List of Past CTC Theory Lunches : 01-Sep-2013 to 31-Dec-2013


Date:   Monday 8-July-2013
Speaker:   Alastair Edge (Durham)
Title:  The Rise and Fall (and Rise again!) of NGC1275 (Special Seminar)

NGC1275/3C84 has held a central role in the long-running saga of cooling flows and AGN Feedback. On the face of it, NGC1275 is a very unusual elliptical galaxy with atypical radio properties. However, on closer inspection it shares many characteristics with the central galaxies in other cooling flow clusters. I will concentrate on the multi-wavelength variability of NGC1275 and discuss its implications for our understanding of AGN Feedback in clusters.


Date:   Tuesday 27-August-2013
Speaker:   Veronique Buat and Denis Burgarella (Marseille)
Title:  Herschel observations: contraints on dust attenuation and star formation histories at high redshift

We will present recent works based on the Herschel Space Observatory in the Far-Infrared.

In the first part, Veronique Buat will report on our recent works on the UV-to-IR SED fitting of distant (z>1) galaxies observed by Herschel. Combining stellar and dust emission in galaxies is found powerful to constrain their dust attenuation as well as their star formation activity. Then, she will briefly present our SED fitting code CIGALE (http://cigale.oamp.fr) and its application to a sample of galaxies from the CDFS observed by Herschel as part of the GOODS-Herschel project. Finally, she will focus on the characterisation of dust attenuation and on the uncertainties on the derivation of the star formation rates and stellar masses, as a function of the range of wavelengths sampled by the data data and of the assumptions made on the star formation histories.

In the second part, Denis Burgarella will report another work that uses new homogeneous luminosity functions in the far-ultraviolet (FUV) from VLT and in the far-infrared (FIR), to study the evolution of the dust attenuation with redshift. With this information, we are able to estimate the redshift evolution of the total (FUV + FIR) star formation rate density (SFRDTOT). By integrating SFRDTOT, we follow the mass building and analyze the redshift evolution of the stellar mass density (SMD). This talk will provide a complete view of star formation from the local Universe to z ~ 4 and, using assumptions on earlier star formation history, compare this evolution with previously published data in an attempt to draw a homogeneous picture of the global evolution of star formation in galaxies.

Finally, a new wide-field 1-5um space project (dubbed WISH) dedicated to the spectroscopic study of the very high redshift universe and to the detection of the first galaxies will be presented.


Date:   Monday 9-Sep-2013
Speaker:   Edmond Cheung (UCSC)
Title:  Galaxy Zoo: Observing Secular Evolution Through Bars

In this talk, I use the Galaxy Zoo 2 dataset to study the behavior of bars in disk galaxies as a function of specific star formation rate (SSFR), and inner galactic structure, i.e., the prominence of the bulge as parameterized by Se#rsic index and central surface stellar mass density. Our sample consists of 13,295 disk galaxies, with an overall bar fraction of 23.6 0.4%, of which 1,154 barred galaxies also have bar length measurements. These samples are the largest ever used to study the role of bars in disk galaxy evolution. I find that the likelihood of a galaxy hosting a bar is anti-correlated with SSFR, regardless of stellar mass or bulge prominence. I find that the trends of bar likelihood with bulge prominence are bimodal with SSFR, i.e., in star-forming galaxies, bulges are more prominent in galaxies more likely to host bars, while in quiescent disk galaxies, bars are less frequent where there are prominent bulges. Our observations of bar length reveal a complex picture. In star-forming disks, longer bars are found where the bulges are more prominent, while in quiescent disks there is a maximum in the average bar length as a function of bulge prominence. I interpret these observations using state-of-the-art simulations of bar evolution which include live halos and the effects of gas and star formation. I suggest our observed trends of bar likelihood with SSFR are driven by the gas fraction of the disks; a factor demonstrated to significantly retard both bar formation and evolution in models. I interpret the bimodal relationship between bulge prominence and bar properties as due to the complicated effects of classical bulges and central mass concentrations on bar evolution, and also to the growth of disky pseudobulges by bar evolution. These results represent empirical evidence for secular evolution driven by bars in disk galaxies. This work suggests that bars are not stagnant structures within disk galaxies, but are a critical evolutionary driver of their host galaxies in the local universe (z < 1).


Date:   Monday 16-Sep-2013
Speaker:   Cole Miller (UMD)
Title:  The Alignment of Supermassive Black Hole Binaries: Chaotic or Lawful?

Most large galaxies appear to have central supermassive black holes, hence when these galaxies merge with each other it is usually assumed that the black holes will eventually coalesce if the mass ratio is not less than ~0.1. If the holes have dimensionless spin parameters drawn from a uniform distribution and the spin orientations are random, then in >10-15% of such coalescences the kick from asymmetric gravitational wave emission is sufficient to eject the remnant from the merged galaxy. However, if the galaxies are gas-rich, interactions of the black holes with the gas can help align the spin axes with each other and with the orbit, which strongly suppresses the kicks. I will discuss recent work in which we demonstrate that when the holes form a gravitationally bound binary, the binary orbit acts as a stabilizer that accelerates this alignment. Thus we expect that black hole coalescence in gas-rich galaxy mergers will have a strong tendency to be aligned and thus that large gravitational wave kicks will be extremely rare. This also suggests that accretion will be efficient, and the supermassive black hole spins will tend to be high, in such environments.


Date:   Monday 23-Sep-2013
Speaker:   Sam Gralla (UMD)
Title:  Force-Free Magnetospheres of Stars and Black Holes

Forty years after the discovery of pulsars and realization that they possess a force-free magnetosphere, their most basic observed feature---pulsed emission---has not been explained. Our attitude is to take a step back and look at the fundamentals of the subject. We have investigated force-free magnetospheres of stars and black holes using some techniques imported from general relativity research. We have found non-stationary, non-axisymmetric exact solutions that should describe the outer magnetosphere of pulsars, including those that are accelerated or torqued. This description includes the exact dynamics of the current sheet, where gamma-ray emission may originate. For an accelerated aligned pulsar, the radiated power is enhanced relative to what would be expected for a magnetic dipole in vacuum. Finally, we have derived the standard cartoon of the aligned pulsar magnetosphere from an explicit, minimal set of assumptions, clarifying what assumptions are actually necessary.


Date:   Monday 30-Sep-2013
Speaker:   Steve Lubow (STScI)
Title:  Dynamics of Disks With Dead Zones

A key issue in modeling protostellar disks is the presence of dead zones, midplane regions with little turbulence. Such regions can lead to nonsteady disk evolution and outbursts. I will describe how such outbursts can be understood in terms of an instability cycle involving gravitational and magnetic forms of turbulence. I will discuss some possible effects of dead zones on protostellar and circumplanetary disks.


Date:   Monday 7-Oct-2013
Speaker:   Taro Shimizu (UMD)
Title:  The Far-IR View of an Ultra-Hard X-ray Selected Sample of AGN

We present early results of the Herschel PACS (70 and 160 microns) and SPIRE (250, 350, and 500 microns) survey of 313 low redshift (z < 0.05), ultra-hard X-ray (14-195 keV) selected AGN from the 58 month Swift/BAT catalog. Selection of AGN from ultra-hard X-rays avoids bias from obscuration that are unavoidable at other wavelengths (eg optical, infrared, and radio) providing the most complete sample of AGN to study the connection between nuclear activity and star formation in host galaxies. With the high angular resolution of PACS, we find that ~35% and ~20% of the sources are 'point-like' at 70 and 160 microns respectively with another 20% that have their flux dominated by a point source located at the nucleus. The inferred star formation rates (SFR) of 0.1 - 100 Msun yr-1 using the 70 and 160 microns flux densities and the calibration of Calzettti et al (2010) are consistent with those inferred from Spitzer NeII fluxes, but we find that 11.25 microns PAH data give ~3x lower SFR. Using GALFIT to measure the size of the FIR emitting regions, we determined the SFR density [Msun yr-1 kpc-2] for our sample, finding a significant fraction to exist above the threshold for star formation driven winds (0.1 Msun yr-1 kpc-2, Heckman 2001). Analysis of the SPIRE colors (250/350 and 350/500) also reveals evidence for the presence of nonthermal synchrotron emission from a radio jet significantly affecting the FIR emission at long wavelengths and altering the shape of the spectral energy distribution (SED).


Date:   Monday 14-Oct-2013
Speaker:   Demos Kazanas (GSFC)
Title:  The supercritical pile gamma-ray burst model: The GRB afterglow steep-decline-and-plateau phase

We present a process that accounts for the steep-decline-and-plateau phase of the Swift-XRT light curves, vexing features of GRB phenomenology. This process is an integral part of the "supercritical pile" GRB model, proposed a few years ago as a means of converting the GRB kinetic energy into radiation with a spectral peak at $E_{\rm pk} \sim m_ec^2$. We compute the evolution of the relativistic blast wave (RBW) Lorentz factor $\Gamma$ to show that the radiation--reaction force due to the GRB emission can produce an abrupt, small ($\sim 25%$) decrease in $\Gamma$ at a radius which is smaller (depending on conditions) than the deceleration radius $R_D$. Because of this reduction, the kinematic criticality criterion of the "supercritical pile" is no longer fulfilled. Transfer of the proton energy into electrons ceases, and the GRB enters abruptly the afterglow phase at a luminosity smaller by $\sim m_p/m_e$ than that of the prompt emission. If the radius at which this slow-down occurs is significantly smaller than $R_D$, the RBW internal energy continues to drive the RBW expansion at a constant (new) $\Gamma$, and its X-ray luminosity remains constant until $R_D$ is reached, at which point it resumes its more conventional decay, thereby completing the "unexpected" XRT light curve phase. If this transition occurs at $R \simeq R_D$, the steep decline is followed by a flux decrease instead of a "plateau", consistent with the conventional afterglow declines. Besides providing an account of these peculiarities, the model suggests that the afterglow phase may in fact begin before the RBW reaches $R \simeq R_D$, thus introducing novel insights into the GRB phenomenology.


Date:   Monday 21-Oct-2013
Speaker:   Tom Statler (Ohio/UMD)
Title:  What Every Astronomer Needs to Know About NSF

The National Science Foundation's Division of Astronomical Sciences funds nearly all US community-wide access to groundbased telescopes and a large fraction of the research grants for astronomy and astrophysics. But rank-and-file astronomers have little opportunity to learn about how this federal agency operates or the major issues that it faces in its efforts to support basic research. This talk will present an overview, based on my observations over 4 years working as an NSF Program Director, of what every astronomer ought to know in order to propose to, communicate with, and talk intelligently about NSF.


Date:   Monday 28-Oct-2013
Speaker:   Daniel Angles-Alcazar (Arizona)
Title:  Torque-Limited Growth of Massive Black Holes in Galaxies

Popular models of black hole growth in a cosmological context rely on the assumption that black hole growth is self-regulated by feedback from the accretion process itself, while often neglecting the angular momentum of the infalling gas at galactic scales. In this talk, I will argue that the assumption of strong self-regulation poses a severe limitation to model predictions given the intrinsic degeneracy between black hole accretion and feedback. I will describe an alternative scenario in which the transport of angular momentum in the galaxy by gravitational instabilities dominates the long-term co-evolution of black holes and star-forming galaxies. I will show that torque-limited growth yields black holes and host galaxies evolving on average towards the observed scaling relations, with no need for mass averaging through mergers or additional self-regulation processes. Strong outflows from the accretion disk play a key role by providing a significant mass loss, but there is no need for coupling to the infalling gas in order to regulate black holes in a non-linear feedback loop. I will end by discussing the main implications of this scenario in the context of current observations.


Date:   Monday 11-Nov-2013
Speaker:   Agnieszka Cieplak (Brookhaven National Labs)
Title:  New Microlensing Constraints of Primordial Black Hole Dark Matter using the First Two Years of Kepler Lightcurve Data

Primordial Black Holes (PBHs) are one of the few Dark Matter (DM) candidates left of the Standard Model of Particle Physics. Here I will present a new method to potentially constrain up to 40% of the remaining PBH DM mass range using the microlensing of source stars monitored by NASA's Kepler satellite. Its photometric precision and the large projected cross sections of the nearby stars allow for the possibility of extending the theoretically detectable PBH DM mass range two orders of magnitude below current limits. I will address how to extract DM properties, such as mass and spatial distribution if PBH microlensing events are detected, as well as present an approximation for estimating the predicted rate of detection for a given star, which could be used for future missions. Finally, I will present the experimental results from the analysis of the first two years of Kepler lightcurve data. With new sources of background error such as stellar flares and comets not predicted by our theoretical calculations, our sensitivity already covers one order of magnitude below the previously existing microlensing limits.


Date:   Monday 18-Nov-2013
Speaker:   Peter Polko (UMD)
Title:  Magnetohydrodynamic jet solutions with gravity

Astrophysical jets can be described best by a magnetohydrodynamic (MHD) flow. I will give a brief overview of MHD, before considering the application to the specific case of a jet. Making further assumptions, such as time independence, axisymmetry, and self-similarity, it is possible to construct a relativistic jet model that can be easily computed. The last part will focus on my work to include gravity and find solutions to the equations that smoothly cross the three singular points present in a relativistic MHD flow.


Date:   Monday 25-Nov-2013
Speaker:   Mikhail Belyaev (Princeton)
Title:  Exciting Waves in Accretion Disk Boundary Layers

In an accretion disk, the boundary layer is the region where the disk attaches to the star. Unlike in the disk proper, the boundary layer has a rotation profile which rises with radius, since the rotation profile must transition from that of the more slowly rotating star to that of the more quickly rotating disk. The rising rotation profile makes the boundary layer (linearly) stable to the MRI instability. Therefore, a different mechanism must be responsible for transporting angular momentum in the boundary layer and decelerating the disk material. The details of this mechanism are observationally significant since up to half of the total disk luminosity comes from the boundary layer region.

Working with Roman Rafikov and James Stone, I have discovered a new instability which is capable of transporting angular momentum and driving accretion in the boundary layer via waves rather than turbulence. This instability is related to the Papaloizou-Pringle class of instabilities, in that it is sourced by a corotation resonance in the boundary layer. However, it is distinct from the traditional Papaloizou-Pringle instability; it is also robust, feeding off the large degree of shear in the boundary layer. I will present both analytical and numerical results, which explain the mechanism of the new instability (which we call the sonic or acoustic instability) and its role in angular momentum transport in accretion disk boundary layers. I will also describe why no reasonable alpha prescription for the viscosity can be reconciled with this instability. This has significant ramifications for semi-analytical models of boundary layers, which typically assume some form of alpha viscosity.


Date:   Monday 2-Dec-2013
Speaker:   Christina Williams (UMass-Amherst)
Title:  The evolution of the compact early-type galaxies at high-redshift

The first galaxies in the universe (e.g. z>2) to quench their star-formation appear to be both the most massive, and also the most compact (in stellar density). The nature of quenching mechanisms are poorly understood at any epoch, however the ubiquity of these quenched compact galaxies at high-redshift implies that quenching at this epoch, during the peak of star-formation activity in the universe, preferentially affects compact galaxies. We have selected a sample of their star-forming progenitors among Lyman-Break Galaxies (LBGs) at z~3 based on the hypothesis that any candidate progenitors must also be compact, since merging and accretion tend to increase the sizes of galaxies. We present results based on analysis of both the spectral energy distributions and available spectroscopy for these compact star-forming progenitors compared to ordinary (in size) LBGs of similar masses. We find that these compact progenitors have redder rest-frame UV colors than ordinary LBGs, but with similar optical colors, consistent with an older burst of star-formation. Using rest-frame UV spectroscopy, we also find potential evidence the compact progenitors have higher metallicities and more powerful winds than ordinary LBGs. These results suggest that compact galaxies may quench more effectively than ordinary galaxies of similar mass, and are consistent with the idea that these compact galaxies undergo a more rapid evolution. Finally, the structural properties of compact galaxies appear inconsistent with those of gas-rich merger remnants, as implied by N-body simulations, providing evidence that these galaxies must form stars through highly dissipative accretion of cold gas.


Date:   Monday 9-Dec-2013
Speaker:   Molly Peeples (STScI)
Title:   What have galaxies done with their metals?

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