Stellar Velocity Dispersion in Dissipative Galaxy Mergers with Star Formation

In order to better understand stellar dynamics in merging systems, such as NGC 6240, we examine the evolution of central stellar velocity dispersion (\sig) in dissipative galaxy mergers using a suite of binary disk merger simulations that include feedback from stellar formation and active galactic nuclei (AGNs). We find that $\sigma_*$ undergoes the same general stages of evolution that were observed in our previous dissipationless simulations: coherent oscillation, then phase mixing, followed by dynamical equilibrium. We also find that measurements of $\sigma_*$ that are based only upon the youngest stars in simulations consistently yield lower values than measurements based upon the total stellar population. This finding appears to be consistent with the so-called "$\sigma_*$ discrepancy," observed in real galaxies. We note that quasar-level AGN activity is much more likely to occur when $\sigma_*$ is near its equilibrium value rather than during periods of extreme $\sigma_*$. Finally, we provide estimates of the scatter inherent in measuring $\sigma_*$ in ongoing mergers.Comment: 17 pages, 10 figures, accepted for publication in Ap

An Empirical Approach to Cosmological Galaxy Survey Simulation: Application to SPHEREx Low-Resolution Spectroscopy

Highly accurate models of the galaxy population over cosmological volumes are necessary in order to predict the performance of upcoming cosmological missions. We present a data-driven model of the galaxy population constrained by deep 0.1-8 $\rm \mu m$ imaging and spectroscopic data in the COSMOS survey, with the immediate goal of simulating the spectroscopic redshift performance of the proposed SPHEREx mission. SPHEREx will obtain over the full-sky $R\sim41$ spectrophotometry at moderate spatial resolution ($\sim6"$) over the wavelength range 0.75-4.18 $\rm \mu m$ and $R\sim135$ over the wavelength range 4.18-5 $\rm \mu m$. We show that our simulation accurately reproduces a range of known galaxy properties, encapsulating the full complexity of the galaxy population and enables realistic, full end-to-end simulations to predict mission performance. Finally, we discuss potential applications of the simulation framework to future cosmology missions and give a description of released data products

Joint Survey Processing I: Compact oddballs in the COSMOS field -- low-luminosity Quasars at z > 6?

The faint-end slope of the quasar luminosity function at z~6 and its implication on the role of quasars in reionizing the intergalactic medium at early times has been an outstanding problem for some time. The identification of faint high-redshift quasars with luminosities of 25mag, with ~30% of sources having their flux contaminated by foreground objects when the seeing resolution is ~0.7". We mitigate these issues by performing a pixel-level joint processing of ground and space-based data from Subaru/HSC and HST/ACS. We create a deconfused catalog over the 1.64 deg² of the COSMOS field, after accounting for spatial varying PSFs and astrometric differences between the two datasets. We identify twelve low-luminosity (M_(UV) ~ -21 mag) z>6 quasar candidates through (i) their red color measured between ACS/F814W and HSC/i-band and (ii) their compactness in the space-based data. We estimate that late-type stars could contribute up to 50% to our sample. Our constraints on the faint end of the quasar luminosity function at z~6.4 suggests a negligibly small contribution to reionization compared to the star-forming galaxy population. The confirmation of our candidates and the evolution of number density with redshift could provide better insights into how supermassive galaxies grew in the first billion years of cosmic time

The Evolution of Stellar Velocity Dispersion During Dissipationless Galaxy Mergers

Using N-body simulations, we studied the detailed evolution of central stellar velocity dispersion, {\sigma}, during dissipationless binary mergers of galaxies. Stellar velocity dispersion was measured using the common mass-weighting method as well as a flux-weighting method designed to simulate the technique used by observers. A toy model for dust attenuation was introduced in order to study the effect of dust attenuation on measurements of {\sigma}. We found that there are three principal stages in the evolution of {\sigma} in such mergers: oscillation, phase mixing, and dynamical equilibrium. During the oscillation stage, {\sigma} undergoes damped oscillations of increasing frequency. The oscillation stage is followed by a phase mixing stage during which the amplitude of the variations in {\sigma} is smaller and more chaotic than in the oscillation stage. Upon reaching dynamical equilibrium, {\sigma} assumes a stable value. We used our data regarding the evolution of {\sigma} during mergers to characterize the scatter inherent in making measurements of {\sigma} in non-quiescent systems. In particular, we found that {\sigma} does not fall below 70% nor exceed 200% of its final, quiescent value during a merger and that a random measurement of {\sigma} in such a system is much more likely to fall near the equilibrium value than near an extremum. Our toy model of dust attenuation suggested that dust can systematically reduce observational measurements of {\sigma} and increase the scatter in {\sigma} measurements.Comment: 18 pages, 8 figure

SPHEREx: an all-sky NIR spectral survey

SPHEREx, a mission in NASA’s Medium Explorer (MIDEX) program recently selected for Phase-A implementation, is an all-sky survey satellite that will produce a near-infrared spectrum for every 6 arcsecond pixel on the sky. SPHEREx has a simple, high-heritage design with large optical throughput to maximize spectral mapping speed. While the legacy data products will provide a rich archive of spectra for the entire astronomical community to mine, the instrument is optimized for three specific scientific goals: to probe inflation through the imprint primordial non-Gaussianity left on today’s large-scale cosmological structure; to survey the Galactic plane for water and other biogenic ices through absorption line studies; and to constrain the history of galaxy formation through power spectra of background fluctuations as measured in deep regions near the ecliptic poles. The aluminum telescope consists of a heavily baffled, wide-field off-axis reflective triplet design. The focal plane is imaged simultaneously by two mosaics of H2RG detector arrays separated by a dichroic beamsplitter. SPHEREx assembles spectra through the use of mass and volume efficient linear variable filters (LVFs) included in the focal plane assemblies, eliminating the need for any dispersive or moving elements. Instead, spectra are constructed through a series of small steps in the spacecraft attitude across the sky, modulating the location of an object within the FOV and varying the observation wavelength in each exposure. The spectra will cover the wavelength range between 0.75 and 5.0 µm at spectral resolutions ranging between R=35 and R=130. The entire telescope is cooled passively by a series of three V-groove radiators below 80K. An additional stage of radiative cooling is included to reduce the long wavelength focal plane temperature below 60K, controlling the dark current. As a whole, SPHEREx requires no new technologies and carries large technical and resource margins on every aspect of the design

The Evolution of Stellar Velocity Dispersion in Galaxy Mergers

Stellar velocity dispersion is a key measurable quantity in galactic astronomy, yet its variation during galaxy mergers is not well-understood theoretically. Thus, while it is fairly common to measure velocity dispersion in galaxies that are in the process of merging, it is unclear how these measurements should be interpreted. In this dissertation, I provide a theoretical analysis of the evolution of stellar velocity dispersion during galaxy mergers. This is done using a set of numerical simulations. The temporal and directional evolution of velocity dispersion are examined in detail for a variety of merger simulations. I also examine the effects that dust attenuation and star formation have on measurements of velocity dispersion by creating detailed, Doppler broadened galaxy spectra. Velocity dispersions are measured from the synthetic spectra using the same technique that is employed for observations of real galaxies. I find that velocity dispersion increases rapidly and significantly as two galaxies pass through one another. As galaxies recede from a collision, their velocity dispersions rapidly decrease and nearly return to their pre-collision values. Velocity dispersion increases in all directions during collisions, however the enhancement is most significant along the collision axis. After the nuclei of the progenitor system coalesce, the velocity dispersion oscillates slightly of the coalesced system oscillated around its final equilibrium value for up to several dynamical timescales. I also find that the mean velocity dispersion of young stars tends to be lower than the velocity dispersion of the galaxy as a whole. The young stars become dynamically heated with time. In most cases, the youngest stars are found in dusty environments. Thus, dust preferen- tially obscures young stars, partially removing them from the flux-weighted velocity dispersion measurement. This causes flux-weighted velocity dispersion measurements to be elevated with respect to mass-weighted measurements because the young stars are dynamically cooler. On the other hand, since young stellar populations are brighter, per unit mass, than older stellar populations, the low dispersion of young stars tends to weight measurements of velocity dis- persion downward when the young stars are not more significantly obscured by dust than the older populations