Ewch i’r prif gynnwys
Dr Sarah Ragan

Dr Sarah Ragan

Marie Skłodowska-Curie Fellow

Ysgol Ffiseg a Seryddiaeth

+44 (0) 29 2087 4289
N/4.06, Adeiladau'r Frenhines - Adeilad y Gogledd, 5 The Parade, Heol Casnewydd, Caerdydd, CF24 3AA
Ar gael fel goruchwyliwr ôl-raddedig


I am a Marie Skłodowska-Curie research fellow and lecturer in the School of Physics and Astronomy. My main research focus is determining the conditions necessary for star formation and whether 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.



  • 2016 - present: Marie Sklowdowska-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

Anrhydeddau a Dyfarniadau

  • 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)

Ymrwymiadau siarad cyhoeddus

  • 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: "Connection 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: "Connection 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.

Pwyllgorau ac adolygu

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


















Here you will find an overview of the modules that I am currently teaching in Cardiff, as well as a description of the type of projects I offer for BSci / MPhys placements. Please feel free to contact me if you need any more information.

BSci (Year 3) Projects

Molecules in dark clouds

Stars form in giant clouds of (predominantly) molecular hydrogen (H2), but at the low characteristic temperatures in these clouds, we can not observe H2 directly. The next most abundant molecule in these clouds that we can observe is carbon monoxide (CO). Using maps of two different "types" (called isotopologues) of CO which emit differently depending on the density and temperature, it is possible to study how the physical conditions in molecular clouds change with their environment. In this project, the student(s) will learn to analyse spectral line data by fitting simple models to the emission profile in order to infer physical properties. They will also need to undertake a literature study to relate the findings from the CO analysis to what is already known about the clouds we are studying.

The work involved will be roughly split between literature review (20%), analysing spectral line data (50%) and writing up the results (30%). This project involves doing Python programming.

MPhys (Year 4) / Master's Projects

The dynamics of star forming filaments

Star cluster formation takes place preferentially within dense filamentary structures in the interstellar medium. Our recent discovery of giant filaments (hundreds of lightyears in length) in the plane of our Milky Way galaxy provides us with a way to study the connection between the scales of individual clusters (a few lightyears in size) to spiral structure in the Galaxy (thousands of lightyears in size). Using archival data, the student(s) will analyse the density and velocity structure of degree-scale filaments and compare the results to new observations of denser gas on smaller scales.

The work involved will be roughly split between literature review (20%), analysing spectral line data (50%) and writing up the results (30%). Important aspects of this project include managing and visualising large datasets, working with IDL or Python analysis tools.

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!


Past projects

  • Supervisor for A. Whitney (BSc) - Feedback from massive embedded protostars - Royal Astronomical Society Bursary, University of Leeds (summer 2015)
  • Supervisor for L. Williamson (BSc) - The onset of infall and outflows in dark clouds - University of Leeds (summer 2015)
  • Co-supervisor for J. Abreu-Vicente (PhD) - Giant Molecular Filaments in the Milky Way - Max Planck Institut für Astronomie (2016)
  • Co-supervisor for S. Zahorecz (MSc) - Plateau de Bure observations of dense gas in infrared-dark clouds - Max Planck Institut für Astronomie (2013)
  • Co-supervisor for J. Pitann (PhD) - Automated extraction of compact sources from Herschel PACS continuum data - Max Planck Institut für Astronomie (2013)

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