Session E4: Astronomy, Astrophysics and Space Science I

Observation of the Stratorotational Instability in Flow between Rotating Concentric Cylinders.

We present a laboratory model of rotating protoplanetary disks. The study is motivated by the need for understanding the fluid instabilities that are involved in the formation of planets. We examine instability in a fluid contained between concentric cylinders as a function of the ratio of cylinder rotation rates, $\mu={\Omega_{outer}}/{\Omega_{inner}}$. This ``Stratorotational Instability'' (SRI) occurs in rotating sheared flows with an axial density dependence. In our experiment a fluid density that decreases exponentially with height is achieved by using water that is very salty at the bottom of the fluid annulus and pure water at the top of the cylinder. Disturbances in this density-stratified fluid oscillate with a natural ``buoyancy frequency'' $N$, which is varied in the range 0.25-1 Hz. We visualize the flow with a suspension of 25 $\mu$m diameter flakes and make digital movies that are analyzed using temporal and spatial Fast Fourier Transforms in order to determine the onset of instability. For each instability we find the axial wavenumber is proportional to $\Omega_{inner} / N$. We also find that, contrary to theory,\footnote{D A Shalybkov, {\it Physics Uspekhi} {\bf 52}, 915 (2009).} the SRI instability occurs for $\mu>\eta$, where $\eta$ is the ratio of the radii of the two cylinders.   

Fluidic Dynamics in Accretion Disk Fragmentation Model of Binary Protostar Twin Formation

Among the models that have been proposed for the development of binary star systems, the binary protostar model has benefitted from affirming evidence in recent astronomical observations. Accretion disk fragmentation surrounding an initial forming star is considered a potential origin for the companion star in binary protostar systems. This research will apply fluidic dynamic analysis to the accretion disks of forming protostars to explain how disk fragmentation forms the companion in the binary protostar model.   

Large Field Polarimetry Measurements using the Texas A\&M Observatory

With the announcement that the BICEP2 polarization signal is due to Milky Way dust instead of the Big Bang, there has been a keen interest recently in properly calibrating for Milky Way dust polarization. At the Texas A\&M Observatory, we have installed a wide-field telescope to measure the polarization of large parts of the sky as a pilot for a long-term project using the 0.8 m telescope at McDonald Observatory with a similar field of view and higher light-gathering power. The telescope is a 110 mm William Optics FLT-110 apochromatic refractor with a $\sim$1 degree field of view that has three optical polarizers at 60 degree intervals and an SBIG ST-8/8E/8XE camera to take data. By recording the polarizations of many galaxies, we can make a polarization map to correct for changes in shear that could be caused by the Milky Way dust. This is especially important since several large galaxy surveys, such as SDSS and DES, coming online need to make precise gravitational shear measurements to measure dark matter content. With this newfound data, we aim to better address the scientific goals of projects relying on polarimetry, such as BICEP, or that may need to take it into account, such as SDSS and DES, in the hopes of learning more about our Universe through the lens of a polarizer.   

Experimental Observation of Internal Gravity Waves

In the oceans, internal gravity waves transport energy and momentum from local generation near the seafloor to the ocean surface level. These internal waves, present for density-stratified fluids in a gravitational field, play a significant role in ocean mixing. Our goal is to determine how internal waves modify the fluid's density field. We investigate internal wave dynamics in a 4-meter long laboratory tank filled with water whose density increases with depth, just as in the oceans. Density perturbations by internal waves distort the light paths through the tank. The optical distortion is examined using a ``synthetic schlieren'' technique, which measures the index of refraction field. Digital movies of light transmitted through the tank are used in the schlieren technique to deduce the time-dependent density gradient field. From the density gradient field we compute the energy flux in the internal gravity waves. Internal wave energy plays a significant but poorly understood role in the energy budget of the oceans.   

Kinematic and Metallicity Comparisons between Dwarf Galaxies and Brightest Cluster Galaxies

Integral Field Unit (IFU) spectroscopy allows us to analyze the entire 2-dimensional surface of a galaxy as apposed to long slit or single fiber methods which provide a more limited view. Using the VIMOS IFU spectrograph on the Very Large Telescope (VLT), we spatially map the kinematic properties of 10 nearby Brightest Cluster Galaxies (BCGs) and 4 nearby companion galaxies at z$<=$0.1. We measure $\lambda_{Re}$ as a proxy for angular momentum, in order to determine whether these galaxies are fast or slow rotators. We find that 30\% (3/10) of the BCGs and 100\% of the BCG companion galaxies (4/4) are fast rotators. We also find that when comparing BCGs to similarly massive early-type galaxies, the ratio of fast rotating galaxies in the two populations is the same, suggesting that mass plays a more important role than environment when determining whether a galaxy is fast or slow rotating. We have also obtained metallicity measurements of these BCGs and find that most exhibit very shallow metallicity gradients. We extend this analysis to low stellar masses with a sample of nearby dwarf galaxies. Current results suggest that although the dwarf galaxies exhibit far lower metallicities, the metallicity gradients are similarly flat in the low mass and high mass regimes.   

A New Star-Formation Rate Calibration from the Polycyclic Aromatic Hydrocarbon Emission Features: Application to High Redshift Lensed Galaxies

Our goal is to calibrate polycyclic aromatic hydrocarbon (PAH) luminosity in the mid-infrared (mid-IR) as a star-formation rate (SFR) indicator that can be used in galaxies that host active galactic nuclei (AGN), where every other SFR indicator is contaminated by the AGN. We use mid-IR spectroscopy from the \textit{Spitzer} Infrared Spectrograph (IRS) and optical spectroscopy from various instruments to calibrate the mid-IR PAH features using (L$_{{\mathrm{H}} \alpha}$ + 0.020$\times L_{24 \mu \mathrm{m}}$) equivalent to dust-corrected H$_{\alpha}$ measurements (Kennicutt et al. 2009). Our sample consists of 226 galaxies corresponding to a range of total IR luminosity, L$_{\mathrm{IR}}$ = L(8-1000$\mu$m) = 10$^{9}$-10$^{12} L_\odot$ over the redshift range from 0$<$ z $<$0.6. We find using a unity relation, fit to the star-forming only galaxies (118 galaxies), correlates linearly to (L$_{\mathrm{H} \alpha}$ + 0.020$\times$L$_{24 \mu \mathrm{m}}$) with a gaussian scatter of $<$0.15 dex. As a result, we present a SFR relation for the PAH luminosity with uncertainties. We then apply our relations to a sample of high-redshift lensed galaxies (1 $<$ z $<$ 3) with previously estimated SFRs from other SFR indicators that are consistent to our PAH SFRs within uncertainties.   

Time Series Photometry with Small Aperture Telescopes

In the past decade small aperture telescopes (d $<$ 20 cm) have been show to produce high quality photometry. These telescopes have advantages over their larger counterparts by being highly reproducible, low cost and highly accessible. Texas A\&M University has been involved in two projects using small aperture telescopes, CSTAR and AggieCam, to study the time series nature of variable stars, exoplanet migration theory and stellar formation. We present the preliminary results of these studies and possible future collaborative efforts to install another such telescope in the Indian Himalayas.   

The Star-Formation Sequence: Tracking the Stellar Mass Growth of Galaxies since 2 Gyr after the Big Bang

Using data from the ZFOURGE Galaxy Evolution Survey in combination with public far-infrared imaging from the $Spitzer$ and $Herschel$ satellites we measure the relation between star-formation rate and stellar mass (SFR-$M_{\odot}$) for galaxies as early as when the universe was 12\% of its current age. Similar to recent work from Whitaker et al. (2014) we find that the slope is not constant with stellar mass but tends to steepen towards lower masses. We also track the evolution of the cosmic star-formation density (SFD) as a function. Despite hosting the largest SFRs amongst star-forming galaxies, massive galaxies ($>10^{11}$ $M_{\odot}$) only constitute at most roughly 13\% of the cosmic star-formation budget. Furthermore, the proportional amount of the SFD as a function of galaxy stellar mass evolves weakly with time. Finally, we compare the mass growth curves for galaxies that follow the average SFR-$M_{\odot}$ relation to the modern technique of tracking galaxy descendants via abundance matching. We find a tension between these two approaches where the predicted mass growth from abundance matching is systematically lower than the integrated star-formation form the SFR-$M_{\odot}$ relations by 0.1-0.3 dex.   

The Star-Formation Rate and Stellar Mass Relation of Distant Galaxies

Distant star-forming galaxies show a correlation between their star-formation rates (SFR) and stellar masses, and this has deep implications for galaxy formation. In this talk, I present a study on the evolution of the slope and scatter of the SFR-stellar mass relation for galaxies at high redshift, z $>$ 3.5, using multi-wavelength photometry from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We find that distant star-forming galaxies follow a nearly unevolving correlation between stellar mass and SFR that follows SFR $\sim$ M$_\star ^\alpha$ with $\alpha \approx$ 0.6. This evolution requires a star-formation history that increases with decreasing redshift (on average, the SFRs of individual galaxies rise with time). The measured scatter in the SFR-stellar mass relation is tight for galaxies with log M$_\star$/M$_\odot >$ 9 dex. This implies that the true intrinsic scatter in the SFR at fixed stellar mass is even smaller, $\sigma$(log SFR)$<$ 0.2 - 0.3 dex. Assuming that the SFR is tied to the net gas inflow rate of galaxies (SFR$\sim$ d(M$_{gas}$)/dt), then this result implies a low scatter in the gas inflow rate, favoring the theory of smooth gas accretion for star-forming galaxies at high redshift.