Eric F Bell, University of Michigan, Ann Arbor
A Lonely Giant: The Sparse Satellite Population of M94 Challenges Galaxy Formation
June 28, 2018 (2:00 PM - 2:20 PM)
PDF, 12.59 MB
The dwarf satellites of 'giant' Milky Way-mass galaxies are our primary probes of low-mass dark matter halos. The number and velocities of the satellite galaxies of the MW and M31 initially puzzled galaxy formation theorists, but are now reproduced well by many models. Yet, are the MW's and M31's satellites representative? Were galaxy formation models 'overfit'? These questions motivate deep searches for satellite galaxies outside the Local Group. We present a deep survey of the 'classical' satellites (M*>4e5 M_sun) of the MW-mass galaxy M94 out to 150kpc projected distance. We find only two satellites, each with M*~1e6 M_sun, compared with 6-12 such satellites in the four other MW-mass systems with comparable data (MW, M31, M81, M101). Using a 'standard' prescription for occupying halos with galaxies, we find that such a sparse satellite population occurs in <0.1% of MW-mass systems in the Millennium-II simulation - a <0.5% probability among a sample of five (known systems + M94). In order to produce an M94-like system more frequently we make satellite galaxy formation much more stochastic than is currently predicted by dramatically increasing the slope and scatter of the stellar mass-halo mass (SMHM) relation. Surprisingly, the SMHM relation must be altered even for halo masses up to 1e11 M_sun - significantly above the mass scales predicted to have increased scatter from current hydrodynamical models. The sparse satellite population of this 'lonely giant' thus advocates for an important modification to ideas of how the satellites around MW-mass galaxies form.
Ana Bonaca, Harvard
What are the tidal streams constraining?
June 27, 2018 (11:00 AM - 11:20 AM)
PDF, 5.07 MB
Cold stellar streams, remnants of tidally disrupted globular clusters, have been employed as tracers of dark matter in the Milky Way. Because of their different positions, different ages, and different levels of observational scrutiny, different streams tell us different things about the Galaxy. We employ a Cramer--Rao or Fisher-matrix approach to understand the quantitative information content in the known streams. In simple, static, analytic models of the Milky Way, streams on eccentric orbits contain the most information about the halo shape. For any individual stream, there are near-degeneracies between dark-matter halo properties and parameters describing the Galactic bulge, disk and the stream progenitor itself, but we find that simultaneous fitting of multiple streams ought to constrain all parameters to a precision of a few percent. At this level, simulated dark matter halos deviate from analytic parametrizations, so we chart the way forward with more flexible models of the Galaxy. Given enough model freedom, streams are most sensitive to the mass enclosed within their current position, and as an ensemble, they can resolve 10% differences in the acceleration field.
Jeffrey L. Carlin, LSST
Near-Field Cosmology with Resolved and Unresolved Stellar Populations Around Low-Mass Local Volume Galaxies
June 28, 2018 (11:50 AM - 12:10 PM)
PDF, 30.74 MB
I will discuss our ongoing observational program to comprehensively map the entire virial volumes of roughly LMC stellar mass galaxies at distances of ~2-4 Mpc. The MADCASH (Magellanic Analog Dwarf Companions And Stellar Halos) survey will deliver the first census of the dwarf satellite populations and stellar halo properties within LMC-like environments in the Local Volume. Our results will inform our understanding of the recent DES discoveries of dwarf satellites tentatively affiliated with the LMC/SMC system. This program will examine whether LMC stellar mass dwarfs outside the Local Group have extended stellar halos, allowing us to assess the relative contributions of in-situ stars vs. merger debris to their stellar populations and halo density profiles. I will outline the constraints on galaxy formation models that can be provided by observations of low-mass galaxy halos and their satellites. This contribution is intended to start a discussion about what can be done to further these goals for more nearby low-mass galaxies that are accessible with DES DR1 data, using resolved stars and unresolved, diffuse objects.
Yumi Choi, University of Arizona
SMASHing the LMC
June 27, 2018 (4:40 PM - 5:00 PM)
PDF, 8.6 MB
As the most massive satellite galaxy, the Large Magellanic Cloud ( LMC) seems to have a significant influence on the Milky Way (MW) disk and its tidal streams. Furthermore, the Magellanic Clouds (MCs) are a unique local laboratory for studying the structure, formation and evolution of dwarf galaxies as a pair and as satellites of the MW in great detail. Understanding the LMC's disk geometry is essential to constrain its tidal interaction history with the Small Magellanic Cloud (SMC) because the LMC's disturbed disk holds the key to the perturbers, such as their tidal orbits and masses. In the LMC main body, we study of the reddening map and 3D structure using red clump stars selected from the SMASH data, and reveal a new 4 kpc warp in the southwestern region likely created by the recent close encounter with the SMC. In addition, we use the SMASH & DES data to study a ring-like stellar overdensity in the LMC disk at R~6 degrees. The stellar overdensity spans at least 270 degrees in position angle around the LMC disk and its amplitude is as large as 2 times higher than the underlying smooth disk. While the origin of this structure is not yet certain, a likely explanation is also the repetitive recent interactions with the SMC. The positions and amplitudes of both the warp and the ring-like overdensity strongly depend on the SMC’s parameters, in particular its impact parameter with the LMC, and our findings will strongly constrain the dynamical evolution of the MCs. Given the sensitivity of the interaction history of the MCs to the tidal field of the MW, it will also constrain the total dark matter halo mass of the MW.
Shany Danieli, Yale
Hunting Low Surface Brightness Galaxies in the Local Volume with the Dragonfly Telephoto Array
June 28, 2018 (2:40 PM - 3:00 PM)
PDF, 11.02 MB
Low surface brightness galaxies (LSBGs) are a significant component of the galaxy population and provide a unique testing ground for theoretical predictions of Cosmology. In particular, the number of low mass dwarf galaxies and Ultra Diffuse Galaxies in the Local Volume provides strong constraints on modern theories of galaxy formation and the nature of dark matter. The lack of observed low mass “field” galaxies, possibly due to their extremely low surface brightnesses which make them difficult to detect, leads to a significant uncertainty in scaling relations constrained by their number. I will present the Dragonfly Wide Field Survey, a 300 sq. deg. imaging survey using the Dragonfly Telephoto Array, down to ultra low surface brightness levels. I will discuss the prospects of finding low mass galaxies in the survey as low surface brightness objects. Data from the Dark Energy Survey is crucial for a complete detection of these LSBGs in the Dragonfly images and for a further study of these galaxies. I’ll discuss the complementary benefits and the synergy between the two surveys.
Peter Ferguson, Texas A&M University
RR-Lyrae in the Dark Energy Survey
June 27, 2018 (5:00 PM - 5:20 PM)
PDF, 1.29 MB
RR-Lyrae stars are important tracers of structure in our galaxy. They provide accurate distances to, and help us understand many interesting objects in the Milky Way such as ultra-faint dwarf galaxies, and tidal streams. The Dark Energy Survey(DES), due to is depth and size probes an interesting parameter space in the Galactic halo that compliments previous datasets. Although it was not optimized for time domain astronomy, containing only 1-10 observations in 5 filters over 3 years, its precise photometry has allowed us to create a catalog of RRab stars. I will discuss the properties of this DES RR-Lyrae catalog, as well as presenting some of my work using this dataset to investigate the 3D structure of Milky Way dwarfs.
Andrew S Graus, University of California, Irvine
Reionization and the radial distribution of satellites
June 28, 2018 (11:10 AM - 11:30 AM)
PDF, 3.65 MB
We use simulations of Milky Way-like halos modeled with a realistic disk potential in order to show the impact of subhalo destruction on the distribution of satellites around the Milky Way. These halos have almost no subhalos within 20 kpc of their centers. Furthermore, there appear to be less halos in the inner regions than dwarf galaxies discovered by SDSS and DES, unless every subhalo with a peak circular velocity of ~ 8 km s^-1 forms a galaxy. This is difficult to do theoretically due to the impact of reionization on the population of dwarf galaxies. We discuss several ways around this issue, including changing the time of reionization, and increasing the number of DES dwarfs that are associated with the LMC.
Carl J Grillmair, Caltech/IPAC
FOSSILs in the Galactic Halo
June 27, 2018 (2:50 PM - 3:10 PM)
PDF, 2.47 MB
We use a matched filter to detect very faint, metal-poor stellar groupings that we term FOSSILs. With size scales on the order of 10 pc to 1 kpc, distances ranging from 2 to 200 kpc, and memberships ranging from a handful to several dozen stars, these FOSSILs stand out significantly from the surrounding field and are presumably signatures of, or debris from, ancient star clusters and dwarf galaxies. They may be localized concentrations of stars within more extensive tidal streams, and in some cases may be the signatures of extant but heretofore undetected ultrafaint galaxies. Using magnitudes and colors from the Pan-STARRs survey, we detect more over a hundred such FOSSILs at 5σ or greater in a 2200 square degree region in the vicinity of the north Galactic pole. A subsample of more populous FOSSILs that could be candidate ultrafaint dwarf galaxies suggests a total population of several hundred such objects within 200 kpc of the Galactic center. Spectroscopic and astrometric follow-up of these FOSSILs will be required to determine the nature of these structures, deepen our understanding of the make-up and accretion history of the Galactic halo, and perhaps solve the missing satellites problem.
Loay Khalifa, DePaul University
Probing the Non Linearity in Galaxy Clusters Through the Analysis of Fractal Dimension Via Wavelet Transform
June 28, 2018 (3:00 PM - 3:20 PM)
PDF, 2.28 MB
The study of large scale structure (LSS) of the universe armed with both all-sky surveys and numerical simulations has become an increasingly important tool to calculate different cosmological parameters. This thesis uses both real and simulated data combined with a unique approach to characterizing LSS to establish some basic cosmology. The main tools of the research are the wavelet transform, and the fractal based-point pro- cesses statistics. Specifically, this thesis calculates the fractal dimension as a function of the cosmological redshift using the Baryon Oscillation Spectroscopic Survey (BOSS). We compare these results to mock data sets produced by the Sloan Digital Sky Survey (SDSS). Taking advantage of the self-similarity and localization properties of the wavelets, allows us to compute the fractal dimension of galaxies in narrow redshift bins. The narrow bins assure that dynamical evolution has not occurred. Because fractal behavior provides us with an indication about linear and non-linear regimes, we believe that we can use this measure to demarcate the point at which dark energy becomes the dominant component governing the evolution of the universe. We developed a computational algorithm to calculate the fractal dimension of galaxy clusters distributions in the redshift range (z=0.4 to z=0.8). We created different two dimensional projections of galaxy clusters distributions at different redshifts. We applied the Wavelet Packet Transform to this data. After that, we calculated the Hurst exponent H, using the log plot of the variance of the Wavelet Packet coefficients. We calculated the fractal dimension D using the relation D = n+1−H for the different galaxy clusters distributions in the redshift range. We presented the evidence that galaxy clusters in the above redshift range behave as fractal systems.
Stacy Y Kim, Ohio State University
There is No Missing Satellites Problem
June 28, 2018 (9:20 AM - 9:40 AM)
PDF, 3.4 MB
A critical challenge to the cold dark matter (CDM) paradigm is that there are fewer satellites observed around the Milky Way than found in simulations of dark matter substructure. We show that there is a match between the observed satellite counts corrected by the detection efficiency of the Sloan Digital Sky Survey (for luminosities $L gtrsim$ 340 L$_odot$) and the number of luminous satellites predicted by CDM, assuming an empirical relation between stellar mass and halo mass. The ``missing satellites problem'', cast in terms of number counts, is thus solved. We also show that warm dark matter (WDM) models with a thermal relic mass smaller than 4 keV are in tension with satellite counts, putting pressure on the sterile neutrino interpretation of recent X-ray observations. Importantly, the total number of Milky Way satellites depends sensitively on the spatial distribution of satellites, possibly leading to a ``too many satellites" problem. Measurements of completely dark halos below $10^8$ M$_odot$, achievable with substructure lensing and stellar stream perturbations, are the next frontier for tests of CDM.
Jennifer Marshall, Texas A&M University
Chemical Abundances of the Milky Way Satellites in DES
June 28, 2018 (10:50 AM - 11:10 AM)
PDF, 1.52 MB
The detailed chemical abundance patterns of stars in ultra-faint dwarf galaxies discovered in modern imaging surveys has shed new light on star formation in the early Universe and production of the most massive elements. In this talk I will review the state of chemical study of the DES-discovered ultra-faint dwarfs and look to the future of this field.
Kristen McQuinn, University of Texas at Austin
Low-Mass Galaxies Discovered in the ALFALFA survey
June 28, 2018 (11:30 AM - 11:50 AM)
PDF, 31.66 MB
The blind extragalactic HI ALFLAFA survey has created a census of the local HI universe over a cosmological volume and has detected HI masses as low as 10^6 Msun. By mining this rich data set, we are discovering and characterizing galaxies at the faint-end of the luminosity function throughout the Local Volume. These extremely low-mass galaxies are predicted to be the most sensitive to processes such as stellar feedback and reionization and can provide unique constraints on galaxy formation and evolution models. I will discuss results from a number of the low-mass galaxies discovered through ALFLAFA including the spin-off SHIELD galaxy sample, and the individual systems Leo P, Leoncino, and Coma P. The low-mass galaxies discovered by ALFALFA are tantalizing evidence of what we might find at the faint-end of the luminosity function with the DES.
Ethan O Nadler, KIPAC/Stanford
Modeling Subhalos and Satellites in Milky Way-like Systems
June 28, 2018 (9:40 AM - 10:00 AM)
PDF, 7.73 MB
High-resolution hydrodynamic simulations of Milky Way-mass halos contain significantly fewer subhalos than dark-matter-only simulations; however, studying a diverse sample of these simulated systems is currently infeasible. I will present a machine learning model that efficiently predicts surviving subhalo populations which are consistent with hydrodynamic results, and I will describe how this model can be used to construct a statistical sample of Milky Way-mass halos. I will discuss applications of this technique in modeling bright satellites around the Milky Way-like hosts observed by SAGA and in modeling the ultra-faint Milky Way satellites relevant to DES.
Heidi Jo Newberg, Rensselaer Polytechnic Institute
The substructure of the stellar halo as measured with SDSS turnoff stars and MilkyWay@home
June 27, 2018 (2:30 PM - 2:50 PM)
PDF, 3.85 MB
We use statistical photometric parallax to derive the underlying density of the stellar halo in the north Galactic cap. In particular we attempt to trace the Sgr dwarf tidal stream, the so-called bifurcated portion of the Sgr stream (also called Stream B), and the Virgo Overdensity/ Virgo Stellar Stream. We find that the streams can be extremely wide, and there is no evidence for a Stream B at all. All of the stars in the region of the Sgr stream can be explained with only a Stream A (the leading tidal tail) and a Stream C. MilkyWay@home is a 1 PetaFLOPS volunteer computing platform.
Robert Nikutta, NOAO
The NOAO Data Lab
June 27, 2018 (2:00 PM - 2:30 PM)
PDF, 5.52 MB
Data Lab (http://datalab.noao.edu) is NOAO's new science exploration platform. It provides open access to large-scale survey data (e.g. DES and Legacy Surveys), images and spectra acquired on NOAO facilities, and allows users to share and publish their own data with collaborators or the wider community. Data Lab furnishes users with compute resources, virtual storage (disk and database) and interfaces to assist in data analysis, filtering, processing and visualization. Additionally, access to local copies of high-value reference data (Gaia, AllWISE, SDSS, etc) as well as external data resources provides an integrated environment that is a great resource for anyone interested in large scale survey science, and especially for researchers seeking readiness for LSST now. I will introduce the Data Lab ecosystem, its functionalities, and database holdings including the newly released uniformly-processed, all-sky NOAO Source Catalog (NSC) of public data obtained with NOAO instruments, with 2.9 billion objects reaching to 23rd magnitude. A demo/tutorial session on Day 3 of the meeting will let participants try Data Lab for themselves by working through several specific science examples ranging from extracting light curves of variable objects, the detection of Milky Way dwarf satellites, to exploring M31. This will hopefully inspire participants to bring their own big-data science questions to an integrated science platform such as Data Lab.
Andrew Pace, Texas A&M University
Proper motions of Milky Way Ultra-Faint satellites with Gaia DR2 × DES DR1
June 28, 2018 (10:00 AM - 10:20 AM)
PDF, 1.35 MB
We present a new, probabilistic method for determining the systemic proper motions of Milky Way (MW) ultra-faint satellites in the Dark Energy Survey (DES). We utilize the superb photometry from the first public data release (DR1) of DES to select candidate members, and cross-match them with the proper motions from Gaia DR2. We model the candidate members with a mixture model (satellite and MW) in spatial and proper motion space. This method does not require prior knowledge of satellite membership, and can successfully determine the tangential motion of most of the DES satellites. With our method we present measurements of the following satellites that lack spectroscopic follow-up: Columba I, Eridanus III, Grus II, Phoenix II, Pictor I, Reticulum III, and Tucana IV, the majority of which are the first systemic proper motion measurements. We compare these to the predictions of Large Magellanic Cloud satellites and to the vast polar structure. With the high precision DES photometry we conclude that most of the newly identified member stars are very metal-poor ([Fe/H] ≲−2) similar to other ultra-faint dwarf galaxies, while Reticulum III is likely more metal-rich. The members we find are excellent targets for future spectroscopic follow-up.
Adriano Pieres, Laboratorio Interinstitucional de e-Astronomia - DES/Brazil
Fitting the Milky Way structural parameters using DES data
June 27, 2018 (3:10 PM - 3:30 PM)
PDF, 2.92 MB
MWFitting is a pipeline that fits the Milky Way structural parameters, comparing the models from Trilegal code to photometric data as the Dark Energy Survey data. I will summarize the results for the implementation of that pipeline in the DES Brazil Portal.
Vinicius Placco, University of Notre Dame
The Age Structure of the Milky Way Halo revealed by DES
June 27, 2018 (4:20 PM - 4:40 PM)
PDF, 35.81 MB
In this talk, I'll present preliminary results on the age structure of the Milky Way halo, as seen from the DES data. Ages are calculated from a sample of blue horizontal branch star candidates. Results are compared with similar efforts using SDSS data, and we argue that age contrast can be used as a complementary method of identifying sub-structures and overdensities in wide-field photometric surveys.
Natalie Price-Jones, University of Toronto
Revealing the evolution of the Milky Way with chemical tagging
June 27, 2018 (4:00 PM - 4:20 PM)
PDF, 1.84 MB
In this era of large surveys of Milky Way stars, it has become possible to explore the history of our Galaxy through a study of its component populations; results achieved through this Galactic archaeology can be applied to more distant galaxies. In particular, performing Galactic archaeology through spectroscopy leads to chemical tagging, the process of dividing stellar populations based on their chemistry. Identifying stars for spectroscopic follow up is an important step of the chemical tagging process, both to select existing structures to test chemical tagging techniques, and to find a broad and complete sample of the Milky Way to which chemical tagging can be applied. I will discuss the ways in which DES DR1 can facilitate chemical tagging, as well as my approach to identifying structure in chemical space. Successful chemical tagging in the Milky Way will uncover the Galaxy’s star formation history, confirm membership of known clusters and stellar streams, and identify remnants of merged satellites. This will offer a deeper understanding of how our Galaxy and its satellites have evolved, allowing extrapolation to the histories of other galaxies.
Adrian Price-Whelan, Princeton University
The Gaia DR2 view of the GD-1 stream
June 27, 2018 (11:40 AM - 12:00 PM)
PDF, 13.07 MB
The exquisite Gaia DR2 astrometry provides a new way to study stellar streams around the Milky Way. Using data from the Gaia combined with Pan-STARRS photometry, we have selected a sample of highly-probable members of the longest cold stream in the Milky Way, GD-1. The resulting map of GD-1: (1) extends the apparent length of the stream by 20 degrees, (2) reveals a plausible location for the progenitor, (3) detects high-contrast gaps along the stream, and (4) indicates the existence of stream members perturbed off the main stream track. These discoveries are only possible because of the exquisite astrometry from Gaia, which permits a clean separation of the stream from Milky Way stars, and is a methodology that can be applied to other streams and photometry datasets. The additional length and a proper treatment of the progenitor will aid in dynamical modeling of GD-1 for mapping the large-scale dark matter distribution. The complex morphology of the stream points to a turbulent history; detailed phase-space properties of the perturbed stream members could potentially constrain dark matter substructure in the Milky Way.
David Sand, University of Arizona
Finding young star-forming galaxies in wide-field imaging datasets
June 28, 2018 (2:20 PM - 2:40 PM)
PDF, 31.74 MB
Motivated by the desire to find dwarf galaxies at the edge of the Local Group and beyond, we will discuss methods for finding `blue’ star forming systems in the DES and other deep imaging data sets. Other groups have done programmatic searches of the SDSS and other archives, while our team has searched for optical counterparts to compact HI sources. Here we will focus on combining GALEX+DECam imaging data to directly find young star forming galaxies and other stellar systems, analogous to Leo P. These techniques will also be useful for finding isolated, stripped remnants in cluster environments, as our group’s work has recently demonstrated in the Virgo Cluster.
Eddie Schlafly, LBL
DECaLS and DECaPS: DECam Surveys of the Northern Galactic Cap and Southern Galactic Plane
June 27, 2018 (9:40 AM - 10:10 AM)
PDF, 11.75 MB
The sensitivity and wide field of the Dark Energy Camera and Blanco telescope have enabled a new generation of deep optical surveys of the sky. The DECam Legacy Survey (DECaLS) has imaged most of the high latitude sky between -15 and 30 degrees of declination, covering 9000 square degrees to a depth of about 24.5 mag in g, 24.0 mag in r, and 23.0 mag in z. The primary science driver of DECaLS is to provide galaxy photometry from which emission line galaxies, quasars, and luminous red galaxies can be selected for spectroscopy by the Dark Energy Spectroscopic Instrument. However, the survey should also enable a rich array of Milky Way and near-field cosomology science. The DECam Plane Survey, by contrast, will observe 2000 square degrees of the southern Galactic plane to depths of 23.7, 22.8, 22.2, 21.8, and 21.0 magnitudes in the grizY bands. Catalogs from the first thousand square degrees are publicly available and contain entries for more than two billion objects, reaching out to the distance of the main-sequence turn-off at the distance to the Galactic center through a reddening E(B-V) of 1.5 magnitudes. Together the surveys cover the full range of different environments in the Milky Way, from the Galactic center to the outer Galactic disk and far out in the Galactic halo.
Denise M Schmitz, California Institute of Technology
Time evolution of intrinsic alignments of galaxies
June 28, 2018 (3:20 PM - 3:40 PM)
PDF, 0.62 MB
Intrinsic alignments (IA), correlations between the intrinsic shapes and orientations of galaxies on the sky, are both a significant systematic in weak lensing and a probe of the effect of large-scale structure on galactic structure and angular momentum. In the era of precision cosmology, it is thus especially important to model IA with high accuracy. Efforts to use cosmological perturbation theory to model the dependence of IA on the large-scale structure have thus far been relatively successful; however, extant models do not consistently account for time evolution. In particular, advection of galaxies due to peculiar velocities alters the impact of IA, because galaxy positions when observed are generally different from their positions at the epoch when IA is believed to be set. I will discuss the evolution of IA from the time of galaxy formation to the time of observation, including the effects of this advection, and show how this process naturally leads to a dependence of IA on the velocity shear. I will then discuss the implications for weak lensing systematics as well as for studies of galaxy formation and evolution. Considering advection introduces nonlocality into the bispectrum, and the degree of nonlocality represents the memory of a galaxy's path from the time of its formation to the time of observation. I will discuss how this result can be used to constrain the redshift at which IA is determined, and provide forecasts for the relevant measurements in DES.
Nora Shipp, University of Chicago
Stellar Streams Discovered in the Dark Energy Survey
June 27, 2018 (10:10 AM - 10:30 AM)
PDF, 1.98 MB
We present the discovery of new stellar streams within the Dark Energy Survey (DES). The DES data covers ∼5000 sq. deg. to a depth of g > 23.5 with a relative photometric calibration uncertainty of < 1%. This data set yields unprecedented sensitivity to the stellar density field in the southern celestial hemisphere, enabling the detection of faint stellar streams to a heliocentric distance of ∼50 kpc. Our analysis of the DES data has led to the discovery of eleven new stellar streams, improved observations of four previously known streams, and revealed evidence of extra-tidal stellar structure associated with four globular clusters: NGC 288, NGC 1261, NGC 1851, and NGC 1904. The study of this ever-growing sample of stellar streams will provide insight into the formation of the Galactic stellar halo, the Milky Way gravitational potential, as well as the large- and small-scale distribution of dark matter around the Milky Way.