SAO: Weekly Science Updates 2015

[bystander]

The Skeleton of the Milky Way
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Dec 18

[img3="A filamentary dark cloud, a possible "bone in the skeleton of the Milky Way," as seen in the infrared with the Spitzer Space Telescope's IRAC camera. This filament stretches over about 45 light-years. Astronomers have found ten such filamentary bones and are looking for more in order to study the skeletal makeup of the galaxy. (Credit: NASA Spitzer/GLIMPSE, Zucker et al.)"]https://www.cfa.harvard.edu/sites/www.c ... 201551.jpg[/img3][hr][/hr]

Our home galaxy, the Milky Way, is a typical barred spiral galaxy, a flattened disk of about a hundred billion stars, gas, and dust that is roughly one hundred thousand light-years in diameter. The galactic disk is surrounded by a large spherical, diffuse halo roughly about five hundred thousand light-years in diameter. Although it is our home, many fundamental questions remain about the Milky Way's structure. For instance, how many major spiral arms does it have, two or four? What are the precise locations and shapes of these arms? What is the nature of the inter-arm structures —are they well-defined spurs of star and gas or more web-like in form? And not least, does it even make sense to describe the Milky Way as a typical spiral?

One difficulty astronomers encounter in answering these and similar questions is the fact that we are embedded within the very galaxy we are attempting to disentangle. The current understanding of the Milky Way’s three dimensional structure stems largely from measurements of the velocity of its gas clouds. Like all spirals the Milky Way rotates, and knowing the general plan of rotation enables astronomers to relate the line-of-sight velocities into distances and hence to construct a rough, three-dimensional model (although it is often difficult to disentangle features at different distances but along the same line-of-sight).

CfA astronomers Catherine Zucker, Cara Battersby and Alyssa Goodman have been trying to trace the "skeletal structure" of the Milky Way as defined by dense, filamentary, and very elongated cold, dark clouds of gas and dust. These structures can stretch over as much as a thousand light-years in length yet are only a few light-years across. The scientists argue that they are part of a kind of spine for the spiral arms in which they reside. The first one was spotted in mid-infrared surveys about five years ago, and the team set out to discover other skeletal features with the aim of using them to help resolve some of the outstanding puzzles of the Milky Ways structure. ...

The Skeleton of the Milky Way - Catherine Zucker, Cara Battersby, Alyssa Goodman

• Astrophysical Journal 815(1):23 (2015 Dec 10) DOI: 10.1088/0004-637X/815/1/23
  arXiv.org > astro-ph > arXiv:1506.08807 > 29 Jun 2015 (v1), 15 Oct 2015 (v2)

[bystander]

Magnetic Fields in Powerful Radio Jets
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Dec 25

[img3="X-ray jets from the galaxy Pictoris A. The greyscale image was taken by the Chandra X-ray Observatory and reveals the detailed X-ray structure of the jets, which extend over nearly one million light-years. The red contours show the radio emission. Astronomers analyzing these and other data have concluded that the X-ray emission is produced by rapidly moving charged particles in magnetic fields.
(Credit: NASA/Chandra, Hardcastle et al.)"]https://www.cfa.harvard.edu/sites/www.c ... 201552.jpg[/img3][hr][/hr]

Super-massive black holes at the centers of galaxies can spawn tremendous bipolar jets when matter in the vicinity forms a hot, accreting disk around the black hole. The rapidly moving charged particles in the jets radiate when they are deflected by magnetic fields; these jets were discovered at radio wavelengths several decades ago. In the most dramatic cases, the energetic particles move at speeds close to the speed of light and extend over hundreds of thousands of light-years, well beyond the visible boundaries of the galaxy. The physical processes that drive these jets and cause them to radiate are among the most important outstanding problems of modern astrophysics.

One of the most significant and unexpected discoveries of the Chandra X-ray Observatory was that bright X-rays are also emitted by these jets. The X-rays are also produced by the acceleration of charged particles, at least according to some models, but there are other possible mechanisms as well. Fast-moving particles can scatter background light, boosting it into the X-ray band. Alternatively, shocks can generate X-ray emission (or at least a significant portion of it), either as the jets interact with stellar winds and interstellar medium or, within the jet, as a consequence of jet variability, instability, turbulence, or other phenomena.

CfA astronomer Aneta Siemiginowska and her colleagues have studied the bright radio jet galaxy Pictoris A, located almost five hundred million light-years away, using very deep Chandra measurements - the observations used an accumulated total of over four days of time, spread over a fourteen year period. These data enabled the first detailed analysis of the spectral character of the emission all along the jets. The emission turns out to be remarkably uniform everywhere, something that is extremely unlikely if scattering were responsible, but which is a natural consequence of the magnetic field process. The scientists therefore reject the scattering model in favor of the latter. However, the jets do have within them many small clumps, internal structures, and lobes. Shocks and/or scattering are possible explanations for the emission in some of these structures. Although these new results represent some dramatic improvements in our understanding of Pic A, high-resolution radio measurements of a large sample of similar jets are now needed to refine and extend the models. Large-scale X-ray jets, for example, have been also detected in very distant quasars. The results from Pic A, together with future Chandra observations, will help astronomers determine the extent to which these distant jets also rely on the same processes, or if they invoke other ones.

Deep Chandra Observations of Pictor A - M. J. Hardcastle et al

• Monthly Notices of the RAS 455(4):3526 (2015 Feb 01) DOI: 10.1093/mnras/stv2553
  arXiv.org > astro-ph > arXiv:1510.08392 > 28 Oct 2015