The potential for Galactic Plane Surveys with LOFAR

1. The potential for Galactic Plane Surveys with LOFAR Wijnholds 2006 Glenn White – The Open University UK, and The Rutherford Appleton Laboratory UK On behalf of the Galactic Astronomy Work Group in the Cosmic Magnetism KSP Wolfgang Reich, Jacco Vink, Dave Green, Huub Rottgering, Glenn White, Richard Strom, Rainer Beck

2. Extragalactic and Galactic sky NEP WSRT SEP ATCA Kassim et al 1999 La Rosa et al 1999 Galactic Nucleus White et al 2008

3. The interstellar medium is the environment in which the processes of galaxy evolution occur • Many states • atomic and ionized hydrogen - relativistic plasma - molecular gas, and dust • A highly disturbed state ISM is maintained by: • energy input from stars at all phases of the stellar life cycle • global, large-scale phenomena, such as viscous dissipation or magnetic stress from Galactic rotation • motion of spiral arm density waves • Interaction with cold gas that is self-gravitating to form stars → Fundamental pre-requisite to understand galaxy evolution, cosmic history of star formation Current (scarce) observations can guide us to the potential for LOFAR surveys – as long as we can deal with depolarisation and scatter broadening - relatively little known so far about the < 300 MHz ISM, except from low resolution studies

4. Magnetic fields and HII regions • H II regions can constrain source electron temperatures, emission measures, and filling factors. Polarisation -> and depolarisation • Synchrotron emission measures the field strength, while its polarization yields the field's orientation in the sky plane and also gives the field's degree of ordering. Faraday rotation yields a full three-dimensional view by providing information on the field component along the line of sight, while the Zeeman effect provides an independent measure of field strength in cold gas clouds. • At lower frequencies HII regions appear as cooler regions against a much hotter Galactic background. Below 100 MHz thermal gas becomes opaque • kinematic distance ambiguities resolved Roy et al 2006 GMRT White et al 2007 - CBI polarisation UTR-2 at 15 MHz: HII regions Sharpless 117 and 119 in absorption, angular resolution ~ 2 degrees - LOFAR will be used to map out the 3-D distribution of the cosmic-ray electron gas

5. SNR and star formation tracers 325 MHz Brogan et al 2006 • Energy spectra of shock accelerated relativistic electrons • Probe the complex modulation of blast-wave physics by the pre-existing environment into which remnants evolve tracing ionised inhomgeneous medium along the line of sight • Extended low density halos – WIM and other phases – magnetised bubbles • Star forming complexes, Super Star Cluster environments Brogan et al 2008

6. Recombination Lines • Radio recombination lines of H, He, C, can trace the temperature, kinematics, and ionization structure as well as abundances of heavy elements in ionised gas. Below ~ 75 MHz, the Galactic background becomes brighter than an average HII region • Strong radio lines are found in HII regions, but narrow, weak lines are also found in the very diffuse, ionized gas that pervades the Warm Ionized Medium WIM • As interstellar carbon recombines into very high Rydberg states (up to n = 768), absorption lines below 150 MHz are generated. The C atoms in these high states are very sensitive to the interstellar environment and permit excellent measurements of density, temperature, and ionisation levels to be carried out (Payne et al. 1994). • Below ~120 MHz, no Hydrogen RRLs are seen and the Carbon RRLs appear in absorption because the excitation temperature approaches the kinetic temperature, permitting direct and accurate measurements of pressure broadening in CII regions (Anantharamaiah, et al, MNRAS, 235, 151; Erickson, W.C. et al. 1995, ApJ, 454, 125), whereas above 120MHz, the population levels are inverted and RRLs are seen in emission 34 MHz 327 MHz

7. LOFAR observations of Galactic emission will provide 1: • Unique diagnostics in an unexplored parameter space able to capitalise on the interaction of radiation with the surrounding plasma – energy deposition/exchange ionised/atomic/molecular • Diffuse Galactic Emission at high latitude where confusion and depolarisation less • Abillity to probe the ionised material edges of protostellar forming regions and feedback processes • Good brightness sensitivity, the cross over with the survey, short baselines • Local interstellar cloud study, which has implications for nearby galaxies, as well as polarisation structure and clumpiness. • Not all galactic sources in the galactic plane may be polarised – some complimentary overlap with the Surveys KSP – but from a different angle. • Use of dynamic processes (shocks, SNRs to probe) the warm and cold ionised medium, and its relationship to star forming material • 260 known remnants, add spectral range -> structure/excitation of synchrotron emission mechanism • Surveys to identify all SNRs – missing young, small and distant remnants, probably due to selection effects such as confusion in the galactic plane on any given line of sight. • PWNe • Wind Nebulae, Pulsars and Nebulae, Old wind nebulae. In x-rays many regions are extended. At low radio frequencies will LOFAR will explore the low energy electron population – interesting to look at the spectral index evolution with LOFAR. A galactic census, providing a different angle on the occurrence and evolution of SNRs, spectral breaks, and search for expected high polarisation.

8. LOFAR observations of Galactic emission will provide 2: • Star Formation • Survey chromospheres around young and active stars • Recombination lines of H, He, C as ionisation tracer • High resolution thermal imaging of HII regions and the photo ionised plasma • Carbon RRLs • Radio Recombination Lines (RRL) are important probes of the ISM uniquely sensitive to physical conditions such as density and temperature and ionisation state • Supershells and massive star formation morphology • Strong evidence for coherent shells in radio emission around the Galactic Centre, and on the large scale through super star clusters and larger galactic scale supershells • Legacy and Synergy to other extant and future surveys • AKARI – HERSCHEL – PLANCK - JCMT

9. Technical Challenges being worked on that are important to this work • Polarisation sensitivity on large scales (diffuse structure) • Beam depolarisation • Data storage – unclear how much processing outside the pipeline may be needed • Brightness sensitivity – optimising array configurations and baseline redundancy • Survey design and integration times • Galactic Depolarisation • Ionospheric Faraday Rotation • Solar maximum

10. Summary • Galactic Observations link to fundamental processes that describe: • Energetics and structure of Galactic Nuclei • Assembly of galactic complexes • Star formation and supernova rates • → central to understanding more distant objects where we lack spatial resolution • Key LOFAR observations (diffuse synchrotron, thermal absorption, RM synthesis) are capable of studying a range of targets that will revolutionise understanding, including 3D tomographic and magnetic field structures of: • HII regions and protostellar disks • Interstellar turbulence on the large scale • SNRs, PWNe and Planetary Nebulae • Polarised emission of the (nearby) diffuse ISM • Carbon recombination lines • Work ongoing within the Cosmic Magnetism KSP to coordinate these studies • LOFAR will a major new facility for the study of Galactic phenomena, that has major implications for all other areas of astronomy on the wider scale

12. 74-330 MHz spectral index grey-scale map of Cas A – dark corresponds to a flatter spectrum, or free-free absorption from unshocked ejecta interior to the reverse shock (Kassim et al 1995). 74-330 MHz spectral index map of W49B revealing the first spatially resolved detection of ISM thermal absorption towards a Galactic SNR (Lacey et al. 2001).

13. At 25MHz, Tint=13 hours, Beam=21’, Number of lines = 20 • At 75MHz, Tint=442 hours, Beam=7’, Number of lines = 16 • At 120MHz, Tint=11 hours, Beam=4.5’, Number of lines = 10 • At 200MHz, Tint=52 hours, Beam=2.5’, Number of lines = 10 • Doeleman 2006

14. Below ~75MHz, the Galactic background becomes brighter than an average HII region (Te~10,000K) and Carbon absorption RRLs can be identified over large sections of the Galactic plane Integration times 10 – 40 hours – difficult as part of a main survey, but feasible and science driven to unique diagnostics (Doelman et al 2006) Roelfsma & Goss 1992 T_e to a few 100 K and n_e a few cm-3 Distribution of low density multiphase ISM