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Dr Sarah Ragan

Dr Sarah Ragan

Astronomy Group

School of Physics and Astronomy

+44 (0) 29 2087 4289
N/3.20, Queen's Buildings - North Building, 5 The Parade, Newport Road, Cardiff, CF24 3AA
Available for postgraduate supervision


I am a Lecturer in the School of Physics and Astronomy.  My main research focus is determining the conditions necessary for star formation and how they vary with galactic environment. I use a wide range of observational techniques to address this question, from molecular line to fine structure line emission to probe the full range of physical conditions in the ISM.

Academic Duties

  • Undergraduate admissions tutor
  • Member of the astronomy postgradute progression panel
  • Astronomy outreach coordinator


Academic Career

  • 2018 - present : Lecturer, Cardiff University, School of Physics and Astronomy, Cardiff, UK
  • 2016 - 2018: Marie Skłodowska-Curie Fellow, Cardiff University, School of Physics and Astronomy, Cardiff, UK
  • 2014 - 2016: Postdoctoral Research Assistant, University of Leeds, School of Physics and Astronomy, Leeds, UK
  • 2011 - 2014: Deutsche Forschungsgemeinschaft (self-funded) Postdoctoral fellow, Max Planck Insitut für Astronomie, Heidelberg, Germany
  • 2010 - 2011: Star and Planet formation postoctoral fellow, Max Planck Institut für Astornomie, Heidelberg, Germany


  • 2009: PhD (Astronomy & Astrophysics) University of Michigan, Ann Arbor, MI, USA
  • 2003: BSc ( [1] Astronomy, [2] Physics and [3] Mathematics ) Drake University, Des Moines, IA, USA


  • September 2017 - Cardiff Galactic Star Formation workshop [chair]
  • EWASS 2017 (Prague) Symposium: Comparing simulations and observations of the varying scales of star formation [SOC]
  • EWASS 2017 (Prague) Special Session: Understanding the environmental dependence of star formation: the importance of Big Data [SOC]
  • EWASS 2016 (Athens) Symposium: The Dynamics of Star and Planet Formation [SOC]
  • EWASS 2015 (Tenerife) Symposium: The Formation and Destruction of Molecular Clouds [SOC]
  • October 2015 - ViaLactea workshop (Leeds): Extended Strutures in Galactic Molecular Clouds [LOC]
  • June 2014 - Early Phases of Star Formation (Ringberg Castle) [LOC]
  • July 2013 - Protostars and Planets VI (Heidelberg)  [LOC]
  • July 2012 - Galactic Scale Star Formation (Heidelberg) [LOC]
  • June 2012 - Early Phases of Star Formation (Ringberg Castle) [LOC]

Honours and awards

  • 2016 Marie Skłodowska-Curie Research Fellowship (Cardiff University, 2 years funding)
  • 2011 Deutsche Forschungsgemeinschaft Grant (Max Planck Institut für Astronomie, 3 years funding)
  • 2009 Ralph Baldwin dissertation award (University of Michigan, prize)
  • 2007 Spitzer Space Telescope archival research grant (1 year funding)
  • 2006 Green Bank Telescope student support grant (1 year funding)

Speaking engagements

  • Colloquium (SFB956): "Linking the Initial Conditions of Star Formation to Galactic Environment", Universität zu Köln, Physikalische Institute, Köln, GERMANY, 27 May 2019
  • Colloquium: "Connecting the initial conditions of star formation to their Galactic origins" University of Kent, Canterbury UK, 13 February 2019
  • Seminar: "Connecting the initial conditions of star formation to their Galactic origins" University of Leicester, Leiceser UK, 16 January 2019
  • Invited review: "Molecular clouds and filaments," Edinburgh, UK, 3rd September 2018. "The Wonders of Star Formation"
  • Contributed talk: "The role of spiral arms in Milky Way star formation", Grenoble, FRANCE, 29th August 2018. "Star Cluster Formation: Mapping the first few Myrs"
  • Seminar: "Connecting the initial conditions of star formation to their Galactic origins" University of Nottingham, 17th January 2018
  • Invited talk: "The Initial Conditions of Stellar Cluster Formation: Gas Observations", San Lorenzo de El Escorial, SPAIN, 12th June 2017. "Star Cluster Formation: Mapping the first few Myrs"
  • Contributed Talk: "The role of spiral arms in star formation in the Milky Way", Firenze, ITALY, 7th June 2017. "Francesco's Legacy: Star Formation in Space and Time"
  • Invited Review: "Giant Molecular Filaments in the Milky Way", Morelia, MEXICO, 4th April 2017. "Multi-scale star formaiton"
  • Contributed Talk: "Connecting the initial conditions of star formation to their Galactic origins", Köln, GERMANY, 14th February 2017."The Physics of the ISM: 6 years of ISM-SPP 1573: What have we learned?"
  • Seminar: "Connecting the initial conditions of star formation to their Galactic origins", University of Sheffield, UK, 7th December 2016.
  • Invited talk: "The role of spiral arms in star formation", Rome, ITALY, 28th September 2016. "VIALACTEA 2016: The Milky Way as a Star Formation Engine"
  • Contributed Talk: "Linking Galactic structure to star formation in the Milky Way", Stockholm, SWEDEN, 23rd August 2016. "How Galaxies Form Stars"
  • Seminar: "Galactic scale trends of star formation in the Milky Way plane", MPIA, Heidelberg, GERMANY, 4th February 2016.
  • Invited Keynote: "Galactic Studies of Fine Structure Lines", Heidelberg, GERMANY, 9th June 2015. "FIR Fine Structure Line Workshop"
  • Seminar: "Observing the life-cycle of star-forming molecular clouds", Cardiff University, UK, 4th March 2015.

Committees and reviewing

  • Astronomy outreach coordinator, Cardiff University
  • Grant reviewer, STFC
  • Journal referee, ApJ, A&A, MNRAS

















  • Module organiser for PX2239: Observing the Universe
  • Module organsier for PX2139: Observational Techniques in Astronomy
  • Deputy module organsier for PX3152: Galaxies and Galaxy Evolution

BSc (Year 3) projects

How do molecular clouds make clusters?

Star clusters form as a result of the gravitational collapse of giant molecular clouds. Recent observations have shown us that the classical theoretical paradigm of star formation— the spherical collapse of pre-stellar cores — leaves out important physical factors, such as the tendency for clouds to show a strong filamentary morphology, that play a central role in how star formation proceeds. New data obtained with the Herschel Space Observatory provides us with the largest ever census of protoclusters embedded in clouds of a range of morphologies. Because we are observing the clouds at the earliest phase of their evolution, we can reliably connect the locations of these embedded protoclusters to the shape and size of the cloud, i.e. before the onset of cloud-destroying stellar feedback.  In the project, the student will statistically analyse the cloud structure and relate it to the spatial distribution of protoclusters, with the aim of determining if and how the spacing and frequency of protoclusters within clouds varies with morphology. This project will require students to use several standard astronomy software packages and apply some basic statistical methods, which will require some basic programming.

MSc projects

The impact of outflows from forming massive stars

When stars are born deeply embedded in their natal molecular clouds, they return some energy to their surroundings in the form of radiation and mechanical feedback from outflows. However, the magnitude and extent of these feedback events are not yet well-constrained by the observations. In order to investigate the signatures of feedback processes in the high-mass case, we have obtained observations of a well-studied embedded high-mass protostar, HMPO 18223-3 in four high-frequency CO transitions using the APEX telescope using the FLASH and CHAMP+ instruments. Each CO transition between adjacent energy levels is excited in gas above a specific temperature, such that the higher CO transitions require higher temperature gas to excite them. Molecular gas entrained in protostellar outflows are well-known examples of where the full range of CO transitions are excited, and they can be used to characterise the physical conditions in the immediate vicinity of protostars, such as the temperature and density structure. These observations, together with a wealth of complementary data from Herschel and ground-based observatories set the stage for a look at how the effects of feedback scale in high-mass protostars in unprecedented detail.

The first task will be to reduce the raw APEX data. This will produce maps of each of the four observed transitions: CO (7-6), (6-5), (4-3) and (3-2). With these maps, it will be immediately possible to measure the spatial extent of warm gas around the protostar. This step will acquaint the student with widely-used sub-millimetre data reduction software (the GILDAS software package). In regions where multiple CO transitions are detected, excitation analysis will enable measurement of the gas temperature and density. The second task involves extracting line properties from the data, running a small grid of basic line radiative transfer models (the use of publicly-available online software is suitable for this problem), and deriving the best fit physical parameters from the set. This part of the project emphasises the importance of marrying observational data with models in order to extract physical information about a problem. This is a self-contained project which takes the student through all steps of the scientific process and will require software development and literature review.

My research: observing Galactic star formation

My main research interests lie in the conditions necessary for star formation. I am involved in several projects with many international collaborators. I highlight below the projects that I've been leading recently. Please contact me if you are interested or want to inquire about possible projects!

Galaxy-scale star formation

Galactic plane surveys enable us to study the nature of star formation throughout the Milky Way over kiloparsec scales. The Herschel Space Observatory has conducted a survey of the entire Milky Way plane in the far-infrared wavelength regime. These wavelengths cover the peak of the spectral energy distribution of thermal emission from cold dust grains. Compact sources at these wavelengths represent the regions in the Galaxy which have the cold, dense conditions necessary for star formation.

In Ragan et al. (2016), we present large-scale trends in the distribution of star-forming objects revealed by the Hi- GAL survey. As a simple metric probing the prevalence of star formation in Hi-GAL sources, we define the fraction of the total number of Hi-GAL sources with a 70 μm counterpart as the ‘star-forming fraction’ or SFF. The mean SFF in the inner galactic disc (3.1 kpc < RGC < 8.6 kpc) is 25 per cent. Despite an apparent pile-up of source numbers at radii associated with spiral arms, the SFF shows no significant deviations at these radii, indicating that the arms do not affect the star-forming productivity of dense clumps either via physical triggering processes or through the statistical effects of larger source samples associated with the arms. Within this range of Galactocentric radii, we find that the SFF declines with RGC at a rate of −0.026 ± 0.002 per kiloparsec, despite the dense gas mass fraction having been observed to be constant in the inner Galaxy. This suggests that the SFF may be weakly dependent on one or more large-scale physical properties of the Galaxy, such as metallicity, radiation field, pressure or shear, such that the dense sub-structures of molecular clouds acquire some internal properties inherited from their environment.

Giant filaments in the Milky Way

Throughout the Milky Way, molecular clouds typically appear filamentary in morphology on what seems like all possible scales. Using the wealth of Galactic plane survey data, we have identified velocity-coherent filaments on up to 100-pc size scales. This discovery enables us to begin connecting the ubiquitous filamentary clouds to Galactic structure. In Ragan et al. (2014) we identify and characterise the first sample of giant molecular filaments (GMFs) in the Galaxy. Many GMFs are clearly aligned with spiral arms but some are squarely in inter-arm regions of the Galactic plane. We find the GMFs in the spiral arms have a higher fraction of their mass in the densest structures, so-called "clumps". GMFs are an important new laboratory in which we can gain a greater understanding of how molecular cloud and star formation depends on their Galactic environment.

Fine structure line cooling in Galactic dark clouds

Stars are born in the densest regions of MCs, but the process appears to be very inefficient, with MCs converting only a few percent of their gas budget into stars per dynamical time. The underlying physical processes that regulate the star formation rate (SFR) in the ISM are still unknown and hotly debated, with candidates ranging from turbulence and magnetic fields to stellar feedback. Further debate stems from the observational evidence that while the “dense” regions in MCs in the solar neighbourhood appear to explain the observed galactic star formation relations, the same approach fails to explain the SFR towards the central molecular zone (CMZ) of the Milky Way. The primary reason for this debate is that we still do not understand how MCs are assembled and destroyed — the two processes that ultimately set the timescale over which a cloud can form stars. The problem is that carbon monoxide (CO), the main tracer of MC structure and dynamics, is only sensitive to the cold interiors of MCs and not their envelopes, and models show that CO may form relatively late in the assembly process. Therefore, alternative tracers that can probe gas in the absence of CO — so-called “CO-dark” gas — are needed to make further progress in understanding MC formation and destruction.

Fine structure line (FSL) tracers such as ionised and atomic carbon ([CII] and [CI]) and atomic oxygen ([OI]) are the key probes of the earliest stages of cloud assembly. These lines are important for several reasons. First, they are able to trace the low-density transition from atomic to molecular gas that marks the boundary from the warm ISM to the cold reservoirs in which stars form. Second, they constitute the main coolants of ISM during this transition, thus providing a way to measure the energetics of the ISM. Finally, due to the different excitation properties of the lines, they allow us to distinguish between different temperature and density regimes in the ISM.

In Beuther, Ragan et al. (2014), we conduct a pilot study of a sample of quiescent molecular clouds in tracers of all carbon phases. Our study revealed that the tracers show a range of behaviours depending on their environment. In one cloud (see figure) the [CII] emission shows intriguing signs of dynamical cloud formation, with strong emission on either side of the dense gas probed with CO. This could be cloud formation "caught in the act"! Our follow-up SOFIA observations of a larger area will help us disentangle the picture further... Stay tuned!

Areas of expertise