arXiv subjects

Raymond G. Carlberg

Publications and source records attributed to Raymond G. Carlberg.

At least 55 records · Page 3Linked to original sources

arXiv2019

Modelling the Effects of Dark Matter Substructure on Globular Cluster Evolution with the Tidal Approximation

We present direct $N$-body simulations of tidally filling 30,000 ${\rm M}_\odot$ star clusters orbiting between 10 kpc and 100 kpc in galaxies with a range of dark matter substructure properties. The time-dependent tidal force is determined based on the combined tidal tensor of the galaxy's smooth and clumpy dark matter components, the latter of which causes fluctuations in the tidal field that can heat clusters. The strength and duration of these fluctuations are sensitive to the local dark matter density, substructure fraction, sub-halo mass function, and the sub-halo mass-size relation. Based on the cold dark matter framework, we initially assume sub-halos are Hernquist spheres following a power-law mass function between $10^5$ and $10^{11} {\rm M}_\odot$ and find that tidal fluctuations are too weak and too short to affect star cluster evolution. Treating sub-halos as point masses, to explore how denser sub-halos affect clusters, we find that only sub-halos with masses greater than $10^{6} {\rm M}_\odot$ will cause cluster dissolution times to decrease. These interactions can also decrease the size of a cluster while increasing the velocity dispersion and tangential anisotropy in the outer regions via tidal heating. Hence increased fluctuations in the tidal tensor, especially fluctuations that are due to low-mass halos, do not necessarily translate into mass loss. We further conclude that the tidal approximation can be used to model cluster evolution in the tidal fields of cosmological simulations with a minimum cold dark matter sub-halo mass of $10^{6} {\rm M}_\odot$, as the effect of lower-mass sub-halos on star clusters is negligible.

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arXiv2019

Butterfly in a Cocoon, Understanding the origin and morphology of Globular Cluster Streams: The case of GD-1

Tidally disrupted globular cluster streams are usually observed, and therefore perceived, as narrow, linear and one-dimensional structures in the 6D phase-space. Here we show that the GD-1 stellar stream ($\approx$ 30 pc wide), which is the tidal debris of a disrupted globular cluster, possesses a secondary diffuse and extended stellar component ($\approx$ 100 pc wide) around it, detected at >5$σ$ confidence level. Similar morphological properties are seen in synthetic streams that are produced from star clusters that are formed within dark matter sub-halos and then accrete onto a massive host galaxy. This lends credence to the idea that the progenitor of the highly retrograde GD-1 stream was originally formed outside of the Milky Way in a now defunct dark satellite galaxy. We deem that in future studies, this newly found $cocoon$ component may serve as a structural hallmark to distinguish between the in-situ and ex-situ (accreted) formed globular cluster streams.

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arXiv2019

Overview of the DESI Legacy Imaging Surveys

The DESI Legacy Imaging Surveys are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image approximately 14,000 deg^2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12 and 22 micorons) observed by the Wide-field Infrared Survey Explorer (WISE) satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project.

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arXiv2019

Tracing the formation of the Milky Way through ultra metal-poor stars

We use Gaia DR2 astrometric and photometric data, published radial velocities and MESA models to infer distances, orbits, surface gravities, and effective temperatures for all ultra metal-poor stars ($\FeH<-4.0$ dex) available in the literature. Assuming that these stars are old ($>11\Gyr$) and that they are expected to belong to the Milky Way halo, we find that these 42 stars (18 dwarf stars and 24 giants or sub-giants) are currently within $\sim20\kpc$ of the Sun and that they map a wide variety of orbits. A large fraction of those stars remains confined to the inner parts of the halo and was likely formed or accreted early on in the history of the Milky Way, while others have larger apocentres ($>30\kpc$), hinting at later accretion from dwarf galaxies. Of particular interest, we find evidence that a significant fraction of all known UMP stars ($\sim26$\%) are on prograde orbits confined within $3\kpc$ of the Milky Way plane ($J_z < 100 \kms \kpc$). One intriguing interpretation is that these stars belonged to the massive building block(s) of the proto-Milky Way that formed the backbone of the Milky Way disc. Alternatively, they might have formed in the early disc and have been dynamically heated, or have been brought into the Milky Way by one or more accretion events whose orbit was dragged into the plane by dynamical friction before disruption. The combination of the exquisite Gaia DR2 data and surveys of the very metal-poor sky opens an exciting era in which we can trace the very early formation of the Milky Way.

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arXiv2018

Pristine Dwarf-Galaxy Survey I: A detailed photometric and spectroscopic study of the very metal-poor Draco II satellite

We present a detailed study of the faint Milky Way satellite Draco II (Dra II) from deep CFHT/MegaCam broadband $g$ and $i$ photometry and narrow-band metallicity-sensitive CaHK observations, along with follow-up Keck II/DEIMOS multi-object spectroscopy. Forward modeling of the deep photometry allows us to refine the structural and photometric properties of Dra II: the distribution of stars in colour-magnitude space implies Dra II is old (13.5 $ \pm 0.5 $ Gyr), very metal poor, very faint ($ L_V = 180 ^{+124}_{-72} L_\odot $), and at a distance $d = 21.5 \pm 0.4$ kpc. The narrow-band, metallicity-sensitive CaHK Pristine photometry confirms this very low metallicity ([Fe/H]$ = -2.7 \pm 0.1$ dex). Even though our study benefits from a doubling of the spectroscopic sample size compared to previous investigations, the velocity dispersion of the system is still only marginally resolved ($σ_{vr}<5.9$ km s$^{-1}$ at the 95 per cent confidence level) and confirms that Dra II is a dynamically cold stellar system with a large recessional velocity ($\langle v_{r}\rangle = -342.5^{+1.1}_{-1.2} $ km s$^{-1}$). We further show that the spectroscopically confirmed members of Dra~II have a mean proper motion of $(μ_α^*,μ_δ)=(1.26 \pm 0.27,0.94 \pm 0.28) $ mas yr$^{-1}$ in the Gaia DR2 data, which translates to an orbit with a pericenter and an apocenter of $21.3 ^{+0.7}_{-1.0}$ and $153.8 ^{+56.7}_{-34.7}$ kpc, respectively. Taken altogether, these properties favour the scenario of Dra~II being a potentially disrupting dwarf galaxy. The low-significance extra-tidal features we map around the satellite tentatively support this scenario.

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arXiv2018

The Pristine Survey IV: Approaching the Galactic metallicity floor with the discovery of an ultra metal-poor star

The early Universe presented a star formation environment that was almost devoid of heavy elements. The lowest metallicity stars thus provide a unique window into the earliest Galactic stages, but are exceedingly rare and difficult to find. Here we present the discovery of an ultra-metal-poor star, Pristine_221.8781+9.7844, using narrow-band Ca H&K photometry from the Pristine survey. Follow-up medium and high-resolution spectroscopy confirms the ultra-metal-poor nature of Pristine_221.8781+9.7844 ([Fe/H] = -4.66 +/- 0.13 in 1D LTE) with an enhancement of 0.3-0.4 dex in alpha-elements relative to Fe, and an unusually low carbon abundance. We derive an upper limit of A(C) = 5.6, well below typical A(C) values for such ultra metal-poor stars. This makes Pristine_221.8781+9.7844 one of the most metal-poor stars; in fact, it is very similar to the most metal-poor star known (SDSS J102915+172927). The existence of a class of ultra metal-poor stars with low(er) carbon abundances suggest that there must have been several formation channels in the early Universe through which long-lived, low-mass stars were formed.

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arXiv2018

Globular Clusters in a Cosmological N-body Simulation

Stellar dynamical model globular clusters are introduced into reconstituted versions of the dark matter halos of the Via-Lactea II (VL-2) simulation to follow the star cluster tidal mass loss and stellar stream formation. The clusters initially evolve within their local sub-galactic halo, later being accreted into the main halo. Stars are continually removed from the clusters, but those that emerged in the sub-galactic halos are dispersed in a wide stream when accreted into the main halo. Thin tidal streams that survive to the present can begin to form once a cluster is in the main halo. A higher redshift start places the star clusters in denser halos where they are subject to stronger tides leading to higher average mass loss rates. A z=3 start leads to a rich set of star streams with nearly all within 100 kpc having a remnant progenitor star cluster in the stream. In contrast, in a z=8 start, all star clusters that are accreted onto the main halo are completely dissolved. These results are compared to the available data on Milky-Way streams, where the majority of streams do not have clearly associated globular clusters. which, if generally true, suggests that there were at least twice as many massive globular clusters at high redshift.

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arXiv2017

The Pristine survey I: Mining the Galaxy for the most metal-poor stars

We present the Pristine survey, a new narrow-band photometric survey focused on the metallicity-sensitive Ca H & K lines and conducted in the northern hemisphere with the wide-field imager MegaCam on the Canada-France-Hawaii Telescope (CFHT). This paper reviews our overall survey strategy and discusses the data processing and metallicity calibration. Additionally we review the application of these data to the main aims of the survey, which are to gather a large sample of the most metal-poor stars in the Galaxy, to further characterise the faintest Milky Way satellites, and to map the (metal-poor) substructure in the Galactic halo. The current Pristine footprint comprises over 1,000 deg2 in the Galactic halo ranging from b~30 to 78 and covers many known stellar substructures. We demonstrate that, for SDSS stellar objects, we can calibrate the photometry at the 0.02-magnitude level. The comparison with existing spectroscopic metallicities from SDSS/SEGUE and LAMOST shows that, when combined with SDSS broad-band g and i photometry, we can use the CaHK photometry to infer photometric metallicities with an accuracy of ~0.2 dex from [Fe/H]=-0.5 down to the extremely metal-poor regime ([Fe/H]<-3.0). After the removal of various contaminants, we can efficiently select metal-poor stars and build a very complete sample with high purity. The success rate of uncovering [Fe/H]SEGUE<-3.0 stars among [Fe/H]Pristine<-3.0 selected stars is 24% and 85% of the remaining candidates are still very metal poor ([Fe/H]<-2.0). We further demonstrate that Pristine is well suited to identify the very rare and pristine Galactic stars with [Fe/H]<-4.0, which can teach us valuable lessons about the early Universe.

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arXiv2017

Star Streams and the Assembly History of the Galaxy

Thin halo star streams originate from the evaporation of globular clusters and therefore provide information about the early epoch globular cluster population. The observed tidal tails from halo globular clusters in the Milky Way are much more shorter than expected from a star cluster orbiting for 10~Gyr. The discrepancy is likely the result of the assumption that the clusters have been orbiting in a non-evolving galactic halo for a Hubble time. As a first step towards more realistic stream histories, a toy model that combines an idealized merger model with a simplified model of the internal collisional relaxation of individual star clusters is developed. On the average, the velocity dispersion increases with distance causing the density of the stream to decline with distance. Consequently, the streams visible in current data will normally be some fraction of the entire stream. Nevertheless, the high surface density segment of the stellar streams created from the evaporation of the more massive globular clusters should all be visible in low obscuration parts of the sky if closer than about 30~kpc. The Pan-STARRS1 halo volume is used to compare the numbers of halo streams and globular clusters.

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arXiv2016

The DESI Experiment Part I: Science,Targeting, and Survey Design

DESI (Dark Energy Spectroscopic Instrument) is a Stage IV ground-based dark energy experiment that will study baryon acoustic oscillations (BAO) and the growth of structure through redshift-space distortions with a wide-area galaxy and quasar redshift survey. To trace the underlying dark matter distribution, spectroscopic targets will be selected in four classes from imaging data. We will measure luminous red galaxies up to $z=1.0$. To probe the Universe out to even higher redshift, DESI will target bright [O II] emission line galaxies up to $z=1.7$. Quasars will be targeted both as direct tracers of the underlying dark matter distribution and, at higher redshifts ($ 2.1 < z < 3.5$), for the Ly-$α$ forest absorption features in their spectra, which will be used to trace the distribution of neutral hydrogen. When moonlight prevents efficient observations of the faint targets of the baseline survey, DESI will conduct a magnitude-limited Bright Galaxy Survey comprising approximately 10 million galaxies with a median $z\approx 0.2$. In total, more than 30 million galaxy and quasar redshifts will be obtained to measure the BAO feature and determine the matter power spectrum, including redshift space distortions.

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arXiv2016

The DESI Experiment Part II: Instrument Design

DESI (Dark Energy Spectropic Instrument) is a Stage IV ground-based dark energy experiment that will study baryon acoustic oscillations and the growth of structure through redshift-space distortions with a wide-area galaxy and quasar redshift survey. The DESI instrument is a robotically-actuated, fiber-fed spectrograph capable of taking up to 5,000 simultaneous spectra over a wavelength range from 360 nm to 980 nm. The fibers feed ten three-arm spectrographs with resolution $R= λ/Δλ$ between 2000 and 5500, depending on wavelength. The DESI instrument will be used to conduct a five-year survey designed to cover 14,000 deg$^2$. This powerful instrument will be installed at prime focus on the 4-m Mayall telescope in Kitt Peak, Arizona, along with a new optical corrector, which will provide a three-degree diameter field of view. The DESI collaboration will also deliver a spectroscopic pipeline and data management system to reduce and archive all data for eventual public use.

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arXiv2016

Detection of a dearth of stars with zero angular momentum in the solar neighbourhood

We report on the detection in the combined $Gaia$-DR1/RAVE data of a lack of disk stars in the solar neighbourhood with velocities close to zero angular momentum. We propose that this may be caused by the scattering of stars with very low angular momentum onto chaotic, halo-type orbits when they pass through the Galactic nucleus. We model the effect in a Milky-Way like potential and fit the resulting model directly to the data, finding a likelihood ($\sim2.7σ$) of a dip in the distribution. Using this effect, we can make a dynamical measurement of the Solar rotation velocity around the Galactic center: $v_{\odot}=239\pm9$ km s$^{-1}$. Combined with the measured proper motion of Sgr A$^*$, this measurement gives a measurement of the distance to the Galactic centre: $R_0=7.9\pm0.3$ kpc.

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arXiv2016

Tidal Debris as a Dark Matter Probe

Tidal debris streams from galaxy satellites can provide insight into the dark matter distribution in halos. This is because we have more information about stars in a debris structure than about a purely random population of stars: we know that in the past they were all bound to the same dwarf galaxy; and we know that they form a dynamically cold population moving on similar orbits. They also probe a different region of the matter distribution in a galaxy than many other methods of mass determination, as their orbits take them far beyond the typical extent of those for the bulk of stars. Although conclusive results from this information have yet to be obtained, significant progress has been made in developing the methodologies for determining both the global mass distribution of the Milky Way's dark matter halo and the amount of dark matter substructure within it. Methods for measuring the halo shape are divided into "predictive methods," which predict the tidal debris properties from the progenitor satellite's mass and orbit, given an assumed parent galaxy mass distribution; and "fundamental methods," which exploit properties fundamental to the nature of tidal debris as global potential constraints. Methods for quantifying the prevalence of dark matter subhalos within halos through the analysis of the gaps left in tidal streams after these substructures pass through them are reviewed.

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arXiv2016

What a Tangled Web We Weave: Hermus as the Northern Extension of the Phoenix Stream

We investigate whether the recently discovered Phoenix stream may be part of a much longer structure that includes the previously discovered Hermus stream. Using a simple model of the Galaxy with a disk, bulge, and a spherical dark matter halo, we show that a nearly circular orbit, highly inclined with respect to the disk, can be found that fits the positions, orientations, and distances of both streams. While the two streams are somewhat misaligned in the sense that they do not occupy the same plane, nodal precession due to the Milky Way disk potential naturally brings the orbit into line with each stream in the course of half an orbit. We consequently consider a common origin for the two streams as plausible. Based on our best fitting orbit, we make predictions for the positions, distances, radial velocities, and proper motions along each stream. If our hypothesis is borne out by measurements, then at ~183 degrees (~235 degrees with respect to the Galactic center) and ~76 kpc in length, Phoenix-Hermus would become the longest cold stream yet found. This would make it a particularly valuable new probe of the shape and mass of the Galactic halo out to ~20 kpc.

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arXiv2016

Dispersal of tidal debris in a Milky-Way-sized dark matter halo

We simulate the tidal disruption of a collisionless N-body globular star cluster in a total of 300 different orbits selected to have galactocentric radii between 10 and 30 kpc in four dark matter halos: (a) a spherical halo with no subhalos, (b) a spherical halo with subhalos, (c) a realistic halo with no subhalos, and (d) a realistic halo with subhalos. This allows us to isolate and study how the halo's (lack of) dynamical symmetry and substructures affect the dispersal of tidal debris. The realistic halos are constructed from the snapshot of the Via Lactea II simulation at redshift zero. We find that the overall halo's lack of dynamical symmetry disperses tidal debris to make the streams fluffier, consistent with previous studies of tidal debris of dwarf galaxies in larger orbits than ours in this study. On the other hand, subhalos in realistic potentials can locally enhance the densities along streams, making streams denser than their counterparts in smooth potentials. We show that many long and thin streams can survive in a realistic and lumpy halo for a Hubble time. This suggests that upcoming stellar surveys will likely uncover more thin streams which may contain density gaps that have been shown to be promising probes for dark matter substructures.

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arXiv2016

Modeling GD-1 Gaps in a Milky-Way Potential

The GD-1 star stream is currently the best available for identifying density fluctuations, "gaps", along its length as a test of the LCDM prediction of large numbers of dark matter sub-halos orbiting in the halo. Density variations of some form are present, since the variance of the density along the stream is three times that expected from the empirically estimated variation in the filtered mean star counts. The density variations are characterized with filters that approximate the shape of sub-halo gravitationally induced stream gaps. The filters locate gaps and measures their amplitude, leading to a measurement of the distribution of gap widths. To gain understanding of the factors influencing the gap width distribution, a suite of collisionless n-body simulations for a GD-1 like orbit in a Milky Way-like potential provides a dynamically realistic statistical prediction of the gap distribution. The simulations show that every location in the stream has been disturbed to some degree by a sub-halo. The small gaps found via the filtering are largely noise. Larger gaps, those longer than 1 kpc, or 10 degrees for GD-1, are the source of the excess variance. The suite of stream simulations shows that sub-halos at the predicted inner halo abundance or possibly somewhat higher can produce the required large sale density variations.

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arXiv2015

An Orphan No Longer? Detection of the Southern Orphan Stream and a Candidate Progenitor

Using a shallow, two-color survey carried out with the Dark Energy Camera, we detect the southern, possibly trailing arm of the Orphan Stream. The stream is reliably detected to a declination of $-38^\circ$, bringing the total known length of the Orphan stream to $108^\circ$. We find a slight offset or "S" shape in the stream at $δ\simeq -14^\circ$ that would be consistent with the transition from leading to trailing arms. This coincides with a moderate concentration of $137 \pm 25$ stars (to $g = 21.6$) that we consider a possible remnant of the Orphan progenitor. The position of this feature is in agreement with previous predictions.

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arXiv2015

Star Streams in Triaxial Isochrone Potentials with Sub-Halos

The velocity, position, and action variable evolution of a tidal stream drawn out of a star cluster in a triaxial isochrone potential containing a sub-halo population reproduces many of the orbital effects of more general cosmological halos but allows easy calculation of orbital actions. We employ a spherical shell code which we show accurately reproduces the results of a tree gravity code for a collisionless star cluster. Streams from clusters on high eccentricity orbits, $e\gtrsim 0.6$, can spread out so much that the amount of material at high enough surface density to stand out on the sky may be only a few percent of the stream's total mass. Low eccentricity streams remain more spatially coherent, but sub-halos both broaden the stream and displace the centerline with details depending on the orbits allowed within the potential. Overall, the majority of stream particles have changes in their total actions of only 1-2\%, leaving the mean stream relatively undisturbed. A halo with 1\% of the mass in sub-halos typically spreads the velocity distribution about a factor of two wider than would be expected for a smooth halo. Strong density variations, "gaps", along with mean velocity offsets, are clearly detected in low eccentricity streams for even a 0.2\% sub-halo mass fraction. Around one hundred velocity measurements per kiloparsec of stream will enable tests for the presence of a local sub-halo density as small as 0.2-0.5\% of the local mass density, with about 1\% predicted for 30 kiloparsec orbital radii streams.

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