2015 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 Starburst-Driven Superwinds with the Cosmic Web Imager

Advisor: 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. Dust carried by superwinds into the intergalactic medium around low-z starbursts could also have important cosmological implications related to our ability to detect high-redshift galaxies. We have recently begun a project to study the optical emission-line nebulae and outflowing superwinds in a large sample of nearby starburst 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 covering 3500-7100 Angstroms, CWI provides us with a unique opportunity for a detailed study of an important piece of the local cosmic web. The successful 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.

Understanding the Role of Stellar Multiplicity in the Formation and Evolution of Planetary Systems

Advisor: David Ciardi

The Kepler Mission, dedicated to the discovery and validation of small, Earth-sized planets has revolutionized our understanding of the size distribution and frequency of planets around other stars. Characterization of the planet host stars and the assessment of the photometric blending of the host stars with bound and background stars is crucial to the accurate determination of the stellar and planetary radii. Without an accurate understanding of the stellar properties and an assessment of the photometric blending of the host stars, an accurate determination of the planet sizes is impossible. Over the past 5 years, we have undertaken a systematic survey of the Kepler Objects of Interest with high spatial resolution adaptive optics and speckle imaging to detect binary companions. We are looking for a student with experience in high spatial resolution imaging to analyse the speckle and AO data for all of the Kepler planet hosting stars (along with a control sample of stars) to determine the stellar multiplicity rates of the Kepler stars, the true sizes of the planetary candidates, and the relationship between stellar multiplicity and planetary occurrence.

The Jansky-VLA Frontier Fields Legacy Survey

Advisor: Eric Murphy

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. Currently, 4 Frontier Fields (4 cluster + 4 parallel) have been approved. 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 two northern-most 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

Advisor: 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.

Obscured star formation in the Virgo cluster

Advisor: Dario Fadda

This project will use my deep Spitzer survey of the Virgo cluster to study the obscured star formation of Virgo member galaxies at an unprecedented depth. In particular, the 24um observations are deep enough to detect dwarf galaxies in the cluster, a population of galaxies very sensitive to environment effects in the cluster. The Spitzer observation covers the region with X-ray diffuse emission encompassing periferic regions with known infalling groups. Starting from a list of candidate members based on spectroscopic and photometric literature redshifts, the student will perform accurate photometry with in-house software using at the same time archival images in the optical, infrared, and UV bands and will study the morphology of the galaxies using SDSS images. Star formation rates, stellar masses, morphological and spectral classifications will be derived for each member and related to the density of the environment. This study on a nearby system with multi-wavelength coverage from different public archives will shed new light on the processes involved in the evolution of galaxies falling into clusters in different density environments.

Local Benchmarks for Cosmic Evolution of Major-Merger Pairs - A Multi-band Study of SFR, Dust and Gas Content

Advisor: Kevin Xu

In the local universe, major mergers of galaxies of nearly equal mass trigger the most extreme starbursts (ULIRGs) and bright AGNs. It has been intensely debated whether there were much more mergers in the earlier universe, and whether mergers play dominant roles in the cosmic evolution of galaxies (mass growth, SFR quenching, etc). We are carrying out a multi-band study for a complete sample of 88 local star-forming major-merger pairs (median redshift 0.04), exploiting data obtained by our own observations using Herschel (dust emission in FIR/sub-mm), IRAM (molecular emission in mm) and GBT (HI emission in radio) and archive data (SDSS, GALEX, WISE, etc). The goal is to set the local benchmarks for the cosmic evolution of the SFR-to-gas relation (the Kennicutt-Schmidt law) for major-merger pairs, complementing a study on the K-S law for high-z mergers in the COSMOS field.

Exploring the Disks around the Nearest Carbon Dwarfs

Advisor: Patrick Lowrance

Carbon dwarfs are currently thought to be a low mass star with an accretion disk supplied by an unseen white dwarf companion during its evolution. Current understanding of the formation and evolution of carbon dwarfs is limited by the small number of objects and observations. We currently have observations of a several of the nearest carbon dwarfs from 1 to 160um using 2MASS, Spitzer IRAC, and Herschel PACS to detect the proposed residual circumstellar disk. The closer distance will enhance the detectability of the disk, and thus the opportunity afforded by this program to add observational proof and exploration is valuable and ground breaking. This program can provide the spectral range and sensitivity to gain the observational understanding of the cold circumstellar disk and dissipation timescales around evolved stars which placing carbon in our ISM‚àí a major building block of life as we know it. We are looking for a student to analyze the photometry from these different infrared instruments and compare with current models to determine parameters of these disks. Experience with infrared data and photometry would be preferred.

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

Advisors: 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 (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.

Search for SNe in Extreme Star-Forming Galaxies using the Palomar Transient Factory

Advisor: Jason Surace

Extreme star-forming galaxies are predicted to have very high supernova rates, typically much greater than one per year. The most extreme star-forming galaxies in the local universe are "luminous infrared galaxies" or LIRGs. These objects have been intensely studied by our group at IPAC, most recently as part of the GOALS project (Great Observatories All-sky LIRG Survey). IPAC is also a partner in the Palomar Transient Factory, a multi-year optical synoptic sky survey which in the course of mapping the sky has also imaged all of the northern hemisphere GOALS objects many tens of times over the course of the last five years. In this project difference imaging will be used to search for optical transients in the LIRGs consistent with SNe. The SNe rate will be compared with that expected from the amount of optically detected star formation seen in HST uv/optical/near-IR imaging of the same targets.

An SED Atlas Spanning X-rays Through Radio Frequencies for the Great Observatories All-sky LIRG Survey

Advisors: Joseph Mazzarella, GOALS Team

The Great Observatories All-sky LIRG Survey (GOALS) is combining the latest imaging and spectroscopic data from nearly all contemporary space telescopes and major ground-based observatories to study a complete sample of Luminous Infrared Galaxies (LIRGs) in the local universe. The GOALS sample, consisting of 202 systems with infrared luminosity greater than 10^{11} L_sun, is pivotal for understanding the detailed properties of the nearest LIRGs and their relation to more numerous counterparts observed at higher redshifts. We are seeking a graduate student interested in constructing and analyzing calibrated spectral energy distributions (SEDs) covering the widest frequency range possible for the entire GOALS sample. In addition to publication of a comprehensive atlas of SEDs that fuses the latest archival data spanning radio through X-ray frequencies (e.g., VLA, ALMA, WISE, Herschel, Spitzer, 2MASS, HST, SDSS, GALEX, Chandra, XMM), the SEDs will be analyzed to better constrain the star forming and AGN properties as a function of galaxy properties and merger stage. By leveraging the cross-matched multiwavelength data in the NASA/IPAC Extragalactic Database (NED), we will attempt to carefully distinguish the SEDs of individual galaxies in pairs and groups from the combined SEDs of the global systems; these were often confused in prior studies due to the wide range of angular separations convolved with varying spatial resolution of the data as a function of wavelength. Where possible, nuclear and disk components will also be separated. The results will provide new insights toward a more global understanding of the energy output of LIRGs, and also provide a fundamental data set for further studies.

The AllWISE2 Motion Survey

Advisor: Davy Kirkpatrick

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. Among these first discoveries were several L-type subdwarfs, a nearby late-M dwarf hidden in the Galactic Plane, and an object -- also found by Luhman (2014a) -- now recognized as a 250K brown dwarf at 2.2 pc from the Sun (Luhman 2014b).

This first catalog of motion discoveries hinted at the potential for other discoveries within the AllWISE Data Release. We have now begun a more exhaustive search (nicknamed "AllWISE2") whose early results indicate that another 40,000 motion objects will be identified by only slightly adjusting the selection criteria outlined in Kirkpatrick et al. (2014). The graduate student will be asked to help scrutinize new candidates to eliminate false motion sources and to help with the characterization of verified motion objects. This characterization will involve associating the verified source with its counterpart in 2MASS, checking whether it is a previously known object using SIMBAD, and using color-color, color-magnitude, and color-reduced motion diagrams to select objects for spectroscopic follow-up. Subprojects already being pursued by the WISE Brown Dwarf team include the study of L subdwarfs, a search for potential microlensing brown dwarfs in the Galactic Plane, the hunt for Y dwarfs missing from photometric selections, and the discovery of new brown dwarfs with kinematics indicative of membership in nearby young moving groups. The student may either join one or more of these subprojects or devise a new one of her/his choosing.

Requirements are a basic knowledge of brown dwarf research and the willingness to work with huge databases. Familiarity with the IRSA Gator service, SIMBAD, TopCat, a programming language such as IDL, and imaging data sets such as DSS, DSS2, SDSS, 2MASS, and WISE is recommended.