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Jielai Zhang

Publications and source records attributed to Jielai Zhang.

At least 37 records · Page 2Linked to original sources
arXiv2021

A Method To Characterize the Wide-Angle Point Spread Function of Astronomical Images

Uncertainty in the wide-angle Point Spread Function (PSF) at large angles (tens of arcseconds and beyond) is one of the dominant sources of error in a number of important quantities in observational astronomy. Examples include the stellar mass and shape of galactic halos and the maximum extent of starlight in the disks of nearby galaxies. However, modeling the wide-angle PSF has long been a challenge in astronomical imaging. In this paper, we present a self-consistent method to model the wide-angle PSF in images. Scattered light from multiple bright stars is fitted simultaneously with a background model to characterize the extended wing of the PSF using a Bayesian framework operating on pixel-by-pixel level. The method is demonstrated using our software elderflower and is applied to data from the Dragonfly Telephoto Array to model its PSF out to 20-25 arcminutes. We compare the wide-angle PSF of Dragonfly to that of a number of other telescopes, including the SDSS PSF, and show that on scales of arcminutes the scattered light in the Dragonfly PSF is markedly lower than that of other wide-field imaging telescopes. The energy in the wings of the Dragonfly point-spread function is sufficiently low that optical cleanliness plays an important role in defining the PSF. This component of the PSF can be modelled accurately, highlighting the power of our self-contained approach.

astro-ph.IM↗
arXiv2021

The Development and Scientific Application of the Dragonfly Telephoto Array

The low surface brightness visible wavelength Universe below 29 mag arcsec$^{-2}$ is teeming with unexplored astrophysical phenomena. Structures fainter than this surface brightness are extremely difficult to image due to systematic errors of sky subtraction and scattered light in the atmosphere and in the telescope. In Chapter 1, I show how The Dragonfly Telephoto Array (Dragonfly for short) addresses these systematics via a combination of hardware and software and is able to image at a level of 30 mag arcsec$^{-2}$ or fainter. In Chapter 2, I describe the Dragonfly Pipeline and how it is optimized for low surface brightness imaging, how it automatically rejects problematic exposures, and its cloud-orchestration. In Chapter 3, I present a study of the outer disk of the nearby spiral galaxy NGC 2841 using Dragonfly as well as archival data in UV from the Galaxy Evolution Explorer Satellite and rest frame 21 cm data using the Very Large Array. While it is commonly accepted that gas dominates over stars in galaxy outer disks, I find that in NGC 2841, this is not the case. The stellar disk extends to five times R25, and there is more stellar than gas mass at all radii. Surprisingly there is a constant ratio of stellar to gas mass beyond 30 kpc, where the disk is also warped. I propose the most likely formation mechanism for this outer disk is co-planar satellite accretion. In Chapter 4, I present a study of thermally emitted and scattered light from dust in the optically thin regions of the Spider HI Cloud, using Dragonfly and Herschel Space Observatory data. In closing the thesis (Chapter 5), I look forward to further improvements in the Dragonfly Pipeline, a population study of the formation mechanisms of galaxy disks and to carrying out tests of dust models.

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arXiv2021

The Deeper, Wider, Faster Program: Exploring stellar flare activity with deep, fast cadenced DECam imaging via machine learning

We present our 500 pc distance-limited study of stellar fares using the Dark Energy Camera as part of the Deeper, Wider, Faster Program. The data was collected via continuous 20-second cadence g band imaging and we identify 19,914 sources with precise distances from Gaia DR2 within twelve, ~3 square-degree, fields over a range of Galactic latitudes. An average of ~74 minutes is spent on each field per visit. All light curves were accessed through a novel unsupervised machine learning technique designed for anomaly detection. We identify 96 flare events occurring across 80 stars, the majority of which are M dwarfs. Integrated are energies range from $\sim 10^{31}-10^{37}$ erg, with a proportional relationship existing between increased are energy with increased distance from the Galactic plane, representative of stellar age leading to declining yet more energetic are events. In agreement with previous studies we observe an increase in flaring fraction from M0 -> M6 spectral types. Furthermore, we find a decrease in the flaring fraction of stars as vertical distance from the galactic plane is increased, with a steep decline present around ~100 pc. We find that ~70% of identified flares occur on short timescales of ~8 minutes. Finally we present our associated are rates, finding a volumetric rate of $2.9 \pm 0.3 \times 10^{-6}$ flares pc$^{-3}$ hr$^{-1}$.

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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

The Outer Stellar Halos of Galaxies: how Radial Merger Mass Deposition, Shells and Streams depend on Infall-Orbit Configurations

Galaxy mergers are a fundamental part of galaxy evolution. To study the resulting mass distributions of different kinds of galaxy mergers, we present a simulation suite of 36 high-resolution isolated merger simulations, exploring a wide range of parameter space in terms of mass ratios (mu = 1:5, 1:10, 1:50, 1:100) and orbital parameters. We find that mini mergers deposit a higher fraction of their mass in the outer halo compared to minor mergers, while their contribution to the central mass distribution is highly dependent on the orbital impact parameter: for larger pericentric distances we find that the centre of the host galaxy is almost not contaminated by merger particles. We also find that the median of the resulting radial mass distribution for mini mergers differs significantly from the predictions of simple theoretical tidal-force models. Furthermore, we find that mini mergers can increase the size of the host disc significantly without changing the global shape of the galaxy, if the impact occurs in the disc plane, thus providing a possible explanation for extended low-surface brightness disks reported in observations. Finally, we find clear evidence that streams are a strong indication of nearly circular infall of a satellite (with large angular momentum), whereas the appearance of shells clearly points to (nearly) radial satellite infall.

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arXiv2018

Spectroscopic Constraints on the Build-up of the Intracluster Light in the Coma Cluster

The stellar content of the intracluster light (ICL) provides unique insight into the hierarchical assembly process of galaxy clusters.However, the ICL is difficult to study due to its low surface brightness and large physical extent. We present optical spectra of three ICL regions in the Coma cluster, located between 100-180kpc from their nearest BCGs: NGC4889 and NGC4874. The mean surface brightness of the three ICL regions are μ$_g$~25.3-26.2mag arcsec$^{-2}$. IFU spectroscopy with 13.5 hr on-source integration time were acquired as part of an ancillary program within the SDSS-IV MaNGA survey. We stacked the 127 individual fiber spectra in each IFU in order to achieve a 1σ limiting surface brightness of 27.9mag arcsec$^{-2}$, corresponding to a mean S/N in the optical of 21.6,9.6,and 11.6Å$^{-1}$. We apply stellar population models to the stacked spectra, and measure the recession velocities, velocity dispersions ($σ$), stellar ages, and [Fe/H]. Our results show that the $σ$ of ICL regions are very high, indicating the stars are tracing the gravitational potential of the cluster, instead of any individual galaxy. The line-of-sight velocities of the three ICL regions are different from each other by ~700km/s, while the velocity of each region is similar to the closest BCG. This suggests that the ICL regions are associated with two distinct subclusters centered on NGC4889 and NGC4874.The stellar populations of these regions are old and metal poor, with ages of 7-12Gyr, and [Fe/H] of -0.8 to -0.6 dex. From the derived age and [Fe/H], the build-up of ICL in Coma is likely to be through the accretion of low mass galaxies or the tidal stripping of the outskirts of massive galaxies that have ended their star formation early on, instead of directly from major mergers of massive galaxies.

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arXiv2018

Low Metallicities and Old Ages for Three Ultra-Diffuse Galaxies in the Coma Cluster

A large population of ultra-diffuse galaxies (UDGs) was recently discovered in the Coma cluster. Here we present optical spectra of three such UDGs, DF7, DF44 and DF17, which have central surface brightnesses of $μ_g \approx 24.4-25.1$ mag arcsec$^{-2}$. The spectra were acquired as part of an ancillary program within the SDSS-IV MaNGA Survey. We stacked 19 fibers in the central regions from larger integral field units (IFUs) per source. With over 13.5 hours of on-source integration we achieved a mean signal-to-noise ratio (S/N) in the optical of $9.5$Å$^{-1}$, $7.9$Å$^{-1}$ and $5.0$Å$^{-1}$, respectively, for DF7, DF44 and DF17. Stellar population models applied to these spectra enable measurements of recession velocities, ages and metallicities. The recession velocities of DF7, DF44 and DF17 are $6599^{+40}_{-25}$km/s, $6402^{+41}_{-39}$km/s and $8315^{+43}_{-43}$km/s, spectroscopically confirming that all of them reside in the Coma cluster. The stellar populations of these three galaxies are old and metal-poor, with ages of $7.9^{+3.6}_{-2.5}$Gyr, $8.9^{+4.3}_{-3.3}$Gyr and $9.1^{+3.9}_{-5.5}$Gyr, and iron abundances of $\mathrm{[Fe/H]}$ $-1.0^{+0.3}_{-0.4}$, $-1.3^{+0.4}_{-0.4}$ and $-0.8^{+0.5}_{-0.5}$, respectively. Their stellar masses are $3$-$6\times10^8 M_\odot$. The UDGs in our sample are as old or older than galaxies at similar stellar mass or velocity dispersion (only DF44 has an independently measured dispersion). They all follow the well-established stellar mass$-$stellar metallicity relation, while DF44 lies below the velocity dispersion-metallicity relation. These results, combined with the fact that UDGs are unusually large for their stellar mass, suggest that stellar mass plays a more important role in setting stellar population properties for these galaxies than either size or surface brightness.

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arXiv2018

The Dragonfly Nearby Galaxies Survey. V. HST/ACS Observations of 23 Low Surface Brightness Objects in the Fields of NGC1052, NGC1084, M96, and NGC4258

We present HST/ACS imaging of twenty-three very low surface brightness ($μ_{e,V}$=25-27.5) galaxies detected in the fields of four nearby galaxy groups. These objects were selected from deep optical imaging obtained with the Dragonfly Telephoto Array. Seven are new discoveries, while most of the others had been identified previously in visual surveys of deep photographic plates and more recent surveys. Few have previously been studied in detail. From the ACS images, we measure distances to the galaxies using both the tip of the red giant branch method and the surface brightness fluctuations method. We demonstrate that the two methods are consistent with each other in the regime where both can be applied. The distances to 15 out of 20 galaxies with stable measurements are consistent with that of the targeted group within errors. This suggests that assuming group membership based solely on projected proximity is ~75% successful in this regime. The galaxies are nearly round, with a median axis ratio of 0.85, and visually resemble dwarf spheroidal galaxies. The objects have a range of sizes, from $R_e$=0.4 kpc to $R_e$=1.8 kpc, with a median of 1.0 kpc. They range in luminosity from $M_V$=-11.4 to $M_V$=-15.6, with a median of -12.4. Galaxies with $R_e$~1 kpc and $M_V$~-12 are fairly rare in the Local Group but we find many of them in this relatively small sample. Four of the objects fall in the class of ultra diffuse galaxies (UDGs), with $R_e$>1.5 kpc and $μ_{0,V}$>24 mag/arcsec^2, including the recently identified dark matter deficient galaxy NGC1052-DF2.

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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.

astro-ph.GA↗
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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arXiv2018

The dragonfly nearby galaxies survey. Iv. A giant stellar disk in ngc 2841

Neutral gas is commonly believed to dominate over stars in the outskirts of galaxies, and investigations of the disk-halo interface are generally considered to be in the domain of radio astronomy. This may simply be a consequence of the fact that deep HI observations typically probe to a lower mass surface density than visible wavelength data. This paper presents low surface brightness optimized visible wavelength observations of the extreme outskirts of the nearby spiral galaxy NGC 2841. We report the discovery of an enormous low-surface brightness stellar disk in this object. When azimuthally averaged, the stellar disk can be traced out to a radius of $\sim$70 kpc (5 $R_{25}$ or 23 inner disk scale lengths). The structure in the stellar disk traces the morphology of HI emission and extended UV emission. Contrary to expectations, the stellar mass surface density does not fall below that of the gas mass surface density at any radius. In fact, at all radii greater than $\sim$20 kpc, the ratio of the stellar to gas mass surface density is a constant 3:1. Beyond $\sim$30 kpc, the low surface brightness stellar disk begins to warp, which may be an indication of a physical connection between the outskirts of the galaxy and infall from the circumgalactic medium. A combination of stellar migration, accretion and in-situ star formation might be responsible for building up the outer stellar disk, but whatever mechanisms formed the outer disk must also explain the constant ratio between stellar and gas mass in the outskirts of this galaxy.

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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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arXiv2017

The Dragonfly Nearby Galaxies Survey. III. The Luminosity Function of the M101 Group

We obtained follow-up HST observations of the seven low surface brightness galaxies discovered with the Dragonfly Telephoto Array in the field of the massive spiral galaxy M101. Out of the seven galaxies, only three were resolved into stars and are potentially associated with the M101 group at $D=7\text{ Mpc}$. Based on HST ACS photometry in the broad F606W and F814W filters, we use a maximum likelihood algorithm to locate the Tip of the Red Giant Branch (TRGB) in galaxy color-magnitude diagrams. Distances are $6.38^{+0.35}_{-0.35}, 6.87^{+0.21}_{-0.30}$ and $6.52^{+0.25}_{-0.27} \text{ Mpc}$ and we confirm that they are members of the M101 group. Combining the three confirmed low luminosity satellites with previous results for brighter group members, we find the M101 galaxy group to be a sparsely populated galaxy group consisting of seven group members, down to $M_V = -9.2 \text{ mag}$. We compare the M101 cumulative luminosity function to that of the Milky Way and M31. We find that they are remarkably similar; In fact, the cumulative luminosity function of the M101 group gets even flatter for fainter magnitudes, and we show that the M101 group might exhibit the two known small-scale flaws in the $Λ\textrm{CDM}$ model, namely `the missing satellite' problem and the `too big to fail' problem. Kinematic measurements of M101$'$s satellite galaxies are required to determine whether the `too big to fail' problem does in fact exist in the M101 group.

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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 Dragonfly Nearby Galaxies Survey. II. Ultra diffuse galaxies near the elliptical galaxy NGC 5485

We present the unexpected discovery of four ultra diffuse galaxies (UDGs) in a group environment. We recently identified seven extremely low surface brightness galaxies in the vicinity of the spiral galaxy M101, using data from the Dragonfly Telephoto Array. The galaxies have effective radii of $10"-38"$ and central surface brightnesses of $25.6-27.7$ mag arcsec$^{-2}$ in g-band. We subsequently obtained follow-up observations with $HST$ to constrain the distances to these galaxies. Four remain persistently unresolved even with the spatial resolution of $HST$/ACS, which implies distances of $D > 17.5$ Mpc. We show that the galaxies are most likely associated with a background group at $\sim 27$ Mpc containing the massive ellipticals NGC 5485 and NGC 5473. At this distance, the galaxies have sizes of $2.6-4.9$ kpc, and are classified as UDGs, similar to the populations that have been revealed in clusters such as Coma, Virgo and Fornax, yet even more diffuse. The discovery of four UDGs in a galaxy group demonstrates that the UDG phenomenon is not exclusive to cluster environments. Furthermore, their morphologies seem less regular than those of the cluster populations, which may suggest a different formation mechanism or be indicative of a threshold in surface density below which UDGs are unable to maintain stability.

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arXiv2016

A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44

Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The apparent survival of these Ultra Diffuse Galaxies (UDGs) in a rich cluster suggests that they have very high masses. Here we present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Keck II telescope. We find a velocity dispersion of 47 km/s, which implies a dynamical mass of M_dyn=0.7x10^10 M_sun within its deprojected half-light radius of r_1/2=4.6 kpc. The mass-to-light ratio is M/L=48 M_sun/L_sun, and the dark matter fraction is 98 percent within the half-light radius. The high mass of Dragonfly 44 is accompanied by a large globular cluster population. From deep Gemini imaging taken in 0.4" seeing we infer that Dragonfly 44 has 94 globular clusters, similar to the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are "failed" galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. We estimate the total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within r=4.6 kpc to enclosed mass profiles of NFW halos. The enclosed mass suggests a total mass of ~10^12 M_sun, similar to the mass of the Milky Way. The existence of nearly-dark objects with this mass is unexpected, as galaxy formation is thought to be maximally-efficient in this regime.

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