Recent Science Highlights
A Supernova Dust Factory
Supernova (SN) explosions are crucial engines driving the evolution of galaxies by shock heating gas, increasing the metallicity, creating dust, and accelerating energetic particles. In 2012, Indebetouw et al. used ALMA to observe SN 1987A, one of the best-observed supernovae, and acquire spatially resolved images at 450µm, 870µm, 1.4mm, and 2.8mm, an important transition wavelength range. The longer wavelength emission is dominated by synchrotron radiation from shock-accelerated particles; the shorter wavelength emission is from the largest mass of dust measured in a supernova remnant (>0.2 M☉). The authors show unambiguously, for the first time, that this dust has formed in the inner ejecta, the cold remnants of the exploded star's core. The dust emission is concentrated to the center of the remnant, so the dust has not yet been affected by the shocks. If a significant fraction survives, and if SN 1987A is typical, SNe are important cosmological dust producers.
Figure: Composite image of SN 1987A. ALMA data (red) shows newly formed dust in the center of the remnant. HST (green) and Chandra (blue) show the expanding shockwave. Credit: R. Indebetouw et al., A. Angelich (NRAO/AUI/NSF); NASA/STScI/CfA/R. Kirshner; NASA/CXC/SAO/PSU/D. Burrows et al.
View Publication: Dust Production and Particle Acceleration in Supernova 1987A Revealed with ALMA, R. Indebetouw (UVA, NRAO), M. Matsuura (Univ College-London), E. Dwek (NASA-Goddard), G. Zanardo (ICRAR), M.J. Barlow (Univ College-London), M. Baes (Gent), P. Bouchet (CEA-Saclay), D.N. Burrows (PSU), R. Chevalier (UVA), G.C. Clayton (LSU), C. Fransson (Stockholm), B. Gaensler (CAASTRO, Sydney), R. Kirshner (CfA), et al., 2014 ApJL, 782, L2.
Added 21 Jan 2014
A Unique Gravitational Laboratory
Gravitationally bound three-body systems show complex orbital interactions, which can constrain the compositions, masses, and interior structures of the bodies and test theories of gravity, if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, PSR B1620-26 – with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades – shows only weak interactions. Ransom et al. report precision timing and multi-wavelength observations of PSR J0337+1715, the only known millisecond pulsar in a triple system, recently discovered as part of a large-scale pulsar survey with the GBT. Strong gravitational interactions are apparent and provide the masses of the pulsar the two white dwarf companions, as well as the inclinations of the orbits. The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory to test the strong equivalence principle of General Relativity.
Figure: A millisecond pulsar (left foreground) is orbited by a hot white dwarf star (center), both of which are orbited by another, more distant and cooler white dwarf (top right). Credit: Bill Saxton; NRAO/AUI/NSF.
View Publication: A Milli-second Pulsar in a Stellar Triple System, S.M. Ransom (NRAO), I.H. Stars (UBC), A.M. Archibald (ASTRON, McGill), J.W.T. Hessles (ASTRON, Amsterdam), D.L. Kaplan (Wisconsin), M.H. van Kerkwijk (Toronto), J. Boyles (WVU, WKU), A.T. Deller (ASTRON), S. Chatterjee (Cornell), A. Schectman-Rook (Wisconsin-Madison), A. Berndsen (UBS), R.S. Lynch (McGill), D.R. Lorimer (WVU), C. Karako-Argaman (McGill), V.M. Kasi (McGill), V.I. Kondratiev (ASTRON, Lebedev), M.A. McLaughlin (WVU), J. van Leeuwen (ASTRON, Amsterdam), R. Rosen (NRAO, WVU), M.S.E. Roberts (Eureka, NYU), and K. Stovall (UT-Brownsville, UNM), published on-line by Nature, 5 January 2014.
Added 21 Jan 2014
Probing the Inner Galaxy
The pulsed radio emission from the Galactic Center (GC) magnetar SGR J1745-29 probes the turbulent, magnetized plasma of the GC hyperstrong scattering screen through both angular and temporal broadening. The angular size of SGR J1745-29 was obtained with the VLBA and VLA at 8.7 and 15.4 GHz. The source sizes are consistent with the scatter-broadened size of Sgr A*, demonstrating that SGR J1745-29 is also located behind the same hyperstrong scattering medium. The distance to the screen is 5.9 ± 0.3 kpc, consistent with being located in the Scutum spiral arm. This is a substantial revision of the previously held model in which the scattering screen is located very close to the GC. This result provides promise for pulsar searches at the GC at 10GHz.
Figure: VLBI mages of J1745-29 at 8 and 15GHz.
View Publication: The Angular Broadening of the Galactic Center Pulsar SGR J1745-29: A New Constraint on the Scattering Medium, Geoffrey C. Bower (Berkeley), Adam Deller (ASTRON), Paul Demorest (NRAO), Andreas Brunthaler (MPIfR), Ralph Eatough (MPIfR), Heino Falcke (ASTRON, MPIfR, Radboud), Michael Kramer (MPIfR), K.J. Lee (MPIfR), and Laura Spitler (MPIfR), 2014 ApJL, 780, L2, (1 January 2014).
Added 21 Jan 2014
Binaries Formed via Disk Fragmentation
Tobin et al. present observations of Class 0/I protostars acquired with the VLA and the Combined Array for Research in Millimeter-wave Astronomy (CARMA). They find that L1165-SMM1 and CB230 IRS1 have probable binary companions at separations of ∼100 AU from detections of secondary peaks at multiple wavelengths. The position angles of these companions are nearly orthogonal to the direction of the observed bipolar outflows, consistent with the expected protostellar disk orientations. They suggest that these companions may have formed from disk fragmentation; turbulent fragmentation would not preferentially arrange the binary companions to be orthogonal to the outflow direction. For L1165-SMM1, the 7.3 mm and 1.3 mm emission show evidence of a large (R >100 AU) disk. For the L1165-SMM1 primary protostar and the CB230 IRS1 secondary protostar, the 7.3mm emission is resolved into structures consistent with ∼20 AU radius disks. For the other protostars, the emission is unresolved, suggesting disks with radii < 20 AU.
Figure: Radio images of the L1165-SMM1 binary protostellar system. Protostar locations are marked with crosses; they are separated by ~ 100 AU. [Left] The shorter wavelength data (1.3 mm/230 GHz) taken with CARMA. [Middle, Right] The longer wavelength data (7.3 mm/41 GHz). The left and middle panels have comparable resolutions of 0.33" or 100 AU; the right panel has 0.1" (30 AU) resolution. For comparison, the orbit of Pluto around the sun has a semi-major axis of 39 AU. The blue and red arrows denote the directions of the outflowing gas from the protostars.
View publication: VLA and CARMA Observations of Protostars in the Cepheus Clouds: Sub-arcsecond Proto-Binaries Formed via Disk Fragmentation, John J. Tobin (NRAO), Claire J. Chandler (NRAO), David J. Wilner (CfA), Leslie W. Looney (Illinois), Laurent Loinard (UNAM), Hsin-Fang Chiang (Hawaii), Lee Hartmann (Michigan), Nuria Calvet (Michigan), Paolo D’Alesso (UNAM), Tyler L. Bourke (CfA), and Woojin Kwon (SRON), 2014, accepted ApJ.
Added 21 Jan 2014
The Dynamics of Massive Starless Cores
Intermediate- and high-mass stars may be born from relatively massive pre-stellar gas cores. The turbulent core accretion model invokes such cores as being in approximate virial equilibrium and in approximate pressure equilibrium with their surrounding clump medium. Alternatively, the competitive accretion model requires strongly sub-virial initial conditions that then lead to extensive fragmentation to the thermal Jeans scale, with intermediate- and high-mass stars forming later by competitive accretion. To test these models, Tan et al. have identified four massive (~100 M☉) clumps from mid-infrared extinction mapping of infrared dark clouds. They present ALMA observations of these four clumps that probe the N2D+ (3–2) line at 2.3 arcsecond resolution, find six N2D+ cores, and determine their dynamical state. The observed velocity dispersions and sizes are broadly consistent with the predictions of the turbulent core model of self-gravitating, magnetized, and virialized cores that are bounded by the high pressures of their surrounding clumps. However, in the most massive cores, their results suggest that moderately enhanced magnetic fields may be needed for the structures to be in virial and pressure equilibrium. Magnetically regulated core formation may thus be important in controlling the formation of massive cores, inhibiting their fragmentation, and thus helping to establish the stellar IMF.
Figure: ALMA data overlaid on an artist’s impression background. The ALMA data show two main cores as imaged by N2D+ emission. The core on the right is particularly bright and rounded, suggesting it is self-gravitating and poised to form a massive, single star. The other core appears more distorted and fragmented, potentially leading to the formation of multiple lower-mass stars. Credit: Bill Saxton & Alexandra Angelich (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO).
View Publication: The Dynamics of Massive Starless Cores with ALMA, Jonathan C. Tan (Florida), Shuo Kong (Florida), Michael J. Butler (Florida), Paola Caselli (Leeds), and Francesco Fontani (INAF), 2014 ApJ, 779, 96.
Added 21 Jan 2014
Detailed Imaging of the Sombrero Galaxy Jet
The Sombrero galaxy (M 104) is associated with one of the nearest low-luminosity active galactic nuclei (AGN). Hada et al. have imaged the radio jet structure of this AGN between 1.4 and 43 GHz using the VLBA down to scales of 0.01pc, or 100 RSchwarzschild. The jet is found to be within 25o of the Earth line-of-sight. The derived pole-on nature of the M 104 jet is in accordance with previous arguments that M 104 contains a true type II AGN, i.e., the broad line region of this nucleus is actually absent or intrinsically weak.
Figure: The VLBA 43GHz image of M104 at 0.2 mas resolution (left), and the spectral index image (right).
View Publication: Evidence for a Nuclear Radio Jet and its Structure Down to <100 Schwarzschild Radii in the Center of the Sombrero Galaxy (M 104, NGC 4594), Kazuhiro Hada (INAF, NAOJ, Akhiro Doi (JAXA), Hiroshi Nagai (NAOJ), Makoto Inoue (ASIAA), Mreki Honma (NAOJ, SOKENDAI), Marcello Giroletti (INAF), and Gabriele Giovannini (INAF, Bologna), 2013 ApJ, 779, 6 (10 Dec 2013).
Added 17 Jan 2014
The Coldest Place in the Universe
The pre-planetary nebula, the Boomerang Nebula, is the coldest known place in the Universe, refrigerated by an expanding molecular outflow (Joule-Thomson effect), to roughly 1 deg K. Planetary nebulae correspond to the dying moments of Solar-analog stars, the transition from the Asymptotic Giant Branch to a planetary nebula. A HST image shows a classic double-lobed hourglass shape with a very narrow central region, expected for expanding gas outflows. ALMA CO observations show the inner double structure, but also a more spherical, high velocity outflow on large scales, and a central dust-lane of millimeter-sized grains.
Figure: The Boomerang Nebula imaged with ALMA and HST. [Blue] HST shows a classic double-lobe shape with a very narrow central region. ALMA shows a more extended CO distribution, plus a narrow dust lane.
View Publication: ALMA Observations of the Coldest Place in the Universe: The Boomerang Nebula, R. Sahai (JPL), W.H.T. Vlemmings (Onsala), P.J. Huggins (NYU), L.-Å Nyman (JAO, ESO), and I. Gonidakis (CSIRO), 2013 ApJ, 777, 92 (10 Nov 2013).
Added 17 Jan 2014
A Magnetized Pulsar near the Galactic Center
The center of our Galaxy hosts a supermassive black hole, Sgr A*. Young, massive stars within 0.5 pc of Sgr A* are evidence of an episode of intense star formation near the black hole a few million years ago, which may have left behind a young neutron star traveling deep into Sgr A*’s gravitational potential. On 25 April 2013, a short X-ray burst was observed in the direction of the Galactic Center. Via observations with Chandra and Swift satellites, the associated magnetar was located and its spin period and derivative were refined. Quasi-simultaneous observations with the Parkes Radio Telescope and the GBT confirmed the existence of this pulsing magnetar at an angular distance of 2.4 ± 0.3 arcsec ≈ 0.07 - 2 pc from Sgr A* and determined a dispersion measure (DM) of 1750 pc cm-3, the highest ever observed for a radio pulsar. The magnetar is likely (~90% probability) in a bound orbit around the black hole.
Figure: Pulse profiles of the three radio observations where the magnetar was detected.
View Publication: A Strongly Magnetized Pulsar within the Grasp of the Milky Way’s Supermassive Black Hole, N. Rea (CSIC-IEEC), P. Espisito (INAF-IASF), J.A. Pons (d’Alacant), R. Turolla (d’Alacant, MSSL-UCL), D.F. Torres (CSIC-IEEC, ICREA), G.L. Israel (INAF-OAR), A. Possenti (INAF-OAC), M. Burgay (INAF-OAC), D. Viganò (d’Alacant), A. Papitto (CSIC-IEEC), R. Perna (JILA) et al., 2013 ApJL, 775, L34 (1 October 2013).
Added 17 Jan 2014
Molecular Gas Dynamics in a Nearby AGN/Starburst
The Seyfert 2 galaxy NGC 1433 was imaged with ALMA in CO 3-2 at the unprecedented spatial resolution of 0.5 arcseconds (24 pc), probing the active galactic nucleus (AGN) feeding and feedback phenomena through the morphology and dynamics of the gas inside the central kpc. ALMA reveals a nuclear bar of 400 pc radius embedded in the large-scale primary bar. The CO map also reveals a nuclear gaseous spiral structure inside the nuclear ring encircling the nuclear stellar bar. Most of the gas shows ordered rotation, although a strong (200 km/s, 7 M⊙/yr) outflow is seen in the nucleus, likely driven by the inner radio jets, possibly boosted by star formation.
Figure: A composite ALMA - HST image of the central parts of the nearby active galaxy NGC 1433. The faint blue background image shows the central dust lanes of this galaxy (HST). The red structures are the dust and molecular gas imaged by ALMA.
View Publication: ALMA Observations of Feeding and Feedback in Nearby Seyfert Galaxies: an AGN-driven Outflow in NGC 1433, F. Combes (Paris), S. García-Burillo (Madrid), V. Casasola (INAF Bologna), L. Hunt (INAF Firenzi), M. Krips (IRAM), A.J. Baker (Rutgers), A. Eckhart (F. Boone (CNRS), A. Eckart (Köln), I. Marquez (Andalucia), R. Neri (IRAM), E. Schinnerer (MPIfA), and L.J. Tacconi (MPIfeP), 2013 A&A, 558, 124 (October 2013).
Added 17 Jan 2014
“Pillars of Creation” in 30 Doradus/Large Magellanic Cloud
Indebetouw et al. present ALMA observations of 30 Doradus and the highest resolution view of molecular gas in an extragalactic star formation region to date (~0.4 pc × 0.6 pc). The 30Dor-10 cloud north of R136 was mapped in 12CO 2–1, 13CO 2–1, C18O 2–1, 1.3 mm continuum, the H30α recombination line, and two H2CO 3–2 transitions. 12CO emission is associated with small filaments and clumps (0.1pc, 103 M⊙), including protostars and “pillars of creation” photoablated by intense radiation from R136. Clumps in 30 Dor follow trends in size, line width, and surface density similar to massive clumps in the Milky Way. The consistency of clump masses calculated from dust continuum, CO, and the virial theorem reveals that the CO abundance in 30 Doradus clumps is not significantly different from the Large Magellanic Cloud average, but the dust abundance may be reduced by ~2x.
Figure: ALMA CO 2-1 contours plus Spitzer multi-color imaging (Red, green, blue: 7.9, 5.8, 3.6 μm, respectively) of 30 Dor. R136 is the blue cluster located south of the ALMA field of view.
View Publication: ALMA Resolves 30 Doradus: Sub-parsec Molecular Cloud Structure near the Closest Super Star Cluster, Remy Indebetouw (NRAO, Virginia), Crystal Brogan (NRAO), C.-H. Rosie Chen (MPIfR), Adam Leroy (NRAO), Kelsey Johnson (Virginia), Erik Muller (NAOJ), Suzanne Madden (Saclay), Diane Cormier (Heidelberg), Frederic Galliano (Saclay), Annie Hughes (MPIfA), Todd Hunter (NRAO), Akiko Kawamura (NAOJ), Amanda Kepley (NRAO), Vianney Lebouteiller (Saclay), Margaret Meixner (STScI), Joana M. Oliveira (Keele), Toshikazu Onishi (Osaka), and Tatiana Vasyunina (Virginia), 2013 ApJ, 774, 73 (1 Sep 2013).
Added 17 Jan 2014
Imaging Large Scale HI structure
The large-scale distribution of neutral hydrogen in the Universe will be luminous through its 21 cm emission. Switzer et al. use data acquired with the GBT that span the redshift range 0.6 < z < 1.0 to detect, for the first time, the auto-power spectrum of 21 cm intensity fluctuations and constrain neutral hydrogen fluctuations at z ∼ 0.8. The synchrotron foregrounds exceed the signal by 103, but have fewer degrees of freedom and can be removed efficiently. Through a Bayesian treatment of signal and foregrounds, they derive an HI baryon density ΩHI bHI= [0.62+0.23-0.15] × 10-3 at 68% confidence, where ΩHI is the neutral hydrogen (HI) fraction of the cosmic closure density, and bHI is the HI bias parameter.
View Publication: Determination of z ~ 0.8 Neutral Hydrogen Fluctuations Using the 21 cm Intensity Mapping Autocorrelation, E.R. Switzer (CITA), K.W. Masui (CITA, Toronto), K. Bandura (McGill), L.-M. Calin (CITA), T.-C. Chang (ASIAA), X.-L. Chen (NAO-CAS, Peking), Y.-C. Li (NAO-CAS), Y.-W. Liao (ASIAA), A. Natarajan (Carnegie Mellon), U.-L. Pen (CITA), J.B. Peterson (Carnegie Mellon), J.R. Shaw (CITA), and T.C. Voytek (Carnegie Mellon), 2013 MNRAS, 434, L46 (1 September 2013).
Added 21 Jan 2014
Measuring the Hubble Constant
A new geometric maser distance has been estimated to the active galaxy NGC 4258 from 18 epochs of Very Long Baseline Interferometry, and 10 years monitoring the velocities of the 22 GHz masers in NGC 4258, using a telescope suite that included the GBT and VLA. A new model includes both disk warping and confocal elliptical maser orbits with differential precession. The distance to NGC 4258 is now determined to be 7.60 ± 0.17 ± 0.15 Mpc, a 3% uncertainty including formal fitting and systematic terms. The resulting Hubble Constant, based on the use of the Cepheid variables in NGC 4258 to recalibrate the Cepheid distance scale, is H0 = 72.0 ± 3.0 km s-1 Mpc-1.
Figure: Covariance plot of H0 and w from the NGC 4258 maser observations.
View Publication: Toward a New Geometric Distance to the Active Galaxy NGC 4258. III. Final Results and the Hubble Constant, Liz Humphreys (ESO, CfA), Mark Reid (CfA), Jim Moran (CfA), Lincoln Greenhill (CfA), and Alice Argon (CfA), 2013 ApJ, 775, 13 (20 September 2013).
Added 21 Jan 2014