Extra-galactic distance through Tip of the Red Giant Branch (TRGB) method

-- with R. Brent Tully (University of Hawaii at Manoa)

The Tip of Red Giant Branch method can be said to be the current gold standard for distance determinations to nearby galaxies. It is both accurate and efficient. Distances of galaxies within 10 Mpc can be obtained with an accuracy of 5% by a single Hubble Space Telescope (HST) orbit. As a comparison, Cepheid period-luminosity relation can measure distances of galaxies farther away with a comparable accuracy, but require multi-epoch observations. Using the TRGB method, we have acquired distances of ~300 galaxies with HST (see Tully et al. 2013 and the Extragalactic Distance Database, EDD), provided a detailed map of 3D structure of the local Universe.

Before, the TRGB method is carried out in optical wavelength. We have extended the TRGB methodology from optical to infrared (Wu et al. 2014). Making the TRGB methodology appicable at infrared enhances the capabiblity of probing objects at low Galactic latitudes, where optical light is heavily attenuated by Galactic dust. With IR TRGB methodology, distances of galaxies in the Zone of Avoidance can also be measured accurately. Furthermore, the next generation space telescope, the James Webb Space Telescope (JWST), works only at infrared wavelength. Our work on IR TRGB will carry this valuable distance determination to the future. Once JWST is operational, the power of TRGB methodology will be enhanced dramatically. Thousands of galaxies will be within reach, allowing accurate distance to be measured to galaxies within the Virgo Cluster and beyond with short exposures.

Star-formation quenching in distant large-scale structures

-- with Roy R. Gal (University of Hawaii at Manoa)

Galaxy clusters and groups are recognized to play a pivot role on galaxy evolution. In present day, galaxies in clusters tend to be passive, with little star formation activites compared to those in the field. This simple observation suggests galaxies in clusters had experienced more rapid evolution and became passive earlier, but how the environment drives galaxy evolutoin still remains unclear. Post-starburst galaxies, whose star-formation activity had shut down recently (within 1 billion years), are ideal proxies tracing the quenching of star formation.

We conduct a data-intensive study of post-starburst galaxies in 20 large-scale structures at z~1, using data of the Observation of Redshift Evolution in Large Scale Environments (ORELSE) Survey (Lubin et al. 2009). For each structure we have >100 spectra of member galaxies from the Keck 10-m telescopes and multi-wavelengh photometry from ground-based and space telescopes (Palomar 5-m, Subaru 8-m, CFHT 3.6-m, UKIRT 3.8-m and Spitzer Space Telescope ). Part of the structures also have resolved images of galaxies from HST, X-ray images from Chandra X-ray Observatory and radio images from the Very Large Array (VLA).

Our first result from a supercluster at z~0.9 shows that, most of post-starburst galaxies are produced by galaxy mergers in groups or while falling into clusters. A minority are resulted from interacting with dense gas in cluster cores, but only in a dynamically evolved cluster. These findings demonstrate how structure formation affects galaxy evolution in high density region of the Universe (Wu et al. 2014). Future analysis on the full sample will draw a more complete picture of the interplay between structure formation and galaxy evolution.

Sub-millimeter interferometric observations of molecular outflows in multiple protostellar systems

-- with Jeremy Lim (University of Hong Kong), Shigehisa Takakuwa (Academia Sinica, Institute of Astronomy and Astrophysics)

Outflows are ubiquitos phenomena in protostellar systems. Using the Sub-millimeter Array (SMA), we had obtained arc-second angular resolution images and the kinematics of CO gas in the central region of nearby multiple protostellar systems. We see the morphology and kinematics of outflows not only correlate with the evolutionary stage of their parent protostars, but also are affected by their neiboring protostars in its parent multiple system (see Wu et al. 2009 for a journal article, also Wu et al. 2009a for a conference poster). Meanwhile, the energetic nature of the outflow can change the properties of its ambient molecular cloud, therefore it can even have an impact on neiboring protostellar systems (Wu et al. 2009b). Our study shows that star formation is a rather complex process in multiple systems than the simple picture in single systems.

This kind of study is only possible with millimeter/sub-millimeter interferometers such as SMA, which provides enough angular resolution. The newly inaugurated interferometer, the Atacama Large Millimeter/sub-millimeter Array (ALMA), is expect to reveal prodigious details of star-forming regions with its unprecedent sensitivity and angular resolution.