Friday, May 28, 2010

Interesting Astrophysics: 17 May to 28 May

The final batch of interesting preprints (and a few accepted papers) for May 2010.

Galaxies and Starbursts

Witnessing the Formation of a Brightest Cluster Galaxy in a Nearby X-ray Cluster
Jesper Rasmussen, John S. Mulchaey, Lei Bai, Trevor J. Ponman, Somak Raychaudhury, Ali Dariush, arXiv:1005.3538 [pdf, ps, other]
Comments: 16 pages, 12 figures. ApJ accepted

UV+IR Star Formation Rates: Hickson Compact Groups with Swift and Spitzer
P. Tzanavaris, A. E. Hornschemeier, S. C. Gallagher, K. E. Johnson, C. Gronwall, S. Immler, A. E. Reines, E. Hoversten, J. C. Charlton, arXiv:1005.4059 [pdf, ps, other]
Comments: Accepted by ApJ. [8 Tables, 16 Figures. Color figures have reduced size for ArXiv - emulateapj v. 2/16/10]
Journal-ref: Astrophysical Journal 716 (2010) 556-573

From their abstract: "We present Swift UVOT (1600-3000A) 3-band photometry for 41 galaxies in 11 nearby (<4500km/s) Hickson Compact Groups (HCGs) of galaxies. We use the uvw2-band (2000A) to estimate the dust-unobscured component, SFR_UV, of the total star-formation rate, SFR_T. We use Spitzer MIPS 24-micron photometry to estimate SFR_IR, the dust-obscured component of SFR_T. We obtain SFR_T=SFR_UV+SFR_IR. Using 2MASS K_s band based stellar mass, M*, estimates, we calculate specific SFRs, SSFR=SFR_T/M*. SSFR values show a clear and significant bimodality, with a gap between low (<~3.2x10^-11 / yr) and high SSFR (>~1.2x10^-10 / yr) systems. All galaxies with MIR activity index a_IRAC <= 0 (>0) are in the high- (low-) SSFR locus, as expected if high levels of star-formation power MIR emission from polycyclic aromatic hydrocarbon molecules and a hot dust continuum. All elliptical/S0 galaxies are in the low-SSFR locus, while 22 out of 24 spirals/irregulars are in the high-SSFR locus, with two borderline cases. ... Unlike HCG galaxies, galaxies in a comparison quiescent SINGS sub-sample are continuously distributed both in SSFR and a_IRAC. Any uncertainties can only further enhance the SSFR bimodality. These results suggest that an environment characterized by high galaxy number-densities and low galaxy velocity-dispersions, such as the one found in compact groups, plays a key role in accelerating galaxy evolution by enhancing star-formation processes in galaxies and favoring a fast transition to quiescence."

Radiation pressure from massive star clusters as a launching mechanism for super-galactic winds
Norman Murray, Brice Ménard, Todd A. Thompson, arXiv:1005.4419 [pdf, ps, other]
Comments: Submitted to ApJ, comments welcome
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA)

Their abstact: "Galactic outflows of low ionization, cool gas are ubiquitous in local starburst galaxies, and in the majority of galaxies at high redshift. How these cool outflows arise is still in question. Hot gas from supernovae has long been suspected as the primary driver, but this mechanism suffers from its tendency to destroy the cool gas as the latter is accelerated. We propose a modification of the supernova scenario that overcomes this difficulty.
Star formation is observed to take place in clusters; in a given galaxy, the bulk of the star formation is found in the ~20 most massive clusters. We show that, for L* galaxies, the radiation pressure from clusters with M>10^6 M_sun is able to expel the surrounding gas at velocities in excess of the circular velocity of the disk galaxy. This cool gas can travel above the galactic disk in less than 2 Myr, well before any supernovae erupt in the driving cluster. Once above the disk, the cool outflowing gas is exposed to radiation, and supernovae induced hot gas outflows, from other clusters in the disk, which drive it to distances of several tens to hundreds of kpc. Because the radiatively driven clouds grow in size as they travel, and because the hot gas is more dilute at large distance, the clouds are less subject to destruction if they do eventually encounter hot gas. Therefore, unlike wind driven clouds, radiatively driven clouds can survive to distances ~50 kpc. We identify these cluster-driven winds with large-scale galactic outflows. Another implication of our model is that only starburst galaxies, where massive clusters reside, are able to drive winds cold outflows on galactic scales via this mechanism. We find that the critical star formation rates above which large scale cool outflows will be launched to be ~0.1 M_sun/yr/kpc^2, which is in good agreement with observations."

I can see a quite a few problems with this line of argumentation. Hopefully I'll get around to addressing them in the monster paper I'm still adding to. Why write three papers when you can just write one absurdly large one?

Black Holes and AGN

"Comets" orbiting a black hole
R. Maiolino, G. Risaliti, M. Salvati, P. Pietrini, G. Torricelli-Ciamponi, M. Elvis, G. Fabbiano, V. Braito, J. Reeves, arXiv:1005.3365 [pdf, ps, other]
Comments: Accepted for publication in A&A. 11 pages, 9 figures

Their abstract: "We use a long (300 ksec), continuous Suzaku X-ray observation of the active nucleus in NGC1365 to investigate the structure of the circumnuclear BLR clouds through their occultation of the X-ray source. The variations of the absorbing column density and of the covering factor indicate that the clouds surrounding the black hole are far from having a spherical geometry (as sometimes assumed), instead they have a strongly elongated and cometary shape, with a dense head (n=10^11 cm^-3) and an expanding, dissolving tail. We infer that the cometary tails must be longer than a few times 10^13 cm and their opening angle must be smaller than a few degrees. We suggest that the cometary shape may be a common feature of BLR clouds in general, but which has been difficult to recognize observationally so far. The cometary shape may originate from shocks and hydrodynamical instabilities generated by the supersonic motion of the BLR clouds into the intracloud medium. As a consequence of the mass loss into their tail, we infer that the BLR clouds probably have a lifetime of only a few months, implying that they must be continuously replenished. We also find a large, puzzling discrepancy (two orders of magnitude) between the mass of the BLR inferred from the properties of the absorbing clouds and the mass of the BLR inferred from photoionization models; we discuss the possible solutions to this discrepancy."

Adaptive optics near infrared integral field spectroscopy of NGC 2992
S. Friedrich, R. I. Davies, E. K. S. Hicks, H. Engel, F. Müller-Sánchez, R. Genzel, L. J. Tacconi, arXiv:1005.4791 [pdf, ps, other]
Comments: 10 pages, 8 figures, accepted for publication in A&amp;A

From their abstract: "NGC 2992 is an intermediate Seyfert 1 galaxy showing outflows on kilo parsec scales which might be due either to AGN or starburst activity. We therefore aim at investigating its central region for a putative starburst in the past and its connection to the AGN and the outflows. Observations were performed with the adaptive optics near infrared integral field spectrograph SINFONI on the VLT, complemented by longslit observations with ISAAC on the VLT, as well as N- and Q-band data from the Spitzer archive. The spatial and spectral resolutions of the SINFONI data are 50 pc and 83 km/s, respectively. ... A simple geometric model of two mutually inclined disks and an additional cone to describe an outflow was developed to explain the observed complex velocity field in H_2 1-0S(1). ... We find a starburst age of 40 Myr - 50 Myr from Br_gamma line diagnostics and the radio continuum; ongoing star formation can be excluded. Both the energetics and the timescales indicate that the outflows are driven by the AGN rather than the starburst. The complex velocity field observed in H_2 1-0S(1) in the central 450 pc can be explained by the superposition of the galaxy rotation and an outflow."

So, the questions remains - is there really an outflow on larger scales too?. The lack of clear cases of purely AGN-driven large scale winds really makes me doubt radiation-driven wind models.

Fading hard X-ray emission from the Galactic Centre molecular cloud Sgr B2
R. Terrier, G. Ponti, G. Belanger, A. Decourchelle, V. Tatischeff, A. Goldwurm, G. Trap, M. R. Morris, R. Warwick, arXiv:1005.4807 [pdf, ps, other]
Comments: Accepted for publication in ApJ. 10 pages, 5 figures

From their abstract: "The centre of our Galaxy harbours a 4 million solar mass black hole that is unusually quiet: its present X-ray luminosity is more than 10 orders of magnitude less than its Eddington luminosity. The observation of iron fluorescence and hard X-ray emission from some of the massive molecular clouds surrounding the Galactic Centre has been interpreted as an echo of a past flare. ... Here we report the observation of a clear decay of the hard X-ray emission from the molecular cloud Sgr B2 during the past 7 years thanks to more than 20 Ms of INTEGRAL exposure. The measured decay time is compatible with the light crossing time of the molecular cloud core . Such a short timescale rules out inverse bremsstrahlung by cosmic-ray ions as the origin of the X ray emission. We also obtained 2-100 keV broadband X-ray spectra by combining INTEGRAL and XMM-Newton data and compared them with detailed models of X-ray emission due to irradiation of molecular gas by (i) low-energy cosmic-ray electrons and (ii) hard X-rays. Both models can reproduce the data equally well, but the time variability constraints and the huge cosmic ray electron luminosity required to explain the observed hard X-ray emission strongly favor the scenario in which the diffuse emission of Sgr B2 is scattered and reprocessed radiation emitted in the past by Sgr A*. Using recent parallax measurements that place Sgr B2 in front of Sgr A*, we find that the period of intense activity of Sgr A* ended between 75 and 155 years ago."

Black Hole Mass, Host galaxy classification and AGN activity
Barry McKernan, K.E.Saavik Ford, Chris Reynolds, arXiv:1005.4907 [pdf, ps, other]
Comments: MNRAS accepted. 14 pages, 11 figures, complete Table 1 in online journal

Theoretical Cosmology

The intergalactic medium over the last 10 billion years I: Lyman alpha absorption and physical conditions
Romeel Davé, Benjamin D. Oppenheimer, Neal Katz, Juna A. Kollmeier, David H. Weinberg, arXiv:1005.2421 [pdf, ps, other]
Comments: 21 pages, submitted to MNRAS

Intergalactic Dust Extinction in Hydrodynamic Cosmological Simulations
Ying Zu, David H. Weinberg, Romeel Davé, Mark Fardal, Neal Katz, Dusan Keres, Benjamin D. Oppenheimer, arXiv:1005.4406 [pdf, ps, other]
Comments: 12 pages, 7 figures, to be submitted to MNRAS

Astrophysical Processes

Secondary ionization and heating by fast electrons
Steven R. Furlanetto and Samuel Johnson Stoever, 2010, MNRAS, 404, 1869
Full Text: HTML, PDF (Size: 720K)

Numerical Astrophysics and Computational Techniques

Searchable Sky Coverage of Astronomical Observations: Footprints and Exposures
Tamas Budavari, Alex Szalay, Gyorgy Fekete, arXiv:1005.2606 [pdf, other]
Comments: 11 pages, 7 figures, submitted to PASP

Their abstract: "Sky coverage is one of the most important pieces of information about astronomical observations. We discuss possible representations, and present algorithms to create and manipulate shapes consisting of generalized spherical polygons with arbitrary complexity and size on the celestial sphere. This shape specification integrates well with our Hierarchical Triangular Mesh indexing toolbox, whose performance and capabilities are enhanced by the advanced features presented here. Our portable implementation of the relevant spherical geometry routines comes with wrapper functions for database queries, which are currently being used within several scientific catalog archives including the Sloan Digital Sky Survey, the Galaxy Evolution Explorer and the Hubble Legacy Archive projects as well as the Footprint Service of the Virtual Observatory."

The Role of Provenance Management in Accelerating the Rate of Astronomical Research

G. Bruce Berriman, Ewa Deelman, arXiv:1005.3358 [pdf, other]
Comments: 8 pages, 1 figure; Proceedings of Science, 2010

Their abstract: "The availability of vast quantities of data through electronic archives has transformed astronomical research. It has also enabled the creation of new products, models and simulations, often from distributed input data and models, that are themselves made electronically available. These products will only provide maximal long-term value to astronomers when accompanied by records of their provenance; that is, records of the data and processes used in the creation of such products. We use the creation of image mosaics with the Montage grid-enabled mosaic engine to emphasize the necessity of provenance management and to understand the science requirements that higher-level products impose on provenance management technologies. We describe experiments with one technology, the "Provenance Aware Service Oriented Architecture" (PASOA), that stores provenance information at each step in the computation of a mosaic. The results inform the technical specifications of provenance management systems, including the need for extensible systems built on common standards. Finally, we describe examples of provenance management technology emerging from the fields of geophysics and oceanography that have applicability to astronomy applications."

Montage: a grid portal and software toolkit for science-grade astronomical image mosaicking
Joseph C. Jacob, et al, arXiv:1005.4454 [pdf, other]
Comments: 16 pages, 11 figures
Journal-ref: Int. J. Computational Science and Engineering. 2009

Their abstract: "Montage is a portable software toolkit for constructing custom, science-grade mosaics by composing multiple astronomical images. The mosaics constructed by Montage preserve the astrometry (position) and photometry (intensity) of the sources in the input images. The mosaic to be constructed is specified by the user in terms of a set of parameters, including dataset and wavelength to be used, location and size on the sky, coordinate system and projection, and spatial sampling rate. Many astronomical datasets are massive, and are stored in distributed archives that are, in most cases, remote with respect to the available computational resources. Montage can be run on both single- and multi-processor computers, including clusters and grids. Standard grid tools are used to run Montage in the case where the data or computers used to construct a mosaic are located remotely on the Internet. This paper describes the architecture, algorithms, and usage of Montage as both a software toolkit and as a grid portal. Timing results are provided to show how Montage performance scales with number of processors on a cluster computer. In addition, we compare the performance of two methods of running Montage in parallel on a grid."

Stars, Supernovae and Planets

Who Pulled the Trigger: a Supernova or an AGB Star?
Alan P. Boss, Sandra A. Keiser, arXiv:1005.3981 [pdf, ps, other]
Comments: 14 pages, 4 figures, 1 table, Astrophysical Journal Letters, in press

Their abstract: "The short-lived radioisotope $^{60}$Fe requires production in a core collapse supernova or AGB star immediately before its incorporation into the earliest solar system solids. Shock waves from a somewhat distant supernova, or a relatively nearby AGB star, have the right speeds to simultaneously trigger the collapse of a dense molecular cloud core and to inject shock wave material into the resulting protostar. A new set of FLASH2.5 adaptive mesh refinement hydrodynamical models shows that the injection efficiency depends sensitively on the assumed shock thickness and density. Supernova shock waves appear to be thin enough to inject the amount of shock wave material necessary to match the short-lived radioisotope abundances measured for primitive meteorites. Planetary nebula shock waves from AGB stars, however, appear to be too thick to achieve the required injection efficiencies. These models imply that a supernova pulled the trigger that led to the formation of our solar system. "

Herschel Observations of the W43 "mini-starburst"
J. Bally, et al, arXiv:1005.4092 [pdf, ps, other]
Comments: 5 pages, 3 figures, accepted for A&amp;A Special Issue

From their abstract: "Aims: To explore the infrared and radio properties of one of the closest Galactic starburst regions. Methods: Images obtained with the Herschel Space Observatory at wavelengths of 70, 160, 250, 350, and 500 microns using the PACS and SPIRE arrays are analyzed and compared with radio continuum VLA data and 8 micron images from the Spitzer Space Telescope. ... Results: The W43 star-forming complex is resolved into a dense cluster of protostars, infrared dark clouds, and ridges of warm dust heated by massive stars. The 4 brightest compact sources with L > 1.5 x 10^4 Lsun embedded within the Z-shaped ridge of bright dust emission in W43 remain single at 4" (0.1 pc) resolution. These objects, likely to be massive protostars or compact clusters in early stages of evolution are embedded in clumps with masses of 10^3 to 10^4 Msun, but contribute only 2% to the 3.6 x 10^6 Lsun far-IR luminosity of W43 measured in a 16 by 16 pc box. The total mass of gas derived from the far-IR dust emission inside this region is ~10^6 Msun. Cometary dust clouds, compact 6 cm radio sources, and warm dust mark the locations of older populations of massive stars. Energy release has created a cavity blowing-out below the Galactic plane."

I wish people would stop calling individual star forming regions, even the biggest ones, starbursts. The term starburst has historically been used to denote significantly enhanced star formation at a galactic scale, and makes most sense when used thusly. See, e.g. Heckman, T., 2005, A&SS, 329, 3.

Evolution of massive stars with pulsation-driven superwinds during the RSG phase
Sung-Chul Yoon, Matteo Cantiello, arXiv:1005.4925 [pdf, ps, other]
Comments: Accepted for publications in ApJ Letters

Interesting to note the significance of pulsation in driving these winds. They're not just steady dust-driven winds. Are there any cases of steady dust-driven winds?

Tuesday, May 18, 2010

3-D supernovae and CP violation

Some cool science news:

1. New simulations of supernova explosions by Müller's group at MPIA show significant differences in behaviour from previous antisymmetric 2-D simulations (MPIA press release, which is also the source of the image shown on the right).

The new computer models of the team at the Max Planck Institute for Astrophysics can now for the first time simulate the complete burst in all three dimensions, from the first milliseconds after the explosion is triggered in the core to a time three hours later, when the shock breaks out of the progenitor star. "We found substantial deviations in our 3D models compared to previous work in 2D," says Nicolay Hammer, the lead author of the paper, "especially the growth of instabilities and the propagation of clumps differ. These are not just minor variations; this effect determines the long-time evolution and ultimately the extent of mixing and observable appearance of core-collapse supernovae."

In the 3D-simulations, metal-rich clumps have much higher velocities than in the 2D case. These "bullets" expand much more rapidly, overtaking material from the outer layers. "With a simple analytic model we could demonstrate that the different geometry of the bullets, toroidal versus quasi-spherical, can explain the differences observed in our simulations," explains co-author Thomas Janka. "While we think that the differences between the 2D- and 3D-models that we found are probably generic, many features will depend strongly on the structure of the progenitor star, the overall energy and the initial asymmetry of the blast."

"We hope that our models, in comparison to observations, will help us to understand how stellar explosions start and what causes them", adds Ewald Müller, the third author of the paper. Investigating a wider variety of progenitor stars and initial conditions will therefore be the focus of future simulation work. In particular, a detailed model that reproduces all observational features of SN 1987A still remains a challenge.
Neat. May also have implications for superwinds, as there again we have a case of the acceleration and motion of dense clumps.

2. The DZero collaboration have found evidence for a 1%-level matter/anti-matter asymmetry (Fermilab press release)., i.e. much larger than previous examples of CP violation.

Friday, May 14, 2010

Interesting Astrophysics: May 03 to May 14

It looks like the Herschel folks have been busy, judged by the deluge of preprints that have appeared on arXiv over the last few weeks. I found Roussel et al's paper on dust in/around M82 and Fischer et al's paper on Markarian 231 (and its molecular outflow) of particular interest.

Nevertheless X-ray-related papers dominate this issue of Interesting Astrophysics, from hot gas in galaxy halos (Mulchaey & Jeltema; Henley et al; Crain et al) to the detection or modelling of the Warm/Hot Intergalactic Medium (Yao et al; Cen & Chisari).

Galaxies and Starbursts

SPIRE imaging of M82: cool dust in the wind and tidal streams
H. Roussel, et al, arXiv:1005.1526 [pdf, ps, other]
Comments: accepted in A&A Herschel special issue

Their abstract in full: "M82 is a unique representative of a whole class of galaxies, starbursts with superwinds, in the Very Nearby Galaxy Survey with Herschel. In addition, its interaction with the M81 group has stripped a significant portion of its interstellar medium from its disk. SPIRE maps now afford better characterization of the far-infrared emission from cool dust outside the disk, and sketch a far more complete picture of its mass distribution and energetics than previously possible. They show emission coincident in projection with the starburst wind and in a large halo, much more extended than the PAH band emission seen with Spitzer. Some complex substructures coincide with the brightest PAH filaments, and others with tidal streams seen in atomic hydrogen. We subtract the far-infrared emission of the starburst and underlying disk from the maps, and derive spatially-resolved far-infrared colors for the wind and halo. We interpret the results in terms of dust mass, dust temperature, and global physical conditions. In particular, we examine variations in the dust physical properties as a function of distance from the center and the wind polar axis, and conclude that more than two thirds of the extraplanar dust has been removed by tidal interaction, and not entrained by the starburst wind."

The Stellar Kinematic Center and the True Galactic Nucleus of NGC253
F. Müller-Sánchez, O. González-Martín, J. A. Fernández-Ontiveros, J. A. Acosta-Pulido, M. A. Prieto, arXiv:1005.1645 [pdf, ps, other]
Comments: 33 pages, 7 figures, Accepted for publication in ApJ, Version with high resolution figures available at this http URL

Herschel PACS Spectroscopic Diagnostics of Local ULIRGs: Conditions and Kinematics in Mrk 231
Jacqueline Fischer, et al, arXiv:1005.2213 [pdf, other]
Comments: Accepted for publication in the Astronomy and Astrophysics Herschel Special Issue, 5 pages, 4 figures

Full abstract, emphasis mine: "In this first paper on the results of our Herschel PACS survey of local Ultraluminous Infrared Galaxies (ULIRGs), as part of our SHINING survey of local galaxies, we present far-infrared spectroscopy of Mrk 231, the most luminous of the local ULIRGs, and a type 1 broad absorption line AGN. For the first time in a ULIRG, all observed far-infrared fine-structure lines in the PACS range were detected and all were found to be deficient relative to the far infrared luminosity by 1 - 2 orders of magnitude compared with lower luminosity galaxies. The deficits are similar to those for the mid-infrared lines, with the most deficient lines showing high ionization potentials. Aged starbursts may account for part of the deficits, but partial covering of the highest excitation AGN powered regions may explain the remaining line deficits. A massive molecular outflow, discovered in OH and 18OH, showing outflow velocities out to at least 1400 km/sec, is a unique signature of the clearing out of the molecular disk that formed by dissipative collapse during the merger. The outflow is characterized by extremely high ratios of 18O / 16O suggestive of interstellar medium processing by advanced starbursts."

Hot Gas Halos in Early-Type Field Galaxies
John S. Mulchaey, Tesla E. Jeltema, arXiv:1004.5376 [pdf, ps, other]
Comments: Accepted for publication in ApJ Letters

Their abstract: "We use Chandra and XMM-Newton to study the hot gas content in a sample of field early-type galaxies. We find that the L_X-L_K relationship is steeper for field galaxies than for comparable galaxies in groups and clusters. The low hot gas content of field galaxies with L_K < L_star suggests that internal processes such as supernovae driven winds or AGN feedback expel hot gas from low mass galaxies. Such mechanisms may be less effective in groups and clusters where the presence of an intragroup or intracluster medium can confine outflowing material. In addition, galaxies in groups and clusters may be able to accrete gas from the ambient medium. While there is a population of L_K < L_star galaxies in groups and clusters that retain hot gas halos, some galaxies in these rich environments, including brighter galaxies, are largely devoid of hot gas. In these cases, the hot gas halos have likely been removed via ram pressure stripping. This suggests a very complex interplay between the intragroup/intracluster medium and hot gas halos of galaxies in rich environments with the ambient medium helping to confine or even enhance the halos in some cases and acting to remove gas in others. In contrast, the hot gas content of more isolated galaxies is largely a function of the mass of the galaxy, with more massive galaxies able to maintain their halos, while in lower mass systems the hot gas escapes in outflowing winds."

A fundamental relation between mass, SFR and metallicity in local and high redshift galaxies

F. Mannucci, G. Cresci, R. Maiolino, A. Marconi, A. Gnerucci, arXiv:1005.0006 [pdf, other]
Comments: 13 pages, submitted to MNRAS
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Their abstract: "We show that the mass-metallicity relation observed in the local universe is due to a more general relation between stellar mass M*, gas-phase metallicity and SFR. Local galaxies define a tight surface in this 3D space, the Fundamental Metallicity Relation (FMR), with a small residual dispersion of ~0.05 dex in metallicity, i.e, ~12%. At low stellar mass, metallicity decreases sharply with increasing SFR, while at high stellar mass, metallicity does not depend on SFR. High redshift galaxies, up to z~2.5 are found to follow the same FMR defined by local SDSS galaxies, with no indication of evolution. The evolution of the mass-metallicity relation observed up to z=2.5 is due to the fact that galaxies with progressively higher SFRs, and therefore lower metallicities, are selected at increasing redshifts, sampling different parts of the same FMR. By introducing the new quantity mu_alpha=log(M*)-alpha log(SFR), with alpha=0.32, we define a projection of the FMR that minimizes the metallicity scatter of local galaxies. The same quantity also cancels out any redshift evolution up to z~2.5, i.e, all galaxies have the same range of values of mu_0.32. At z>2.5, evolution of about 0.6 dex off the FMR is observed, with high-redshift galaxies showing lower metallicities. The existence of the FMR can be explained by the interplay of infall of pristine gas and outflow of enriched material. The former effect is responsible for the dependence of metallicity with SFR and is the dominant effect at high-redshift, while the latter introduces the dependence on stellar mass and dominates at low redshift. The combination of these two effects, together with the Schmidt-Kennicutt law, explains the shape of the FMR and the role of mu_0.32. The small metallicity scatter around the FMR supports the smooth infall scenario of gas accretion in the local universe."

A fundamental plane for field star-forming galaxies
M.A. Lara-López, J. Cepa, A. Bongiovanni, A.M. Pérez García, A. Ederoclite, H. Castañeda, M. Fernández Lorenzo, M. Póvic, M. Sánchez-Portal, arXiv:1005.0509 [pdf, ps, other]
Comments: Submitted to A&amp;A as a letter to the Editor on April 15, 2010. 4 pages, 4 Figures

Their abstract: "Star formation rate (SFR), metallicity and stellar mass are within the important parameters of star--forming galaxies that characterize their formation and evolution. They are known to be related to each other at low and high redshift in the mass--metallicity, mass--SFR, and metallicity--SFR relations. In this work we demonstrate the existence of a plane in the 3D space defined by the axes SFR [log(SFR)(M_sun yr^-1)], gas metallicity [12+log(O/H)], and stellar mass [log(M_star/M_sun)] of star-forming galaxies. We used star--forming galaxies from the "main galaxy sample" of the Sloan Digital Sky Survey--Data Release 7 (SDSS-DR7) in the redshift range 0.04 < z < 0.1 and r-magnitudes between 14.5 and 17.77. Metallicities, SFRs, and stellar masses were taken from the Max-Planck-Institute for Astrophysics-John Hopkins University (MPA-JHU) emission line analysis database. From a final sample of 44214 galaxies, we find for the first time a fundamental plane for field galaxies relating the SFR, gas metallicity, and stellar mass for star--forming galaxies in the local universe. One of the applications of this plane would be estimating stellar masses from SFR and metallicity. High redshift data from the literature at redshift ~2.2 and 3.5, do not show evidence for evolution in this fundamental plane."

Our Milky Way as a Pure-Disk Galaxy -- A Challenge for Galaxy Formation
Juntai Shen, R. Michael Rich, John Kormendy, Christian D. Howard, Roberto De Propris, Andrea Kunder, arXiv:1005.0385 [pdf, ps, other]
Comments: 5 pages; emulateapj format; submitted to ApJL

Massive star formation in Wolf-Rayet galaxies. V: Star formation rates, masses and the importance of galaxy interactions
Angel R. Lopez-Sanchez
Comments: 33 pages, 21 figures, accepted for publication by A&A

From the abstract: "We have performed a comprehensive analysis of a sample of 20 starburst galaxies, most of them classified as Wolf-Rayet galaxies. In this paper, the last of the series, we analyze the global properties of our galaxy sample using multiwavelength data (X-ray, FUV, optical, NIR, FIR, and radio). The agreement between our Ha-based SFR and those provided by indicators at other wavelengths is remarkable, but we consider that the new Ha-based calibration provided by Calzetti et al. (2007) should be preferred over older calibrations. The FUV-based SFR provides a powerful tool to analyze the star-formation activity in both global and local scales independently to the Ha emission. We provide empirical relationships between the ionized gas mass, neutral gas mass, dust mass, stellar mass, and dynamical mass with the B-luminosity. ... Considering all data, we found that 17 up to 20 galaxies are clearly interacting or merging with low-luminosity dwarf objects or HI clouds. We conclude that interactions do play a fundamental role in the triggering mechanism of the strong star-formation activity observed in dwarf starburst galaxies."

The Dearth of Chemically Enriched Warm-Hot Circumgalactic Gas
Y. Yao, Q. D. Wang, S. V. Penton, T. M. Tripp, J. M. Shull, J. T. Stocke, arXiv:1005.0923 [pdf, ps, other]
Comments: 10 pages, 7 figures, and 5 tables. Accepted for publication in the ApJ, June 2010 - 20 v716 issue.

Argues against a local group origin for the z~0 X-ray absorbers, as have others. Also "These results indicate that the putative CGM [Circum Galactic Medium] in the temperature range of 10^{5.5}-10^{6.3} K may not be able to account for the missing baryons unless the metallicity is less than 10% solar." Of course its probably more accurate to say the mire astronomers believe the missing baryons to be in the warm-hot inter galactic medium, and not within galactic halos at all.

The Origin of the Hot Gas in the Galactic Halo: Confronting Models with XMM-Newton Observations
David B. Henley, Robin L. Shelton, Kyujin Kwak, M. Ryan Joung, Mordecai-Mark Mac Low, arXiv:1005.1085 [pdf, ps, other]
Comments: 18 pages, 13 figures. Submitted to the Astrophysical Journal
Subjects: Galaxy Astrophysics (astro-ph.GA)

Full abstract: "We compare the predictions of three physical models for the origin of the hot halo gas with the observed halo X-ray emission, derived from 26 high-latitude XMM-Newton observations of the soft X-ray background between $l=120\degr$ and $l=240\degr$. These observations were chosen from a much larger set of observations as they are expected to be the least contaminated by solar wind charge exchange emission. We characterize the halo emission in the XMM-Newton band with a single-temperature plasma model. We find that the observed halo temperature is fairly constant across the sky (~1.8e6-2.4e6 K), whereas the halo emission measure varies by an order of magnitude ($\sim$0.0005-0.006 cm^-6 pc), including significant sightline-to-sightline variation on scales as small as a few degrees. When we compare our observations with the model predictions, we find that most of the hot gas observed with XMM-Newton does not reside in an extended hot halo (predicted by disk galaxy formation models), nor is it contained within isolated extraplanar supernova remnants - both these models are at least an order of magnitude too faint in the XMM-Newton band. A model of a supernova-driven interstellar medium, one feature of which is a fountain of hot gas from the disk into the halo, gives the best agreement with the observed 0.4-2.0 keV surface brightness. This model overpredicts the halo X-ray temperature by a factor of ~2. However, there are a several plausible explanations for this discrepancy. Therefore, our general conclusion is that the hot halo gas observed with XMM-Newton originates in a fountain driven into the halo by disk supernovae."

Can galaxy outflows and re-accretion produce a downsizing in the specific star-formation rate of late-type galaxies?
C. Firmani, V. Avila-Reese and A. Rodríguez-Puebla, 2010, MNRAS, 404, 1100
Full Text: HTML, PDF (Size: 685K)

Theoretical Cosmology

Star Formation Feedback and Metal Enrichment History Of The Intergalactic Medium
Renyue Cen, Nora Elisa Chisari, arXiv:1005.1451 [pdf, other]
Comments: 52 pages, 26 figures, submitted to ApJ, all comments welcome

From their abstract: "Using the state-of-the-art cosmological hydrodynamic simulations we compute the metal enrichment history of the intergalactic medium (IGM). Overall, we show that galactic superwind feedback from star formation is able to transport metals to the IGM that matches a broad range of observations. We find ... (6) While gravitational shocks from large-scale structure formation dominate the energy budget (80-90%) for turning about 50% of IGM to the warm-hot intergalactic medium (WHIM) by z=0, galactic superwind feedback shocks are energetically dominant over gravitational shocks at z>1-2. (7) Most of the so-called "missing metals" at z=2-3 are hidden in a warm-hot (T=10^{4.5-7}K) gaseous phase. (8) Approximately (37,46,10,7)% of the total metals at z=0 are in (stars, WHIM, X-ray gas, cold gas); the distribution stands at (23,57,2,18)% and (14,51,4,31)% at z=2 and z=4, respectively."

X-ray coronae in simulations of disc galaxy formation
Robert A. Crain, Ian G. McCarthy, Carlos S. Frenk, Tom Theuns, Joop Schaye, arXiv:1005.1642 [pdf, other]
Comments: 21 pages, 16 figures. Accepted for publication in MNRAS. Accompanying visualisations at this http URL

Their abstract: "The existence of X-ray luminous gaseous coronae around massive disc galaxies is a long-standing prediction of galaxy formation theory in the cold dark matter cosmogony. This prediction has garnered little observational support, with non-detections commonplace and detections for only a relatively small number of galaxies which are much less luminous than expected. We investigate the coronal properties of a large sample of bright, disc-dominated galaxies extracted from the GIMIC suite of cosmological hydrodynamic simulations recently presented by Crain et al. Remarkably, the simulations reproduce the observed scalings of X-ray luminosity with K-band luminosity and star formation rate and, when account is taken of the density structure of the halo, with disc rotation velocity as well. Most of the star formation in the simulated galaxies (which have realistic stellar mass fractions) is fuelled by gas cooling from a quasi-hydrostatic hot corona. However, these coronae are more diffuse, and of a lower luminosity, than predicted by the analytic models of White & Frenk because of a substantial increase in entropy at z ~ 1-3. Both the removal of low entropy gas by star formation and energy injection from supernovae contribute to this increase in entropy, but the latter is dominant for halo masses M_200 <~ 10^(12.5) Msun. Only a small fraction of the mass of the hot gas is outflowing as a wind but, because of its high density and metallicity, it contributes disproportionally to the X-ray emission. The bulk of the X-ray emission, however, comes from the diffuse quasi-hydrostatic corona which supplies the fuel for ongoing star formation in discs today. Future deep X-ray observations with high spectral resolution (e.g. with NeXT/ASTRO-H or IXO) should be able to map the velocity structure of the hot gas and test this fundamental prediction of current galaxy formation theory."

Numerical Astrophysics

Multi-layered configurations in differentially-rotational equilibrium
Kenta Kiuchi, Hiroki Nagakura, Shoichi Yamada, arXiv:1005.2236 [pdf, ps, other]
Comments: To appear on APJ, high-resolution figures are available in the published version.

Full abstract: "We present a new formula to numerically construct configurations in rotational equilibrium, which consist of multiple layers. Each layer rotates uniformly or differentially according to cylindrical rotation-laws that are different from layer to layer. Assuming a different barotropic equation of state (EOS) for each layer, we solve the Bernoulli equation in each layer separately and combine the solutions by imposing continuity of the pressure at each boundary of the layers. It is confirmed that a single continuous barotropic EOS is incompatible with the junction condition. Identifying appropriate variables to be solved, we construct a convergent iteration scheme. For demonstration, we obtain two-layered configurations, each layer of which rotates rapidly with either an "$\Omega$-constant law" or a "$j$-constant law" or a "$v$-constant law". Other rotation laws and/or a larger number of layers can be treated similarly. We hope that this formula will be useful in studying the stellar evolution in multi-dimension with the non-spherical configuration induced by rotation being fully taken into account."

Stars, Supernovae and Planets

On the evolution of a star cluster and its multiple stellar systems following gas dispersal
Nickolas Moeckel and Matthew R. Bate, 2010, MNRAS, 404, 712
Full Text: HTML, PDF (Size: 3726K)

Bursting SN 1996cr's Bubble: Hydrodynamic and X-ray Modeling of its Circumstellar Medium
Vikram V. Dwarkadas, Daniel Dewey, Franz Bauer, arXiv:1005.1090 [pdf, ps, other]
Comments: Accepted to MNRAS. 21 pages, 8 Figures, 6 in color. For a version with higher resolution colour figures see this http URL
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

From their abstract: "SN1996cr is one of the five closest SNe to explode in the past 30 years. Due to its fortuitous location in the Circinus Galaxy at ~ 3.7 Mpc, there is a wealth of recently acquired and serendipitous archival data available to piece together its evolution over the past decade, including a recent 485 ks Chandra HETG spectrum. In order to interpret this data, we have explored hydrodynamic simulations, followed by computations of simulated spectra and light curves under non-equilibrium ionization conditions, and directly compared them to the observations. Our simulated spectra manage to fit both the X-ray continuum and lines at 4 epochs satisfactorily, while our computed light curves are in good agreement with additional flux-monitoring data sets. These calculations allow us to infer the nature and structure of the circumstellar medium, the evolution of the SN shock wave, and the abundances of the ejecta and surrounding medium. The data imply that SN 1996cr exploded in a low-density medium before interacting with a dense shell of material about 0.03pc away from the progenitor star."

Radio and X-ray Observations of the Type Ic SN 2007gr Reveal an Ordinary, Non-relativistic Explosion
Alicia M. Soderberg, Andreas Brunthaler, Ehud Nakar, Roger A. Chevalier, arXiv:1005.1932 [pdf, ps, other]
Comments: 14 pages, 6 figures, submitted to ApJ

Again, evidence that the supernova blastwave is expanding into circum-stellar medium shaped by stellar winds.

Generation of radiative knots in a randomly pulsed protostellar jet II. X-ray emission
R. Bonito, S. Orlando, M. Miceli, J. Eislöffel, G. Peres, F. Favata, arXiv:1005.2125 [pdf, ps, other]
Comments: Accepted for publication in Astronomy and Astrophysics

Their abstract: "Protostellar jets are known to emit in a wide range of bands, from radio to IR to optical bands, and to date also about ten X-ray emitting jets have been detected, with a rate of discovery of about one per year. We aim at investigating the mechanism leading to the X-ray emission detected in protostellar jets and at constraining the physical parameters that describe the jet/ambient interaction by comparing our model predictions with observations. We perform 2D axisymmetric hydrodynamic simulations of the interaction between a supersonic jet and the ambient. The jet is described as a train of plasma blobs randomly ejected by the stellar source along the jet axis. We explore the parameter space by varying the ejection rate, the initial jet Mach number, and the initial density contrast between the ambient and the jet. We synthesized from the model the X-ray emission as it would be observed with the current X-ray telescopes. The mutual interactions among the ejected blobs and of the blobs with the ambient medium lead to complex X-ray emitting structures within the jet: irregular chains of knots; isolated knots with measurable proper motion; apparently stationary knots; reverse shocks. The predicted X-ray luminosity strongly depends on the ejection rate and on the initial density contrast between the ambient and the jet, with a weaker dependence on the jet Mach number. Our model represents the first attempt to describe the X-ray properties of all the X-ray emitting protostellar jets. The comparison between our model predictions and the observations can provide a useful diagnostic tool necessary for a proper interpretation of the observations. In particular, we suggest that the observable quantities derived from the spectral analysis of X-ray observations can be used to constrain the ejection rate, a parameter explored in our model that is not measurable by current observations."

Monday, May 03, 2010

Problems with SALT

SciAm online has a short article discussing SALT's (Southern African Large Telescope) ongoing problems with spherical aberration: "Southern Hemisphere's Largest Telescope Hamstrung by Optical Problems" by Bruce Dorminey.

[Image: SALT's 11 meter diameter segmented primary mirror. From the SALT website telescope overview.]