Wine and Cheese Fall 2021

Wine and Cheese sessions with one speaker will have a 50 minute talk with 10 minutes for questions. Sessions with two speakers will have two 25 minute talks, each with 5 minutes for questions. Sessions in the Graduate Student Series will have three 15 minute talks, each with 5 minutes for questions.

20 September

Yunyang Li (JHU)

The Sunyaev-Zel'dovich Effect, Clusters, and ACT
The Sunyaev-Zel'dovich (SZ) effect introduces a specific distortion of the blackbody spectrum of the cosmic microwave background (CMB) radiation when it scatters off hot gas in clusters of galaxies. The frequency dependence of the distortion is only independent of the cluster redshift when the evolution of the CMB radiation is adiabatic. Using 370 clusters within the redshift range 0.07-1.4 from the largest SZ-selected cluster sample to date from the Atacama Cosmology Telescope, we provide new constraints on the deviation of CMB temperature evolution from the standard model $\alpha=0.017^{+0.029}_{-0.032}$, where $T(z)=T_0(1+z)^{1-\alpha}$. This result is consistent with no deviation from the standard adiabatic model. Attributing deviation from adiabaticity to the decay of dark energy, this result constrains its effective equation of state $w_\mathrm{eff}=-0.998^{+0.008}_{-0.010}$.

Isu Ravi (JHU)

Lyman-alpha Filter Prototype to Enable Astronomical Photometry in the Lyman Ultraviolet
Observations of astronomical objects in the far ultraviolet (FUV wavelengths span 900-1800 A) from earth’s orbit has been impeded due to bright Lyman-α geocoronal emission. The Johns Hopkins Rocket Group is developing a hydrogen absorption cell that would act as a narrow band Lyman-α rejection filter to enable space-based photometric observation in bandpasses that span over the Lyman ultraviolet region shortward of the geocoronal line. While this technology has been applied to various planetary missions with single element photomultiplier detectors it has yet to be used on near earth orbiting satellites with a multi-element detector. We are working to develop a cell that could be easily incorporated into future Lyman ultraviolet missions. The prototype cell is a low-pressure (∼ few torr) chamber sealed between a pair of MgF2 windows allowing transmission down to 1150 ̊A. It is filled with molecular hydrogen that is converted to its neutral atomic form in the presence of a hot tungsten filament, which allows for the absorption of the Lyman-α photons. Molecular hydrogen is stored in a fully saturated non-evaporable getter module (St707TM), which allows the cell pressure to be increased under a modest application of heat (a 20 degree rise from room temperature has produced a rise in pressure from 0.6 to 10 torr). Testing is now underway using a vacuum ultraviolet monochromator to characterize the cell optical depth to Lyman-α photons as functions of pressure and tungsten filament current. We will present these results, along with a discussion of enabled science in broadband photometric applications.

Gabriela Sato-Polito (JHU)

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27 September

Arshia Jacob (JHU)

Small Molecules, Big Impact: Investigating Hydrides in the Interstellar Medium
The nature and distribution of atomic and molecular gas in the interstellar medium (ISM) is of great interest to astrophysicists because it is this gas that provides the raw material for the formation of new stars. My work concentrates on investigating hydride molecules in the ISM and their use as diagnostics of it's different phases. Particular emphasis is given to the central CH radical, a probe of diffuse and translucent molecular clouds, including those not traced by the otherwise common CO. Following which I will detail our search for, and address questions regarding the origin of another molecule, CH2, which despite being chemically associated with the ubiquitous CH, has largely remained elusive. I will also discuss the transition from the diffuse atomic to the diffuse and translucent molecular phases of the ISM by extending our view of the chemistry of argonium, ArH+, a tracer of almost purely atomic gas. And lastly introduce the audience to HyGAL, a SOFIA Legacy Project aimed at characterising the Galactic ISM with observations of hydrides and other small molecules.

Ayan Acharyya (JHU)

“Mockulus reparo” – to Fix the Effects on Metallicity Gradient Measurements Due to Our Insufficient “Seeing”
Gas-phase metallicity gradients of galaxies are a crucial ingredient for understanding the chemical evolution of galaxies. However, measurements of gradients are often affected by limited spatial resolution. The extent of this effect may be different across observations, making cross-observation comparisons unreliable. We produce mock IFU datacubes from hydrodynamic simulations of an isolated galaxy and use those to quantify the said effect. I will present the application of our models on current IFU surveys — MaNGA, SAMI and CALIFA — by correcting the observed metallicity gradients for the effects of spatial resolution. I will demonstrate that while the mean trend of metallicity gradient versus stellar mass is largely unaffected, individual galaxies can undergo significant corrections. Our proposed method of correcting metallicity gradients by using isolated galaxy simulations already enables meaningful cross-survey comparisons. However, the next goal is to produce mock datacubes of cosmological simulations, in order to sample a broad range of galaxy properties. In this regard, I will briefly highlight some of my current efforts to study the metallicity distributions in the FOGGIE simulations.