My interest: Star formation at high resolution in the nearby universe

My general science interests are star formation and galaxy evolution which I examine with the tools of sub-/milliter/radio astronomy and interferometry. In particular, I study the extreme conditions in starbursts and intensely star forming regions in the nearby universe at very high resolution. Observations of in the submillimeter to radio wavelengths cover a wide variety of ISM ingredient and a large spectrum of physical processes detectable through their characteristic emission.

Some background on what I am interested in ...

What is sub-/millimeter/radio interferometry?

In the long wavelength (from submilliter over centimeter, meter up to kilometers) regime of the electro-magnetic spectrum, photons cannot be measured individually as in optical CCDs due to the low energies. Instead, the voltage induced by the electrical field of a photon is detected. This comes with it's own set of challenges but also advantages.

ALMA
Atacama Large Millimeter/Submillimeter Array.
(Credit: ESO/C. Malin; distributed under CC4.0; source)

On the downside, the achievable resolution is low compared to shorter wavelength (optical or infrared). Due to resolution = λ/D, the long wavelengths lambda require large telescope apertures D which become impractible for telescope dishes of hundreds of meters. Furtunately, interconnecting a number of small antennas to behave like a single giant antenna is possible. This method is called interferometry. Placing the antennas far apart, from hundreds of meters to different continents, we can achieve very high resolution in far away targets. This method comes at the cost of increased computational effort during observations when correlating the observed signals from the individual antennas and also enhanced complexity in recreating an actual image of the source. For very high resolution data or superior image fidelity, the task of creating an image itself, before any analysis, is a major effort and requires skills and experience. Especially so when pushing to the limits of resolution and sensitivity. Since working on my Bachelor’s thesis, I have been deeply involved in technical aspects of data reduction and processing. This has been crucial to achieve the high resolution and image fidelity that is needed to study small-scale effects in the context of large galaxies.

On the upside, detecting photon electric fields instead of particles allows to measure signals of many frequencies simultaneously. This is equivalent to measuring many colors of light at the same time for optical intruments. As such sub-/millimeter/radio observations yield not just the distribution of light on the sky but also the velocity structure. The complicated kinematic information contained within a 3D data cube (position, position, velocity information) requires complex methods for analysis and interpretation.

Star formation over cosmic times and the relation to local starburst

Antennae
In the antennae galaxies, a starburst is triggered by the major merger.
(Credit: NASA, ESA, STSci; public domain; source)

The evolution of the cosmic star formation rate density shows that the majority of today’s stars formed at redshifts z~2 when the universe was only ~3.5 Gyr old. A typical star forming galaxies at that time was significantly smaller in size but shows SFRs higher by ~2 orders of magnitude. This is due to the redshift evolution of the main sequence of star forming galaxies.

The large distance to these high-redshift galaxies prevent current and also future facilities to resolve the spatial scales relevant to star formation, i.e. from the ~100 pc scales of giant molecular clouds (GMCs) down to ~1 pc scales of stellar clusters. Lukily, the physical conditions for star formation similar to typical z~2 galaxies also occur in extreme regions in the local universe, in particular galactic starbursts. These rare starburst systems can therefore be used as analogs to high-redshift star forming galaxies and are close enough that current instruments can achieve the required spatial resolution.

Outflows and feedback

An immediately related question that is also of crucial importance for the galaxy evolution process is the role of stellar feedback on galactic scales. Strong stellar feedback caused by high SFR densities can launch outflows of ionized, neutral and molecular gas that potentially can escape the main body of a galaxy. Consequently, such outflowing gas removes the potential fuel for future star formation. Therefore, outflows can suppress and quench star formation.

M82 starburst
The prominent ionized outflow (Hα, red) in the M82 starburst.
(Credit: NASA, ESA, STSci; public domain; source)

Outflows driven by star formation are thought to be a crucial driver of galaxy evolution. Strong stellar feedback caused by high star formation rate densities can launch outflows of ionized, neutral and molecular gas that potentially can escape the main body of a galaxy. Consequently, such outflowing gas removes the potential fuel for future star formation. Therefore, outflows can suppress and quench star formation, as also demonstrated by theoretical predictions and simulations. Depending on the velocity of the outflow and a galaxy’s escape velocity, outflowing gas can be re–accreted at later cosmic times (the so–called 'galactic fountain') or leave the system altogether. This process thus has the potential to enrich the galactic disk and circum–galactic medium with metals.

Galactic outflows are a multi–phase phenomenon and are observed across the electro–magnetic spectrum from X-ray, UV, optical like Hα to IR, cold dust, PAH emission, and sub-millimeter to radio including Hi. Typically, large-scale outflow features at high relative velocity (100s-1000s km s−1) are observed in the ionized and neutral gas, whereas molecular outflows often appear as smaller, more compact features. The latter are nonetheless important as they dominate the mass budget. In some galaxies, the gas phases seem to be stratified with an inner ionized outflow cone, a surrounding neutral shell, and molecular gas situated along the outer edge. Typically, the outflows originate from an extended region, so the apparent outflow cone has its tip cut-off.

Molecular outflows are thus closely intertwined with feedback processes and star formation. The high-resolution structure and kinematic properties of (molecular) outflows are not studied in detail yet, primarily due to the lack of high resolution and high sensitivity observations. Starburst galaxies are the obvious target to study star formation-driven outflows due to the high star formation rates in these system. Active galactic nuclei also exert feedback on galaxies and need to be considered as an outflow engine when present. Molecular outflows have been studied over the past years in a few nearby starbursts (M82, NGC253, NGC1808, ESO320-G030) but to the level of detail that we can achieve now, e.g. in NGC253.

My research ...

My work over the past years has focused on the different aspects of star formation (feedback) and its relation to the ISM in local systems at high spatial and spectral resolution.

Galactic center and the Central Molecular Zone

SWAG detail
Detail of the ammonia (3,3) map obtained with SWAG. The 100x60 pc2 cut-out features Sgr B2 to the right and a part of the 1.3° cloud. (Krieger et al. 2017)

One focus of my work is on the Milky Way’s Galactic center (GC), the most nearby galactic center at only 8 kpc distance for which sub-pc resolution can easily be achieved. It shows enhanced star formation compared to the Galactic disk and increased mass and density of the molecular gas, the fuel for star formation. For a long time the GC has been considered a good analog for high-redshift star formation regions. Although the GC is forming stars at a relatively high rate, it is still lower by an order of magnitude compared to what is expected from current models given the observed ISM properties.

To address this problem, we conducted the "Survey of Water and Ammonia in the Galactic Center" (SWAG, PI: J. Ott), a 750 h survey at ~25 GHz with the Australia Telescope Compact Array (ATCA). It is designed to provide detailed maps of the physical and chemical properties of the ISM in the GC at 0.9 pc resolution. As a core member of the survey team, I developed the data reduction and imaging pipeline and spent 4 weeks observing at the ATCA. In Krieger et al. (2017), we use SWAG to confirm that orbital dynamics on ~100 pc scales can considerably affect the evolution of molecular clouds and can probably trigger SF.

Read more details on the Central Molecular Zone and the SWAG survey.

The nuclear starburst NGC253

The other galaxy that I focused my work on so far is the nearby galaxy NGC253. It is considered the prototypical nuclear starburst in the Southern hemisphere and an ideal target for ALMA observations. In its central ~500 pc, the star formation rate surface density locally reaches >100 M yr-1 kpc-2 very similar to what has been found in high-redshift star forming galaxies on the main sequence. Due to the close proximity of only 3.5 Mpc, current instruments can achieve pc-scale resolution in the optical, IR and sub-/millimeter wavelengths in NGC253.

NGC253 CO
CO(3-2) in the central ~800 pc of NGC253. The colors show molecular gas approaching (blueshifted, blue) and receding (redshifted, red) relative to the systemic velocity (green). (Krieger et al. 2019a)

During my PhD work, I have extensively studied the molecular gas in and around the central region of NGC253 with ALMA observations of unprecedented sensitivity and spatial resolution (2.5 pc). In my work I performed a kinematic decomposition of the molecular gas and could separate emission emerging from the galaxy’s disk from the starburst-driven molecular outflow (Krieger et al. 2019a). This detailed investigation of the molecular outflow shows that the molecular phase is the dominant outflow phase for mass and likely also energy and momentum. The molecular mass is equally distributed in localized coherent features and diffuse molecular gas.
In Krieger et al. (2019b), we present a detailed study of the forming super star clusters (SSCs) in the center of NGC253 in which the majority of the star formation rate is located. These SSCs provide the energy source and potential launching site of the outflows. In ALMA band 7 (~350 GHz) spectra, we detect an incredible wealth of molecular spectral lines to determine the physical and chemical conditions of the dense ISM. We measure very high dense gas fractions in the SSCs and find that all SSCs are consistent with chemistry driven by photon-dominated regions with no evidence for X-ray dominated region as may have been expected by the presence of an (currently unknown) AGN. The detection of vibrationally excited species implies strong IR radiation fields due to a greenhouse effect. These findings constrain the environment that lie at the heart of the feedback and launching of the outflows.

Find more details on my work on NGC253.

How do the different high-redshift analogs compare?

Both, NGC253 and the Milky Way Galactic center have been proposed as high-redshift analogs. Indeed, they show similar properties of the molecular gas mass and distribution. However, they vary significantly in their star forming properties such as the star foramtion rate. We are currently exploring to what degree the gas kinematics drive this difference using matched observations of the molecular gas.

What's next?

Massive star formation at high redshift covers a range of properties, presumably as a function of environment, as do their local analogs. In order to deepen our understanding of the star formation and related feedback processes, we need to expand the current pilot studies. One important step is to expand our high-resolution analysis to further local galaxies where the necessary resolution can be achieved. Currently, we are observing the molecular gas in M82 with NOEMA (PI: N. Krieger) in a large 73h project.

For a complete picture of star formation, feedback and outflow driving, we also need complementary data of the ionized and deutral gas phases of the ISM and stellar properties. At the moment, we are in the process of obtaining those information from complementary optical and near infrared observations.