## Friday, April 02, 2010

### Interesting Astrophysics: Mar 23 to Mar 31

Dwarfs and giants compete in this edition of Interesting Astrophysics. The dwarfs (or dwarves, as Tolkien would have us write it) are dwarf starburst galaxies and the giants are very very luminous starbursting galaxies.

There are two very nice papers on the very popular (if somewhat atypical) dwarf starburst galaxy NGC 1569 (Kepley et al, and Westmoquette et al), both of interest with respect to its galactic wind. Monreal-Ibero et al present a paper on NGC 5253 (possibly the second most popular dwarf starburst galaxy judged by number of papers published). And yes, that region of nitrogen enrichment that is the one clear case of localized chemical enrichment by recently formed massive stars is still there.

Younger et al discuss the physical sizes of the star forming regions in submillimeter galaxies, and argue that the data supports an interpretation as merger-driven starbursts rather than simply over-active disks. This is not particularly surprising, as the SMGs have long been thought to be semi-equivalent to local Ultraluminous IR galaxies (ULIRGs are merger driven starbursts).

Not content with merely Ultraluminosity, Rowan-Robinson et al present a catalog of 179 Hyperluminous IR galaxies, and discuss their spectral energy distributions and star formation rates.

Last but not least I'll mention Chen et al's work on probing outflows from star forming galaxies with Na I absorption in SDSS spectra. Its a big paper, but for me the results that seem coolest are that their results appear to confirm the Lehnert & Heckman star formation per unit area threshold above which winds (or extra-planar gas) appear, and that outflows really do seem to have a prefer opening angle of ~60 degrees.

Galaxies and Starbursts

The Role of the Magnetic Field in the Interstellar Medium of the Post-Starburst Dwarf Irregular Galaxy NGC 1569
Amanda A. Kepley, Stefanie Mühle, John Everett, Ellen G. Zweibel, Eric M. Wilcots and Uli Klein, 2010, ApJ, 712, 536

This is a really nice piece of work. Their are relatively few studies of magnetic field structures and strengths in starbursts with superwinds that answer the kind of questions that this paper addresses, so this paper is a very valuable addition to the literature.

Their abstract (emphasis mine): "NGC 1569 is a nearby dwarf irregular galaxy which underwent an intense burst of star formation 10-40 Myr ago. We present observations that reach surface brightnesses 2-80 times fainter than previous radio continuum observations and the first radio continuum polarization observations of this galaxy at 20 cm, 13 cm, 6 cm, and 3 cm. These observations allow us to probe the relationship of the magnetic field of NGC 1569 to the rest of its interstellar medium (ISM). We confirm the presence of an extended radio continuum halo at 20 cm and see for the first time the radio continuum feature associated with the western Hα arm at wavelengths shorter than 20 cm. Although, in general, the spectral indices derived for this galaxy steepen as one moves into the halo of the galaxy, there are filamentary regions of flat spectral indices extending to the edge of the galaxy. The spectral index trends in this galaxy support the theory that there is a convective wind at work in this galaxy. There is strong polarized emission at 3 cm and 6 cm and weak polarized emission at 20 cm and 13 cm. We estimate that the thermal fraction is 40%-50% in the center of the galaxy and falls off rapidly with height above the disk. Using this estimate, we derive a total magnetic field strength of 38 μG in the central regions and 10-15 μG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ~3-9 μG and is strongest in the halo. Using our total magnetic field strength estimates and the results of previous observations of NGC 1569, we find that the magnetic pressure is the same order of magnitude but, in general, a factor of a few less than the other components of the ISM in this galaxy. The uniform magnetic field in NGC 1569 is closely associated with the Hα bubbles and filaments. We suggest that a supernova-driven dynamo may be operating in this galaxy. Based on our pressure estimates and the morphology of the magnetic field, the outflow of hot gas from NGC 1569 is clearly shaping the magnetic field, but the magnetic field in turn may be aiding the outflow by channeling gas out of the disk of the galaxy. Dwarf galaxies with extended radio continuum halos like that of NGC 1569 may play an important role in magnetizing the intergalactic medium."

Mapping the roots of the galactic outflow in NGC1569
Mark S. Westmoquette, Linda J. Smith, Jay S. Gallagher and Katrina M. Exter, 2009, Ap&SS, 324, 187

The physical scale of the far-infrared emission in the most luminous submillimetre galaxies II: evidence for merger-driven star formation
J. D. Younger, G. G. Fazio, M. L. N. Ashby, F. Civano, M. Elvis, M. A. Gurwell, J.-S. Huang, D. Iono, A. B. Peck, G. R. Petitpas, K. S. Scott, D. J. Wilner, G. W. Wilson, M. S. Yun,
Comments: 9 pages, 3 Figures, submitted to MNRAS

From their abstract: "These relatively compact sizes (<5-10 kpc) argue strongly for merger-driven starbursts, rather than extended gas-rich disks, as the preferred channel for forming SMGs. For the most luminous objects, the derived sizes may also have important physical consequences; under a series of simplifying assumptions, we find that these two objects in particular are forming stars close to or at the Eddington limit for a starburst."

Hyperluminous infrared galaxies from IIFSCz
Michael Rowan-Robinson, Lingyu Wang,
Comments: 11 pages, 11 figures: accepted for publication in MNRAS.

From their abstract: "We present a catalogue of 179 hyperluminous infrared galaxies (HLIRGs) from the Imperial IRAS-FSS Redshift (IIFSCz) Catalogue. Of the 92 with detections in at least two far infrared bands, 62 are dominated by an M82-like starburst, 22 by an Arp220-like starburst and 8 by an AGN dust torus. On the basis of previous gravitational lensing studies and an examination of HST archive images for a further 5 objects, we estimate the fraction of HLIRGs that are significantly lensed to be 10-30%. ... The implied star-formation rates, even after correcting for lensing magnification, are in excess of 1000 Mo /yr."

A study of the interplay between ionized gas and star clusters in the central region of NGC 5253 with 2D spectroscopy
A. Monreal-Ibero, J. M. Vilchez, J. R. Walsh, C. Munoz-Tunon,
Comments: accepted in Astronomy & Astrophysics. 21 pages, 22 figures

From their abstract: "A detailed 2D study of the central region of NGC5253 has been performed to characterize the stellar and ionized gas structure as well as the extinction distribution, physical properties and kinematics of the ionized gas in the central ~210pc x 130pc. ... The [SII]l6717/[SII]l6731 map shows an electron density (N_e) gradient declining from the peak of emission in Ha (790cm^-3) outwards, while the argon line ratio traces areas with $N_e~4200 - 6200cm^(-3). The area polluted with extra nitrogen, as deduced from the excess [NII]/Ha, extends up to distances of 3.3" (~60pc) from the maximum pollution, which is offset by ~1.5" from the peak of continuum emission. Wolf-Rayet features are distributed in an irregular pattern over a larger area (~100pc x 100pc) and associated with young stellar clusters. ... The line profiles are complex. Up to three emission components were needed to reproduce them. One of them, associated with the giant HII region, presents supersonic widths and [NII] and [SII] emission lines shifted up to 40km/s with respect to Ha. Similarly, one of the narrow components presents offsets in the [NII] line of <~20km/s. This is the first time that maps with such velocity offsets for a starburst galaxy have been presented." Absorption-line probes of the prevalence and properties of outflows in present-day star-forming galaxies Yan-Mei Chen, Christy A. Tremonti, Timothy M. Heckman, Guinevere Kauffmann, Benjamin J. Weiner, Jarle Brinchmann, Jing Wang, Comments: 18 pages, 18 figures, submitted to AJ Their abstract: "We analyze star forming galaxies drawn from SDSS DR7 to show how the interstellar medium (ISM) Na I 5890, 5896 (Na D) absorption lines depend on galaxy physical properties, and to look for evidence of galactic winds. We combine the spectra of galaxies with similar geometry/physical parameters to create composite spectra with signal-to-noise ~300. The stellar continuum is modeled using stellar population synthesis models, and the continuum-normalized spectrum is fit with two Na I absorption components. We find that: (1) ISM Na D absorption lines with equivalent widths EW > 0.8A are only prevalent in disk galaxies with specific properties -- large extinction (Av), high star formation rates (SFR), high star formation rate per unit area ($\Sigma_{\rm SFR}$), or high stellar mass (M*). (2) the ISM Na D absorption lines can be separated into two components: a quiescent disk-like component at the galaxy systemic velocity and an outflow component; (3) the disk-like component is much stronger in the edge-on systems, and the outflow component covers a wide angle but is stronger within 60deg of the disk rotation axis; (4) the EW and covering factor of the disk component correlate strongly with dust attenuation, highlighting the importance that dust shielding may play the survival of Na I. (5) The EW of the outflow component depends primarily on$\Sigma_{\rm SFR}$and secondarily on Av; (6) the outflow velocity varies from ~120 to 160km/s but shows little hint of a correlation with galaxy physical properties over the modest dynamic range that our sample probes (1.2 dex in log$\Sigma_{\rm SFR}$and 1 dex in log M*)." Massive star formation in Wolf-Rayet galaxies. III: Analysis of the O and WR populations Angel R. Lopez-Sanchez, Cesar Esteban, Comments: Accepted in A&A, 19 pages, 13 figures Black Holes and AGN Infrared Excess Sources: Compton Thick QSOs, low luminosity Seyferts or starbursts? A. Georgakakis, M. Rowan-Robinson, K. Nandra, J. Digby-North, P. G. Perez-Gonzalez, G. Barro, Comments: Accepted by MNRAS Full abstract: "We explore the nature of Infrared Excess sources (IRX), which are proposed as candidates for luminous L_X(2-10keV)>1e43erg/s Compton Thick (N_H>2e24cm^{-2}$) QSOs at z~2. Lower redshift, z~1, analogues of the distant IRX population are identified by firstly redshifting to z=2 the SEDs of all sources with secure spectroscopic redshifts in the AEGIS (6488) and the GOODS-North (1784) surveys and then selecting those that qualify as IRX sources at that redshift. A total of 19 galaxies are selected. The mean redshift of the sample is $z\approx1$. We do not find strong evidence for Compton Thick QSOs in the sample. For 9 sources with X-ray counterparts, the X-ray spectra are consistent with Compton Thin AGN. Only 3 of them show tentative evidence for Compton Thick obscuration. The SEDs of the X-ray undetected population are consistent with starburst activity. There is no evidence for a hot dust component at the mid-infrared associated with AGN heated dust. If the X-ray undetected sources host AGN, an upper limit of L_X(2-10keV) =1e43erg/s is estimated for their intrinsic luminosity. We propose that a large fraction of the $z\approx2$ IRX population are not Compton Thick QSOs but low luminosity [L_X(2-10keV)<1e43erg/s], possibly Compton Thin, AGN or dusty starbursts. It is shown that the decomposition of the AGN and starburst contribution to the mid-IR is essential for interpreting the nature of this population, as star-formation may dominate this wavelength regime. "

Numerical Methods and Hydrodynamics

A method for reconstructing the PDF of a 3D turbulent density field from 2D observations
Christopher M. Brunt, Christoph Federrath, Daniel J. Price,
Comments: 5 pages, 2 figures, accepted for publication in MNRAS Letters

Theoretical Cosmology

Toward the Formation of Realistic Galaxy Disks
Alyson Brooks,
Comments: Proceedings of the Frank N. Bash Symposium 2009: "New Horizons in Astronomy." Comments welcome.

Stars, Supernovae and Planets

Direct Confirmation of the Asymmetry of the Cas A SN Explosion with Light Echoes
A. Rest, et al,
Comments: 12 pages, 7 figures, submitted to ApJ

Spectral Classification; Old and Contemporary
Sunetra Giridhar,
Comments: To be published in "Principles and Perspectives in Cosmochemistry" Lecture Notes on Kodai School on Synthesis of Elements in Stars: Ed Aruna Goswami &amp; Eswar Reddy, Springer Verlag, 2009, 17 pages, 10 figures

Updating the Historical Sunspot Record
Leif Svalgaard,
Comments: SOHO23 Conference Proceedings, Astronomical Society of the Pacific

Abstract: "We review the evidence for the argument that Rudolf Wolf's calibration of the Sunspot Number is likely to be correct and that Max Waldmeier introduced an upwards jump in the sunspot number in 1945. The combined effect of these adjustments suggests that there has been no secular change in the sunspot number since coming out of the Maunder Minimum ~1700."