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Dynamics and Detection of Tidal Debris
Tidal debris structures are striking evidence of hierarchical assembly -- the premise that the Milky Way and galaxies like it have been built over cosmic time through the coalescence of many smaller objects. In the prevailing Lambda -- Cold Dark Matter cosmology, the vast majority of mergers by number are minor; one dark matter halo, hosting a larger galaxy, dominates the interaction and a smaller object, the satellite, is stripped of mass by tidal forces. When the luminous component of the satellite is disrupted the debris may form structures such as stellar tidal streams or shells, depending on the parameters of the interaction. In this Thesis we examine the properties of this debris left behind by minor mergers theoretically, computationally, and observationally, making strides towards a more complete understanding of what tidal debris can tell us about the history of galaxy formation in the Universe.
Around the Milky Way itself we have examined the properties of the Orphan Stream, a stellar tidal stream so named due to uncertainty about the position and current state of its progenitor. Using 3.6 um observations taken as part of the Spitzer Merger History and Shape of the Galactic Halo program, the latest period--luminosity--metallicity relations, and archival data, we compute precise distances to RR Lyrae stream members with state--of--the--art 2.5% relative uncertainties. Fitting an orbit to the data, we measure an enclosed mass for the Milky Way that is in good agreement with other recent results, once the biases in orbit fitting are taken into account. By applying the same technique to N--body simulations we determined that the Orphan progenitor is most likely similar to the classical dwarf spheroidal satellites.
We also examined tidal debris more generally, in particular by investigating the source of the morphological dichotomy between shells and streams. We find that the transition from a stream--like to a shell--like morphology occurs when the differential azimuthal precession between the orbits of stars exceeds the position angle subtended by individual petals of the progenitor orbit's rosette. This statement is cast more precisely in terms of scaling relations that control the dispersion of energy and angular momentum in the debris, and we find that the observed morphology can be predicted for a given host, orbit, and mass ratio. This leads us to the idea that the observed occurrence rates of different morphologies can be used to recover, at the population statistics level, the progenitor satellites' orbital infall distribution. This a part of the cosmological accretion history that is otherwise inaccessible. To achieve this in practice requires an unbiased and automated method to detect and classify substructure; we have developed just such a tool and demonstrate its effectiveness. In the upcoming era of LSST and WFIRST the methods and insights developed in this Thesis will be useful in decoding the information about the current state and assembly of galaxies encoded in tidal debris
SMHASH: anatomy of the Orphan Stream using RR Lyrae stars
Stellar tidal streams provide an opportunity to study the motion and structure of the disrupting galaxy as well as the gravitational potential of its host. Streams around the Milky Way are especially promising as phase space positions of individual stars will be measured by ongoing or upcoming surveys. Nevertheless, it remains a challenge to accurately assess distances to stars farther than 10 kpc from the Sun, where we have the poorest knowledge of the Galaxy’s mass distribution. To address this, we present observations of 32 candidate RR Lyrae stars in the Orphan tidal stream taken as part of the Spitzer Merger History and Shape of the Galactic Halo (SMHASH) program. The extremely tight correlation between the periods, luminosities, and metallicities of RR Lyrae variable stars in the Spitzer IRAC 3.6μm band allows the determination of precise distances to individual stars; the median statistical relative distance uncertainty to each RR Lyrae star is 2.5 per cent. By fitting orbits in an example potential, we obtain an upper limit on the mass of the Milky Way interior to 60 kpc of 5.6^(+1.2)_(−1.1)×10^(11) M⊙, bringing estimates based on the Orphan Stream in line with those using other tracers. The SMHASH data also resolve the stream in line-of-sight depth, allowing a new perspective on the internal structure of the disrupted dwarf galaxy. Comparing with N–body models, we find that the progenitor had an initial dark halo mass of approximately 3.2 × 109 M⊙, placing the Orphan Stream’s progenitor amongst the classical dwarf spheroidals
ECO and RESOLVE: Galaxy Disk Growth in Environmental Context
We study the relationships between galaxy environments and galaxy properties
related to disk (re)growth, considering two highly complete samples that are
approximately baryonic mass limited into the high-mass dwarf galaxy regime, the
Environmental COntext (ECO) catalog (data release herein) and the B-semester
region of the REsolved Spectroscopy Of a Local VolumE (RESOLVE) survey. We
quantify galaxy environments using both group identification and smoothed
galaxy density field methods. We use by-eye and quantitative morphological
classifications plus atomic gas content measurements and estimates. We find
that blue early-type (E/S0) galaxies, gas-dominated galaxies, and UV-bright
disk host galaxies all become distinctly more common below group halo mass
~10^11.5 Msun, implying that this low group halo mass regime may be a preferred
regime for significant disk growth activity. We also find that blue early-type
and blue late-type galaxies inhabit environments of similar group halo mass at
fixed baryonic mass, consistent with a scenario in which blue early types can
regrow late-type disks. In fact, we find that the only significant difference
in the typical group halo mass inhabited by different galaxy classes is for
satellite galaxies with different colors, where at fixed baryonic mass red
early and late types have higher typical group halo masses than blue early and
late types. More generally, we argue that the traditional
morphology-environment relation (i.e., that denser environments tend to have
more early types) can be largely attributed to the morphology-galaxy mass
relation for centrals and the color-environment relation for satellites.Comment: 26 pages and 28 figures; v2 contains minor figure and text updates to
match final published version in ApJ; ECO data table release now available at
http://resolve.astro.unc.edu/pages/data.ph
SMHASH: anatomy of the Orphan Stream using RR Lyrae stars
Stellar tidal streams provide an opportunity to study the motion and structure of the disrupting galaxy as well as the gravitational potential of its host. Streams around the Milky Way are especially promising as phase space positions of individual stars will be measured by ongoing or upcoming surveys. Nevertheless, it remains a challenge to accurately assess distances to stars farther than 10 kpc from the Sun, where we have the poorest knowledge of the Galaxy’s mass distribution. To address this, we present observations of 32 candidate RR Lyrae stars in the Orphan tidal stream taken as part of the Spitzer Merger History and Shape of the Galactic Halo (SMHASH) program. The extremely tight correlation between the periods, luminosities, and metallicities of RR Lyrae variable stars in the Spitzer IRAC 3.6μm band allows the determination of precise distances to individual stars; the median statistical relative distance uncertainty to each RR Lyrae star is 2.5 per cent. By fitting orbits in an example potential, we obtain an upper limit on the mass of the Milky Way interior to 60 kpc of 5.6^(+1.2)_(−1.1)×10^(11) M⊙, bringing estimates based on the Orphan Stream in line with those using other tracers. The SMHASH data also resolve the stream in line-of-sight depth, allowing a new perspective on the internal structure of the disrupted dwarf galaxy. Comparing with N–body models, we find that the progenitor had an initial dark halo mass of approximately 3.2 × 109 M⊙, placing the Orphan Stream’s progenitor amongst the classical dwarf spheroidals
The Origin of Faint Tidal Features Around Galaxies in the RESOLVE Survey
We study tidal features (TFs) around galaxies in the REsolved Spectroscopy of a Local VolumE (RESOLVE) survey. Our sample consists of 1048 RESOLVE galaxies that overlap with the DECam Legacy Survey, which reaches an r-band 3σ depth of ∼27.9 mag arcsec−2 for a 100 arcsec2 feature. Images were masked, smoothed, and inspected for TFs like streams, shells, or tails/arms. We find TFs in 17±2% of our galaxies, setting a lower limit on the true frequency. The frequency of TFs in the gas-poor (gas-to-stellar mass ratio < 0.1) subsample is lower than in the gas-rich subsample (13±3% vs. 19±2%). Within the gas-poor subsample, galaxies with TFs have higher stellar and halo masses, ∼3× closer distances to nearest neighbors (in the same group), and possibly fewer group members at fixed halo mass than galaxies without TFs, but similar specific star formation rates. These results suggest TFs in gas-poor galaxies are typically streams/shells from dry mergers or satellite disruption. In contrast, the presence of TFs around gas-rich galaxies does not correlate with stellar or halo mass, suggesting these TFs are often tails/arms from resonant interactions. Similar to TFs in gas-poor galaxies, TFs in gas-rich galaxies imply 1.7x closer nearest neighbors in the same group; however, TFs in gas-rich galaxies are associated with diskier morphologies, higher star formation rates, and higher gas content. In addition to interactions with known neighbors, we suggest that TFs in gas-rich galaxies may arise from accretion of cosmic gas and/or gas-rich satellites below the survey limit
LSST Cadence Optimization White Paper in Support of Observations of Unresolved Tidal Stellar Streams in Galaxies beyond the Local Group
Deep observations of faint surface brightness stellar tidal streams in
external galaxies with LSST are addressed in this White Paper contribution. We
propose using the Wide--Fast--Deep survey that contains several nearby galaxies
(at distances where the stars themselves are not resolved, i.e., beyond 20
Mpc). In the context of hierarchical galaxy formation, it is necessary to
understand the prevalence and properties of tidal substructure around external
galaxies based on integrated (i.e., unresolved) diffuse light. This requires
collecting observations on much larger samples of galaxies than the Milky Way
and M31. We will compare the observed structures to the predictions of
cosmological models of galactic halo formation that inform us about the number
and properties of streams around Milky Way-like galaxies. The insight gained
from these comparisons will allow us to infer the properties of stream
progenitors (masses, dynamics, metallicities, stellar populations). The changes
in the host galaxies caused by the interactions with the dissolving companion
galaxies will be another focus of our studies. We conclude by discussing
synergies with WFIRST and Euclid, and also provide concrete suggestions for how
the effects of scattered light could be minimized in LSST images to optimize
the search for low surface brightness features, such as faint unresolved
stellar tidal streams.Comment: 13 pages, 2 figures, submitted to Call for White Papers on LSST
Cadence Optimizatio
Tidal Features at 0.05<z<0.45 in the Hyper Suprime-Cam Subaru Strategic Program: Properties and Formation Channels
We present 1,201 galaxies at that host tidal features, detected
from the first deg of imaging from the Hyper Suprime-Cam
Subaru Strategic Program (HSC-SSP). All galaxies in the present sample have
spectroscopic observations from the Sloan Digital Sky Survey (SDSS)
spectroscopic campaigns, generating a sample of 21208 galaxies. Of these
galaxies, we identify 214 shell systems and 987 stream systems. For 575 of
these systems, we are additionally able to measure the colors of the
tidal features. We find evidence for star formation in a subset of the streams,
with the exception of streams around massive ellipticals, and find that stream
host galaxies span the full range of stellar masses in our sample. Galaxies
which host shells are predominantly red and massive: we find that observable
shells form more frequently around ellipticals than around disc galaxies of the
same stellar mass. Although the majority of the shells in our sample are
consistent with being formed by minor mergers, of shell host
galaxies have colors as red as their host galaxy, consistent with being
formed by major mergers. These "red shells" are additionally preferentially
aligned with the major axis of the host galaxy, as previously predicted from
simulations. We suggest that although the bulk of the observable shell
population originates from fairly minor mergers, which preferentially form
shells that are not aligned with the major axis of the galaxy, major mergers
produce a significant number of observable shells.Comment: 24 pages, 14 figures. Submitted to Ap
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