|Sep 26||Adam Miller||
The falling costs of computing and CCD detectors has led to a great boom in wide-field time-domain optical surveys during the past ~decade, with several new surveys expected prior to the arrival of LSST. This observational boon, however, comes with a catch: the data rates from these surveys are so large that discovery techniques heavily dependent on human intervention are becoming unviable. Identifying the extraordinary within large data sets requires an understanding and model of the more common sources. In this talk I will detail new methods, which utilize semi-supervised machine-learning algorithms, to automatically classify the light curves of time-variable sources. Using these methods, we have produced a data-driven probabilistic catalog of variables found in the All Sky Automated Survey (ASAS). We identify true outliers within the data via the clustering of sources which are not classified with high significance. This has led to the discovery of several rare systems: including four new members of the R Coronae Borealis class, which are thought to be the product of low mass white dwarf mergers, but are underrepresented in the Milky Way relative to the LMC. Finally, I will describe how a machine-learned robot discovered two outbursting young stars which are changing the way we understand star formation.
|Oct 3||Anastasia Fialkov||
In this talk I will show that first stars at z~20 leave detectable signature in the redshifted 21-cm line of neutral hydrogen. Two effects previously omitted in similar studies are responsible for a substantial enhancement in the expected signal: the relative motion between the dark matter halos and the in-falling gas, and the negative Lyman-Werner feedback of the starlight on star formation. I will shortly describe the underlying semi-numerical method, which combines linear evolution on large scales and application of analytical models and the results of small-scale numerical simulations on non-linear scales. Finally, I will explain how the two effects boost the power spectrum of the 21-cm signal thus making its detection easier.
|Oct 10||Smadar Naoz||
The search for extra-solar planets has led to the surprising discovery of many Jupiter-like planets in very close proximity to their host star, the so-called ``hot Jupiters'' (HJs). Even more surprising, many of these HJs have orbits that are eccentric or highly inclined with respect to the equator of the star, and some (about 25%) even orbiting counter to the spin direction of the star. This poses a unique challenge to all planet formation models. We show that secular interactions between Jupiter-like planet and another perturber in the system can easily produce retrograde HJ orbits. We show that in the frame of work of secular hierarchical triple system (the so-called Kozai mechanism) the inner orbit's angular momentum component parallel to the total angular momentum (i.e., the z-component of the inner orbit angular momentum) need not be constant. In fact, it can even change sign, leading to a retrograde orbit. A brief excursion to very high eccentricity during the chaotic evolution of the inner orbit allows planet- star tidal interactions to rapidly circularize that orbit, decoupling the planets and forming a retrograde hot Jupiter. We estimate the relative frequencies of retrograde orbits and counter to the stellar spin orbits using Monte Carlo simulations, and find that the they are consistent with the observations. The high observed incidence of planets orbiting counter to the stellar spin direction may suggest that three body secular interactions are an important part of their dynamical history.
|Oct 17||Mike McCourt||
Galaxy clusters are filled with a hot, tenuous plasma known as the intracluster medium, or ICM. I will discuss recent work aimed at understanding how the plasma can cool to form multi-phase gas, and what implications this cooling has for the structure of the ICM. I apply this result to the observed non-self-similarity (analogous to the missing baryon problem) in clusters. I will also discuss a project to understand the temperature profiles in the ICM and what implications this has for convection and turbulent pressure support in clusters.
|Oct 24||Anne Medling||
Major mergers are thought to be an important stage of galaxy evolution, and in gas-rich galaxies can instigate intense star formation and nuclear activity. Much of this activity goes on in the inner kiloparsec, which has been difficult to study due to resolution and extinction concerns. We present < 0.1"-resolution near-infrared integral field spectroscopy of the nuclear regions of nearby (U)LIRGs in late stages of merging. Our observations were taken with the OSIRIS spectrograph on the Keck II telescope, with laser guide star adaptive optics, and provide stellar and gas kinematics on scales of ~20-80 pc. In our sample, we find nuclear disks of stars and/or gas on scales of a few hundred parsecs around one or both nuclei in all nine of the merging systems observed. Binary black hole merger simulations suggest that the presence of these nuclear disks may be important for angular momentum loss and final coalescence. By studying the kinematics of these nuclear disks, we can also measure black hole masses in the midst of major mergers and trace galaxies' paths along black hole scaling relations to assess galaxy-bulge growth relations during this important stage of galaxy evolution. Our gas-rich mergers tend to lie above black hole scaling relations. To illustrate the richness of our dataset, we also present an in-depth picture for Mrk 273, in which we confirm the AGN found with Chandra and conclude that the second nucleus likely contains an obscured AGN as well.
|Oct 31||Kenneth Wong||
We investigate whether lines of sight containing multiple cluster-scale halos are the best cosmic telescopes for lensing high-redshift (z~10) sources into detectability. For lines of sight of fixed angular size and total mass, we test how the lensing cross section and the number of faint galaxies detected at high redshift change as that mass is distributed among multiple halos, as well as which physical properties of the halos are most important. We find that multiple projected halos are can result in improvement in the detection of faint, high-z sources compared to single halos of equivalent total mass due to the interactions among the lensing potentials when the projected halos overlap. Using integrated LRG luminosity density as a tracer of mass, we have identified lines of sight in the SDSS that are likely to contain the largest total integrated masses. These fields contain a diversity of single massive clusters and chance alignments of multiple halos in projection, and are likely to be among the best gravitational lenses known.
|Nov 7||Michele Trenti||
Until now, investigating the early stages of galaxy formation has been mostly the realm of theoretical modeling and computer simulations, which require many physical ingredients and are challenging to test observationally. However, the latest Hubble observations in the near infrared are shedding new light on the properties of galaxies within the first billion years after the Big Bang, including our recent discovery of the most distant proto-cluster of galaxies at redshift z~8. With the dynamic range of simulations and observations approaching each other, there is a great advantage in bridging the gap between them. I will show how simulations and theory drive the design of successful observations and, in turn, how observations not only test predictions, but also provide insight to direct further modeling. With many open questions, a large range of astrophysics involved, and an array of future computational and observational facilities coming online, the field is rich and promising for future progresses.
|Nov 14||Will Dawson||
DLSCL J0916.2+2951 (Musket Ball Cluster) is a galaxy cluster merger where the dark matter (DM) and galaxies have become dissociated from the collisional gas. Because the time since first pass-through is longer than for any other such merger, this system is the one most sensitive to a nonzero DM self-interaction cross-section, which would be observed as the DM trailing the galaxies. We detect an offset of 19" between the weak lensing mass centroid and the galaxy density centroid consistent with expectations of a DM cross-section between 0.2 and 1.0 cm^2/g. We are able to rule out the hypothesis of a zero DM cross-section at ~85% confidence. In addition to discussing this measurement I will present our ongoing efforts to create and analyze a sample of dissociative cluster mergers capable of testing the null hypothesis to greater than 99% confidence.
|Nov 19||Jonathan Stern||
We study the properties of low redshift broad line AGN, and their relation to their host galaxies, based on a new sample derived from the SDSS survey. The sample is supplemented by data from the GALEX, ROSAT, and 2MASS surveys. We find the following. The average AGN hosts are regular non emission line galaxies (NEG), which become bluer with increasing AGN luminosity, suggesting a correlation of the AGN luminosity and the host star formation rate. The observed AGN optical-UV emission is subject to some reddening, and the intrinsic emission is blue, consistent with accretion disk model predictions. The narrow emission lines reveal that the covering factor of circumnuclear gas (10s - 100s pc) decreases with increasing AGN luminosity, and the gas metallicity follows the host mass, similar to the mass - metallicity relation of normal galaxies. The metallicity of the broad line region (0.01s - 0.1s pc) also appears to be related to the host mass.
|Nov 21||Chiara Spiniello||
In this talk I present the first results from The X-Shooter Lens Survey (XLENS). XLENS targets the internal structures and the mass profiles of a sample of massive lens galaxies from the SLACS (Sloan Lens ACS Survey) with velocity dispersions greater than 250 km/s using the X-Shooter spectrograph on ESO's VLT. We are able to disentangle the contributions of stellar and dark matter in the inner regions of the galaxies by combining strong gravitational lensing, dynamical models, and spectroscopic stellar population analysis. Recent observations indicate that the internal dark-matter fraction increases rapidly with galaxy mass in very massive ETGs, although some hints for a varying initial mass function (IMF) have also been suggested where the low-mass end of the stellar IMF steepens with galaxy mass. With XLENS we are able, for the first time ever, to probe directly the structure and dynamical evolution of ETGs and simultaneously to constrain the low mass end of the IMF normalization and slope. By combining precise lensing and dynamical constraints on the mass distribution (few % accuracy) with high signal-to-noise spectroscopy in the entire rest-frame visible to the NIR, and mass-to-light ratios with 10-20% absolute accuracy, we are able to obtain the most precise dark-matter mass fractions to date. I will present the first XLENS cases showing the power of our survey for understanding the interplay of stellar and dark mass in massive ETGs.
|Dec 12||Guillermo Barro||
The remarkably small and compact sizes of massive quiescent galaxies at z~2 has fueled multiple studies that investigate different evolutionary scenarios to explain how these galaxies formed. A missing part of the puzzle is the nature of their progenitors. Such progenitors are expected to be massive, compact, star-forming galaxies at higher redshifts. However, direct evidence for such counterparts has proven difficult to obtain using only the HST optical images, which probe the rest-frame UV at redshifts z > 2. UV morphologies can easily miss large, massive, red hosts. Such camouflaged components would, however, be easily visible in the near-IR. Using the deepest HST WFC3/F160W imaging data from the CANDELS survey, that probes the optical rest-frame bands at z>2, we are able to identify a significant population of galaxies with similar structural properties as the quiescent population but without fully suppressed star-formation. The number density of these sources account for the observed increment in the density of massive quiescent galaxies between z=2 and 3, while their estimated luminosity-weighted ages are consistent with a formation epoch of ~1 Gyr. For some of these objects we detect prominent Balmer breaks and Balmer absorption lines that supports the post-starburst hypothesis. Interestingly enough, we also find a high rate of X-ray detections among these galaxies (> 40%) indicating that the triggering of an AGN could play a fundamental role in the quenching process.