arXiv subjects

Khyati Malhan

Publications and source records attributed to Khyati Malhan.

48 records · Page 3Linked to original sources

arXiv2019

Constraining the Milky Way Halo Potential with the GD-1 stellar stream

We use ESA/Gaia astrometry together with SEGUE and LAMOST measurements of the GD-1 stellar stream to explore the improvement on the Galactic gravitational potential that these new data provide. Assuming a realistic universal model for the dark matter halo together with reasonable models of the baryonic components, we find that the orbital solutions for GD-1 require the circular velocity at the Solar radius to be $V_{\rm circ}(R_\odot) =244 \pm 4\,{\rm km\, s^{-1}}$, and also that the density flattening of the dark halo is $q_ρ=0.82^{+0.25}_{-0.13}$. The corresponding Galactic mass within 20kpc was estimated to be $M_{\rm MW}(<20kpc)=2.5 \pm 0.2 \times 10^{11}\, M_\odot$. Moreover, Gaia's excellent proper motions also allowed us to constrain the velocity dispersion of the GD-1 stream in the direction tangential to the line of sight to be $<2.30\,{\rm km\, s^{-1}}$ (95 % confidence limit), confirming the extremely cold dynamical nature of this system.

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arXiv2019

Identification of the Long Stellar Stream of the Prototypical Massive Globular Cluster $ω$ Centauri

Omega Centauri is the most massive globular cluster of the Milky Way, and possesses many peculiar properties. In particular, the cluster contains distinct multiple stellar populations, with a large spread in metallicity and different kinematics as a function of light elements abundance, implying that it formed over an extended period of time. This has lead to the suggestion that $ω$ Cen is the remnant core of an accreted dwarf galaxy. If this scenario is correct, $ω$ Cen should be tidally limited, and one should expect to find tidal debris spread along its orbit. Here we use N-body simulations to show that the recently-discovered `Fimbulthul' structure, identified in the second data release (DR2) of the Gaia mission, is the long sought-for tidal stream of $ω$ Cen, extending up to $28°$ from the cluster. Follow-up high-resolution spectroscopy of 5 stars in the stream show that they are closely-grouped in velocity, and have metallicities consistent with having originated in that cluster. Guided by our N-body simulations, we devise a selection filter that we apply to Gaia data to also uncover the portion of the stream in the highly-contaminated and crowded field within $10°$ of $ω$ Cen. Further modelling of the stream may help to constrain the dynamical history of the dwarf galaxy progenitor of this disrupting system and guide future searches for its remnant stars in the Milky Way.

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arXiv2019

The Streams of the Gaping Abyss: A population of entangled stellar streams surrounding the Inner Galaxy

We present the discovery of a large population of stellar streams that surround the inner Galaxy, found in the Gaia DR2 catalog using the new STREAMFINDER algorithm. Here we focus on the properties of eight new high-significance structures found at Heliocentric distances between 1 and 10 kpc and at Galactic latitudes $|b|>20°$, named Slidr, Sylgr, Ylgr, Fimbulthul, Svöl, Fjörm, Gjöll and Leiptr. Spectroscopic measurements of seven of the streams confirm the detections, which are based on Gaia astrometry and photometry alone, and show that these streams are predominantly metal-poor. The sample possesses diverse orbital properties, although most of the streams appear to be debris of inner-halo globular clusters. Many more candidate streams are visible in our maps, but require follow-up spectroscopy to confirm their nature. We also explain in detail the workings of the algorithm, and gauge the incidence of false detections by running the algorithm on a smooth model of the Gaia catalog.

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arXiv2018

A-type stars in the Canada-France Imaging Survey II. Tracing the height of the disc at large distances with Blue Stragglers

We present the kinematics of Blue Straggler (BS) stars identified in the Canada-France-Imaging-Survey (CFIS), covering 4000 deg$^2$ on the sky in the $u$-band. The BS sample, characterised through CFIS and Pan-STARRS photometry, has been kinematically decomposed into putative halo and disc populations after cross-matching with Gaia astrometry and SDSS/SEGUE/LAMOST spectroscopy. This decomposition clearly reveals the strong flaring of the outer Milky Way disc. In particular, we show that we can detect this flaring up to a vertical height of $|Z| \simeq 8$ kpc at a Galactocentric distance of $R\sim $27 kpc. While some small level of flaring is expected for extended discs built up by radial migration, we demonstrate that this observed very strong flaring of the Milky Way disc is more likely consistent with it being dynamically heated by the repeated passage of the Sagittarius dwarf spheroidal galaxy through the midplane.

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arXiv2018

A-type stars in the Canada-France Imaging Survey I. The stellar halo of the Milky Way traced to large radius by blue horizontal branch stars

We present the stellar density profile of the outer halo of the Galaxy traced over a range of Galactocentric radii from $15< R_{GC} < 220$ kpc by blue horizontal branch (BHB) stars. These stars are identified photometrically using deep $u-$band imaging from the new Canada-France-Imaging-Survey (CFIS) that reaches 24.5 mag. This is combined with $griz$ bands from Pan-STARRS 1 and covers a total of $\sim4000$ deg$^2$ of the northern sky. We present a new method to select BHB stars that has low contamination from blue stragglers and high completeness. We use this sample to measure and parameterize the three dimensional density profile of the outer stellar halo. We fit the profile using (i) a simple power-law with a constant flattening (ii) a flattening that varies as a function of Galactocentric radius (iii) a broken power law profile. We find that outer stellar halo traced by the BHB is well modelled by a broken power law with a constant flattening of $q=0.86 \pm 0.02$, with an inner slope of $γ=4.24 \pm 0.08$. This is much steeper than the preferred outer profile that has a slope of $β=3.21\pm 0.07$ after a break radius of $r_b=41.4^{+2.5}_{-2.4}$ kpc. The outer profile of the stellar halo trace by BHB stars is shallower than that recently measured using RR Lyrae, a surprising result given the broad similarity of the ages of these stellar populations.

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arXiv2018

Phlegethon, a nearby $75°$-long retrograde stellar stream

We report the discovery of a $75°$ long stellar stream in Gaia DR2 catalog, found using the new STREAMFINDER algorithm. The structure is probably the remnant of a now fully disrupted globular cluster, lies $\approx 3.8 \, {\rm kpc}$ away from the Sun in the direction of the Galactic bulge, and possesses highly retrograde motion. We find that the system orbits close to the Galactic plane at Galactocentric distances between $4.9$ and $19.8 \, {\rm kpc}$. The discovery of this extended and extremely low surface brightness stream ($Σ_G\sim 34.3 \, {\rm mag \, arcsec^{-2}}$) with a mass of only $2580\pm140 \, {\rm\,M_\odot}$, demonstrates the power of the STREAMFINDER algorithm to detect even very nearby and ultra-faint structures. Due to its proximity and length we expect that Phlegethon will be a very useful probe of the Galactic acceleration field.

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arXiv2018

Ghostly Tributaries to the Milky Way: Charting the Halo's Stellar Streams with the Gaia DR2 catalogue

We present a panoramic map of the stellar streams of the Milky Way based upon astrometric and photometric measurements from the Gaia DR2 catalogue. In this first contribution, we concentrate on the halo at heliocentric distances beyond 5 kpc, and at Galactic latitudes $|b|>30°$, using the STREAMFINDER algorithm to detect structures along plausible orbits that are consistent with the Gaia proper motion measurements. We find a rich network of criss-crossing streams in the halo. Some of these structures were previously-known, several are new discoveries, but others are potentially artefacts of the Gaia scanning law and will require confirmation. With these initial discoveries, we are starting to unravel the complex formation of the halo of our Galaxy.

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arXiv2018

STREAMFINDER II: A possible fanning structure parallel to the GD-1 stream in Pan-STARRS1

STREAMFINDER is a new algorithm that we have built to detect stellar streams in an automated and systematic way in astrophysical datasets that possess any combination of positional and kinematic information. In Paper I, we introduced the methodology and the workings of our algorithm and showed that it is capable of detecting ultra-faint and distant halo stream structures containing as few as $\sim 15$ members ($Σ_{\rm G} \sim 33.6\, {\rm mag \, arcsec^{-2}}$) in the Gaia dataset. Here, we test the method with real proper motion data from the Pan-STARRS1 survey, and by selecting targets down to $r_{0}=18.5$ mag we show that it is able to detect the GD-1 stellar stream, whereas the structure remains below a useful detection limit when using a Matched Filter technique. The radial velocity solutions provided by STREAMFINDER for GD-1 candidate members are found to be in good agreement with observations. Furthermore, our algorithm detects a $\sim 40^°$ long structure approximately parallel to GD-1, and which fans out from it, possibly a sign of stream-fanning due to the triaxiality of the Galactic potential. This analysis shows the promise of this method for detecting and analysing stellar streams in the upcoming Gaia DR2 catalogue.

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arXiv2018

STREAMFINDER I: A New Algorithm for detecting Stellar Streams

We have designed a powerful new algorithm to detect stellar streams in an automated and systematic way. The algorithm, which we call the STREAMFINDER, is well suited for finding dynamically cold and thin stream structures that may lie along any simple or complex orbits in Galactic stellar surveys containing any combination of positional and kinematic information. In the present contribution we introduce the algorithm, lay out the ideas behind it, explain the methodology adopted to detect streams and detail its workings by running it on a suite of simulations of mock Galactic survey data of similar quality to that expected from the ESA/Gaia mission. We show that our algorithm is able to detect even ultra-faint stream features lying well below previous detection limits. Tests show that our algorithm will be able to detect distant halo stream structures $>10^{\circ}$ long containing as few as $\sim 15$ members ($Σ_{\rm G} \sim 33.6\, {\rm mag \, arcsec^{-2}}$) in the Gaia dataset.

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arXiv2017

The Canada-France Imaging Survey: First results from the u-band component

The Canada-France Imaging Survey (CFIS) will map the northern high Galactic latitude sky in the $u$-band ("CFIS-u", 10,000$\, {\rm deg^2}$) and in the $r$-band ("CFIS-r", 5,000$\, {\rm deg^2}$), enabling a host of stand-alone science investigations, and providing some of the ground-based data necessary for photometric redshift determination for the Euclid mission. In this first contribution we present the $u$-band component of the survey, describe the observational strategy, and discuss some first highlight results, based on approximately one third of the final area. We show that the Galactic anticenter structure is distributed continuously along the line of sight, out to beyond 20 kpc, and possesses a metallicity distribution that is essentially identical to that of the outer disk sampled by APOGEE. This suggests that it is probably a buckled disk of old metal-rich stars, rather than a stream or a flare. We also discuss the future potential for CFIS-u in discovering star-forming dwarf galaxies around the Local Group, the characterization of the white dwarf and blue straggler population of the Milky Way, as well as its sensitivity to low-surface brightness structures in external galaxies.

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arXiv2017

Chemical Mapping of the Milky Way With The Canada-France Imaging Survey: A Non-parametric Metallicity-Distance Decomposition of the Galaxy

We present the chemical distribution of the Milky Way, based on 2,900$\, {\rm deg^2}$ of $u$-band photometry taken as part of the Canada-France Imaging Survey. When complete, this survey will cover 10,000$\, {\rm deg^2}$ of the Northern sky. By combing the CFHT $u$-band photometry together with SDSS and Pan-STARRS $g,r,$ and $i$, we demonstrate that we are able to measure reliably the metallicities of individual stars to $\sim 0.2$ dex, and hence additionally obtain good photometric distance estimates. This survey thus permits the measurement of metallicities and distances of the dominant main-sequence population out to approximately 30 kpc, and provides much higher number of stars at large extraplanar distances than have been available from previous surveys. We develop a non-parametric distance-metallicity decomposition algorithm and apply it to the sky at $30°< |b| < 70°$ and to the North Galactic Cap. We find that the metallicity-distance distribution is well-represented by three populations whose metallicity distributions do not vary significantly with vertical height above the disk. As traced in main-sequence stars, the stellar halo component shows a vertical density profile that is close to exponential, with a scale height of around 3 kpc. This may indicate that the inner halo was formed partly from disk stars ejected in an ancient minor merger.

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arXiv2017

Measuring the Sun's Motion with Stellar Streams

We present a method for measuring the Sun's motion using the proper motions of Galactic halo star streams. The method relies on the fact that the motion of the stars perpendicular to a stream from a low-mass progenitor is close to zero when viewed from a non-rotating frame at rest with respect to the Galaxy, and that the deviation from zero is due to the reflex motion of the observer. The procedure we implement here has the advantage of being independent of the Galactic mass distribution. We run a suite of simulations to test the algorithm we have developed, and find that we can recover the input Solar motion to good accuracy with data of the quality that will soon become available from the ESA/Gaia mission.

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