arXiv subjects

Zili Shen

Publications and source records attributed to Zili Shen.

At least 19 recordsLinked to original sources

arXiv2026

A Third Galaxy Missing Dark Matter along a Trail of Galaxies in the NGC 1052 Field

While most dwarf galaxies are strongly dark matter dominated, two remarkable objects in the NGC 1052 field, DF2 and DF4, appear to lack dark matter. DF2 and DF4 were recently found to be part of a trail of low luminosity galaxies that follow a linear relation between their position and radial velocity. If the other galaxies on this trail formed together with DF2 and DF4, e.g., from gas that was separated from dark matter through a 'bullet dwarf' collision, they may lack dark matter as well. Here we constrain the mass of DF9, the galaxy on the trail that most closely resembles DF2 and DF4. Using Keck/KCWI we find that DF9's stellar velocity dispersion is $6.5^{+3.9}_{-4.3}$ km s$^{-1}$. This is consistent with the $8.3^{+0.9}_{-1.4}$ km s$^{-1}$ dispersion that is expected from DF9's $1.4\times 10^8$ M$_\odot$ stellar mass alone, and we conclude that - like DF2 and DF4 - dark matter is not required to explain the kinematics of DF9. The dispersion is far below the $24\pm3$ km s$^{-1}$ expected if DF9 had a $1.4\times 10^{10}$ M$_\odot$ dark matter halo falling on the stellar mass-halo mass relation. As demonstrated in Appendix B, these results are not sensitive to the assumed distance of the trail. Our results are further evidence that the trail of low mass galaxies in the NGC 1052 field formed together in a unique galaxy formation channel, and are consistent with the prediction of the bullet dwarf scenario that other trail galaxies should show the same lack of dark matter as DF2 and DF4.

astro-ph.GA↗

arXiv2025

Quantitative Morphology of Galactic Cirrus in Deep Optical Imaging

Imaging of optical Galactic cirrus, the spatially resolved form of diffuse Galactic light, provides important insights into the properties of the diffuse interstellar medium (ISM) in the Milky Way. While previous investigations have focused mainly on the intensity characteristics of optical cirrus, their morphological properties remain largely unexplored. In this study, we employ several complementary statistical approaches -- local intensity statistics, angular power spectrum / $\Delta$-variance analysis, and wavelet scattering transform analysis -- to characterize the morphology of cirrus in deep optical imaging data. We place our investigation of optical cirrus into a multi-wavelength context by comparing the morphology of cirrus seen with the Dragonfly Telephoto Array to that seen with space-based facilities working at longer wavelengths (Herschel 250 $\mu m$, WISE 12 $\mu m$, and Planck radiance), as well as with structures seen in the DHIGLS HI column density map. Our statistical methods quantify the similarities and the differences of cirrus morphology in all these datasets. The morphology of cirrus at visible wavelengths resembles that of far-infrared cirrus more closely than that of mid-infrared cirrus; on small scales, anisotropies in the cosmic infrared background and systematics may lead to differences. Across all dust tracers, cirrus morphology can be well described by a power spectrum with a common power-law index $\gamma\sim-2.9$. We demonstrate quantitatively that optical cirrus exhibits filamentary, coherent structures across a broad range of angular scales. Our results offer promising avenues for linking the analysis of coherent structures in optical cirrus to the underlying physical processes in the ISM that shape them. Furthermore, we demonstrate that these morphological signatures can be leveraged to distinguish and disentangle cirrus from extragalactic light.

astro-ph.GA↗

arXiv2025

Kinematic Confirmation of a Remarkable Linear Trail of Galaxies in the NGC 1052 Field, Consistent with Formation in a High-Speed Bullet Dwarf Collision

A unique linear trail of diffuse galaxies was recently identified in the NGC 1052 field. This trail includes the remarkable, ultra-diffuse galaxies DF2 and DF4 which lack dark matter and host unusually luminous globular clusters. It has been proposed that the trail formed via a high-speed collision between two gas-rich dwarf galaxies. This scenario predicts that the trail galaxies are kinematically connected and follow a specific trend in radial velocity as a function of position, based on the known velocities and positions of DF2 and DF4. To test this hypothesis, we measured radial velocities for seven additional galaxies on the trail. While the galaxies' low surface brightnesses presented observational challenges, we employ several methods to obtain measurements for galaxies with effective surface brightnesses up to 28.6 mag arcsec$^{-2}$, including a narrow slit placed over globular clusters and a novel wide slit mode on Keck/LRIS, as well as a 'light bucket' mode on Keck/KCWI. We find that five of our seven targets follow the precise velocity trend predicted by DF2 and DF4, to a degree with just a 2% chance of randomly occurring. Moreover, the trail galaxies' radial velocities are significantly higher than those of the NGC 1052 group, setting it apart as a separate, kinematically connected system. Our findings support the theory that this trail of galaxies, including DF2 and DF4, formed together in a single event. A 'bullet dwarf' collision remains the only known explanation for all the unusual properties of DF2, DF4, and the associated trail of galaxies.

astro-ph.GA↗

arXiv2025

Investigating the Ultra-diffuse Galaxy NGC5846_UDG1 through the Kinematics of its Rich Globular Cluster System

Recent studies of ultra-diffuse galaxies (UDGs) have shown their globular cluster (GC) systems to be central in unveiling their remarkable properties and halo masses. Deep HST imaging revealed 54 GC candidates around the UDG NGC5846_UDG1 (UDG1), with a remarkable 13 per cent of the stellar light contained in the GC system. We present a kinematic analysis of UDG1's GC system from observations with the integral field spectrograph KCWI on the Keck II telescope. We measure recessional velocities for 19 GCs, confirming them as members of UDG1, giving a total of 20 confirmed GCs when combined with literature. Approximately 9 per cent of the stellar light are contained just in the confirmed GCs. We determine the GC system's velocity dispersion to be $σ_{\rm GC}$=29.8$^{+6.4}_{-4.9}$ km s$^{-1}$. We find that $σ_{\rm GC}$ increases with increasing magnitude, consistent with predictions for a GC system that evolved under the influence of dynamical friction. The GC system velocity dispersion is constant out to $\sim1R_{\rm eff}$. Using $σ_{\rm GC}$, we calculate $M_{\rm dyn}$=$2.09^{+1.00}_{-0.64}\times$10$^{9}$M$_{\odot}$ as the dynamical mass enclosed within $\sim$2.5 kpc. The dark matter halo mass suggested by the GC number-halo mass relationship agrees with our dynamical mass estimate, implying a halo more massive than suggested by common stellar mass-halo mass relationships. UDG1, being GC-rich with a massive halo, fits the picture of a failed galaxy.

astro-ph.GA↗

arXiv2025

Fuzzy Galaxies or Cirrus? Decomposition of Galactic Cirrus in Deep Wide-Field Images

Diffuse Galactic cirrus, or Diffuse Galactic Light (DGL), can be a prominent component in the background of deep wide-field imaging surveys. The DGL provides unique insights into the physical and radiative properties of dust grains in our Milky Way, and it also serves as a contaminant on deep images, obscuring the detection of background sources such as low surface brightness galaxies. However, it is challenging to disentangle the DGL from other components of the night sky. In this paper, we present a technique for the photometric characterization of Galactic cirrus, based on (1) extraction of its filamentary or patchy morphology and (2) incorporation of color constraints obtained from Planck thermal dust models. Our decomposition method is illustrated using a $\sim$10 deg$^2$ imaging dataset obtained by the Dragonfly Telephoto Array, and its performance is explored using various metrics which characterize the flatness of the sky background. As a concrete application of the technique, we show how removal of cirrus allows low surface brightness galaxies to be identified on cirrus-rich images. We also show how modeling the cirrus in this way allows optical DGL intensities to be determined with high radiometric precision.

astro-ph.GA↗

arXiv2024

Testing the Bullet Dwarf Collision Scenario in the NGC 1052 Group Through Morphologies and Stellar Populations

NGC 1052-DF2 and -DF4 are two ultra-diffuse galaxies that have been reported as deficient in dark matter and associated with the same galaxy group. Recent findings suggest that DF2 and DF4 are part of a large linear substructure of dwarf galaxies that could have been formed from a high-velocity head-on encounter of two gas-rich galaxies, known as a bullet dwarf collision. Based on new observations from the Hubble Space Telescope, combined with existing imaging from the u band to mid-infrared, we test the bullet dwarf scenario by studying the morphologies and stellar populations of the trail dwarfs. We find no significant morphological differences between the trail dwarfs and other dwarfs in the group, while for both populations, their photometric major axes unexpectedly align parallel with the trail. We find that the trail dwarfs have significantly older ages and higher metallicities than the comparison sample, supporting the distinctiveness of the trail. These observations provide key constraints for any formation model, and we argue that they are currently best explained by the bullet dwarf collision scenario, with additional strong tests anticipated with future observations.

astro-ph.GA↗

arXiv2024

First Results from the Dragonfly Ultrawide Survey: the Largest Eleven Quenched Diffuse Dwarf Galaxies in 3100 deg$^2$ with Spectroscopic Confirmation

The Dragonfly Telephoto Array employs a unique design to detect very large and diffuse galaxies, which might be missed with conventional telescopes. The Dragonfly Ultrawide Survey (DFUWS) is a new wide-field survey which will cover 10,000 deg$^2$ of the northern sky, and it provides an ideal dataset to find these large diffuse galaxies. From 3100 deg$^2$ of DFUWS data, we identified eleven large, low surface brightness galaxies as a pilot sample for spectroscopic follow-up. These are the largest galaxies in the examined area that appear smooth and isolated, with effective radii of 12"-27". Eight are below 24 $\mathrm{mag\,arcsec^{-2}}$ in central $g$-band surface brightness. Keck Cosmic Web Imager (KCWI) spectra of the diffuse light show that all eleven galaxies in this sample are quiescent, and seven qualify as ultra-diffuse galaxies (UDGs). Eight galaxies have distances between 15 and 30 Mpc, while the other three are in the Pegasus cluster at 50 Mpc. Their spectra show evidence of a $\sim 1$Gyr old stellar population in addition to an even older stellar population. The intermediate-age component is present in group and satellite galaxies but not in the Pegasus cluster UDGs. All galaxies in this sample are detected in both Dragonfly and Legacy imaging, and the sample partially overlaps with existing UDG catalogs. This pilot sample provides an excellent training set for our analysis of the upcoming full 10,000 deg$^2$ DFUWS data, from which we may expect to discover even larger, previously-unknown galaxies.

astro-ph.GA↗

arXiv2024

Realizing the potential of the Dragonfly Spectral Line Mapper: Calibration methods and on-sky performance

The Dragonfly Spectral Line Mapper is an innovative all-refracting telescope designed to carry out ultra-low surface brightness wide-field mapping of visible wavelength line emission. Equipped with ultranarrowband (0.8 nm bandwidth) filters mounted in Dragonfly Filter-Tilter instrumentation, the Dragonfly Spectral Line Mapper maps H$α$, [NII]$λ$6583, and [OIII]$λ$5007 line emission produced by structures with sizes ranging from $\sim$1 to 1000 kpc in the local Universe. These spatial scales encompass that of the exceedingly diffuse and faintly radiating circumgalactic medium, which is singularly difficult to detect with conventional mirror-based telescope instrumentation. Extremely careful control of systematics is required to directly image these large scale structures, necessitating high fidelity sky background subtraction, wavelength calibration, and specialized flat-fielding methods. In this paper, we discuss the on-sky performance of the Dragonfly Spectral Line Mapper with these methods in place.

astro-ph.IM↗

arXiv2024

The Dragonfly Spectral Line Mapper: Completion of the 120-lens array

The Dragonfly Spectral Line Mapper is a mosaic telescope comprising 120 Canon telephoto lenses, based on the design of the Dragonfly Telephoto Array. With a wide field of view, and the addition of the "Dragonfly Filter-Tilter" instrumentation holding ultra narrow bandpass filters in front of each lens, the Dragonfly Spectral Line mapper is optimized for ultra low surface brightness imaging of visible wavelength line emission. The Dragonfly Spectral Line Mapper was constructed and commissioned in four phases from March 2022 to November 2023. During this time, four individual mounts of 30 lenses each were constructed and commissioned. The commissioning of the telescope included the deployment of the "Dragonfly StarChaser" which carries out image stabilization corrections in the telephoto lens, to enable hour-long exposures to be taken. In addition, we introduced new instrumentation such as a film to cover the optics to keep the filters clean. Here we describe the updated design of the complete 120-lens array, and the implementation of the instrumentation described above. Additionally, we present updated characterization of the cameras and filter transmission for the full array. Finally, we reflect on the construction and commissioning process of the complete 120-lens array Dragonfly Spectral Line Mapper, and remark on the feasibility of a larger 1000-lens array.

astro-ph.IM↗

arXiv2024

Software infrastructure for the highly-distributed semi-autonomous Dragonfly Spectral Line Mapper

The Dragonfly Spectral Line Mapper (DSLM) is a semi-autonomous, distributed-aperture based telescope design, featuring a modular setup of 120 Canon telephoto lenses, and equal numbers of ultra-narrowband filters, detectors, and other peripherals. Here we introduce the observatory software stack for this highly-distributed system. Its core is the Dragonfly Communication Protocol (DCP), a pure-Python hardware communication framework for standardized hardware interaction. On top of this are 120 REST-ful FastAPI web servers, hosted on Raspberry Pis attached to each unit, orchestrating command translation to the hardware and providing diagnostic feedback to a central control system running the global instrument control software. We discuss key features of this software suite, including docker containerization for environment management, class composition as a flexible framework for array commands, and a state machine algorithm which controls the telescope during autonomous observations.

astro-ph.IM↗

arXiv2024

Analysis of Galaxies at the Extremes: A Kinematic Analysis of the Virgo Cluster Dwarfs VCC 9 and VCC 1448 using the Keck Cosmic Web Imager

We present spatially resolved Keck Cosmic Web Imager stellar spectroscopy of the Virgo cluster dwarf galaxies VCC 9 and VCC 1448. These galaxies have similar stellar masses and large half-light radii but very different globular cluster (GC) system richness ($\sim$25 vs. $\sim$99 GCs). Using the KCWI data, we spectroscopically confirm 10 GCs associated with VCC 1448 and one GC associated with VCC 9. We make two measurements of dynamical mass for VCC 1448 based on the stellar and GC velocities respectively. VCC 1448's mass measurements suggest that it resides in a halo in better agreement with the expectation of the stellar mass -- halo mass relationship than the expectation from its large GC counts. For VCC 9, the dynamical mass we measure agrees with the expected halo mass from both relationships. We compare VCC 1448 and VCC 9 to the GC-rich galaxy Dragonfly 44 ($\sim74$ GCs), which is similar in size but has $\sim 1$ dex less stellar mass than either Virgo galaxy. In dynamical mass -- GC number space, Dragonfly 44 and VCC 1448 exhibit richer GC systems given their dynamical mass than that of VCC 9 and other `normal' galaxies. We also place the galaxies in kinematics -- ellipticity space finding evidence of an anticorrelation between rotational support and the fraction of a galaxy's stellar mass in its GC system. i.e., VCC 9 is more rotationally supported than VCC 1448, which is more rotationally supported than Dragonfly 44. This trend may be expected if a galaxy's GC content depends on its natal gas properties at formation.

astro-ph.GA↗

arXiv2023

Astronomy as a Field: A Guide for Aspiring Astrophysicists

This book was created as part of the SIRIUS B VERGE program to orient students to astrophysics as a broad field. The 2023-2024 VERGE program and the printing of this book is funded by the Women and Girls in Astronomy Program via the International Astronomical Union's North American Regional Office of Astronomy for Development and the Heising-Simons Foundation; as a result, this document is written by women in astronomy for girls who are looking to pursue the field. However, given its universal nature, the material covered in this guide is useful for anyone interested in pursuing astrophysics professionally.

astro-ph.IM↗

arXiv2023

Confirmation of an anomalously low dark matter content for the galaxy NGC1052-DF4 from deep, high resolution continuum spectroscopy

NGC1052-DF4 was found to be the second "galaxy lacking dark matter" in the NGC1052 group, based on its velocity dispersion of $σ_{\rm gc}=4.2^{+4.4}_{-2.2}$ km/s as measured from the radial velocities of seven of its globular clusters. Here we verify this result by measuring the stellar velocity dispersion of the galaxy. We observed the diffuse stellar light in NGC1052-DF4 with the Keck Cosmic Web Imager (KCWI) in its highest resolution mode, with $σ_{\mathrm{instr}}\approx 7$ km/s. With a total science + sky exposure time of 34hrs, the resulting spectrum is exceptional both in its spectral resolution and its S/N ratio of 23Å$^{-1}$. We find a stellar velocity dispersion of $σ_{\rm stars} = 8.0^{+2.3}_{-1.9}$ km/s, consistent with the previous measurement from the globular clusters. Combining both measurements gives a fiducial dispersion of $σ_{\rm f} = 6.3_{-1.6}^{+2.5}$ km/s. The implied dynamical mass within the half-light radius is $8_{-4}^{+6} \times 10^7 M_{\odot}$. The expected velocity dispersion of NGC1052-DF4 from the stellar mass alone is $7 \pm 1$ km/s, and for an NFW halo that follows the stellar mass -- halo mass relation and the halo mass -- concentration relation, the expectation is $\sim 30$ km/s. The low velocity dispersion rules out a normal NFW dark matter halo, and we confirm that NGC1052-DF4 is one of at least two galaxies in the NGC1052 group that have an anomalously low dark matter content. While any viable model for their formation should explain the properties of both galaxies, we note that NGC1052-DF4 now poses the largest challenge as it has the most stringent constraints on its dynamical mass.

astro-ph.GA↗

arXiv2023

A Recipe for Unbiased Background Modeling in Deep Wide-Field Astronomical Images

Unbiased sky background modeling is crucial for the analysis of deep wide-field images, but it remains a major challenge in low surface brightness astronomy. Traditional image processing algorithms are often designed to produce artificially flat backgrounds, erasing astrophysically meaningful structures. In this paper, we present three ideas that can be combined to produce wide-field astronomical data that preserve accurate representations of the background sky: (1) Use of all-sky infrared/sub-mm data to remove the large-scale time-varying components while leaving the scattered light from Galactic cirrus intact, with the assumptions of (a) the underlying background has little power on small scales, and (b) the Galactic cirrus in the field is optically thin on large scales; (2) Censoring of frames contaminated by anomalously prominent wings in the wide-angle point-spread function; and (3) Incorporation of spatial covariance in image stacking that controls the local background consistency. We demonstrate these methods using example datasets obtained with the Dragonfly Telephoto Array, but these general techniques are prospective to be applied to improve sky models in data obtained from other wide-field imaging surveys, including those from the upcoming Vera Rubin Telescope.

astro-ph.IM↗

arXiv2023

A candidate runaway supermassive black hole identified by shocks and star formation in its wake

The interaction of a runaway supermassive black hole (SMBH) with the circumgalactic medium (CGM) can lead to the formation of a wake of shocked gas and young stars behind it. Here we report the serendipitous discovery of an extremely narrow linear feature in HST/ACS images that may be an example of such a wake. The feature extends 62 kpc from the nucleus of a compact star-forming galaxy at z=0.964. Keck LRIS spectra show that the [OIII]/H$β$ ratio varies from ~1 to ~10 along the feature, indicating a mixture of star formation and fast shocks. The feature terminates in a bright [OIII] knot with a luminosity of 1.9x10$^{41}$ ergs/s. The stellar continuum colors vary along the feature, and are well-fit by a simple model that has a monotonically increasing age with distance from the tip. The line ratios, colors, and the overall morphology are consistent with an ejected SMBH moving through the CGM at high speed while triggering star formation. The best-fit time since ejection is ~39 Myr and the implied velocity is v~1600 km/s. The feature is not perfectly straight in the HST images, and we show that the amplitude of the observed spatial variations is consistent with the runaway SMBH interpretation. Opposite the primary wake is a fainter and shorter feature, marginally detected in [OIII] and the rest-frame far-ultraviolet. This feature may be shocked gas behind a binary SMBH that was ejected at the same time as the SMBH that produced the primary wake.

astro-ph.GA↗

arXiv2022

Monochromatic globular clusters as a critical test of formation models for the dark matter deficient galaxies NGC1052-DF2 and NGC1052-DF4

It was recently proposed that the dark matter-deficient ultra-diffuse galaxies DF2 and DF4 in the NGC1052 group could be the products of a "bullet dwarf" collision between two gas-rich progenitor galaxies. In this model DF2 and DF4 formed at the same time in the immediate aftermath of the collision, and a strong prediction is that their globular clusters should have nearly identical stellar populations. Here we test this prediction by measuring accurate F606W-F814W colors from deep HST/ACS imaging. We find that the clusters are extremely homogeneous. The mean color difference between the globular clusters in DF2 and DF4 is $-0.003\pm 0.005$ mag and the observed scatter for the combined sample of 18 clusters with $M_V<-8.6$ in both galaxies is $0.015 \pm 0.002$ mag. After accounting for observational uncertainties and stochastic cluster-to-cluster variation in the number of red giants, the remaining scatter is $0.008^{+0.005}_{-0.006}$ mag. Both the color difference and the scatter are an order of magnitude smaller than in other dwarf galaxies, and we infer that the bullet scenario passes an important test that could have falsified it. No other formation models have predicted this extreme uniformity of the globular clusters in the two galaxies. We find that the galaxies themselves are slightly redder than the clusters, consistent with a previously-measured metallicity difference. Numerical simulations have shown that such differences are expected in the bullet scenario, as the galaxies continued to self-enrich after the formation of the globular clusters.

astro-ph.GA↗

arXiv2022

A Standardized Framework for Collecting Graduate Student Input in Faculty Searches

We present a procedure designed to standardize input received during faculty searches with the goal of amplifying student voices. The framework was originally used to collect feedback from graduate students, but it can be adapted easily to collect feedback from undergraduate students, faculty, staff or other stakeholders. Implementing this framework requires agreement across participating parties and minimal organization prior to the start of faculty candidate visits.

astro-ph.IM↗

arXiv2022

Tidal Distortions in NGC1052-DF2 and NGC1052-DF4: Independent Evidence for a Lack of Dark Matter

Two ultra diffuse galaxies in the same group, NGC1052-DF2 and NGC1052-DF4, have been found to have little or no dark matter and to host unusually luminous globular cluster populations. Such low mass diffuse objects in a group environment are easily disrupted and are expected to show evidence of tidal distortions. In this work we present deep new imaging of the NGC1052 group, obtained with the Dragonfly Telephoto Array, to test this hypothesis. We find that both galaxies show strong position angle twists and are significantly more elongated at their outskirts than in their interiors. The group's central massive elliptical NGC1052 is the most likely source of these tidal disturbances. The observed distortions imply that the galaxies have a low total mass or are very close to NGC1052. Considering constraints on the galaxies' relative distances, we infer that the dark matter halo masses of these galaxies cannot be much greater than their stellar masses. Calculating pericenters from the distortions, we find that the galaxies are on highly elliptical orbits, with a ratio of pericenter to present-day radius Rperi/R0~0.1 if the galaxies are dark matter-free and Rperi/R0~0.01 if they have a normal dark halo. Our findings provide strong evidence, independent of kinematic constraints, that both galaxies are dark matter deficient. Furthermore, the similarity of the tidal features in NGC1052-DF2 and NGC1052-DF4 strongly suggests that they arose at comparable distances from NGC1052. In Appendix A, we describe sbcontrast, a robust method to determine the surface brightness limit of images.

astro-ph.GA↗