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Deborah Lokhorst

Publications and source records attributed to Deborah Lokhorst.

30 records · Page 2Linked to original sources
arXiv2020

The Dragonfly Edge-on Galaxies Survey: Shaping the outer disc of NGC 4565 via accretion

We present extremely deep imaging of the well-known edge-on galaxy NGC 4565 (the "Needle Galaxy"), observed as part of the Dragonfly Edge-on Galaxies Survey. Our imaging traces starlight over scales of ~100 kpc to surface brightnesses of ~32 mag/arcsec^2 in azimuthally averaged radial profiles. In narrow slice profiles along the major axis (with bin sizes ranging from 1.7 x 0.5 kpc to 1.7 x 7.8 kpc), we achieve a depth of ~29-30 mag/arcsec^2. We confirm the previously observed asymmetric disc truncation. In addition, the sharp northwest truncation turns over to a shallower component that coincides with a fan-like feature seen to wrap around the northwest disc limb. We propose that the fan may be a tidal ribbon, and qualitatively replicate the fan with simple simulations. Alternative explanations of the fan and the disc's asymmetry are considered as well. We conclude that we are most likely seeing evidence for accretion-based outer disk growth in NGC 4565.

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arXiv2020

Multi-resolution filtering: an empirical method for isolating faint, extended emission in Dragonfly data and other low resolution images

We describe an empirical, self-contained method to isolate faint, large-scale emission in imaging data of low spatial resolution. Multi-resolution filtering (MRF) uses independent data of superior spatial resolution and point source depth to create a model for all compact and high surface brightness objects in the field. This model is convolved with an appropriate kernel and subtracted from the low resolution image. The halos of bright stars are removed in a separate step and artifacts are masked. The resulting image only contains extended emission fainter than a pre-defined surface brightness limit. The method was developed for the Dragonfly Telephoto Array, which produces images that have excellent low surface brightness sensitivity but poor spatial resolution. We demonstrate the MRF technique using Dragonfly images of a satellite of the spiral galaxy M101, the tidal debris surrounding M51, two ultra-diffuse galaxies in the Coma cluster, and the galaxy NGC5907. As part of the analysis we present a newly-identified very faint galaxy in the filtered Dragonfly image of the M101 field. We also discuss variations of the technique for cases when no low resolution data are available (self-MRF and cross-MRF). The method is implemented in mrf, an open-source MIT licensed Python package.

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arXiv2020

The Dragonfly Wide Field Survey. I. Telescope, Survey Design and Data Characterization

We present a description of the Dragonfly Wide Field Survey (DWFS), a deep photometric survey of a wide area of sky. The DWFS covers 330 $\mathrm{deg}^2$ in the equatorial GAMA fields and the Stripe 82 fields in the SDSS $g$ and $r$ bands. It is carried out with the 48-lens Dragonfly Telephoto Array, a telescope that is optimized for the detection of low surface brightness emission. The main goal of the survey is to study the dwarf galaxy population beyond the Local Group. In this paper, we describe the survey design and show early results. We reach $1σ$ depths of $μ_g\approx 31$ mag arcsec$^{-2}$ on arcminute scales and show that Milky Way satellites such as Sextans, Bootes, and Ursa Major should be detectable out to $D\gtrsim 10$ Mpc. We also provide an overview of the elements and operation of the 48-lens Dragonfly telescope and a detailed description of its data reduction pipeline. The pipeline is fully automated, with individual frames subjected to a rigorous series of quality tests. The sky subtraction is performed in two stages, ensuring that emission features with spatial scales up to $\sim 0.^{\circ}9 \times 0.^{\circ}6$ are preserved. The DWFS provides unparalleled sensitivity to low surface brightness features on arcminute scales.

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arXiv2019

Dragonfly imaging of the galaxy NGC5907: a revised view of the iconic stellar stream

In 2008 it was reported that the stellar stream of the edge-on spiral NGC5907 loops twice around the galaxy, enveloping it in a giant corkscrew-like structure. Here we present imaging of this iconic object with the Dragonfly Telephoto Array, reaching a $1σ$ surface brightness level of $μ_g\approx 30.5$ mag/arcsec$^2$ on spatial scales of 1' (the approximate width of the stream). We find a qualitatively different morphology from that reported in the 2008 study. The Dragonfly data do not show two loops but a single curved stream with a total length of 45' (220 kpc). The surface brightness of the stream ranges from $μ_g \approx 27.6$ mag/arcsec$^2$ to $μ_g\approx 28.8$ mag/arcsec$^2$, and it extends significantly beyond the region where tidal features had previously been detected. We find a density enhancement near the luminosity-weighted midpoint of the stream which we identify as the likely remnant of a nearly-disrupted progenitor galaxy. A restricted N-body simulation provides a qualitative match to the detected features. In terms of its spatial extent and stellar mass the stream is similar to Sagittarius, and our results demonstrate the efficacy of low surface brightness-optimized telescopes for obtaining maps of such large streams outside the Local Group. The census of these rare, relatively high mass events complements the census of common, low mass ones that is provided by studies of streams in the Milky Way halo.

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arXiv2019

On the detectability of visible-wavelength line emission from the local circumgalactic and intergalactic medium

We describe a new approach to studying the intergalactic and circumgalactic medium in the local Universe: direct detection through narrow-band imaging of ultra-low surface brightness visible-wavelength line emission. We use the hydrodynamical cosmological simulation EAGLE to investigate the expected brightness of this emission at low redshift ($z$ $\lesssim$ 0.2). H$α$ emission in extended halos (analogous to the extended Ly$α$ halos/blobs detected around galaxies at high redshifts) has a surface brightness of $\gtrsim700$ photons cm$^{-2}$ sr$^{-1}$ s$^{-1}$ out to $\sim$100 kpc. Mock observations show that the Dragonfly Telephoto Array, equipped with state-of-the-art narrow-band filters, could directly image these structures in exposure times of $\sim$10 hours. H$α$ fluorescence emission from this gas can be used to place strong constraints on the local ultra-violet background, and on gas flows around galaxies. Detecting H$α$ emission from the diffuse intergalactic medium (the "cosmic web") is beyond current capabilities, but would be possible with a hypothetical 1000-lens Dragonfly array.

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arXiv2018

A Revised Velocity for the Globular Cluster GC-98 in the Ultra Diffuse Galaxy NGC1052-DF2

We recently published velocity measurements of luminous globular clusters in the galaxy NGC1052-DF2, concluding that it lies far off the canonical stellar mass - halo mass relation. Here we present a revised velocity for one of the globular clusters, GC-98, and a revised velocity dispersion measurement for the galaxy. We find that the intrinsic dispersion $σ=5.6^{+5.2}_{-3.8}$ km/s using Approximate Bayesian Computation, or $σ=7.8^{+5.2}_{-2.2}$ km/s using the likelihood. The expected dispersion from the stars alone is ~7 km/s. Responding to a request from the Editors of ApJ Letters and RNAAS, we also briefly comment on the recent analysis of our measurements by Martin et al. (2018).

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arXiv2018

A galaxy lacking dark matter

Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio M_{halo}/M_{stars} has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5x10^{10} solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2x10^8 solar masses. We infer that its velocity dispersion is less than 10.5 kilometers per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4x10^8 solar masses. This implies that the ratio M_{halo}/M_{stars} is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.

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arXiv2018

An enigmatic population of luminous globular clusters in a galaxy lacking dark matter

We recently found an ultra diffuse galaxy (UDG) with a half-light radius of R_e = 2.2 kpc and little or no dark matter. The total mass of NGC1052-DF2 was measured from the radial velocities of bright compact objects that are associated with the galaxy. Here we analyze these objects using a combination of HST imaging and Keck spectroscopy. Their average size is = 6.2+-0.5 pc and their average ellipticity is <ε> = 0.18+-0.02. From a stacked Keck spectrum we derive an age >9 Gyr and a metallicity of [Fe/H] = -1.35+-0.12. Their properties are similar to ω Centauri, the brightest and largest globular cluster in the Milky Way, and our results demonstrate that the luminosity function of metal-poor globular clusters is not universal. The fraction of the total stellar mass that is in the globular cluster system is similar to that in other UDGs, and consistent with "failed galaxy" scenarios where star formation terminated shortly after the clusters were formed. However, the galaxy is a factor of ~1000 removed from the relation between globular cluster mass and total galaxy mass that has been found for other galaxies, including other UDGs. We infer that a dark matter halo is not a prerequisite for the formation of metal-poor globular cluster-like objects in high redshift galaxies.

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arXiv2017

Extensive Globular Cluster Systems Associated with Ultra Diffuse Galaxies in the Coma Cluster

We present Hubble Space Telescope imaging of two ultra diffuse galaxies (UDGs) with measured stellar velocity dispersions in the Coma cluster. The galaxies, Dragonfly 44 and DFX1, have effective radii of 4.7 kpc and 3.5 kpc and velocity dispersions of $47^{+8}_{-6}$ km/s and $30^{+7}_{-7}$ km/s, respectively. Both galaxies are associated with a striking number of compact objects, tentatively identified as globular clusters: $N_{\rm gc}=74\pm 18$ for Dragonfly 44 and $N_{\rm gc}=62\pm 17$ for DFX1. The number of globular clusters is far higher than expected from the luminosities of the galaxies but is consistent with expectations from the empirical relation between dynamical mass and globular cluster count defined by other galaxies. Combining our data for these two objects with previous HST observations of Coma UDGs we find that UDGs have a factor of $6.9^{+1.0}_{-2.4}$ more globular clusters than other galaxies of the same luminosity, in contrast to a recent study of a similar sample by Amorisco et al. (2017), but consistent with earlier results for individual galaxies. The Harris et al. (2017) relation between globular cluster count and dark matter halo mass implies a median halo mass of $M_{\rm halo}\sim 1.5\times 10^{11}\,{\rm M}_{\odot}$ for the sixteen Coma UDGs that have been observed with HST so far, with the largest and brightest having $M_{\rm halo}\sim 5\times 10^{11}\,{\rm M}_{\odot}$.

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arXiv2016

Future Prospects: Deep Imaging of Galaxy Outskirts using Telescopes Large and Small

The Universe is almost totally unexplored at low surface brightness levels. In spite of great progress in the construction of large telescopes and improvements in the sensitivity of detectors, the limiting surface brightness of imaging observations has remained static for about forty years. Recent technical advances have at last begun to erode the barriers preventing progress. In this Chapter we describe the technical challenges to low surface brightness imaging, describe some solutions, and highlight some relevant observations that have been undertaken recently with both large and small telescopes. Our main focus will be on discoveries made with the Dragonfly Telephoto Array (Dragonfly), which is a new telescope concept designed to probe the Universe down to hitherto unprecedented low surface brightness levels. We conclude by arguing that these discoveries are probably only scratching the surface of interesting phenomena that are observable when the Universe is explored at low surface brightness levels.

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arXiv2016

The Next Generation Virgo Cluster Survey. XIX. Tomography of Milky Way Substructures in the NGVS Footprint

The Next Generation Virgo Cluster Survey (NGVS) is a deep $u^*giz$ survey targeting the Virgo cluster of galaxies at 16.5~Mpc. This survey provides high-quality photometry over an $\sim$ 100 deg$^2$ region straddling the constellations of Virgo and Coma Berenices. This sightline through the Milky Way is noteworthy in that it intersects two of the most prominent substructures in the Galactic halo: the Virgo Over-Density (VOD) and Sagittarius stellar stream (close to its bifurcation point). In this paper, we use deep $u^*gi$ imaging from the NGVS to perform tomography of the VOD and Sagittarius stream using main-sequence turnoff (MSTO) stars as a halo tracer population. The VOD, whose centroid is known to lie at somewhat lower declinations ($α\sim 190^\circ$, $δ\sim -5^\circ$) than is covered by the NGVS, is nevertheless clearly detected in the NGVS footprint at distances between $\sim$ 8 and 25~kpc. By contrast, the Sagittarius stream is found to slice directly across the NGVS field at distances between 25 and 40~kpc, with a density maximum at $\simeq$~35~kpc. No evidence is found for new substructures beyond the Sagittarius stream, at least out to a distance of $\sim$~90 kpc --- the largest distance to which we can reliably trace the halo using MSTO stars. We find clear evidence for a distance gradient in the Sagittarius stream across the $\sim 30$~deg of sky covered by the NGVS and its flanking fields. We compare our distance measurements along the stream to those predicted by leading stream models.

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