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Sarah Ragan

Dr Sarah Ragan

Senior Lecturer
Astronomy Group
Cardiff Hub for Astrophysics Research and Technology

School of Physics and Astronomy

Users
Available for postgraduate supervision

Overview

I've been a member of staff in the School of Physics and Astronomy since 2016, and I am currently a Senior Lecturer. Since earning my PhD in 2009 at the University of Michigan, Ann Arbor, I have held reserach positions at Max Planck Institute for Astronomy in Heidelberg, the University of Leeds and Cardiff University. I took on an academic position in 2018. 

My research is on the early phases of star formation, and I use a combination of observational and modeling techniques to understand what conditions in molecular clouds are necessary for star formation, which of those conditions promote the formation of high mass stars in particular, and what causes the formation of the molecular clouds in the first place. Our research has shown that these topics are all interlinked, and connected to the broader environment in the Galaxy as well!

Publication

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Articles

Research

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!

Teaching

  • Module organiser of PX1120 Mathematical Methods for Physicists I
  • Module organiser of PX2155 Observational Techniques in Astronomy, the second-year undergraduate astronomy laboratory module.

Biography

I've been a member of staff in the School of Physics and Astronomy since 2016, and I am currently a Senior Lecturer. Since earning my PhD in 2009 at the University of Michigan, Ann Arbor, I have held reserach positions at Max Planck Institute for Astronomy in Heidelberg, the University of Leeds and Cardiff University. I took on an academic position in 2018. 

My research is on the early phases of star formation, and I use a combination of observational and modeling techniques to understand what conditions in molecular clouds are necessary for star formation, which of those conditions promote the formation of high mass stars in particular, and what causes the formation of the molecular clouds in the first place. Our research has shown that these topics are all interlinked, and connected to the broader environment in the Galaxy as well!

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)

Academic positions

Academic positions

  • 2021 - present: Senior Lecturer, Cardiff University, School of Physics and Astronomy, Cardiff, UK
  • 2018 - 2021 : Lecturer, Cardiff University, School of Physics and Astronomy, Cardiff, UK

Research positions

  • 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

Education

  • 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

Committees and reviewing

  • Director of Undergraduate Studies, School of Physics and Astronomy, Cardiff University (2021-24)
  • Member of the Equity, Diversity and Inclusion committee, School of Physics and Astronomy, Cardiff University (current)
  • Grant reviewer, STFC
  • Journal referee, ApJ, A&A, MNRAS

Contact Details

Email RaganSE@cardiff.ac.uk
Telephone +44 29208 74289
Campuses Queen's Buildings - North Building, Room N/2.15A, 5 The Parade, Newport Road, Cardiff, CF24 3AA

Research themes

Specialisms

  • Data science
  • Astronomical sciences
  • Astronomical instrumentation
  • Galactic astronomy

External profiles