increasingly large samples of ground-based, high-redshift galaxies/quasars with spectroscopic data from programs with the 8-/10-meter telescopes (e.g., Keck, Steidel et al. 2010, VLT, Bordoloi et al. 2011) and the SDSS III/IV BAO surveys (e.g., Zhu et al. 2014) now allow the detection of weak absorption signals in composite spectra that are orders of magnitude weaker than the strong absorption lines in individual spectra. As an example, the figure below shows the galaxy-gas (LRG-Mg II) correlation by cross-correlating one million LRG locations from SDSS-III with Mg II absorption in 100 thousands of quasars spectra from SDSS-I/II (Zhu et al. 2014). We detected Mg II around LRGs over all scales up to 20 Mpc (and beyond) and there is no sudden break at any given scale.


Our results show that even around massive quiescent galaxies (LRGs), there is a considerable amount of cool gas. The COS-Halos program also showed similar results around less massive quiescent galaxies at lower redshifts (e.g., Thom et al. 2012, Werk et al. 2014). Moreover, what is most puzzling is the kinematics of this cool gas. The figure below shows that the velocity dispersion of the cool gas clouds traced by Mg II around LRGs is about half of velocity dispersion expected from gravitational potential and virial theorem given the host dark matter halo mass (10^13.5 solar mass). Similar results were also obtained from COS observations for less massive galaxies at lower redshifts (e.g., Stocke et al. 2013).


The fast development in this new field raises more questions than answers. One good example is that why the cool gas around quiescent galaxies, which is moving slower than expected from virial theorem, does not fall in onto the galaxies and feed more star formation. The goal eventually is to form a comprehensive understanding of the cosmic baryon cycle, i.e., how galaxies obtain gas, form stars, and return baryons into the CGM. From an observer’s perspective, there are three fundamental observables:


  • Composition, or what is in the CGM

  • Kinematics, or the motion of the gas and its implications for where the gas came from and where it is going

  • Distribution, or where are different components located


The HST COS is a unique (and irreplaceable) instrument for detailed observations of multi-phase gas at low redshift. The next-generation dark energy surveys (SDSS-IV/eBOSS, DESI, PFS, LSST, WFIRST) will provide an unprecedentedly large amount of data. To extract physical information from the unstructured data, it requires creative thinking and we are actively developing innovative techniques that will maximize the potential of the dark energy surveys for astrophysical implications.


★ This work has been funded by the NSF grant AST-1109665 and #HST-HF2-51351

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