Galaxy Evolution

Image Credit: The Royal Astronomical Society
Cappellari 2016 (ARA&A)

The visible Universe we see today is the collection of various shapes and sizes of the gravitationally-bound systems of stars, gas, dust, and dark matter, which are so-called galaxies. While the most massive among them host supermassive black holes at their centers, this picture is not clear in their smaller cousins. Studies of the galaxies' formation and evolution over cosmic time reveal their structures and the underlying physics that operate the World we are living in.

Our research on galaxy evolution at Phenikaa University focuses on the local Universe where evolution clues can be revealed through the resolved photometry and kinematics. This information can be obtained from the high resolution of imaging, integral field spectroscopy, and the help of state of the art of orbital dynamical modelings of stars or gas. We also search for the evolution signatures through the stellar populations and the star formation histories of galaxies by optimizing their spectra or photometry to models of stellar populations.

We are involved in many leading surveys to study galaxy evolution includes the High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI) on the Extremely Large Telescope (ELT) and the Vera C. Rubin Telescope (formerly known as Large Synoptic Survey Telescope or LSST). Also, we employ a multi-wavelength approach using state-of-the-art observations with ground-based (e.g., Atacama Large Millimeter/submillimeter Array (ALMA), Very Large Telescope (VLT), and Gemini) and space (e.g., Hubble Space Telescope (HST) and James Webb Space Telescope, JWST) observatories.

Supermassive Black Holes Formation and Evolution

Supermassive black holes are a crucial ingredient for galaxy evolution. They closely correlate with galaxy properties (e.g., velocity dispersion or stellar mass of the bulge), indicating the coevolution of these two components throughout Cosmic history. We seek to understand the formation and coevolution mechanisms of the two, which may be a key to shape the Universe we see today. Our interests mainly cover the following topics:

  • Black holes: feedback, demographics, seeds origin

  • Black hole assembly in various environments

  • Co-evolution of galaxies and supermassive black holes

  • Developed codes for black hole dynamical modelings

Stellar Clusters - Building Blocks of Galaxies

A star cluster is a set of gravitationally-bound stars or a collection of stars with a mass density large enough to resist tidal disruption from Solar Neighborhoods and numerous enough to avoid evaporation within a few 100 million years. Our group is interested in a variety of observational aspects of star clusters (i.e., globular clusters and nuclear star clusters) that are summarized below:


HST observations revealed that globular clusters not only formed more than 13 Gyr years ago but are also still forming in the Universe today. High-quality images and spectroscopies in Milky Way and nearby has located clouds that have properties expected for the progenitors of massive stellar clusters. Synergies of ALMA, JWST, and ELT will lead to a significant breakthrough for stellar-cluster formation.

Multiple Populations

A variety of photometric and spectroscopic surveys have been accumulating evidence that globular clusters have experienced anomalies abundance known as “multiple populations,” which contradicted the once thought of their single population. This intrinsically inhomogeneous abundance has put a challenge to their current evolution theory of globular clusters. Image Credit: NASA/ESA.

IMF Variation

The stellar initial mass function (IMF) influences most of the observables properties of stars and the star formation process in galaxies. Recent surveys also hint the IMF may not universal but sensitive to the environment. Thus, accurate constraining such an IMF variation will provide deep insights into the evolution of galaxies. Figure adopted from Bastian (2012).

Intermediate-mass Black Holes - the missing link

Black holes in the Universe around us come in two main types. We understand the origin of black holes with masses of a few times the mass of our Sun. These are born when massive stars explode as supernovae. However, we do not understand the origin of the so-called supermassive black holes, which can be several billion times more massive than the Sun. These are found at the centers of massive galaxies like our Milky Way, and their formation remains a mystery. It may be possible to grow supermassive black holes from stellar-mass “seeds,” or perhaps these enigmatic objects were born already massive in the extreme conditions after the Big Bang. Our group studies and hunts for “intermediate” mass black holes (IMBH), an elusive relic black hole that may be the “missing link” between these two populations in various stellar systems.

ELT will be crucial for hunting such missing IMBHs. Given the unprecedented sensitivity and resolution to zoom into the hearts of low-mass galaxies (i.e., nuclear star clusters) and globular clusters, ELT reveals the motions and distribution of stars and

warm gas (e.g., with its powerful HARMONI and MICADO+MAORY instruments) in the regions dominated by the black holes' gravitational potentials and weigh their masses. ELT, TMT, and MAVIS on VLT will be the powerful discovery machines that will be pivotal for pushing this field forward to the regime that even the current JWST cannot achieve.