MIT Astrophysics Brown Bag Lunch Series: Spring 2016

Selected Mondays at 12:05 PM in Marlar Lounge, 37-252 (unless otherwise noted)
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA

The Brown Bag lunch is a forum for visiting astronomers/astrophysicists to speak about their research. Talks begin at 12:05 and speakers should plan 40 minutes of material, to leave room for questions during and after the presentation.  Owing to lighter job talk traffic, BBL talks during the Spring semester are arranged on a "by request" basis.  Weeks not listed below are assumed to have no talk yet scheduled.  If you are visiting MIT and would like to give a lunch talk during one of our open weeks, you may either contact the organizers directly or have your local colleagues arrange a time. The present organizers of the series are Brendan Griffen, Michael McDonald, and Paul Torrey.

Monday March 7 -- two talks

Germanium CCDs

Christopher Leitz
Lincoln Laboratory

Host: Mark Bautz

The short-wave infrared (SWIR) band, spanning approximately 1 – 3 μm, is of great importance for night vision, wavefront sensing, and hyper-spectral imaging.  While silicon is the material of choice for visible imaging, it is insensitive to wavelengths longer than 1.1 μm.  The dominant imaging technology for wavelengths from 0.9 – 1.7 μm consists of InGaAs diode arrays bump-bonded to silicon readout integrated circuits (ROICs).  In contrast to silicon, these detectors are expensive, fragile, and limited to 150-mm diameter or smaller substrates.  Bump bonding limits the minimum pixel pitch to about 15 µm.  Furthermore, back-illumination is performed on the chip- rather than wafer-level, and response cuts off below 920 nm due to absorption in the InP substrate.  Finally, the largest InGaAs devices are limited to 1.3 Mpixel.  Arrays based on HgCdTe are also available and cover a larger portion of the SWIR band but require substantial cooling and are not as widely used as those of InGaAs.  
Compared to these imager materials, bulk germanium offers several compelling advantages.  Germanium is commercially available in wafer diameters up to 200-mm and can be processed in the same semiconductor fabrication tools used to build silicon imaging devices.  This enables fabrication of large-format imaging devices with a narrow pixel pitch.  Recent advances in surface passivation have yielded the high-quality metal-oxide interface required for fabricating CCDs, providing the same advantages (uniformity, noiseless binning, TDI) of silicon CCDs.  Next, germanium offers wider sensitivity than any of these materials, with high quantum efficiency from 0.3 – 1.8 μm, enabling both SWIR and hyper-spectral imaging.  Finally, because it can be grown ultra-pure and has a high atomic number, germanium is also an efficient X-ray absorber with sensitivity to 30 keV and beyond, enabling further applications in medical and scientific imaging.
In this talk, we describe our progress in fabricating germanium CCDs, outline our performance goals for a germanium imaging device, and highlight potential applications of these devices.


The abundance and spatial distribution of ultra-diffuse galaxies in nearby galaxy clusters

Remco van der Berg
CEA Saclay, France

Host: Rongmon Bordoloi

Recent observations have shown that Ultra-Diffuse Galaxies (UDGs, which have the luminosities of dwarfs but sizes of giants) are surprisingly abundant in local clusters of galaxies. The origin of UDGs remains puzzling, as the interpretation of these studies has been hindered by the (partly) subjective selection of these galaxies, and the limited study of only the Coma and Virgo clusters. I’ll present results of the first systematic search for UDGs in eight nearby clusters. I’ll focus on the abundance of UDGs as a function of halo mass, and their radial distribution in the clusters. The latter indicates that they were likely accreted by the cluster several Gyrs ago, but are still hosted by massive dark-matter haloes. I’ll discuss implications and future prospects to learn more about their properties.


Monday April 4 -- Two Talks!


Relativistic Effect in Galaxy Clustering as a Novel Probe of Inflationary Cosmology

Jaiyul Yoo
University of Zurich

Host: Mark Vogelsberger

Precision measurements in current and future galaxy surveys bring in new challenges, demanding substantial advances in theoretical modeling. I will discuss the recent theoretical advances in formulating galaxy clustering in a general relativistic context. The relativistic effect in galaxy clustering or the deviation from the standard Newtonian description becomes substantial on large scales, in which modified gravity theories deviate from general relativity and the fingerprint of the inflationary epoch remains in its pristine form. I will discuss how the subtle relativistic effect in cosmology can be used to test general relativity and probe signatures of the early Universe.


Talk #2 -- Using galaxies as natural telescopes: Disentangling the quasars’ X-ray emission regions with microlensing

Ana Mosquera
US Naval Academy

Host: Monica Turner

As accreting supermassive black holes, AGNs are among the most physically interesting objects in the Universe. They also help to regulate star formation and the evolution of galaxies. One of their defining characteristics is their X-ray emission, but the origin and spatial extent of this emission is still unknown in large part because we have lacked the means to resolve the central engine.  However, microlensing in multiply imaged gravitationally lensed quasars allows us to zoom in on in the structure of AGN and explore their physics in more detail. Quantitative microlensing X-ray size constraints exist for 6 systems, setting the first upper limits for these emitting regions. Obtaining these pioneering results was possible due to the high resolution and sensitivity of the CXO. The next frontier is to explore possible correlations of the X-rays sizes with black hole mass and spectral index, as well as to set constraints on the scaling of the size with X-ray energy. In my talk I will briefly describe our methodology and results, and discuss key aspects of future surveys that would help us to obtain unprecedented results in the field.

Monday April 11 

Runaway Planets, Hypervelocity Stars, Sonic Booms, and our Galactic Center

Idan Ginsburg
Harvard Smithsonian Center for Astrophysics

Host: Brendan Griffen

The center of our galaxy is home to a supermassive black hole. It is also home to a number of stars that orbit surprisingly close to the black hole. I will describe how these stars got into such unexpected orbits, what new science they can teach us, and how we may be able to find even more of them. I will also discuss the possibility of planets around the innermost region of the Milky Way, why this is exciting and important, and how we might be able to detect them.

Tuesday April 19 -- note special date due to April 18 holiday

DLAs in the Star Forming Disks of z<0.15 Galaxies on Top of QSOs in the SDSS

Lorrie Straka
Leiden University

Host: Monica Turner

Understanding the distribution of gas in galaxies and its interaction with the CGM is crucial in order to complete the picture of galaxy evolution. At all redshifts, absorption features seen in QSO spectra serve as a unique probe of the gaseous content of foreground galaxies and the CGM, extending out to ~200 kpc. Studies show that star formation history is intimately related to the co-evolution of galaxies and the CGM. In order to study the environments traced by absorption systems and the role of inflows and outflows, it is critical to measure the emission properties of host galaxies and their halos. However, the success rate for detecting host galaxies is still incredibly low (in some cases, less than 30%), particularly for the highest column density systems known as Damped Lyman-alpha absorbers (DLAs). In recent years, great strides have been made towards overcoming the deficit of confirmed host galaxies compared to the number of observed absorption systems, but it remains unclear how DLAs are related to morphology and inferred characteristics of galaxies. SDSS provides the wide coverage at low redshift necessary to probe configurations of galaxies and QSOs in imaging, which has allowed us to compile a sample of galaxies on top of QSOs (GOTOQs). New data from HST/COS of QSOs probing the star forming disks of z<0.15 galaxies from the SDSS and preliminary results will be presented.


Monday April 25 -- TBA

Wednesday May 4   (Please note that this talk will take place on a Wednesday)

Measuring the bolometric luminosity of exoplanet beta Pictoris b via direct-imaging with Magellan AO

Katie Morzinski
Steward Observatory

Host: Amanda Bosh

Physical characteristics of exoplanets remain difficult to measure.  One well-studied planet is beta Pictoris b, which is 9 AU from and embedded in the debris disk of the young nearby A6V star beta Pictoris.  We imaged the system with Magellan Adaptive Optics (MagAO) instruments VisAO and Clio, at 5 wavelengths from 0.9-5 microns.  I will present a complete spectral energy (SED) distribution with the new MagAO data plus literature data from Gemini and VLT.  Atmosphere models demonstrate the planet is cloudy but are degenerate in effective temperature and radius.  As the measured SED now covers >80% of the planet's energy, we can determine the bolometric luminosity empirically by combining the 22 independent measurements at 16 passbands from 0.99-4.8 microns.  The empirically-determined luminosity is in agreement with values from atmospheric models, but brighter than values from the field-dwarf bolometric correction.  Thus, beta Pic b is another example where directly comparing young exoplanets to old brown dwarfs gives inaccurate physical characteristics like luminosity.  Understanding exoplanet properties is a data-limited field, so directly measuring the luminosity for more exoplanets is critical to developing a fuller picture of their evolution.


Monday May 9 -- Two Talks

Linking the most massive galaxies to their hot gas halos

Andy Goulding
Princeton University

Host: Esra Bulbul

Abstract:  Studies of the physical properties of local elliptical galaxies (e.g., gas temperatures, halo masses, stellar kinematics) are shedding new light on galaxy formation. I will present our recent investigations into the X-ray and optical properties of nearby, massive early-type galaxies. We harness the exquisite spatial resolution and sensitivity of the Chandra X-ray observatory, combined with integral field optical spectroscopy, to provide the most complete picture to date, of the influence of large and small scale environment on the evolution of elliptical galaxies.


Escape fraction of ionizing photons - the impact of binary stars

Xiangcheng Ma

Host: Paul Torrey

Abstract: The escape fraction of Lyman Continuum photons needs to be ~20% in order to ionize the universe. We use the high-resolution cosmological zoom-in simulations from the FIRE project, which include detailed models of the multi-phase ISM, star formation, and stellar feedback, and Monte Carlo radiative transfer code, to study the escape fraction from dwarf galaxies at z>6. Using 'standard' stellar population models from STARBURST99, we find that the time-averaged escape fraction is ~5%, far below the requirement of reionization. This is because most of the absorption comes from individual star-forming clouds, while the timescale for stellar feedback to destroy these birth clouds is comparable to the lifetime of massive stars that produce most of the ionizing photons. However, stellar population models that include binary interaction (e.g. the BPASS model) predict much more ionizing photons than single-star models from STARBURST99 at late times. We find that using BPASS model, the predicted escape fraction can be boosted by a factor of 4-10, which matches the requirement of deionization.