3/1/18 LETTER. Instructors: Jim Cordes & Shami Chatterjee. Reading: as indicated in Syllabus on web

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1 Astro 2299 The Search for Life in the Universe Lecture 9 Last time: Star formation Formation of protostars and planetary systems This time A few things about the epoch of reionization and free fall times Magnetic fields in the universe, including planets Reading: as indicated in Syllabus on web Assignment 3 will be posted next week Midterm exam March 15 Instructors: Jim Cordes & Shami Chatterjee 1 LETTER 1 MARCH 2018 VOL 555 NA TUR E 67 doi: /nature25792 reserved. An absorption profile centred at 78 megahertz in the sky-averaged spectrum Judd D. Bowman 1, Alan E. E. Rogers 2, Raul A. Monsalve 1,3,4, Thomas J. Mozdzen 1 & Nivedita Mahesh 1 After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz 1. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting fullwidth at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars; however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions 2. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude 3. The lowfrequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later. Observations with the Experiment to Detect the Global Epoch of Brightness temperature, T 21 (K) H1 H2 H3 H4 H5 H6 P Age of the Universe (Myr) 22 Redshift, z Figure 2 Best-fitting 21-cm absorption profiles for each hardware case. Each profile for the brightness temperature T21 is added to its residuals and plotted against the redshift z and the corresponding age of the Universe. The thick black line is the model fit for the hardware and analysis configuration with the highest signal-to-noise ratio (equal to 52; H2; see Methods), processed using MHz and a four-term polynomial (see equation (2) in Methods) for the foreground model. The thin solid lines are the best fits from each of the other hardware configurations (H1, H3 H6). The dash-dotted line (P8), which extends to z > 26, is reproduced from Fig. 1e and uses the same data as for the thick black line (H2), but a different foreground model and the full frequency band. properties of an individual cloud. The observed absorption profile is

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4 High velocity neutron stars Palomar H image Hubble H images The Guitar Nebula Bow shock from high velocity motion through the ISM ~ 1000 km/s High velocity NS will escape the Galaxy High velocity from asymmetry in the supernova collapse Magnetic Fields 11 4

5 Relevance of Magnetic Fields Magnetic fields are important for formation and evolution of protoplanetary disks (transferring angular momentum so that the disk can contract Cosmic rays are trapped in the Galaxy by its magnetic field; some CRs cause mutations in the biosphere Earth s magnetic field protects the biosphere from most (but not all) CRs and from particles from the Sun Navigation: some birds (magnetite in their brains) human Magnetic fields Neutron stars G (Gauss) White dwarfs up to 10 8 G Sun ~ kg sunspots Earth ~ 1 G Interplanetary medium ~ 10-5 G Interstellar medium ~ few x 10-6 G Intergalactic medium ~ 10-9 G 1 Tesla = 10 4 G 5

6 An accreting neutron star (emission = gravitationally driven) 14 Neutron star as a radio pulsar (rotation driven)

7 Magnetic Fields of Planets Measuring Magnetic Fields Polarization of starlight Optical light is polarized by interstellar dust grains that are aligned by the interstellar magnetic field, B Polarized radio waves emitted by relativistic electrons in interstellar B (synchrotron radiation) Zeeman splitting of spectral lines Atoms in magnetic fields have spectral lines that divide according to B and the orbital angular momentum of the electrons (e.g. 21 cm atomic line of H) Faraday rotation Rotation of the angle of polarization as radio waves travel through magnetized, ionized gas 17 7

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14 Cosmic ray intensities (left) compared with predictions (right) from IBEX. The similarity between these observations and predictions as evidenced by the similar color regions supports the local galactic magnetic field direction determined from IBEX observations made from particles at vastly lower energies than the cosmic ray observations shown here. The blue area represents regions of lower fluxes of cosmic rays. The gray and white lines separate regions of different energies lower energies above the lines, high energies below. Credit: Nathan Schwadron, UNH-EOS. Read more at: 14

15 Main points from recent lectures Stars and planets form out of molecular clouds The Jeans mass is the minimum mass core or sub-cloud that can collapse. It depends on the temperature and size of the core. Molecular clouds are cold and dense (by necessity) so the Jeans mass ~few solar masses Collapse requires getting past the centrifugal barrier imposed by angular momentum This requires that mass in the inner parts of the region must transfer its angular momentum to the outer parts. This occurs via magnetic fields that thread the gas and also by jets that carry away some of the angular momentum Disks and jets are generic aspects of collapse and accretion on galactic scales (super-massive black holes [M87]) and protostars. Also neutron stars and stellar-mass black holes Planets form bottom-up : Rocky planets: planetesimals à proto-planets à planets Gas giants: rocky cores + accreted gas The final configuration of planets is related to the Hill sphere size of each planet and gravitational encounters in the stages leading to the final configuration 15

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17 Simulations of Planet Formation Phil Armitage 17