Hi! I'm currently a UC Chancellor's Fellow at the University of California, Riverside (UCR) and the University of California, Irvine (UCI). My main research interests focus on infrared galaxies, interacting galaxies, active galactic nuclei, broad line region physics, and the broader field of galaxy formation and evolution. And really, where did we, our planet, our star, and our galaxy come to be the way we are today?
Department of Physics and Astronomy
4129 Frederick Reines Hall
University of California
Irvine, CA 92697
vivianu [at] uci [dot] edu
Office: FRH 2154
UC Chancellor's Fellow, UC Riverside / UC Irvine 2015 - 2017
TMT Postdoctoral Scholar, UC Riverside 2012 - 2014
Ph.D. in Astronomy, Institute for Astronomy, University of Hawaii at Manoa 2012
M.S. in Astronomy, Institute for Astronomy, University of Hawaii at Manoa 2008
B.S. in Astrophysics, California Institute of Technology 2006
Publications and Curriculum Vitae
Here is my CV in pdf format: cv_vu.pdf (Updated Aug 2017)
I am interested in galaxy formation and evolution in the very broad sense, particularly via infrared galaxies, galaxy mergers, and AGN.
1. Local Luminous Infrared Galaxies
Star formation has been occurring ever since the universe was only about 600 million years old, but it did not reach its peak activity level until 3 billions years later. Why star formation took place so rapidly during that epoch has been nebulous. The present paradigm of galaxy evolution suggests that the key lies in a group of objects discovered by the Infrared Astronomical Satellite due to their extreme brightness in the infrared: the luminous and ultraluminous infrared galaxies (LIRGs: LIR[8-1000um] > 1011 Lsun). Though relatively rare locally, (U)LIRGs are responsible for much of the star formation during the peak of star formation history (e.g. Le Floc’h et al. 2005). Therefore, knowledge of the processes that drive this enormous energy in (U)LIRGs is pivotal to comprehend the evolution of galaxies as part of NASA’s Cosmic Origins Program.
Global properties provide a basic understanding of these (U)LIRGs as a whole, including how luminous, star-forming, and massive they are. To this end, we have taken new optical images and, combined with multi-wavelength data from the X-ray to the radio bands in the literature, constructed spectral energy distributions which we then modeled to derive various physical properties (U et al. 2012). We found the mean dust temperature to be ~33.2K, about 10K cooler than previous measurements, dust mass Mdust = 2×107 M⊙, and stellar mass M⋆ = 6×1010 M⊙. Using different indicators, we also found that ~60% of the ULIRGs and 25% of the LIRGs are classified to host active galactic nuclei (AGN). The compiled photometry along with the derived physical quantities provide an important benchmark of (U)LIRGs quoted in detailed studies of nearby interstellar medium and compact starbursts properties (Kamenetzky et al. 2014; Murphy et al. 2013) as well as surveys of high-redshift galaxies (Willott et al. 2013; Wang et al. 2013).
1.2 Nuclear Disks and Black Hole Masses
- Medling, U et al. "Stellar and Gaseous Nuclear Disks Observed in Nearby (U)LIRGs." 2014, ApJ, 784, 70
- Medling, U et al. "Following Black Hole Scaling Relations through Galaxy Mergers." 2015b, ApJ, 803, 61
An understanding of the global properties of the (U)LIRGs prompts detailed investigations on the nature of black hole accretion and star formation in their nuclei that power their enormous luminosities. We have undertaken a high-resolution observing campaign using the near-infrared integral-field spectrograph OSIRIS with adaptive optics on the Keck Telescopes to study gas and stellar dynamics in the central regions. We discovered rotating gas and stellar disks (~200 pc in size) in nearly all 17 of our galaxies (Medling, U et al. 2014), which provided a means to model the Keplerian motion of material around the black hole and infer its mass (Medling, U et al. 2015b). Black hole masses are found to be in the range 0.3–1×109M?, lying on or above scaling relations that link black hole masses and galaxy bulge properties. The implication is that black holes grow rapidly during the early stages of mergers, outpacing its corresponding bulge formation.
1.3 AGNs and Molecular Outflows
Our observations further enabled several case studies where we disentangle the nuclear physical conditions and explore the link between AGNs and starburst as powering engines in (U)LIRGs. Analyzing H2, Brg, and coronal line [Si VI] in AGN-host ULIRG Mrk 273, we observed outflowing molecular gas from what was thought as a quiescent nucleus, adding it to the small list of spectroscopically confirmed nearby dual AGNs (U et al. 2013). In IRAS F17207, we identified shocked gas originating from outflows or collision between two gaseous star-forming disks (Medling, U et al. 2015a). Our H2 survey has found small-scale outflows associated with shocked molecular gas from elevated H2/Brg ratios and H2 velocity dispersion in the nuclei (U et al. in prep). We have instigated more integral-field programs to explore the small- and large-scale gas dynamics in (U)LIRGs that are at different stages of interaction in order to trace outflows through the merging process. These insights advance our understanding of the interplay between stars, gas, and black holes as the smoking gun signature for large-scale galactic winds that play a key role in regulating star formation within galaxies and enriching the circumgalactic medium.
1.4 The Morphology and Kinematics of Cold Molecular Gas in (U)LIRGs
- Wang, J. et al. "Fast and Furious: Shock Heated Gas as the Origin of Spatially Resolved Hard X-ray Emission in the Central 5 kpc of the Galaxy Merger NGC 6240." 2014, ApJ, 785, 55
- Tan, Q.; Gao, Y.; Wang, Z.; U, Vivian. "High-resolution SMA imaging of (ultra)-luminous infrared galaxies." 2012, IAUS, 284, 471
- U, et al. "High-resolution Mapping of CO(3-2) in NGC 6240." 2011, ASPC, 446, 97
Molecular gas plays an important role in fueling nuclear activity in extreme environments in the nuclei of (U)LIRGs, but its morphology and correlation to stars and dust have been difficult to distinguish given the lack of high-resolution submillimeter observations previously. Our SMA campaign to build on the archival work of Wilson et al. (2008) and improve on sample size and spatial resolution has been motivated to determine the role of mergers in the state of the nuclear CO gas and in triggering nuclear starbursts and/or AGN. The CO gas emission in NGC 6240 has been highlighted as a special case for the interesting structure that it displays.
1.5 Other Selected Publications from the Great Observatory All-Sky LIRG Survey
- Larson, K. et al. "Morphology and Molecular Gas Fractions of Local Luminous Infrared Galaxies as a Function of Infrared Luminosity and Merger Stage." 2016, ApJ, 825, 128
- Davies, R.; Medling, A.; U, Vivian; et al. "Reconstructing merger timelines using star cluster age distributions: the case of MCG+08-11-002." 2016, MNRAS, 458, 158
- Stierwalt, S. et al. "Mid-infrared Properties of Nearby Luminous Infrared Galaxies. II. Probing the Dust and Gas Physics of the GOALS Sample." 2014, ApJ, 790, 124
- Stierwalt, S. et al. "Mid-infrared Properties of Nearby Luminous Infrared Galaxies. I. Spitzer Infrared Spectrograph Spectra for the GOALS Sample." 2013, ApJS, 206, 1
- Mazzarella, J. et al. "Investigation of Dual Active Nuclei, Outflows, Shock-heated Gas, and Young Star Clusters in Markarian 266." 2012, AJ, 144, 125
- Iwasawa, K.; Sanders, D. B.; Teng, S. H.; U, Vivian; et al. "C-GOALS: Chandra observations of a complete sample of luminous infrared galaxies from the IRAS Revised Bright Galaxy Survey." 2011 A&A, 529, 106
- Iwasawa, K. et al. "The location of an active nucleus and the soft X-ray shadowing by a tidal tail in the ULIRG Mrk 273." 2011, A&A, 528, 137
- Petric, A. et al. "Mid-Infrared Spectral Diagnostics of Luminous Infrared Galaxies." 2011, ApJ, 730, 28
- Haan, S. et al. "The Nuclear Structure in Nearby Luminous Infrared Galaxies: HST NICMOS Imaging of the GOALS Sample." 2011, AJ, 141, 100
- Inami, H. et al. "The Buried Starburst in the Interacting Galaxy II Zw 096 as Revealed by the Spitzer Space Telescope." 2010, AJ, 140, 63
- Howell, J. et al. "The Great Observatories All-Sky LIRG Survey: Comparison of Ultraviolet and Far-Infrared Properties." 2010, ApJ, 715, 572
- Armus, L. et al. "GOALS: The Great Observatories All-sky LIRG Survey." 2009, PASP, 121, 559
2. Reverberation Mapping of Nearby Seyfert I Galaxies: LAMP 2016
The technique of reverberation mapping has been used to estimate virial black hole masses and, more fundamentally, to probe the broad line region structure in Seyfert I galaxies. Efforts from the previous Lick AGN Monitoring Project (LAMP) campaigns and other studies to date have culminated in a large sample of reverberation mapped AGNs and measurements of their black hole masses, which in turn enabled major improvement to various AGN scaling relations. However, the high-luminosity end of such relations remains poorly constrained; this is because of observational challenges presented by the weaker continuum flux variations and longer time dilation in these sources. To this end, we have initiated a new LAMP2016 campaign to target AGNs with luminosities of 10^44 erg/s, with predicted H-beta lags of ~20 - 60 days or black hole masses of 10^7 - 10^8.5 Msun. Designed to monitor ~20 AGNs biweekly from April 2016 till May 2017 with the Kast spectrograph on the 3-m Shane Telescope at Lick Observatory, we aim to probe luminosity-dependent trends in broad line region structure and dynamics, improve calibrations for single-epoch estimates of high-redshift quasar black hole masses, and test photoionization models for the radially-stratified structure of the broad line region.
3. Lyman Alpha Emitting sources at 3 < z < 7
- U et al. "A Correlation Between Lyα Spectral Line Profile and Rest-Frame UV Morphology." 2015, ApJ, 815, 57
High-redshift star-forming galaxies are often probed using Lyα 1216 emission as the hydrogen line is redshifted into visible wavelengths easily accessible from ground-based optical telescopes. The spectral profile of Lyα is indicative of the origin and escape path of Lyα photons, as well as the extinction by neutral hydrogen and dust in the inter-stellar medium (Tapken et al. 2007; Yamada et al. 2012). Here we explore the relationship between the spectral shape of the Lyα emission and the UV morphology of the host galaxy using a sample of 304 Lyα-emitting BV i-dropouts at 3 < z < 7 in the GOODS and COSMOS fields. Using our extensive reservoir of high-quality Keck DEIMOS spectra combined with HST WFC3 data, we measure the Lyα line asymmetries for individual galaxies and compare them to axial ratios measured from observed J- and H-band (restframe UV) images. We find that the Lyα skewness exhibits a large scatter at small elongation (a/b < 2), and this scatter decreases as axial ratio increases. Comparison of this trend to radiative transfer models and various results from literature suggests that these high-redshift LAEs are not likely to be intrinsically round and symmetric disks, but they probably host galactic outflows traced by Lyα emitting clouds. The ionizing sources are centrally located, with the optical depth a good indicator of the absorption and scattering events on the escape path of Lyα photons from the source. Our results found no evidence for evolution in Lyα asymmetry or axial ratio with look-back time.
4. Thirty Meter Telescope: Infrared Multiobject Spectrograph
- Check out the IRMS webpage!
- Try out the preliminary ETC for IRMS here! (Start from the README file in the tarball; contact me if you have any questions.)
The nature of high-redshift galaxies is best probed by large, spectroscopic surveys of the deep fields; a suitable instrument would be an infrared multi-object spectrograph on a large telescope, like the IRMS on TMT. As a core member of the Science Team working closely with Project Scientist Dr. Bahram Mobasher, I have developed its exposure time calculator to quantify the sensitivity of the instrument as aided by NFIRAOS, the adaptive optics systems on the TMT, in order to optimize its design parameters. The experience from making contributions to the IRMS Operation Concept Design Document as well as the TMT Detailed Science Cases (as an ISDT member) is excellent preparation for the era of next-generation instrumentation.
5. Galaxies at Intermediate Redshifts
As an undergrad, I did my senior thesis with Dr. Richard Ellis and Dr. Lauren MacArthur at Caltech on spiral bulges with 0.1<z<1.2 in the GOODS field. My very first summer research experience (e.g. SURF) with Dr. Chris Conselice on distant Extremely Red Objects had secured my interests in extragalactic research ever since my early college years!
- MacArthur, L. A.; Ellis, R. S.; Treu, T.; U, Vivian et al. "The Evolutionary History of Galactic Bulges: Photometric and Spectroscopic Studies of Distant Spheroids in the GOODS Fields." 2008, ApJ, 680, 70
- Conselice, C. J.; Bundy, K.; U, Vivian et al. "The faint and extremely red K-band selected galaxy population in the DEEP2/Palomar fields." 2008, MNRAS, 383, 1366
6. Stellar Atmospheres of Blue Supergiants in M33
While the cosmological distance ladder is extended to very high redshifts with hitherto unknown precision, we are confronted with the fact that distances to many nearby galaxies where individual stars and HII regions can be resolved are disturbingly uncertain. The Triangulum Galaxy M33 suffers from such a dilemma where the distance moduli obtained with different techniques differ by as much as 30% in linear distance (Bonanos et al. 2006). Applying the flux-weighted gravity– luminosity relationship (FGLR; Kudritzki et al. 2008, Urbaneja et al. 2008) to the quantitative spectral analysis of blue supergiants, we computed a new distance modulus of 24.93±0.11 mag for M33 (U et al. 2009). This significance of this work is three-fold: 1. Our distance has been widely adopted (e.g. McConnachie et al. 2010; Sanchez et al. 2010); it refines the FGLR calibration as a basis for wide application of this distance determination method. 2. Our distance has been compared to that from HST observations of Cepheids and with the tip of the red giant branch using HST ACS images to constrain reddening assumptions. 3. Our measurement of metal abundance and metallicity gradient have notable implications for those who study the resolved stellar properties in galaxies (e.g. Magrini et al. 2010, Verley et al. 2010, Barker et al. 2011).
Half the fun of astronomy is discovery; the other half is telling folks about it! I have been involved in outreach activities since my graduate school days in Hawaii, and am committed to diversifying the future STEM workforce as well as promoting astronomy in general.
Although I couldn't travel to the totality path for the Great American Eclipse 2017, it was nonetheless a blast volunteering at the UCI Eclipse Viewing Party on campus serving a huuuge crowd of 1000+ from within the community! All the activities from telescope viewing to science demos and kids craft station were immensely popular. It was also a great opportunity just to enhance public interests in science in general.
As a TMT affiliate, I participated in the Journey through the Universe (JTTU) event that took place in Hilo, Hawaii in early March, 2015. It was the annual astronomy outreach event jointly hosted by the 'Imiloa Astronomy Center and all the other observatories. As part of the eventful week, I held a family-centered discussion with a public talk on Colliding Galaxies, and went into a high school classroom as an astronomy ambassador to talk about optics and planets.
In December 2014, I gave a public talk at UC Riverside to ~60 people on how merging galaxies give rise to supermassive black holes. Questions about black holes from the audience are almost always as fun as giving the talk itself! This series of public talks as given by faculty and postdocs (sometimes in Spanish!) seeks to educate the public about the breadth of astronomy.
As part of the UCI Women in Physics group and the Astronomy Allies, I advocate for the inclusion and equal opportunities for women and underrepresented minorities (URM), particularly victims of harassment, in physics and astronomy. Changes to improving the field's climate for women and URM start with raising awareness and should be followed by actionable items, which include communication of specific issues and application of the corresponding remedies on the part of those in power. I am also eager to advise junior students on e.g. work-life balance, the delicacy of which is often cited as a reason why many young women leave the field.
As the astronomy outreach program at UCR takes off, I have had to chance to participate in several activities, including eclipse viewing (only do so under safe conditions!) and folding origami stars with high school female students.
When I was a graduate student at the IfA, our outreach committee was very involved in spreading astronomy throughout the island, including the annual IfA Open House, stargazing, and starlab. As a city, Honolulu suffered sufficient light pollution that the starlab shows we presented with our portable planetarium at various schools remained the best option for children to learn about the night sky.