2016 IPAC Visiting Graduate Fellowship

Program Description

The Infrared Processing and Analysis Center (IPAC) at Caltech announces the availability of six-month graduate student fellowships. The program is designed to allow students from other U.S. or international institutions to visit IPAC-Caltech and perform astronomical research in close association with an IPAC scientist. Eligible applicants are expected to have completed preliminary course work in their graduate program and be available for research during the period of the award. Funding from IPAC will be provided for a 6-month period via monthly stipends, plus relocation expenses. Several students are expected to be accepted each year, subject to the availability of funding. Students are expected to be at IPAC during the duration of the Fellowship, nominally January to July, with some flexibility on the starting and ending dates.

Description of Research Projects

The Importance of Turbulence and Shocks in Violently-colliding Galaxies in Dense Environments

Advisor: Phil Appleton

Our team has isolated a sub-set of violently interacting galaxies in Hickson Compact Groups which show enhanced warm molecular-hydrogen emission through previous observations with Spitzer. The boosted H2 signal cannot come from star formation, and we strongly suspect that shock-waves from high-velocity gas-on-gas collisions within the groups, or AGN winds could be responsible for heating the warm molecular component. We have obtained visible-light 3-d spectroscopy from the McDonald Observatory IFU system (Virus-P) of a dozen of our prime "shocked-gas" candidates, and this project would be to help reduce these data and help interpret the results. We would build BPT emission-like diagnostic diagrams across the face of the galaxies to determine the excitation of the diffuse ionized gas, and compare the results with recently obtained Herschel IFU data on the same systems in C+ and [OI]. We would therefore be interested in working with someone who has experience with optical spectroscopy of galaxies, but the project is self-contained and we are open to those who are new to spectroscopy. The project will help us map the importance of shock-excitation in galaxies across space and time.

Observing Galactic Superwinds with the Cosmic Web Imager

Lee Armus

Galactic-scale winds, or 'Superwinds', driven by the collective effect of massive stars and supernovae, have been invoked as a source for the heating and metal-enrichment of both the intra-cluster and inter-galactic medium, as a critical factor in the evolution of galaxies, and as the source of the mass-metallicity relation for galaxies. It has also been suggested that superwinds are a natural and necessary transition phase in the evolution from an IR luminous galaxy to QSO. We have recently begun a project to study the optical emission-line nebulae and outflowing superwinds in a sample of nearby starburst and infrared luminous galaxies using the Cosmic Web Imager - a new wide-field, image slicing, optical integral field spectrograph on the Palomar Hale 200-inch Telescope. With an acrminute field of view and a resolution of R~5000, CWI provides us with a unique opportunity for a detailed study of an important piece of the local cosmic web. We are combining these CWI data with Keck/OSIRIS near-infrared LGS/AO observations to trace the circum-nuclear ionized atomic and warm molecular gas down to scales of 50-100pc. The student will participate in the observing, data reduction and analysis, and publication of our CWI data, in collaboration with members of the proposing team at IPAC, Caltech, The Johns Hopkins University, and UC Santa Barbara.

The Jansky-VLA Frontier Fields Legacy Survey

Eric Murphy & Vandana Desai

Untangling the physical processes that govern galaxy formation and evolution remains a key challenge for astrophysics in the 21st century. Recent observations have pushed galaxy detections to within 500Myrs of the Big Bang, in which the Hubble Space Telescope (HST) and the Spitzer Space Telescope (Spitzer) have played a crucial role. This has been done in a large part by leveraging massive clusters that can be used to magnify intrinsically faint systems at high redshifts that would otherwise go undetected. In March 2013, an ad-hoc advisory committee unanimously recommended that HST and Spitzer undertake a program of six deep fields centered on strong lensing galaxy clusters in parallel with six deep blank field. While the HST and Spitzer data provide information on the stellar populations, stellar masses, and un-obscurred (rest-frame UV) star formation activity, it is only by having our newly observed Janksy VLA radio data that we will be able to characterize the obscured properties of detected systems.

Using the Janksy VLA, we have presently imaged the 3 of the Frontier Fields down to 1uJy at 3 and 6GHz as part of our ongoing JVLA Frontier Field Legacy Survey. At 6GHz, these data reach an angular resolution of 0.3", similar to the resolution delivered by HST/WFC3. The average magnification of these two strong lensing clusters is a factor of ~7 over the full HST/WFC3 field of view, providing access to intrinsically faint sources at high redshift. Consequently, these data will provide unobscured, integrated star formation rates out to z~8; radio morphologies of L* galaxies out to z~3; sub-kpc resolution for highly magnified sources at z > 1; and AGN diagnostics via polarization maps and radio spectral indices. Together with the HST and Spitzer data, these new radio images will inform a variety of extragalactic topics, including the importance of dusty star-forming galaxies at high redshift; the evolution of supermassive black holes; the nature of starburst galaxies out to z~3; and the rapid evolution of galaxies in the lensing clusters themselves.

The Herschel EDGE-on galaxy Survey (HEDGES): Investigating dust Processing in Galaxy Halos

Eric Murphy

The study of edge-on spiral galaxies provides a dimension to our understanding of galaxy structure and evolution unobtainable in any other way. In particular, observationally characterizing the role of feedback processes resulting from the accretion, expulsion, and/or cycling of material between galaxy halos and disks is best carried out for such objects. Gaseous halos are both the depository of galaxy feedback processes (e.g., from AGN and supernovae), and the interface between the disk's interstellar medium (ISM) and the intergalactic medium, through which infall, required for lasting star formation in disks, occurs. To date, we have a fairly good picture of how warm and hot ionized gas is distributed in halos, and a rapidly improving picture for the HI, however the distribution of dust in halos, and their role in dust evolution, remain highly uncertain. Specifically, there are a number of outstanding questions regarding the physical processes governing the interchange of disk/halo material such as: (1) How does halo dust content relate to disk star formation activity? (2) What are the physical characteristics of halo dust (i.e., temperature(s), mass, emissivity, PAH mass fraction, and what does their variation with height from the plane tell us about grain modification by the energetic processes responsible for disk-halo cycling? (3) What can dust and radio continuum halos tell us about transport effects that are important for understanding the far-infrared (FIR)--Radio correlation? (4) How does the distribution of halo dust compare to that of other gas tracers, and hence what can we learn about how such dust is associated with various gas phases?

To investigate these outstanding questions we have acquired deep Herschel imaging with PACS and SPIRE to measure variations in their FIR spectral energy distribution (SED) as a function of disk height and energetics. Using these new Herschel data, we will be able to properly sample the peak of the halo dust SEDs (i.e., 6 bands between 70-500um) across many resolution elements, allowing for the first time the ability to map and characterize the distribution and evolution of halo dust (warm and cold components) for a sample of energetically diverse edge-on spirals.

Mining PTF Orion for Exoplanets

Julian van Eyken

The PTF Orion photometric time-series survey ran during the winters of 2009 and 2010 as part of the Palomar Transient Factory (PTF), with the goal of searching for extremely young transiting planets in the young Orion associations (van Eyken et al. 2011). Young stars tend to be very active, and their intrinsic stellar noise makes searching for the signatures of planets challenging. Although a few young planets have been found to date via direct imaging, these planets necessarily lie at large distances from their host stars. A young transit search probes a largely unexplored part of exoplanet parameter space: young planets on small, rapid orbits. A transiting planet could provide unique opportunities for characterization, and, therefore, could be very valuable for informing planet formation and evolution models. So far, one candidate object orbiting a young T-Tauri star has been found in the survey data, PTFO 8-8695, which we are currently pursuing (van Eyken et al. 2012, Barnes et al. 2013). However, the survey yielded light curves for a total of ~110,000 stars, and we have only investigated a small fraction of the data. There is much more to be mined, and this is the basis for this project. There are numerous avenues to pursue, depending on the applicant's interests, but particular goals are to search for further T-Tauri planets in the dataset to see how common they might be, and whether they share similar characteristics; and to search for more conventional main-sequence planets lying in the background or foreground of the association.

Dust Chemistry, Variability, and Mass-Loss Histories of Massive Hot Stars

Patrick Morris and Schuyler Van Dyk

Stars with initial masses above 25 solar masses experience one or more episodes of significant mass loss from the stellar surfaces, in evolutionary transition from main sequence OB-type star to luminous blue variable (LBV) or Wolf-Rayet star, before exploding as a supernova. These mass loss events may extend over several thousand years or in bursts, losing up to 2-3 solar masses in extreme cases, such as the famous LBV eta Carinae, observable in circumstellar nebula with often spectacular displays of interactions between the present day stellar wind, previous mass loss events, and the ISM. The chemistry and physical properties of the dust formed and the mass loss history itself are tied to the evolution of the central star, and these are poorly known for more than only a couple of well known examples. Yet the near- and mid-IR colors of these objects can be used to identify visually-obscured candidate Wolf-Rayet stars and LBVs from all sky surveys, such as 2MASS and WISE, to reconcile the deficit of observed Wolf-Rayet stars versus evolution model predictions of numbers and lifetimes of this rare stellar class. The student would either: (1) refine the current near-infrared-based color selection scheme for Wolf-Rayet and progenitor LBV-type stars in the Galaxy by including the WISE all-sky survey, incorporating mid-IR colors which are influenced by line emission from the stellar wind and thermal properties of the circumstellar nebulae and interstellar reddening. Or, (2) examine the fundamental properties of known Wolf-Rayet stars described and quantified from imaging and spectroscopic observations taken with the Spitzer Space Telescope, the Infrared Space Observatory, and the Herschel Space Observatory. Or, (3) based on ground-based optical and near-infrared monitoring data of candidate LBVs, determine whether these stars are actually variables. The results will be strongly influential in the interpretations of massive stars and their energetics and chemical enrichment of the ISM in the Milky Way and other galaxies.

Probing Cosmology With Photometric Supernovae from the LSST

Yun Wang

Accurate and precise photometric redshifts of Type Ia supernovae (SNe Ia) can enable the use of SNe Ia, measured only with photometry, to probe cosmology. This dramatically increases the science return of supernova surveys planned for the Large Synoptic Survey Telescope (LSST). The student will learn to make realistic simulations of SNe expected from the LSST, and study how the type and redshift of the SNe can be determined using photometry only. Based on this work, we will investigate the cosmological constraints that can be obtained using photometric SNe Ia from the LSST.

Unveiling cluster galaxy transformations in the epoch of increased cosmic star formation with Multi-Object Near-Infrared IFU Spectroscopy.

Alessandro Rettura and Ranga-Ram Chary

Emerging from the cosmic web, galaxy clusters are the most massive, bound structures in the universe. Understanding how and when galaxies therein assemble their stellar mass, stop forming stars, and acquire their observed morphological properties is still an outstanding problem. In the densest cluster environment, a large number of mechanisms have been proposed to both trigger star formation and then quench it. These include: galaxy-galaxy merging within groups that are falling into the cluster core for the first time; galaxy-galaxy harassment within the core of the clusters as galaxies pass each other at high velocities; and collisional compression and ram pressure stripping of gas in galaxies by the hot intra-cluster medium. These processes would also alter the morphology of galaxies, driving them into the the tight red-sequence of passive spheoridal galaxies observed at low redshift. To understand exactly how these galaxies reach their current states, we need to analyze the morphological and kinematic properties of galaxy disks with active star formation and unveil the signatures associated with galaxy-galaxy or galaxy-intracluster medium interactions.

In recent years, the advent of powerful integral field unit (IFU) spectroscopy coupled with HST imaging has enabled, for the first time, detailed study of both the internal kinematics and the morphologies of galaxies at high redshift. Our team is conducting an ambitious VLT/KMOS IFU spectroscopic survey of more than 120 late-type galaxies in 6 massive high redshift clusters at 0.8 < z < 1.6. We aim to detect the distribution of ionized gas via the H_alpha emission line and perform a dynamical analysis where the kinematics of the ionized gas will be revealed and studied as a function of environment and cosmic time. The student will participate in the data reduction and analysis, and publication of our KMOS data, in collaboration with members of the proposing team at IPAC and Universidad the Concepcion, Chile. The wealth of spectroscopic data available for all clusters will allow us to measure cluster masses via velocity dispersion measurement, apt for direct comparison with estimates based on Xray observations and/or weak lensing. The student will also derive H_alpha maps to assess the presence of asymmetries that can be associated with cluster processes such as ram-pressure stripping and galaxy-galaxy interactions. IFU observations, combined with morphological information and color maps derived from the HST/ACS and WFC3 imaging data already available for all the clusters in our sample, will allow us, for the first time at these redshift, to relate the distribution and kinematics of the ionized gas to the global and local environment in an epoch of increased cosmic star formation.

In Search of the Coldest Atmospheres: Identifying Companions to the Latest WISE Brown Dwarfs Using HST

Chris Gelino

Brown dwarfs are a class of objects with masses intermediate between stars and planets. Brown dwarfs with estimated effective temperatures below 500 K are just now starting to be discovered, filling in the gap between brown dwarfs and Jupiter-like planets (Teff=128K). The Wide-field Infrared Survey Explorer mission (WISE) is uncovering the closest, brightest examples of these objects, making them the best targets for further study. We are using the Keck II telescope in Hawaii and the Hubble Space Telescope (HST) to obtain high-resolution images of WISE brown dwarfs with spectral types T8 and later (Teff<700K) in order to search for companions down to effective temperatures of 300K. We have HST images in one or more filters of 36 brown dwarfs. The binaries discovered by this program will provide critical information for characterizing the properties of objects at the very bottom of the Main Sequence thus helping to bridge the modeling efforts between brown dwarfs and extrasolar planets. We are looking for a student to process and analyze the HST data using a combination of existing and to-be-written software. Experience with Python and/or IDL is preferred.

Spectroscopic Follow-Up of AllWISE High Motion Surveys

Sergio Fajardo-Acosta

The AllWISE Data Release provides apparent motion measurements for all sources detected in coadded images made from the WISE (All-Sky + 3-Band Cryo) and NEOWISE (Post-Cryo) missions. A first catalog of 22,000 motion objects from this Data Release was published in Kirkpatrick et al. (2014) and included 3,525 discoveries over the entire sky. At the same time, Luhman (2014) discovered 762 objects from standard WISE processing. These data hinted at the potential for even more discoveries, after modification of AllWISE selection criteria for bona fide high motion sources. We have recently completed a new all-sky search, dubbed, "AllWISE2," using improved selection criteria, and have discovered thousands of additional high motion objects not previously known from AllWISE.

From our list of AllWISE2 discoveries, we have identified candidates for spectroscopic follow-up and characterization, first using color-color, color-mag, and color-motion space. These candidates may point to unusual systems, such as cold white dwarfs of spectral varieties not previously seen. Other systems may be binaries of brown dwarfs of peculiar abundances. We may also be seeing single nearby stars with atmospheric phenomena of an entirely new nature. The graduate student may also choose a topic of study of his/her own for these high motion objects.

We are carrying out optical and near infrared spectroscopy of the above candidates, using facilities such as the Double Spectrograph at the Hale Telescope at Palomar Observatory, and the SpeX near-infrared spectrograph at the NASA Infrared telescope Facility, among others. We expect the graduate student to assist with these observations, and with interpretation and publication of results.

Requirements are a basic knowledge of brown dwarf theory, and familiarity with optical and near-infrared spectroscopy. Familiarity with programming languages such as IDL and IRAF, the IRSA catalog search tools, and imaging datasets such as WISE and 2MASS is recommended.

Understanding Luminous Infrared Galaxies with Infrared Spectroscopy and Spectral Energy Distributions

Joe Mazzarella, Lee Armus, and the GOALS Team

Luminous Infrared Galaxies (LIRGs) generate their enormous power through intense starbursts and the fueling of Active Galactic Nuclei (AGN). With the Great Observatories All-sky LIRG Survey (GOALS), we are characterizing a sample of 202 low-redshift LIRGs across the electromagnetic spectrum (see A key part of GOALS is the study of the mid and far-infrared spectra and spectral energy distributions (SEDs) of LIRGs as a function of power source, merger stage, and gas content. The infrared spectra are rich in emission and absorption features from dust, molecules, and neutral and ionized atomic gas, as well as powerful continuum emission from dust at a wide variety of temperatures. The successful candidate will work with the GOALS team at IPAC to analyze Spitzer, Herschel, HST and ground-based data in order to constrain the energy sources and physical conditions in these rapidly evolving galaxies. Multiwavelength data in the NASA/IPAC Extragalactic Database (NED) will also be used to analyze SEDs spanning the widest frequency range possible (X-rays through radio) for each system. The results will provide new insights toward understanding the global properties of LIRGs, while placing them in context with lower luminosity systems in the local universe (e.g., the KINGFISH/SINGS sample) and also with high-redshift LIRGs that will be studied with ALMA and JWST.

Finding the Coolest, Closest Brown Dwarfs

Davy Kirkpatrick and C. Beichman

We are seeking to form the most comprehensive, three-dimensional view to date of the Solar Neighborhood by adding recently discovered, very cold brown dwarfs to the known census of warmer brown dwarfs and stars. In addition to filling out our knowledge base of the Solar Neighborhood, these objects give us the most leverage in discerning the shape of the low-mass end of the field mass function as well as defining the low-mass cutoff for "star" formation. Acquiring distances for these coldest objects, however, is hampered by the fact that they are exceedingly difficult to detect from ground-based observatories. We have therefore embarked on a program to measure parallaxes for the brightest late-T and Y dwarfs using the Spitzer Space Telescope. All images from the Spitzer parallax program are already in hand, so the student will be charged with reducing and analyzing the data. This will involve accurate measurement of the centroid of the object in each image while accounting for effects such as distortion in the Spitzer/IRAC arrays and inter-pixel effects that bias the positions. With those extractions in hand, the student will also help with the building of code to find the best parallax and proper motion solution using the measured astrometry.

A second aspect of this program will be to carry out near-IR spectroscopy of the coolest objects using the NIRES and/or NIRSPEC instruments on Keck telescope and TripleSpec at Palomar. Combining atmospheric information from Keck spectroscopy, bolometric luminosities and space motions obtainable from Spitzer astrometry will lead to important constraints on the mass, age and composition of these systems.