Interesting Astrophysics: Jun 22 - Jun 30

The following set of astrophysical preprints from arXiv attracted my attention in the last week. Beyond the usual predominance of galaxy-related papers (in particular detection of molecular P-Cygni profiles in Arp 220's core) there are a number of interesting star/star cluster related papers (in particular Krumholz & Matzner; Allison et al; van Loon).

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Galaxies and Starbursts

Star Formation Rates for Starburst Galaxies from Ultraviolet, Infrared, and Radio Luminosities
Authors: Lusine A. Sargsyan, Daniel W. Weedman, arXiv:0906.3688 [ps, pdf, other]
Comments: Accepted for publication in The Astrophysical Journal

Origin and Dynamical Support of Ionized Gas in Galaxy Bulges
Authors: Luis C. Ho, arXiv:0906.4103 [ps, pdf, other]
Comments: To appear in ApJ, 11 pages, 6 figures

P-Cygni Profiles of Molecular Lines toward Arp 220 Nuclei
Authors: Kazushi Sakamoto, Susanne Aalto, David J. Wilner, John H. Black, John E. Conway, Francesco Costagliola, Alison B. Peck, Marco Spaans, Junzhi Wang, Martina C. Wiedner, arXiv:0906.5197 [ps, pdf, other]
Comments: ApJL in press, 6 pages

Quest for truly isolated galaxies
Authors: Noah Brosch, arXiv:0906.4212 [ps, pdf, other]
Comments: Contribution for the "Galaxies in Isolation" conference

Evolution of the X-ray Luminosity in Young HII Galaxies
Authors: D. Rosa González, E. Terlevich, E. Jiménez Bailón, R. Terlevich, P. Ranalli, A. Comastri, E. Laird, K. Nandra, arXiv:0906.4461 [ps, pdf, other]
Comments: Accepted by MNRAS

The Discovery of a Large Lyman-alpha+HeII Nebula at z~1.67: A Candidate Low Metallicity Region?
Authors: M. K. M. Prescott, A. Dey, B. T. Jannuzi, arXiv:0906.4785 [ps, pdf, other]
Comments: Accepted for publication in ApJ; emulateapj format, 17 pages, 7 figures, 3 tables

An Ultraviolet Study of Star-Forming Regions in M31
Authors: Yongbeom Kang, Luciana Bianchi, Soo-Chang Rey, arXiv:0906.4839 [ps, pdf, other]
Comments: 39 pages, 14 figures, accepted for publication in ApJ

From their abstract: "When star formation detected from IR measurements of the heated dust is added to the UV-measured star formation (from the unobscured populations) in the recent few Myrs, we find the SFR has slightly decreased in recent epochs, with a possible peak between 10 and 100 Myrs, and an average value of SFR ~ 0.6 or 0.7 M_{sun} yr^{-1} (for metallicity Z=0.02 or 0.05 respectively) over the last 400 Myrs."

The Cosmic Web of Baryons
Authors: Joel N. Bregman, arXiv:0906.4984 [pdf]
Comments: Submitted as a science white paper to the Decadal Survey (Astro2010). In pdf format only. 7 pages

The Missing Baryons in the Milky Way and Local Group
Authors: Joel N. Bregman, arXiv:0906.4993 [pdf]
Comments: A White Paper submitted to The Galactic Neighborhood Science Frontiers Panel of Astro2010 (Decadal Survey). 7 pages in pdf format only

Faint Lyman-Break galaxies as a crucial test for galaxy formation models
Authors: Barbara Lo Faro, Pierluigi Monaco, Eros Vanzella, Fabio Fontanot, Laura Silva, Stefano Cristiani, arXiv:0906.4998 [ps, pdf, other]
Comments: 17 pages, 13 figures, 1 table; accepted for publication by MNRAS

A Physical Model of Lyman Alpha Emitters
Authors: Vithal Tilvi, Sangeeta Malhotra, James E. Rhoads, Evan Scannapieco, Robert J. Thacker, Ilian T. Iliev, Garrelt Mellema, arXiv:0906.5159 [ps, pdf, other]
Comments: 9 pages, 6 figures, Comments welcome

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The Interstellar Medium

The Dynamics of Radiation Pressure-Dominated HII Regions
Authors: Mark R. Krumholz, Christopher D. Matzner, arXiv:0906.4343 [ps, pdf, other]
Comments: 10 pages, 2 figures, emulateapj format, submitted to ApJ

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Stars

Dynamical mass segregation on a very short timescale
Authors: R. J. Allison, S. P. Goodwin, R. J. Parker, R. de Grijs, S. F. Portegies Zwart, M. B. N. Kouwenhoven, arXiv:0906.4806 [ps, pdf, other]
Comments: ApJ Letter, accepted

The effects of red supergiant mass loss on supernova ejecta and the circumburst medium
Authors: Jacco Th. van Loon, arXiv:0906.4855 [ps, pdf, other]
Comments: Invited review, in "Hot and Cool: Bridging Gaps in Massive Star Evolution", Pasadena (USA), November 2008. ASP Conference Series, 10 pages

A Possible Supernova Remnant high above the Galactic Disk
Authors: David B. Henley, Robin L. Shelton, arXiv:0906.4937 [ps, pdf, other]
Comments: 16 pages, 11 figures. Accepted for publication in the Astrophysical Journal

Hard X-ray flux from low-mass stars in the Cygnus OB2 Association
Authors: M. Caramazza, J.J. Drake, G. Micela, E. Flaccomio, arXiv:0906.5222 [ps, pdf, other]
Comments: 5 pages

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Numerical and Statistical Methods

Fisher Matrices and Confidence Ellipses: A Quick-Start Guide and Software
Authors: Dan Coe, arXiv:0906.4123 [pdf, other]

Comments: 4 pages, 1 figure, not submitted to any journal. Comments welcome. Software will be made available at this http URL

Frank James Low, 1933-2009

Frank J. Low, a pioneer in infra-red astronomical detector design, passed away on June 10th, 2009.

Of particular note is his work on detectors for Kuiper Airborne Observatory, IRAS and the Spitzer Space Telescope.

The NYT has an obituary by Dennis Overbye: "Frank J. Low, Who Helped Drive Field of Infrared Astronomy, Dies at 75".

Another biography of him associated with his winning of the ASp's 2006 Bruce Medal can be found here.

Interesting Astrophysics: Jun 15 - Jun 19

A quiet week on the journals and astro-ph. Of the papers that did strike my eye quiet a few were on moderate-to-high redshift galaxies (Hainline et al; Quider et al; Nesvadba et al) and/or the IGM (Chen & Muchaey, Cooksey et al).

There is also an interesting paper by Mellot et al re-examining the often reoccuring and always controversial claims of links between terrestrial mass extinctions and the Solar system's crossing of Milky Way spiral arms.

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Galaxies and Starbursts

Rest-Frame Optical Spectra of Three Strongly Lensed Galaxies at z~2
Kevin N. Hainline, Alice E. Shapley, Katherine A. Kornei, Max Pettini, Elizabeth Buckley-Geer, Sahar S. Allam, Douglas L. Tucker, arXiv:0906.2197 [ps, pdf, other]
Comments: 16 pages, 8 figures. Accepted for publication in the Astrophysical Journal

The Ultraviolet Spectrum of the Gravitationally Lensed Galaxy `The Cosmic Horseshoe': A Close-up of a Star-forming Galaxy at z = 2
Anna M. Quider, Max Pettini, Alice E. Shapley, Charles C. Steidel, arXiv:0906.2412 [ps, pdf, other]
Comments: 17 pages, 9 Figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society

Dwarf Galaxies in the NGC 1023 Group
Neil Trentham, Brent Tully, arXiv:0906.2540 [ps, pdf, other]
Comments: MNRAS, in press

Probing the IGM-Galaxy Connection at z < 0.5 I: A Galaxy Survey in QSO Fields and a Galaxy-Absorber Cross-Correlation Study
Hsiao-Wen Chen, John S. Mulchaey, arXiv:0906.3293 [ps, pdf, other]

Comments: 27 journal-style pages, 15 figures; ApJ in press; full-resolution version is available at this http URL

The Last Eight-Billion Years of Intergalactic CIV Evolution
Kathy L. Cooksey, Christopher Thom, J. Xavier Prochaska, Hsiao-Wen Chen, arXiv:0906.3347 [ps, pdf, other]
Comments: 63 pages, 18 figures, 10 tables; submitted to ApJ

Effects of Galactic fountains and delayed mixing in the chemical evolution of the Milky Way

E. Spitoni, F. Matteucci, S. Recchi, G. Cescutti, A. Pipino, arXiv:0906.3400 [ps, pdf, other]

Comments: Accepted by A&A

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The Interstellar Medium

Supershells in the Multi-Phase Milky Way: Insights from HI Synthesis Imaging and CO Surveys
J. R. Dawson, N. M. McClure-Griffiths, Y. Fukui, arXiv:0906.2630 [pdf, other]
Comments: 4 pages, to appear in conference proceedings "The Role of Disk-Halo Interaction in Galaxy Evolution, Outflow vs. Infall?" ed. M. A. de Avillez, European Astronomical Society Publications, 2009 in press

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Black Holes and AGN

Giant outflows in z~2 radio galaxies: The smoking gun of AGN feedback in the early universe
N. P. H. Nesvadba, arXiv:0906.2900 [ps, pdf, other]
Comments: To appear in the proceedings of the workshop "The Interface between Galaxy Formation and AGN", Vulcano (Messina), Italy

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Stars, Supernovae and Planets

Spectral Evolution of the Extraordinary Type IIn Supernova 2006gy
Nathan Smith, Ryan Chornock, Jeffrey M. Silverman, Alexei V. Filippenko, Ryan J. Foley, arXiv:0906.2200 [ps, pdf, other]
Comments: Really long. 30 pages, 26 figs, appendix. Submitted to ApJ - v2 corrected one reference

Testing the link between terrestrial climate change and Galactic spiral structure
Adrian L. Melott, Andrew C. Overholt, Martin K. Pohl, arXiv:0906.2777 [pdf]

Their abstract: "We confront past suggestions of a close link between terrestrial climate change and the Sun's transit of spiral arms in its path through the Milky Way galaxy. These links produced concrete fits, deriving the unknown spiral pattern speed from terrestrial climate correlations. We use new information on spiral structure based on CO data that does not make simplifying assumptions about symmetry and circular rotation. Comparing the times of these transits to changes in the climate of Earth, not only do the claimed correlations disappear, but also we find that they cannot be resurrected for any reasonable pattern speed."

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Other

Radio quiet, please! - protecting radio astronomy from interference
W. van Driel, arXiv:0906.2268 [pdf, other]
Comments: To appear in IAU Symposium 260 The Role of Astronomy in Society and Culture, 8 pages

Its about time! A new radio supernova in Messier 82

Brunthaler et al (2009, A&A, 499, L17) have just reported the discovery of a new radio transient in the nucleus of the nearby starburst galaxy Messier 82 (M82).

In short, radio telescopes see something now that wasn't there two years ago, and this is most probably a radio supernova, indeed, one of the closest supernova to Earth in the last five years.

But why is this scientifically interesting?

M82 has been called the "exploding galaxy" with good reason: huge filaments and arcs of very hot gas extend out ~12 kiloparsecs (~40,000 light years) perpendicular the plane of this roughly edge-on disk galaxy, connected to the dusty and optically obscured nuclear regions (The image to the right shows the galaxy in cyan, hot X-ray-emitting gas in blue, hydrogen emission in yellow, and dust emission in red).

Spectroscopic observations reveal that the hot gas visible in optical light (e.g. as observed with the Hubble Space Telescope) is flowing outward from the nucleus at a speed of ~600 kilometres per second (~1,350,000 miles per hour), and this is only one tracer of the multi-phase galaxy-sized wind (aka superwind) outflowing from this galaxy. Such superwinds were a particularly common occurrence earlier in the Universe when most galaxies and stars were forming, and indeed winds may have had significant effects on how galaxies formed and evolved.

M82 was first recognized as a peculiar and interesting galaxy in the early 1960's (see e.g. Lynds & Sandage 1963) but it only in the late 1970's and early 1980's that astronomers realized both that it represented a wind rather than a single explosive event, and that the wind was powered by a high rate of core collapse supernovae hidden from optical view in a dusty obscured "starburst" nucleus (M82 is not host an AGN, and does not appear to have a central supermassive black hole, despite many attempts by astronomers to find one).

Indeed, although M82 is perhaps only a tenth the mass of our own Milky Way galaxy it is thought to have a star formation (and hence supernova) rate 2-3 times higher than the entire Milky Way. Furthermore all of this activity is concentrated down within the central 500 parsecs (~1,500 light years) of M82 (by way on contrast, the Milky Way is ~100,000 light years across). Because M82 is one of the closest powerful starbursts it has become an important astrophysical laboratory in which to study the physics of star formation under extreme conditions.


Estimates based on the infra-red and/or radio luminosity of M82 suggest that one supernova should occur in M82's center every twenty years or so, but no-one has ever seen one go off, despite ~40 years of observation. Traditionally most supernova are detected at optical wavelengths, but optical wavelength observations of M82's starburst region are difficult, given the high obscuration. At radio wavelengths the dust that obscures the optical light is effectively transparent, so radio observations have been the primary means of studying many starburst regions, including that of M82. These radio observations reveal approximately 50 compact sources (e.g. see the MERLIN/VLA image shown above, taken from the Jodrell Bank website) that are thought to be the remnants of supernova that occurred in the recent past (although there is some debate as to how young or old they are). However they are not direct detections of stars exploding now.


Brunthaler et al were using the VLA radio telescope (the Very Large Array) to observe M82 a number of times in 2007 and 2008, and noticed the sudden appearance of bright radio transient in the center of M82, which must have occurred some time between two of their observations 2007 October 29 and 2008 March 24 (see the image above, taken from their press release). Initially very bright, the source has faded rapidly by a factor of 10 within a year, roughly exponentially. Brunthaler discuss possible interpretations for this object, and conclude that it is most probably a radio supernova (i.e. its a supernova that is a bright radio source), resulting from the core collapse and subsequent explosion of a massive star. On that basis it is has been given the identifier SN 2008iz.

Indeed, following up from the work described in the paper, observations with Very Long Baseline Array (VLBA) have actually resolved the ring-like expanding remnant.

If so this is the first supernova directly detected in M82 seen observations began (there have been claims that a peculiar source called 41.5+597, that has since disappeared, may have been a radio supernova, but the evidence in not sufficient to make a confident identification of its nature).

However there is more to Brunthaler's detection than just a confirmation that supernova are exploding in M82. The nature of the 50 or so well known compact radio sources in M82, often presumed to be young supernova remnants (SNRs) in free expansion, has been under challenge.

The compact radio sources in M82 have long been assumed to be young SNRs because they appear to fall on the trend of radio surface brightness vs diameter established for older, larger, SNRs within our own galaxy: the so-called Σ-D relation. These sources have diameters in the range ~0.5 to 8 pc, which would imply ages of a few hundred years for the largest sources if they were expanding at ~10,000 km/s. However, if they really are rapidly expanding young SNRs one would expect to see them fade as they expanded, even within the few decades of observations available to astromers on Earth. Yet the M82 sources don't appear to be fading!

As summarized by Brunthaler et al:

Kronberg & Sramek (1985) and Kronberg et al. (2000) monitored the flux densities of 24 radio sources in M 82 from 1980 until 1992. Most sources (75%) remained surprisingly constant. There is some controversy about how the fluxes of these compact radio sources can be stable. Models of supernova remnants expanding into a dense medium may explain this (Chevalier & Fransson 2001). Seaquist & Stankovic (2007) argue that the radio emission could arise from wind-driven bubbles. Studying the evolution of a young source could be very important for understanding these models.

The Chevalier & Franson model posits that the compact sources are actually much older SNRs that are compact because they have been physically confined by high density gas in the starburst region - thus they can't expand and fade quickly. But this causes other problems. SNRs in dense surrounding eventually lose all their expansion energy via radiation, so they cannot contribute to powering the large scale superwind from M82. One is there forced to posit two populations of SNRs in M82, one that creates radio sources but doesn't contribute to the observed superwind, and another population that freely expands to mingle with and power the wind. Furthermore, the relative fraction of these two hypothesized classes aren't known. which complicates any attempt to assess the energy budget of the superwind.

Just to complicate matters further, two of the brightest and most compact radio sources have been monitored with the VLBI for about a decade now, and have been see to expand, one at 1500-2000 km/s and one at 9000-11000 km/s (Beswick et al, 2006). These objects may indeed be young remnants, but then why do all the other compact sources not fade as expected?

SN2008iz can help us resolve some of these issues in the decades to come because we know it is now a very young SN, with a moderately well determined explosion date. Observations of how this young SN remnant expands may help us to understand both the nature, age and environment of the other compact radio sources in M82, and hence the properties of the interstellar medium into which these supernova are exploding. That, in turn, will improve our understanding of how superwinds are created and powered.

Other links to this story:

• The MPIfR press release.
• A SPACE.COM article based on the press release.
  

Other References:
Beswick, R.J., et al, 2006, MNRAS, 369, 1221.
Chevalier, R. A., & Fransson, C. 2001, ApJ, 558, L27 [NASA ADS]
Lynds, C., and Sandage, A., 1963, ApJ, 137, L1005.
Kronberg, P. P., & Sramek, R. A. 1985, Science, 227, 28 [NASA ADS]
Kronberg, P. P., Sramek, R. A., Birk, G. T., et al. 2000, ApJ, 535, 706 [NASA ADS]
Seaquist, E. R., & Stankovic, M. 2007, ApJ, 659, 347 [NASA ADS]

Brunthaler, A., Menten, K., Reid, M., Henkel, C., Bower, G., & Falcke, H. (2009). Discovery of a bright radio transient in M 82: a new radio supernova? Astronomy and Astrophysics, 499 (2) DOI: 10.1051/0004-6361/200912327