arXiv subjects

Imad Pasha

Publications and source records attributed to Imad Pasha.

At least 37 records · Page 2Linked to original sources

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.

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arXiv2024

The Heavy Metal Survey: The Evolution of Stellar Metallicities, Abundance Ratios, and Ages of Massive Quiescent Galaxies Since z~2

We present the elemental abundances and ages of 19 massive quiescent galaxies at $z\sim1.4$ and $z\sim2.1$ from the Keck Heavy Metal Survey. The ultra-deep LRIS and MOSFIRE spectra were modeled using a full-spectrum stellar population fitting code with variable abundance patterns. The galaxies have iron abundances between [Fe/H] = -0.5 and -0.1 dex, with typical values of $-0.2$ [$-0.3$] at $z\sim1.4$ [$z\sim2.1$]. We also find a tentative $\logσ_v$-[Fe/H] relation at $z\sim1.4$. The magnesium-to-iron ratios span [Mg/Fe]$\,=0.1$\,--\,$0.6$ dex, with typical values of $0.3$ [$0.5$] dex at $z\sim1.4$ [$z\sim2.1$]. The ages imply formation redshifts of $z_{\rm form}=2-8$. Compared to quiescent galaxies at lower redshifts, we find [Fe/H] was $\sim0.2$ dex lower at $z=1.4-2.1$. We find no evolution in [Mg/Fe] out to $z\sim1.4$, though the $z\sim2.1$ galaxies are $0.2$ dex enhanced compared to $z=0-0.7$. A comparison of these results to a chemical evolution model indicates that galaxies at higher redshift form at progressively earlier epochs and over shorter star-formation timescales, with the $z\sim2.1$ galaxies forming the bulk of their stars over 150 Myr at $z_{\rm form}\sim4$. This evolution cannot be solely attributed to an increased number of quiescent galaxies at later times; several Heavy Metal galaxies have extreme chemical properties not found in massive galaxies at $z\sim0.0-0.7$. Thus, the chemical properties of individual galaxies must evolve over time. Minor mergers also cannot fully account for this evolution as they cannot increase [Fe/H], particularly in galaxy centers. Consequently, the build-up of massive quiescent galaxies since $z\sim2.1$ may require further mechanisms such as major mergers and/or central star formation.

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arXiv2024

The Heavy Metal Survey: Star Formation Constraints and Dynamical Masses of 21 Massive Quiescent Galaxies at $z=1.3-2.3$

In this paper, we present the Heavy Metal Survey, which obtained ultradeep medium-resolution spectra of 21 massive quiescent galaxies at $1.3<z<2.3$ with Keck/LRIS and MOSFIRE. With integration times of up to 16\,hr per band per galaxy, we observe numerous Balmer and metal absorption lines in atmospheric windows. We successfully derive spectroscopic redshifts for all 21 galaxies and for 19 we also measure stellar velocity dispersions ($σ_v$), ages, and elemental abundances, as detailed in an accompanying paper. Except for one emission-line active galactic nucleus, all galaxies are confirmed as quiescent through their faint or absent H$α$ emission and evolved stellar spectra. For most galaxies exhibiting faint H$α$, elevated [NII]/H$α$ suggests a non-star-forming origin. We calculate dynamical masses ($M_{\rm dyn}$) by combining $σ_v$ with structural parameters obtained from HST/COSMOS(-DASH), and compare them with stellar masses ($M_*$) derived using spectrophotometric modeling, considering various assumptions. For a fixed initial mass function (IMF), we observe a strong correlation between $M_{\rm dyn}/M_*$ and $σ_v$. This correlation may suggest that a varying IMF, with high-$σ_v$ galaxies being more bottom heavy, was already in place at $z\sim2$. When implementing the $σ_v$-dependent IMF found in the cores of nearby early-type galaxies \textit{and} correcting for biases in our stellar mass and size measurements, we find a low scatter in $M_{\rm dyn}/M_*$ of 0.14 dex. However, these assumptions result in unphysical stellar masses, which exceed the dynamical masses by 34%. This tension suggests that distant quiescent galaxies do not simply grow inside-out into today's massive early-type galaxies and the evolution is more complicated.

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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.

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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.

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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.

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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.

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arXiv2024

A robust and simple method for filling in masked data in astronomical images

Astronomical images often have regions with missing or unwanted information, such as bad pixels, bad columns, cosmic rays, masked objects, or residuals from imperfect model subtractions. In certain situations it can be essential, or preferable, to fill in these regions. Most existing methods use low order interpolations for this task. In this paper a method is described that uses the full information that is contained in the pixels just outside masked regions. These edge pixels are extrapolated inwards, using iterative median filtering. This leads to a smoothly varying spatial resolution within the filled-in regions, and ensures seamless transitions between masked pixels and good pixels. Gaps in continuous, narrow features can be reconstructed with high fidelity, even if they are large. The method is implemented in maskfill, an open-source MIT licensed Python package. Its performance is illustrated with several examples, and compared to several alternative interpolation schemes.

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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.

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arXiv2023

pysersic: A Python package for determining galaxy structural properties via Bayesian inference, accelerated with jax

A standard practice in extragalactic population studies is the fitting of parametric models to galaxy images. From such fits, key structural parameters of galaxies such as total flux and effective radius (size) can be extracted. One of the most popular parametric forms is that of the Sérsic profile, which is flexible enough to reasonably fit the light distribution of nearly all galaxies. Here we present pysersic, a Bayesian framework created to facilitate the inference of structural parameters from galaxy images. Pysersic is written in pure Python, and is built using the jax framework, allowing for just-in-time compilation, auto-differentiation and seamless execution on CPUs, GPUs or TPUs. Inference is performed with the numpyro package using gradient based methods, e.g., No U-Turn Sampling, for efficient and robust posterior estimation in only a few minutes on a modern laptop. Pysersic is designed to have a user-friendly interface, allowing users to fit single or multiple sources in a few lines of code, while also being flexible enough for integration into current and future analysis pipelines. In addition to sampling, pysersic can produce point estimates of the best model via optimization in several seconds, and approximate the posterior via stochastic variational inference. The use of the numpyro probabilistic language provides future extensibility to arbitrary models beyond the Sérsic.

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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.

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arXiv2022

The Dragonfly Spectral Line Mapper: Design and First Light

The Dragonfly Spectral Line Mapper (DSLM) is the latest evolution of the Dragonfly Telephoto Array, which turns it into the world's most powerful wide-field spectral line imager. The DSLM will be the equivalent of a 1.6m aperture $f$/0.26 refractor with a built-in Integral Field Spectrometer, covering a five square degree field of view. The new telescope is designed to carry out ultra-narrow bandpass imaging of the low surface brightness universe with exquisite control over systematic errors, including real-time calibration of atmospheric variations in airglow. The key to Dragonfly's transformation is the "Filter-Tilter", a mechanical assembly which holds ultra-narrow bandpass interference filters in front of each lens in the array and tilts them to smoothly shift their central wavelength. Here we describe our development process based on rapid prototyping, iterative design, and mass production. This process has resulted in numerous improvements to the design of the DSLM from the initial pathfinder instrument, including changes to narrower bandpass filters and the addition of a suite of calibration filters for continuum light subtraction and sky line monitoring. Improvements have also been made to the electronics and hardware of the array, which improve tilting accuracy, rigidity and light baffling. Here we present laboratory and on-sky measurements from the deployment of the first bank of lenses in May 2022, and a progress report on the completion of the full array in early 2023.

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arXiv2022

The pathfinder Dragonfly Spectral Line Mapper: Pushing the limits for ultra-low surface brightness spectroscopy

The pathfinder Dragonfly Spectral Line Mapper is a distributed aperture telescope based off of the Dragonfly Telephoto Array with additional instrumentation (the Dragonfly "Filter-Tilter") to enable ultranarrow bandpass imaging. The pathfinder is composed of three redundant optical tube assemblies (OTAs) which are mounted together to form a single field of view imaging telescope (where the effective aperture diameter increases as the square-root of the number of OTAs). The pathfinder has been on sky from March 2020 to October 2021 equipped with narrowband filters to provide proof-of-concept imaging, surface brightness limit measurements, on sky testing, and observing software development. Here we describe the pathfinder telescope and the sensitivity limits reached along with observing methods. We outline the current limiting factors for reaching ultra-low surface brightnesses and present a comprehensive comparison of instrument sensitivities to low surface brightness line emission and other methods of observing the ultra-faint line emission from diffuse gas. Finally, we touch on plans for the upcoming 120-OTA Dragonfly Spectral Line Mapper, which is currently under construction.

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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.

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arXiv2022

Quenching in Cosmic Sheets: Tracing the Impact of Large Scale Structure Collapse on the Evolution of Dwarf Galaxies

Dwarf galaxies are thought to quench primarily due to environmental processes most typically occurring in galaxy groups and clusters or around single, massive galaxies. However, at earlier epochs, ($5 < z < 2$), the collapse of large scale structure (forming Zel'dovich sheets and subsequently filaments of the cosmic web) can produce volume-filling accretion shocks which elevate large swaths of the intergalactic medium (IGM) in these structures to a hot ($T>10^6$ K) phase. We study the impact of such an event on the evolution of central dwarf galaxies ($5.5 < \log M_* < 8.5$) in the field using a spatially large, high resolution cosmological zoom simulation which covers the cosmic web environment between two protoclusters. We find that the shock-heated sheet acts as an environmental quencher much like clusters and filaments at lower redshift, creating a population of quenched, central dwarf galaxies. Even massive dwarfs which do not quench are affected by the shock, with reductions to their sSFR and gas accretion. This process can potentially explain the presence of isolated quenched dwarf galaxies, and represents an avenue of pre-processing, via which quenched satellites of bound systems quench before infall.

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arXiv2022

A giant shell of ionized gas discovered near M82 with the Dragonfly Spectral Line Mapper pathfinder

We present the discovery of a giant cloud of ionized gas in the field of the starbursting galaxy M82. Emission from the cloud is seen in H$α$ and [NII]$λ$6583 in data obtained though a small pathfinder instrument used to test the key ideas that will be implemented in the Dragonfly Spectral Line Mapper, an upcoming ultranarrow-bandpass imaging version of the Dragonfly Telephoto Array. The discovered cloud has a shell-like morphology with a linear extent of $0.8^{\circ}$ and is positioned $0.6^{\circ}$ northwest of M82. At the heliocentric distance of the M81 group, the cloud's extent corresponds to 55 kpc and its projected distance from the nucleus of M82 is 40 kpc. The cloud has an average H$α$ surface brightness of $2\times10^{-18}$ $\mathrm{erg} ~\mathrm{cm}^{-2} ~\mathrm{s}^{-1} ~\mathrm{arcsec}^{-2}$. The [NII]$λ$6583/H$α$ line ratio varies from [NII]/H$α$ $\sim0.2$ to [NII]/H$α$ $\sim1.0$ across the cloud, with higher values found in its eastern end. Follow-up spectra obtained with Keck-LRIS confirm the existence of the cloud and yield line ratios of [NII]$λ$6583/H$α$ = 0.340 $\pm$ 0.003 and [SII]$λ$6716,6731/H$α$ = 0.64 $\pm$ 0.03 in the cloud. This giant cloud of material could be lifted from M82 by tidal interactions or by its powerful starburst. Alternatively, it may be gas infalling from the cosmic web, potentially precipitated by the superwinds of M82. Deeper data are needed to test these ideas further. The upcoming Dragonfly Spectral Line Mapper will have 120 lenses, $40\times$ more than in the pathfinder instrument used to obtain the data presented here.

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arXiv2021

A Nascent Tidal Dwarf Galaxy Forming within the Northern HI Streamer of M82

We identify a $\sim$600 pc-wide region of active star formation located within a tidal streamer of M82 via $Hα$ emission ($F_{Hα}\sim 6.5\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$), using a pathfinder instrument based on the Dragonfly Telephoto Array. The object is kinematically decoupled from the disk of M82 as confirmed via Keck / LRIS spectroscopy, and is spatially and kinematically coincident with an overdensity of HI and molecular hydrogen within the "northern HI streamer" induced by the passage of M81 several hundred Myr ago. From HI data, we estimate that $\sim5\times10^7$ M$_{\odot}$ of gas is present in the specific overdensity coincident with the $Hα$ source. The object's derived metallicity (12+$\log(O/H)\simeq 8.6$), position within a gas-rich tidal feature, and morphology (600 pc diameter with multiple star forming clumps), indicate that it is likely a tidal dwarf galaxy in the earliest stages of formation.

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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.

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