In my research as an astrophysicist I study the Universe, in particular the largest objects in it, called galaxy clusters. They are crucial to understand the formation of structure over cosmic time, and are the biggest laboratories to study particles at extreme energies.
If you want to know more about my research, please visit the research section.
I grew up in small town in northern Bavaria (Germany), feeling very connected to nature. In order to pursue my interest in physics, technology, and of course, astronomy, I moved to the beautiful city of Bonn and started my Physics, and later Astronomy studies. After more than 8 years I decided that some change is necessary, and took the great oportunity to start my first PostDoc at the Center for Astrophyics in Cambridge, MA.
In the meanwhile I have explored already many parts of the United States and collected great memories.
X-ray calibration uncertainties need to be taken into account for any science results.
Cosmology
Cosmology explores the interior design of the Universe. With the help of galaxy clusters, the largest
gravitationally relaxed objects, we trace the large scale structure in the Universe, and witness the
structure formation history. We aim to solve some of the most fundamental questions in science: What is the Universe made of? How does it evolve in
the future? Our current findings imply that the two most important components in terms of energy today are
Dark Matter and Dark Energy, which we both understand very poorly. Galaxy clusters play a key role in
understanding the Universe. Any cosmological theory needs has to explain the distribution and composition of galaxy clusters.
Since galaxy clusters are excellent probes to trace the formation and growth of structure in the Universe, my collaborators and I
have used - for the first time - a complete sample of galaxy clusters (HIFLUGCS), to determine individual masses that allow
quantifying the various biases. We are in the process of expending this to a larger sample (eeHIFLUGCS). My main papers following
this study are: HICOSMO I - Determination of the cluster masses HICOSMO II - Cosmological application
These publications are based on results obtained during my PhD,
for which I was awarded with the PhD Prize by the
Foundation for Physics and Astronomy in Bonn.
At this time the field of cosmology had already made several discoveries with enormous impact on how
we see the Universe, like the cosmological redshift implying an expanding Universe, or
the claim for an unknown type of matter (Dark Matter), which was introduced by Fritz Zwicky when he
noticed the high velocity dispersions of galaxies in galaxy clusters (Zwicky, 1933). Zwicky did not
know about the X-ray bright gas in galaxy clusters, which is one order of magnitude more massive then all
the member galaxies, when introducing the new type of matter. His claim was confirmed later by detailed
X-ray observations measuring the ICM mass from the X-ray surface brightness, because it turned out that
neither the galaxies nor the ICM are the dominant matter component in galaxy clusters, but an invisible,
most likely non baryonic type of matter, whose mystery is still not solved.
Clusters are filled with a hot plasma that outweighs all their member galaxies and cools within a billion
years. They are fascinating laboratories that enable a variety of different studies:
The amount of baryonic matter (mostly hot, X-ray bright gas) in galaxy clusters reflects the
amount of baryonic matter in the Universe. We used this "f-gas test" in our
HICOSMO II paper.
The enrichment and distribution of heavy elements in the Universe is traced very well in galaxy
clusters, since, unlike galaxies, they retain all the gas. Elements can be traced in galaxy clusters and
their distribution allows to draw conclusions on the origin and how they are removed from stellar
systems and galaxies.
We discovered an infalling galaxy into a nearby cluster which leaves a long trail of gas behind,
possibly the longest X-ray tail ever observed. A deeper
follow-up study
confirms our findings.
Some emission lines of elements can only be detected under these extreme conditions, like in the hot
intracluster gas. This gives new insights to atomic physics. With very high spectral resolution
emission lines from Oxygen, Carbon or Iron can also be used to trace the extremely faint gaseous halos
of galaxies. We
evaluated the detectability of such halos with a potential, future X-ray instrument. The extent and
structure of the X-ray bright halo around galaxies is important to understand the galaxy ecosystem,
interplay of stellar and AGN feedback.
The most energetic collisions of matter happen when galaxy clusters collide. This is a unique test
for Dark Matter models to study the gravitational, and maybe even non-gravitational interactions.
Furthermore, the frequency of these events challenges the cosmological model.
We presented a in-depth study of the
quadrupel-merger Abell 1758
, which will eventually form the
most massive object in the Universe.
Giant supermassive black holes located in the centers of the dominant galaxies are thought to
provide the reheating mechanism that balances the cooling: As the cooling gas is accreted by
the black hole, jets of relativistic particles are ejected that reheat the surrounding plasma. My
novel
approach combines accretion and jet emission models over a broad spectral energy distribution,
allowing for the first time to link the macroscopic effects of reheating, and connect it with the
ondensation of molecular gas from the cooling X-ray phase, all the way to the jets from the central
supermassive black hole.
Future X-ray instruments will no longer limited by statistical uncertainties, instead systematics in the
calibration and the background will play a crucial role for any science application. I have demonstrated
that the current major X-ray observatories, Chandra and XMM-Newton,
are not consistent
due to uncertainties in the effective area calibration, and quantified the effect on cosmological
measurements. As part of the IACHEC collaboration I develop effective area calibration schemes. The
particle background from high-energetic cosmic ray particles present in current X-ray instruments
marks an irreducible threshold of 5-10% due to the uncertainties connected with it (variability, energy
signatures). Using background-reduction techniques where I exploit the spatial correlation
between cosmic-ray particle tracks and secondary events, I verify simulations, and reach a
deeper
understanding of the various background components.
The ultimate fate of the gas that is cooling in the centers of clusters and groups has not
been fully understood. The previous Section (11.C) motivated already why galaxy
groups are the ideal objects to study
AGN feedback due to their increased
cooling and shallower gravitational
potentials. NGC5044 is the best object to
understand all the interplay of the various
processes (triggering of AGN outbursts
or star formation, reheating of the gas,
feeding the central AGN with cooling
gas), since it is very nearby and the X-ray
brightest group in the sky. Together with
my
collaborators at SAO, I have analyzed
deep ALMA CO data in various
configurations, as well as ACA and
ALMA total power array data, to
understand the amount, distribution and
dynamics of the cold molecular gas, which can be accreted and feed the AGN in the
center. We find that most of the cold gas is missed by ALMA since it is too diffuse and
extended, the ACA with its larger beam can better sample the sky and locate all the
molecular gas that was found with single dish instruments. It also allows us to create a
3D representation of the distribution (figure to the right. The third axis is the velocity
with respect to the central galaxy). It was revealed that most of this cold gas is diffuse
and on large scales, while its velocity distribution generally follows the hotter H
phase. We were also able to look more closely at each of the detected clouds, which
contain about 70% of all the CO that has been found: With their their typical mass of
and, and velocity range to , they fit very well into the
predictions of the chaotic cold accretion model (CCA), which predicts the
condensation of cold gas out of the hot phase through turbulence in the gas. We also
find an absorption features in the continuum spectrum of the central AGN and can
relate it to two small clouds that fall onto the supermassive black hole, located most
likely already within its sphere of influence. These results were published in ApJ (894,
72; Schellenberger et al. 2020), and have been presented at two AAS conferences.
Driven by these amazing discoveries, we pushed further and tried to understand the
ongoing processes on the smallest scales around the central black hole. Deep VLBA
observations in two bands (PI: Schellenberger) allowed us to detect parsec-scale jets
from the AGN. However, their direction appears perpendicular (in projection) to the
signs of past AGN activity imprinted on larger scales:
Through our multi-wavelength analysis from low frequency radio to sub-mm, we
concluded that the feedback works in an advection dominated mode, which is
radiatively inefficient and most energy is transported through ions inwards to the black
hole. This modeling is a major step forward in understanding the feedback on the
smallest scales, and links it to macroscopic processes, such as cold molecular clouds.
These results have been published in ApJ (906, 16; Schellenberger et al., 2021).
As next steps we decided to proceed two-fold: On the one hand we try to answer the
open questions related to NGC5044, such as the impact of time-variability and shape
of jets and compact radio source on even smaller scales. But we also don’t want to be
limited just to a single source, and have recently established a sample of clusters and
groups to be analyzed in a homogenous way comparing the small VLBA jets and X-ray
cavities in the hot gas.
For the deeper follow-up on NGC5044, we have an ongoing, and highly successful
SMA monitoring program (PI Schellenberger, 4 successful proposals so far) to
understand the time variability of the 230GHz continuum flux on timescales of weeks to
years. We have already captured two peaks in the 230GHz continuum flux, that can be
linked to changes in the accretion. We also have obtained high frequency VLA and
VLBA observations at 20 and 40GHz (PI: Schellenberger for both) to resolve the jets
and accretion disk, and better parametrize the SED at these frequencies.
The newly constructed sample of small jets in cool core clusters and groups is
currently being analyzed by Francesco Ubertosi, a visiting student from Bologna, Italy,
under the supervision of Ewan O’Sullivan and myself.
We have submitted more requests for additional data of galaxy groups similar to
NGC5044, in the mm-band using the Large Millimeter Telescope Alfonso Serrano (LMT,
Mexico), and for funding through the SI Scholarly Studies program.
„THE KEY MISSING LINK IN UNVEILING THE PHYSICS DRIVING GALAXY
GROWTH IS TO MEASURE THE PROPERTIES OF THE DIFFUSE GAS WITHIN,
SURROUNDING, AND BETWEEN GALAXIES“ — ASTRO2020 DECADAL
The Astro2020 Decadal Survey
has clearly identified the mapping
of the circumgalactic medium
(CGM, gaseous halo around
galaxies that extents to the viral
radius) as a key objective. An X-ray microcalorimeter for the
2030s that can address this, and many other outstanding
questions concerning structure formation in the Universe.
I am analyzing mock observations of simulated galaxy halos. This will help to understand how
different feedback models (supernovae vs. AGN) included in the simulations can be
distinguished by LEM. Since no standard analysis software exists, I wrote all code in
python, mostly from scratch, to a) make surface brightness profiles of prominent
emission lines in the CGM, b) test line ratios (e.g., oxygen VII and VIII) as a proxy for
gas temperatures, c) quantify azimuthal asymmetry and substructure in the halos, and
d) create spectral maps of the halos to measure outflows and rotation features through
the temperature and velocity structure.
Schellenberger et al. (2024) "Mapping the Imprints of Stellar and Active Galactic Nucleus Feedback in the Circumgalactic Medium with X-Ray Microcalorimeters"
The cosmological analysis of the HIFLUGCS galaxy clusters ("HICOSMO") provided reliable cosmological parameters
such as the matter energy density, ΩM, and the σ8. The former quantifies the amount of matter in the Universe which is available to build structure, while the latter defines the amount of initial fluctuations in the density field of the early Universe, that enables the collapse of overdense regions to eventually form galaxies and clusters.
Our study (blue ellipse) defines a small parameter space for the cosmological parameters. Which can be combined with other probes, like primary CMB anisotropies (red ellipse), to further achieve high precision cosmology.
The selection of eeHIFLUGCS follows a careful re-analysis of the ROSAT All-Sky survey data, with a particular emphasis
of selecting only real clusters with verified redshifts above a flux limit of 5x10-12 erg/s/cm2
eeHIFLUGCS is
Spatial distribution of the clusters in Galactic coordinates (blue are "old" HIFLUGCS clusters).
eeHIFLUGCS will allow us to increase the statistics for many aspects of galaxy clusters and provide the ability to challenge current models of cosmology (e.g., the largest sample to analyze biases on cosmological parameters), scaling relations (e.g., to test non-Gaussianity of the scatter), cross calibration (e.g., time evolution), cool-core fraction (e.g., sample selection and influence on cosmology), and feedback (e.g., radio AGN power, star formation and cooling time).
Cosmology prediction for eHIFLUGCS compared to the current reference.
The most massive, but very rare, galaxy cluster are not only very useful for cosmology,
but also help to understand structure formation and cluster evolution. Abell 1758
consists of two separate clusters, a northern and a southern cluster separated by
about 6 million light years, although it was originally discovered as a single object. Both
of these clusters appear to consist of two sub clusters themselves, which are currently
merging — the northern one in a past core-passage stage, the southern one in an
earlier stage. Eventually the northern and southern clusters will merge and form the
most massive object in the universe.
Together with my collaborators at the CfA, I performed a study of this intriguing object,
to answer some fundamental questions: Are the two clusters already beginning to
merge, and can the relative motion of the individual galaxies tell us if there are
enormous filaments, along which the clusters are aligned? Deep Chandra
observations revealed a shock front associated with the merger in the
northern cluster and a merging velocity of about 60% the sound speed of the
gas, however weaker than expected. Also the lack of accelerated charged
particles (typically seen as radio relics in the low frequency radio band)
indicates that the merger energy may not be transferred into the gas as
efficiently as we might assume. The southern cluster shows even fewer
signs of the merger. Locations and velocities of the individual galaxies observed
in the optical regime, shows in-falling subgroups onto the major clusters.
Some of these subgroups could be identified with X-ray emission. This shows that
there might be a large filament present along which smaller groups of galaxies form
and fall onto the massive clusters, which is aligned perpendicular to the anticipated
merger axis of the two large clusters. These interesting results have been published in
ApJ (Volume 882, 59, Schellenberger
et al., 2019), and were featured in several press releases, including an interview with
Bruce Dorminey for Forbes online,
Astronomy Now, and a
Chandra CXC image release. It was also selected as the
Chandra Science Highlight for October 2019. In the future we need to conduct similar
studies on massive, merging clusters, to be able to make statements on the formation of clusters within filaments.
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NGC 6338: The most energetic group merger
Galaxy groups, smaller systems than galaxy clusters with only a few tens of
galaxies, are the ideal environment to study the feedback cycle. In these
objects, the X-ray bright gas cools even faster than in clusters, and a
reheating process provided by the central active galactic nucleus (AGN)
has to be well-tuned to establish an equilibrium.
The galaxy group NGC6338 is a massive group-group merger. Chandra
and XMM-Newton data revealed shock-heated gas shown in red in the
figure on the right (provided by the CXC team in their image release, see
right figure), and cooler gas shown in blue. Despite the separation of the two colliding
galaxy groups, the merger happens mostly along the line of sight.
Despite the very hostile environment, together with Ewan O’Sullivan and other
collaborators, I studied signs for ongoing feedback with the
Giant Metrewave Radio Telescope (GMRT) and found
several cycles of outbursts imprinted through radio lobes
(red contours show the 650MHz emission in figure to the right). These lobes have
corresponding X-ray cavities in the hot ICM. We also found an actively ongoing feedback
cycle indicate through small parsec-scale jets seen in VLBA observations. Dedicated
Submillimeter Array (SMA) observations
(PI Schellenberger) help to understand the
spectral energy distribution (SED) up to 230GHz. We conclude that the feedback cycle
continues as in any normal galaxy group, while NGC6338 is undergoing the most energetic
group merger. These results have recently been published in
MNRAS (Volume 488, 2925; O’Sullivan, Schellenberger, et al. 2019), and ApJ (Schellenberger et al., 2023).
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NGC 741: Mergers and AGN Feedback
While AGN and mergers are thought to play important roles in group and cluster evolution, their effects in galaxy groups are poorly understood. We show results from an analysis of deep Chandra and XMM-Newton observations of NGC 741, which provides an excellent example of a group with multiple concurrent phenomena: both an old central radio galaxy and a spectacular infalling head-tail source, strongly-bent jets, a 100kpc radio trail, intriguing narrow X-ray filaments, and gas sloshing features. Supported principally by X-ray and radio continuum data, we address the merging history of the group, the nature of the X-ray filaments, the extent of gas stripping from NGC 742, the character of cavities in the group, and the roles of the central AGN and infalling galaxy in heating the intra-group medium. Recently, we have published deep, wideband multifrequency radio observations (144 MHz‑8 GHz) of the remarkable galaxy group NGC 741, which yield crucial insights into the interaction between the infalling head-tail radio galaxy (NGC 742) and the main group.
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Galaxy clusters, the largest virialized objects in the Universe, are excellent probes to
trace the formation and growth of structure. For a cosmological application a detailed
understanding of the underlying selection effects is crucial. For example, comparing
observed and derived quantities (e.g., luminosity and total cluster mass) often assumes
that the clusters are in a similar stage of their dynamical and chemical evolution.
However, the formation process, including major and minor mergers, can cause
deviations from the expected behaviors, and the effect on cosmological parameters
has to be taken into account for cosmological X-ray studies using galaxy clusters (e.g.,
as demonstrated in Schellenberger & Reiprich, 2017).
I have started an investigation of selected galaxy clusters, that appear under-luminous
in the X-ray band, compared to the expectation from the Sunyaev-Zeldovich (SZ)
effect. These objects are likely in an early stage of their formation process. I
successfully proposed for 143ks of XMM-Newton observing time to study a selected
sample of 13 interesting objects that will help to answer the important questions: Is
there a class of objects that is missed or not included in standard scaling relations?
How efficient is this selection in finding clusters with radio relics and shocks, which are
rare still not well understood? The obtained X-ray data will help to quantify the energy
distribution, study thermal heat transport and enrichment evolution. Furthermore, I
successfully observed X radio features with Giant Metrewave Radio Telescope (GMRT,
total of 20 hours observing time).
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Abell 2108
In a detailed pilot study (Schellenberger et al. 2022 ApJ 925, 1, 91) I focuse on one of the most extreme outliers in
this sample, Abell 2108. It turns out that this object is a highly disturbed galaxy cluster
merger (see image above; X-ray filled contours in blue, radio contours in red, with
regions of interest marked). Mergers between galaxy clusters often drive shocks into
the intracluster medium, the effects of which are sometimes visible via temperature and
density jumps in the X-rays, and via radio emission from relativistic particles energized
by the shock’s passage. The XMM-Newton data (PI: Schellenberger) confirmed it to be
a merging low-mass cluster featuring two distinct sub-clusters, both with a highly
disturbed X-ray morphology. The deep, pointed GMRT data in 3 bands (covering 120–
750 MHz, all PI: Schellenberger) show an extended radio feature resembling a radio
relic near the location of a temperature discontinuity in the X-rays. Abell 2108 has been
discovered to be one of the few low-mass mergers which likely hosting a radio relic
with an exceptionally steep spectrum. To account for the shock/relic offset, we
propose a scenario in which the shock created the relic by re-accelerating a cloud of
pre-existing relativistic electrons and then moved away, leaving behind a fading relic.
The electron-aging timescale derived from the high-frequency steepening in the relic
spectrum is consistent with the shock travel time to the observed X-ray discontinuity.
Recently, we were awarded deep Chandra follow-up observations to locate the X-ray edge.
Observations are planned to start in 2025.
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X-ray observations of galaxy clusters provide the unique potential of measuring the mass of the hot intra
cluster medium and the total gravitating mass assuming hydrostatic equilibrium. This information can be
used to infer cosmology and give an independent measurement of parameters like ΩM and σ8 . But any
interpretation of X-ray data depends crucially on the accuracy of the instrumental calibration. If those
uncertainties are not explicitly incorporated, unknown biases will be introduced.
Despite the expensive ground calibrations before launch and regular observation of calibration targets,
there are still significant differences between the current major X-ray instruments as shown in Nevalainen
et al. (2010) and Kettula et al. (2013). These studies have only shown the pure effective area calibration
uncertainties and the impact and temperature and flux measurements with small samples (N = 11).
In Schellenberger et al. (2015) we have shown in detail the calibration uncertainties between the five X-ray instruments,
EPIC-MOS1/MOS2/PN onboard XMM-Newton and ACIS-I/S onboard Chandra and quantified the influence
on temperature measurements and the cosmological parameters.
The stacked residuals ratio quantifies the energy dependence of the calibration uncertainties between two instruments, Chandra ACIS and XMM-Newton PN in this case. The steep gradient between 1 and 3 keV results in the observed temperature differences.
Similar studies have recently been performed to add NuSTAR, Potter et al. (2023), and eROSITA, Migkas et al. (2024).
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HRC Operations for the Chandra X-ray Observatory — diagnosis and innovative anomaly resolution, health and safety monitoring, calibration techniques, algorithms, and analyses.
Student/PostDoc advisory, (Co-)Investigator on grants/projects totalling over $1.2M
Communication
education/lecturing – numerous press releases including Astronomy Now, Chandra CXC, Chandra Science Highlight, and interviews with Quanta and Forbes online
Synergy
Member on advisory committees invited expert on time allocation committees
Skills
Programming Python/C/IDL – writing papers, proposals – presentations – projects involving AI techniques – analysis of large and noisy datasets (X-ray, radio, sub-mm)
Contact
email: schellengerrit.science
Academic career
Center for Astrophysics | Harvard & Smithsonian
since 2023: Chandra HRC Instrument Project Scientist
2021 - 2023: Astrophysicist
2016 - 2021: SAO PostDoc Fellow
2016: PostDoc Bridge by Bonn-Cologne Graduate School of Physics and Astronomy
Our study (blue ellipse) defines a small parameter space for the cosmological parameters. Which can be combined with other probes, like primary CMB anisotropies (red ellipse), to further achieve high precision cosmology.
Spatial distribution of the clusters in Galactic coordinates (blue are "old" HIFLUGCS clusters).
Cosmology prediction for eHIFLUGCS compared to the current reference. 
The stacked residuals ratio quantifies the energy dependence of the calibration uncertainties between two instruments, Chandra ACIS and XMM-Newton PN in this case. The steep gradient between 1 and 3 keV results in the observed temperature differences. 
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