Research
The question of how galaxies evolve is one of the most fundamental in all of astronomy. My passion in astronomy is to reveal how stars and gas interact to set the fate of the chemical history galaxies over cosmic time.
My work involves individual components in the star formation cycle -- stars and ionized/molecular gas -- to examine the relation between molecular gas that forms stars and the star clusters as the engines that ionize their HII regions.
Publications
Projects I'm involved with
I was a finalist for ANU's 2020 Vice Chancellor Award for Early Career Academics.
Spatial variations of HII region properties in local galaxies with the TYPHOON survey
Grasha, Chen, Battisti+22, ApJ, 929, 118
The content of heavy elements in a galaxy is one of the key properties for understand its formation and evolutionary history. Heavy elements are illuminated by gas-phase metallicity abundance variations in the ISM, a consequence of the full chemical enrichment history at the location in which star formation is occurring, and therefore, where HII regions are created. Using local spiral galaxies in the TYPHOON survey, we find that while spiral patterns play a role in organizing the ISM, the uniform distribution of enhanced abundances across the entire disks of these galaxies suggests that metal abundance enhancements are more likely driven by strong correlations with the local physical conditions.
Figure below: false-color RGB image of NGC 5068 (left), narrow-band image highlighting the emission lines (middle), and gas-phase oxygen abundance maps of the HII regions (right).
Stromlo stellar tracks: non-solar scaled abundances for massive stars
Grasha, Roy, Sutherland, and Kewley 2021, ApJ, 908, 241
Until now, all stellar evolution tracks are computed at solar, scaled-solar, or alpha-element enhanced abundances, and none of these models correctly represent observe elemental abudnaces (Galactic Concordance) observed in nearby HII regions. We present the first implementation of Galactic Concordance abundances to stellar evolution models called the Stromlo stellar tracks. The Stromlo tracks cover massive stars (10-300 M☉) over a finely sampled grid of metallicities (-2.0 < [Z/H] < +0.5; Δ[Z/H] = 0.1) evolved from the pre-main sequence to the end of Carbon burning.
The implementation of non-solar scaled abundances is critical for the evolution of main-sequence, massive hot stars in order to estimate accurate stellar outputs (L, Teff, g), which, in turn, have a significant impact on determining the ionizing photon luminosity budgets.
These non-solar (and non-alpha enhanced) abundance patterns necessitates current stellar population techniques applied to high-redshift galaxies as nearly all stellar population synthesis models assume solar abundance ratios. This will be extremely important for accurate analyses with the upcoming JWST and the Stromlo models are a vital first step in order to derive high-resolution metallicity diagnostics for galaxy evolution studies.
The Stromlo Stellar Tracks are available online.
Figure: evolution of ionizing radiation Q for the Stomlo tracks (pink) and MIST solar tracks (black; Choi et al. 2016). The assumption of the elemental abundances has a dramatic impact for energies higher than the hydrogen ionization photon rate and will have considerable impact for photoionization contributions at high redshift.
Systematic searches for cold neutral gas with 21 cm absorption
Grasha, Darling, Leroy, and Bolatto 2020, MNRAS, 498, 883
Grasha, Darling, Bolatto, Leroy, and Stocke 2019, ApJS, 245, 3
The universe is composed mostly of hydrogen gas and represents the raw material for the formation of stars. Spectroscopy of the atomic and molecular matieral in giant clouds of hydrogen help us to understand the evolution of the star-forming reservoirs of gas over cosmic time and is crucial in our understanding of how present-day galaxies came to be.
Using radio observations with the Green Bank Telescope, we undertook a large search of cold neutral gas to study the hydrogen content of galaxies from over the past 11 Gyr to present day. This represents the first survey of its kind to measure the evolution of neutral gas via HI 21cm absorption continusously from z=0 to z~3 and is competitive with prior low-reshift measurements.
Correlating the products of star formation to the reservoirs of molecular gas
Grasha, Calzetti, Adamo+19, MNRAS, 483, 4707
Grasha, Calzetti, Bittle+18, MNRAS, 481, 1016
The impact of spiral structure and feedback from stellar populations on molecular clouds has broad-ranging implications for star formation and the evolution of galaxies over cosmic time.
My latest work aims at constraining the timescales over which star formation is associated with molecular clouds, the effect on galaxy dynamics in impacting the onset of star formation from molecular gas, and the role of star formation in the gas turbulence. This science requires combining the high-resolution UV/optical cluster catalogs of LEGUS with ALMA to resolve the GMCs.
I constrain the timescale for the association of star clusters with their natal gas reservoirs through an examination of the median age of star clusters as a function of distance from their nearest GMC. The analysis has been done for the star clusters and GMCs with the PAWS survey in M51 and in NGC 7793 with ALMA observations.
Turbulence as a driver of the observed hierarchy in the positions/ages of star clusters
Grasha, Elmegreen, Calzetti+17, ApJ, 842, 25
A core belief of our understanding of star formation is that the hierarchical structure arises from a fragmenting turbulent interstellar medium. In such a turbulent media, the size and age relation between pairs of star clusters should be related via well-defined power laws. We examined this behavior for 8 of the LEGUS galaxies, finding the expected power laws as expected from turbulence, signifying that star clusters are born in a hierarchy and clusters that are born closer together are more similar in age than clusters born further apart.
In each galaxy, we find that there is a maximum scale where the age becomes randomized within each galaxy, and we use this location to derive the maximum size and age of correlated star forming regions in each galaxy.
Perhaps the most surprising part of the study is that we take the randomization velocity -- the size scale over the time scale, which signifies how fast the cluster components must travel to diffuse the complex -- and find that the derived velocity is always 3-5 times greater than the shear velocity experienced by each complex.
There are two main implications:
(1) Star formation is scale-free on a local crossing time, as traced by clusters.
(2) There is no bulk collapse from a large-scale gravitational instability, and instead, shear determines the largest scales for the coherence of star formation (quite possibly by destroying the larger scales).
The hierarchical structure in the positions of young star clusters
Grasha, Calzetti, Adamo+17, ApJ, 840, 113
Grasha, Calzetti, Adamo+15, ApJ, 815, 93
Star clusters are expected to inherit the hierarchical structure of the ISM in their spatial distribution at birth, mirroring the clustered distributions from the GMCs from which they originate.
I quantify the hierarchy in the distribution of the star clusters using the two-point correlation function. In a study of 6 of the LEGUS galaxies, I examined the sizescales and timescales for the coherence of star clusters, examining how large typical star-forming complexes are and how long such star-forming clumps survive before random motion causes the large-scale hierarchies to dissolve. The main findings can be summarized below:
(1) Star clusters are born in large correlated star-forming complexes of a few hundred parsecs in size and their dispersal from the complexes is also a hierarchical process.
(2) The observed clustering increases significantly in the youngest (<10 Myr) clusters, signifying that the correlated structures last for relatively short timescales, dispersing on the order of ~10s of Myr.
(3) The strength of the clustering is measured with the TPCF slope and different slopes imply different fractal dimensions and hierarchies for different galaxies, signifying that each galaxy has its own influence in the formation and survival of structure.
Quantifying the scatter in the IRX-beta relation
Grasha, Calzetti, Andrews+13, ApJ, 773, 174
Dust grains are a fundamental constituent of galaxies, efficiently absorbing optical/UV light and re-emitting the energy at far-infrared wavelengths. The correlation that links the deficit UV light to the IR excess is called the IRX-beta relation. The IRX-beta relation is robust for starburst galaxies, allowing it to be used to estimate the total UV attenuation, however, normal star-forming galaxies deviate from starburst galaxies in terms of their positions in the IRX-beta plane.
I performed a study on investigating the scatter in the IRX-beta relation with 98 quiescent galaxies within the LVL survey. These galaxies were chosen to be low in metallicity and thus relatively low in dust content. I found that the mean age of the stellar population appears to be the an important parameter that drives galaxies away from the starburst trend due to the older and less massive stars contributing to the near-UV emission of galaxies.