Dwarf Galaxy Research Group



We seek to understand the environmentally influenced evolution of small, "dwarf" galaxies, which are so abundant in the present-day Universe. We investigate the structure and internal dynamics of dwarf galaxies through multiwavelength imaging and spectroscopic observing campaigns like SMAKCED (Stellar content, MAss and Kinematics of Cluster Early-type Dwarf galaxies) and FDS (Fornax Cluster Deep Survey), the influence of environment on dwarf galaxies and their progenitors through an analysis of galaxy clusters and groups with different characteristics, and the connection between the dark matter halo population and the visible dwarf galaxies, based on cosmological simulations and galaxy evolution models.


→ Metals in Dwarf Galaxies: Indications for Environmental Influence (07 May 2018)

→ The Puzzle of Ultra-Diffuse Galaxies (21 Aug 2017)

→ Access our posters and talks that we prepared for the conference "Galaxy Clusters Across Cosmic Time" (05 July 2017)

→ 14 PhD positions in EU network SUNDIAL, joins computer science and astronomy for galaxy evolution studies (23 Nov 2016)

→ Dwarf galaxy workshop focuses on combining knowledge, fostering exchange, and discussing future projects (04 Oct 2016)

→ Destroyed dwarf galaxies or massive star clusters? New insights into compact stellar systems (23 May 2016)

→ European Commission approves new Innovative Training Network SUNDIAL (02 May 2016)

Metals in Dwarf Galaxies: Indications for Environmental Influence

7 May 2018

Galaxies show many characteristics that depend on their total mass. For example, more massive galaxies are typically larger in size, their stars are more densely packed, and they produced a higher percentage of chemical elements heavier than helium, commonly subsumed under the term "metallicity". The observed correlation of stellar metallicity and stellar mass is one of the fundamental relations of galaxies. However, in the present-day universe, the relation is found to be rather broad, with a large scatter in metallicity at a given galaxy mass. The actual cause for this appearance can be difficult to reconstruct observationally. Therefore, we approach the problem from the simulation side.

By using the state-of-the-art cosmological simulation Illustris, we were able to directly examine the evolution of almost 500 dwarf galaxies in a cluster environment. In order to identify the processes responsible for the broad relation today, we determined the mass-metallicity relation with the smallest intrinsic scatter. As it turned out, the most narrow relation appeared when we considered each galaxy at a different point of time: namely when it reached the peak of its stellar mass. This relation would not be accessible by observational means.

But what happens after the galaxies reach their respective peak mass? Once they enter a massive cluster environment, the galaxies get stripped of most of their gas, leaving them unable to form any new stars. More importantly, the tidal forces that pull on the galaxies after they fell into the cluster eventually cause the stripping of stars and thus the decrease of stellar mass. We find that this process plays a dominant role in shaping the present-day mass-metallicity relation. So when trying to understand the evolution of galaxies from the Big Bang until today, we obviously need to take into account what the environment did to them.

Mass-metallicity relations (MZR) for Illustris low-mass galaxies residing in a cluster of 3.8·1014 Msun at redshift z = 0, color-coded by early (z >= 0.76, lookback time >= 6.65 Gyr) and late infall into a group of at least 1012 Msun . Left: MZR of all galaxies at z = 0. Right: MZR using masses and metallicities at the galaxies' individual times of peak stellar mass. (Credit: Christoph Engler, ZAH).

Original publication:
C. Engler, T. Lisker, A. Pillepich: On the Scatter of the Present-day Stellar Metallicity–Mass Relation of Cluster Dwarf Galaxies.
2018, Research Notes of the American Astronomical Society, Vol. 2, Art. 6

The Puzzle of Ultra-Diffuse Galaxies

21 Aug 2017
→ Heidelberg University Press Release          → ING News Release                        

Heidelberg astronomers have discovered large, yet barely shining galaxies where nobody would have expected them – in the central region of a huge galaxy cluster. This discovery of so-called ultra-diffuse galaxies is both spectacular and puzzling.

Our solar system is located in a spiral galaxy composed of billions of stars, the Milky Way. With the naked eye, we can see some 3.000 stars in a dark night. However, if Earth would reside within an ultra-diffuse galaxy, we would only spot a few dozen stars on the sky. Galaxies of this type were either not able to produce more stars in the first place, or they got stripped of them by tidal forces.

Intriguingly, though, larger telescopes and improved imaging techniques have recently led to the discovery of many ultra-diffuse galaxies in the harshest environments possible: galaxy clusters. "We have been asking ourselves how those fragile objects are able to survive among such dense, massive accumulations of hundreds of large and small galaxies," explains Carolin Wittmann, PhD student at the Astronomisches Rechen-Institut (ARI) of the Zentrum für Astronomie der Universität Heidelberg (ZAH). Using very deep optical images obtained in 2012 with the Prime Focus Camera of the 4.2-metre William Herschel Telescope on the Canary Island of La Palma, Ms. Wittmann identified about 90 such galaxies in the core of the Perseus Cluster, 240 million lightyears away.

"Surprisingly, most galaxies appear intact – only very few show signs of ongoing disruption," emphasizes Dr. Thorsten Lisker, who initiated the project. If this means that the ultra-diffuse galaxies can withstand the strong tidal field of the Perseus Cluster, then they must contain a large amount of unseen mass – dark matter – whose gravitational attraction acts as a binding force. Along with international partners, the researchers are now hoping to obtain data of similar quality on the outskirts of the Perseus Cluster, where the environmental influence would have been less strong, preserving more of the original structure of the galaxies.


Fig. 1: The central region of the Perseus galaxy cluster. This mosaic image is composed of many "tiles", individual deep exposures which were taken with the 4.2-metre William Herschel Telescope on La Palma. Ultra-diffuse galaxies are hard to spot, which is illustrated in the enlarged region containing one of the newly discovered faint objects. Projected on the sky the entire image has about the diameter of the full moon (Credit: Carolin Wittmann, ZAH).

Fig. 2: A gallery of several ultra-diffuse galaxies discovered in the Perseus galaxy cluster. These objects are barely visible against the background. Diffuse bright spots are foreground stars in the Milky Way (Credit: Carolin Wittmann, ZAH).

Original publication:
C. Wittmann, T. Lisker, L. Ambachew Tilahun, E.K. Grebel, C. Conselice, et al.: A population of faint low surface brightness galaxies in the Perseus cluster core.
2017, Monthly Notices of the Royal Astronomical Society, 470, 1512

14 PhD positions in EU network SUNDIAL, joins computer science and astronomy for galaxy evolution studies

23 Nov 2016

Our new EU Innovative Training Network "SUNDIAL" currently offers 14 PhD positions across Europe in astronomy and computer science. The application deadline is December 15, 2016:
SUNDIAL website
AAS Job Register entry
It covers topics such as detecting ultrafaint galaxy signals, developing automated models for galaxy recognition and classification, and developing new methods to compare observations and galaxy simulations as well as visualization.

Dwarf galaxy workshop focuses on combining knowledge, fostering exchange, and discussing future projects

04 Oct 2016

A discussion-intensive workshop on the present and future research on dwarf galaxies will be hosted at ESO headquarters (Garching, Germany) from October 10 to 13, 2016, organised by Drs. Eric Emsellem (ESO) and Thorsten Lisker (ZAH). The workshop format will be focusing on interaction and discussion within a small group of 35 invited, international participants, with the intention of promoting open exchange of ideas and defining new challenges for the future. In particular, the current and upcoming capabilities of simulations and instrumentation will be reviewed in the light of our advancing understanding of dwarf galaxy evolution. More information is provided at

Destroyed dwarf galaxies or massive star clusters? New insights into compact stellar systems

23 May 2016

Globular clusters can not only be found around the Milky Way. Thousands of them form vast systems around the huge elliptical galaxies in the centers of galaxy clusters. There, astronomers discovered a new type of object in 1999: massive stellar systems that appear like oversized globular clusters. Are these already galaxies, rather than star clusters? At first, many scientists favoured a scenario in which these objects are remnants of disrupted dwarf galaxies – until a few years ago a plain statistical approach showed that the majority is consistent with simply being the largest members of the globular cluster population. However, we do know from observations and computer simulations that small galaxies occasionally get destroyed in the core regions of galaxy clusters. It thus remained open what fraction the surviving nuclei of such galaxies contribute to the compact stellar systems we observe.

In a recently published study, we analysed several hundred compact stellar systems in the Fornax galaxy cluster, 65 million lightyears from us. We used deep optical images that we acquired in Chile, at the 2.2m telescope of the Max Planck Society and the European Southern Observatory. About two dozen of the objects we observed exhibit an extended and partly distorted outer distribution of stars (Fig. 1), which is probably due to the tidal forces exerted by the galaxy cluster. Surprisingly, these peculiar objects are not located in the very central cluster area – where those forces would be strongest – but they seem to group around it (Fig. 2). Due to their location and motion, we believe that they do not belong to the population of central star clusters, but that they instead are part of the galaxy population. They were originally dwarf galaxies whose orbit carried them too close to the galaxy cluster center, eventually leading to their disruption. Only their stellar nuclei remained, as well as a little bit of material from the former galaxy body, which we are now observing as the faint outer extensions of these objects. To ultimately confirm this hypothesis, we are now hoping for high-quality spectroscopic measurements and high-resolution imaging data of the Hubble Space Telescope, in order to probe the compact stellar systems' age, metal content, and core structure.


Fig. 1: Selection of compact objects with extended and/or distorted structures. To the right of each object we show a reference image for how an unperturbed star cluster-like object would appear at the same location in our data.

Fig. 2: Spatial distribution of objects with extended structures (red), compared to dwarf galaxies (blue) and globular clusters (grey; larger symbols represent the most massive of them).

Original publication:
C. Wittmann, T. Lisker, A. Pasquali, M. Hilker & E.K. Grebel: Peculiar compact stellar systems in the Fornax cluster
2016, Monthly Notices of the Royal Astronomical Society, 459, 4450

European Commission approves new Innovative Training Network SUNDIAL

02 May 2016

Nine academic institutions and five partner organisations from nine European countries will form the new Innovative Training Network SUNDIAL, which the European Commission has now approved for implementation in 2017. SUNDIAL, the SUrvey Network for Deep Imaging Analysis & Learning, brings together a team of leading European astronomers and computational scientists who will embark on a broad and data science intensive research programme. As a multi-disciplinary collaboration, led by the University of Groningen, SUNDIAL will develop innovative computational methods and apply them to large astronomical datasets acquired in observational surveys, thereby training a new generation of computational/astronomy/data scientists. Our dwarf galaxy research group is leading Heidelberg University's involvement in the network, with a project on "Studying galaxy evolution in cluster environments through comparing observed and simulated galaxy and cluster properties."

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