I study the interstellar medium and star formation process in nearby galaxies using a wide range of multi-wavelength observations. Below you can find an (incomplete) overview of topics that I work on.
I'm interested in the composition of galaxies: their stellar population, star formation activity, and the gas and dust between stars. Most of my work has been on the atomic and molecular interstellar medium - the material that typically dominates the galaxy's mass budget - with the aim to understand their properties and relationship with the current star formation activity. Lately, I'm focusing on determining the density, temperature, and chemical structure of the atomic and molecular gas phases and consider how this is linked to galactic properties and stellar population.
Over large scales in galaxies the gas components and star formation activity establish a balance in which the many interrelated physical processes reach a statistical equilibrium. This balance depends on galactic environment, differs within and among galaxies, and evolves slowly with time. A main goal of my work is to obtain robust observational measurements of the gas components, star formation rate, stellar population, and environmental parameters to determine their exact balances across a wide range of nearby galaxies. These observations provide the galactic context for more detailed studies (within the Milky Way and as discussed in the "cloud-scale evolution" section) and link nearby galaxies to the galaxy population across the universe. At the same time, they define a critical benchmark for theoretical models of star formation and galaxy evolution.
I am utilizing ground-based telescopes operating at millimeter and radio wavelengths to obtain new observations of the interstellar medium in galaxies. This includes the Very Large Array (VLA) to observe the 21cm line of atomic hydrogen (HI) gas and the IRAM 30m to map carbon monoxide (CO) at 1 or 3mm - an easily visible tracer of molecular (H2) gas. These data I combine with multi-wavelength observations from the infrared to the ultraviolet to also trace dust, old stars, and current star formation. Thanks to new instrumentation at the IRAM 30m and the Green Bank Telescope (GBT), I am now also pursuing sensitive surveys of much rarer and thus fainter molecules such as HCN or HCO+ at 4mm tracing the densest regions of molecular gas.
I'm involved in or lead several big surveys of nearby galaxies. This includes the THINGS (PI Fabian Walter) and LITTLE THINGS (PI Deidre Hunter) HI surveys at the VLA and the HERACLES CO survey (PI Adam Leroy) at the IRAM 30m (for which I had a central role in data acquisition and interpretation). Over the past years, we significantly extended these initial surveys with the everyTHINGS (PI Karin Sandstrom) and everyHERACLES (lead by me) surveys to now sample ~80 nearby spiral galaxies and ~40 dwarf galaxies. With several hundred resolution elements per galaxies and ~100,000 independent regions in total, we have created a one-of-a-kind database of ~kpc-resolved measurements representative of the entire nearby galaxy population. The latest addition are surveys of dense gas tracers such as EMPIRE (PI Frank Bigiel) at the IRAM 30m or the GBT (PI Amanda Kepley), though due to their intrinsic faintness for smaller sample sizes.
Research highlights from these surveys include the unambiguous finding that the star formation rate scales with the molecular gas mass that even holds true in the outer disks of spiral galaxies where the atomic gas outweighs the molecular gas (Schruba et al 2011); the robust measure of the (large-scale) relationship between molecular gas and star formation rate (Bigiel, Schruba et al 2011; Leroy, Schruba et al 2013) and its emergence as ensemble average of many individual star-forming regions at distinct evolutionary states (Schruba et al 2012); and the determination of environmental variations of the dense gas fraction and star formation efficiency in the dense gas (Usero, Schruba et al 2015; Bigiel, Schruba et al 2016; Gallagher, Schruba et al 2018). We calibrated various star formation rate tracers (Leroy, Schruba et al 2012); developed the use of dust as alternative tracer of molecular gas (Sandstrom, Schruba et al 2013); found that the molecular gas has a disk as thick as the atomic gas (Caldu-Primo, Schruba et al 2013); and inferred the dark matter distribution from the gas kinematics (Oh, Schruba et al 2015). Currently, we are investigating the balance of atomic and molecular gas, a ratio that depends on multiple environmental properties, and inevitably controls how much of the gas in a galaxy is able to form stars.
The Local Group galaxy Andromeda (M31) is the best target to study the physical processes that control the formation of molecular clouds and stars. Currently a large group of astronomers employs the world’s most powerful telescopes (HST, Spitzer, Herschel, CARMA, VLA) to obtain a multi-wavelength view on Andromeda ranging from ultraviolet to radio. The HST observations by the PHAT team quantify the properties of individual stars, the local radiation field, and gas mass by extinction maps (Dalcanton et al 2012; Dalcanton, Schruba et al 2015). The infrared data from Spitzer and Herschel allow the determination of dust mass and temperature which is a proxy of the gas mass and the local radiation field (Draine et al 2014, Gordon et al 2016). With the VLA we mapped the atomic gas and radio continuum emission (Lee, Leroy, Schruba et al, in prep), and at CARMA I have lead a large survey to map CO emission, a tracer of molecular gas, across several spiral arms of Andromeda (Schruba et al 2019b).
We are using these data to study the atomic-molecular gas balance on cloud scales, the properties of molecular clouds across the galaxy, and the effect of feedback that young stars have on the surrounding interstellar medium. First results include the separation in line width of the atomic gas into a tenuous warm neutral medium and a clumpy cold neutral medium that is correlated with molecular gas tracers and ongoing star formation (Lee, Leroy, Schruba et al, in prep). Also the molecular gas is found to separate into two morphologically distinct components: a tenuous component that is well mixed with the atomic gas and a clumpy component that resides in molecular clouds (Caldu-Primo & Schruba 2016). These studies are the first of a kind that identify the star-forming gas phase by morphology instead of choice of (a single) gas tracer and highlight the distinct, non-coeval thermal, gravitational, and chemical evolution of the interstellar medium.
The resolved HST observations by PHAT of hundred million of individual stars provide a detailed characterization of the recent star formation history (Lewis, Schruba et al, 2015) and young stellar clusters (Johnson et al, 2015) that we use to study the time evolution of the gas-star cycle. An observational study of the (non-) overlap of molecular clouds and young stellar clusters indicates that the star formation process within molecular clouds is fast (~few Myr) and only a short episode in the lifetime of molecular clouds (~15-30 Myr; Beerman, Schruba et al, in prep); while theoretical modeling of the de-correlation of gas and star formation tracers as function of spatial scale and evolutionary state of individual clouds further provides first constraints on the duration of cloud dispersal and the efficiency of stellar feedback (Kruijssen & Longmore 2014; Kruijssen, Schruba et al 2018). Within the MUSTANG collaboration, we are currently employing this theoretical model on high-resolution observations of a large set of nearby galaxies. We have employed ALMA to obtain a complete census of molecular clouds in the nearby low-mass spiral galaxy NGC300 and find that these clouds live only about one dynamical timescale (~10 Myr) at which point newly formed stars destroy their natal birth cloud by their strong feedback (Kruijssen, Schruba et al 2019; to appear in Nature). Currently we are working on applying the method to galaxies observed by the ALMA Large Program PHANGS (co-PI Schruba) which systematiclly studies the molecular gas and star formation process across the local galaxy population.
A first glimpse of such detailed studies of the interplay between molecular gas properties and star formation have been possible in the neighboring Triangular galaxy (M33). Comparing the local molecular gas mass and star formation rate for apertures centered on either GMCs or HII regions and varying the size of the apertures, showed that the tight scaling of star formation rate and molecular gas mass observable on large scales including many star-forming regions breaks down as only a single such region is considered reflecting the evolution of star-forming regions (Schruba et al, 2010).
To understand the formation history of massive galaxies requires us to understand the formation and growth of low mass galaxies and in particular the conversion of gas to star within them. Their low metal abundance and thus low molecule abundance however severaly hinders observational studies. In addition, low dust content depreciates molecule formation and is less affective at shielding molecules from dissociating radiation. For a long time this has impeded the detection of carbon monoxide (CO) in galaxies with less then ~20% solar metallicity and kept the actual molecular gas content and star formation process in low mass galaxies elusive.
The HERACLES CO survey at the IRAM 30m includes 15 low mass galaxies. While CO emission remained undetected in these galaxies, the large field of view of the HERACLES maps together with a newly developed stacking technique allowed to derive strong upper limits on their CO emission (Schruba et al 2012). We found their CO luminosity per unit star formation rate to be
With the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) it became possible to shed light on the faintness of CO emission in low mass galaxies. With two deep exposures targeting star-forming complexes in the nearby galaxy WLM, we detected CO emission for the first time at 1/8 solar metallicity. The CO emission originates from a few small parsec-sized cores - about a tenth in size of Galactic molecular clouds (Elmegreen, Schruba et al 2013; Nature).
Recently, with ALMA observations I could confirm the finding of small parsec-sized CO cores in four star-forming complexes of the Local Group, low metallicity galaxy NGC 6822 (Schruba et al 2017). These CO-bright cores fill only a small area of large cloud complexes as traced by dust emission. While the complexes likely harbor significant molecular hydrogen (H2), they are largely devoid of CO molecules. Interestingly, the CO-bright cores share the same dynamical properties as equally sized CO-bright structures within our Galaxy. This is todays best observational evidence that the star formation process is not fundamentally different in the low metallicity (20% solar) environment of NGC 6822. Future ALMA observations of ionized and neutral carbon will shed further light on the chemical composition and star formation process at low metallicity. The following press releases have been published on this work: ESO, MPE, NOVA.