arXiv subjects

Dennis Zaritsky

Publications and source records attributed to Dennis Zaritsky.

At least 55 records · Page 3Linked to original sources

arXiv2024

LIGHTS. Survey Overview and a Search for Low Surface Brightness Satellite Galaxies

We present an overview of the LIGHTS (LBT Imaging of Galactic Halos and Tidal Structures) survey, which currently includes 25 nearby galaxies that are on average $\sim$ 1 mag fainter than the Milky Way, and a catalog of 54 low central surface brightness (24 $< μ_{0,g}$/mag arcsec$^{-2} < 28$) satellite galaxy candidates, most of which were previously uncatalogued. The depth of the imaging exceeds the full 10-year depth of the Rubin Observatory's Legacy Survey of Space and Time (LSST). We find, after applying completeness corrections, rising numbers of candidate satellites as we approach the limiting luminosity (M$_r \sim -8$ mag) and central surface brightness ($μ_{0,g} \sim 28$ mag arcsec$^{-2}$). Over the parameter range we explore, each host galaxy (excluding those that are in overdense regions, apparently groups) has nearly 4 such candidate satellites to a projected radius of $\sim$ 100 kpc. These objects are mostly just at or beyond the reach of spectroscopy unless they are H I rich or have ongoing star formation. We identify 3, possibly 4, ultra-diffuse satellite galaxies (UDGs; effective radius $ > 1.5$ kpc). This incidence rate falls within expectations of the extrapolation of the published relationship between the number of UDGs and host halo mass. Lastly, we visually identify 12 candidate satellites that host a nuclear star cluster (NSC). The NSC occupation fraction for the sample (12/54) matches that published for satellites of early-type galaxies, suggesting that the parent's morphological type plays at most a limited role in determining the NSC occupation fraction.

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arXiv2024

Properties of Nuclear Star Clusters in Low Surface Brightness Galaxies

Using the SMUDGes and SDSS catalogs, and our own reprocessing of the Legacy Surveys imaging, we investigate the properties of nuclear star clusters (NSCs) in galaxies having central surface brightnesses as low as 27 mag arcsec$^{-2}$. We identify 273 (123 with known redshift) and 32 NSC-bearing galaxies in the two samples, respectively, where we require candidate NSCs to have a separation of less than 0.10$r_e$ from the galaxy center. We find that galaxies with low central surface brightness ($μ_{0,g} > 24$ mag arcsec$^{-2}$) are more likely to contain an NSC if 1) they have a higher stellar mass, 2) a higher stellar to total mass ratio, 3) a brighter central surface brightness, 4) a larger axis ratio, or 5) lie in a denser environment. Because of the correlations among these various quantities, it is likely that only one or two are true physical drivers. We also find scaling relations for the NSC mass with stellar mass ($M_{NSC}/$\Msol$ = 10^{6.02\pm0.03}(M_{*,gal}/10^{8} $\Msol$)^{0.77\pm0.04}$) and halo mass ($M_{NSC}/$\Msol$ = 10^{6.11\pm0.05}(M_{h,gal}/10^{10} $\Msol$)^{0.92\pm0.05}$), although it is the scaling with halo mass that is consistent with a direct proportionality. In galaxies with an NSC, $M_{NSC} \approx 10^{-4}M_{h,gal}$. This proportionality echoes the finding of a direct proportionality between the mass (or number) of globular clusters (GCs) in galaxies and the galaxy's total mass. These findings favor a related origin for GCs and NSCs.

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arXiv2024

Extending the Chemical Reach of the H3 Survey: Detailed Abundances of the Dwarf-galaxy Stellar Stream Wukong/LMS-1

We present the first detailed chemical-abundance analysis of stars from the dwarf-galaxy stellar stream Wukong/LMS-1 covering a wide metallicity range ($-3.5 < \rm[Fe/H] \lesssim -1.3$). We find abundance patterns that are effectively indistinguishable from the bulk of Indus and Jhelum, a pair of smaller stellar streams proposed to be dynamically associated with Wukong/LMS-1. We confirmed a carbon-enhanced metal-poor star ($\rm[C/Fe] > +0.7$ and $\rm[Fe/H] \sim -2.9$) in Wukong/LMS-1 with strong enhancements in Sr, Y, and Zr, which is peculiar given its solar-level [Ba/Fe]. Wukong/LMS-1 stars have high abundances of $α$ elements up to $\rm[Fe/H] \gtrsim -2$, which is expected for relatively massive dwarfs. Towards the high-metallicity end, Wukong/LMS-1 becomes $α$-poor, revealing that it probably experienced fairly standard chemical evolution. We identified a pair of N- and Na-rich stars in Wukong/LMS-1, reminiscent of multiple populations in globular clusters. This indicates that this dwarf galaxy contained at least one globular cluster that was completely disrupted in addition to two intact ones previously known to be associated with Wukong/LMS-1, which is possibly connected to similar evidence found in Indus. From these $\geq$3 globular clusters, we estimate the total mass of Wukong/LMS-1 to be ${\approx}10^{10} M_\odot$, representing ${\sim}1$% of the present-day Milky Way. Finally, the [Eu/Mg] ratio in Wukong/LMS-1 continuously increases with metallicity, making this the first example of a dwarf galaxy where the production of $r$-process elements is clearly dominated by delayed sources, presumably neutron-star mergers.

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arXiv2024

The Faint Satellite System of NGC 253: Insights into Low-Density Environments and No Satellite Plane

We have conducted a systematic search around the Milky Way (MW) analog NGC 253 (D=3.5 Mpc), as a part of the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) - a Magellan+Megacam survey to identify dwarfs and other substructures in resolved stellar light around MW-mass galaxies outside of the Local Group. In total, NGC 253 has five satellites identified by PISCeS within 100 kpc with an absolute V-band magnitude $M_V<-7$. We have additionally obtained deep Hubble Space Telescope imaging of four reported candidates beyond the survey footprint: Do III, Do IV, and dw0036m2828 are confirmed to be satellites of NGC 253, while SculptorSR is found to be a background galaxy. We find no convincing evidence for the presence of a plane of satellites surrounding NGC 253. We construct its satellite luminosity function, which is complete down to $M_V$$\lesssim$$-8$ out to 100 kpc and $M_V$$\lesssim$$-9$ out to 300 kpc, and compare it to those calculated for other Local Volume galaxies. Exploring trends in satellite counts and star-forming fractions among satellite systems, we find relationships with host stellar mass, environment, and morphology, pointing to a complex picture of satellite formation, and a successful model has to reproduce all of these trends.

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arXiv2023

Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes). VI. Nuclear Star Clusters

We present our photometric search for potential nuclear star clusters (NSCs) in ultra-diffuse galaxies (UDGs) as an extension of the SMUDGes catalog. We identify 325 SMUDGes galaxies with NSCs and, from the 144 with existing distance estimates, identify 33 NSC hosts as UDGs ($μ_{0,g}$ $\ge$ 24 mag arcsec$^{-2}$, $r_e \ge 1.5$ kpc). The SMUDGes with NSCs lie on the galaxy red sequence, satisfy the NSC-host galaxy stellar mass relationship, have a mean NSC stellar mass fraction of 0.02 but reach as high as 0.1, have NSCs that are displaced from the host center with a standard deviation of 0.10$r_e$, and weakly favor higher density environments. All of these properties are consistent with previous results from higher surface brightness galaxy samples, allowing for at most a relatively weak dependence of NSC behavior with host galaxy surface brightness.

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arXiv2023

When the Well Runs Dry: Modeling Environmental Quenching of High-mass Satellites in Massive Clusters at \boldmath$z \gtrsim 1$

We explore models of massive ($\gt 10^{10}~{\rm M}_{\odot}$) satellite quenching in massive clusters at $z\gtrsim1$ using an MCMC framework, focusing on two primary parameters: $R_{\rm quench}$ (the host-centric radius at which quenching begins) and $τ_{\rm quench}$ (the timescale upon which a satellite quenches after crossing $R_{\rm quench}$). Our MCMC analysis shows two local maxima in the 1D posterior probability distribution of $R_{\rm quench}$ at approximately $0.25$ and $1.0~R_{\rm{200}}$. Analyzing four distinct solutions in the $τ_{\rm quench}$-$R_{\rm quench}$ parameter space, nearly all of which yield quiescent fractions consistent with observational data from the GOGREEN survey, we investigate whether these solutions represent distinct quenching pathways and find that they can be separated between \textquote{starvation} and \textquote{core quenching} scenarios. The starvation pathway is characterized by quenching timescales that are roughly consistent with the total cold gas (H$_{2}$+H{\scriptsize I}) depletion timescale at intermediate $z$, while core quenching is characterized by satellites with relatively high line-of-sight velocities that quench on short timescales ($\sim 0.25$ Gyr) after reaching the inner region of the cluster ($\lt 0.30~R_{\rm{200}}$). Lastly, we break the degeneracy between these solutions by comparing the observed properties of transition galaxies from the GOGREEN survey. We conclude that only the \textquote{starvation} pathway is consistent with the projected phase-space distribution and relative abundance of transition galaxies at $z \sim 1$. However, we acknowledge that ram pressure might contribute as a secondary quenching mechanism.

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arXiv2023

Pavo: Discovery of a star-forming dwarf galaxy just outside the Local Group

We report the discovery of Pavo, a faint ($M_V = -10.0$), star-forming, irregular, and extremely isolated dwarf galaxy at $D\approx2$ Mpc. Pavo was identified in Dark Energy Camera Legacy Survey imaging via a novel approach that combines low surface brightness galaxy search algorithms and machine learning candidate classifications. Follow-up imaging with the Inamori-Magellan Areal Camera & Spectrograph on the 6.5 m Magellan Baade telescope revealed a color--magnitude diagram (CMD) with an old stellar population, in addition to the young population that dominates the integrated light, and a tip-of-the-red-giant-branch distance estimate of $1.99^{+0.20}_{-0.22}$ Mpc. The blue population of stars in the CMD is consistent with the youngest stars having formed no later than 150 Myr ago. We also detected no H$α$ emission with SOAR telescope imaging, suggesting we may be witnessing a temporary low in Pavo's star formation. We estimate the total stellar mass of Pavo to be $\log M_\ast/\mathrm{M_\odot} = 5.6 \pm 0.2$ and measure an upper limit on its HI gas mass of $1.0 \times 10^6\,\mathrm{M_\odot}$ based on the HIPASS survey. Given these properties, Pavo's closest analog is Leo P ($D=1.6$ Mpc), previously the only known isolated, star-forming, Local Volume dwarf galaxy in this mass range. However, Pavo appears to be even more isolated, with no other known galaxy residing within over 600 kpc. As surveys and search techniques continue to improve, we anticipate an entire population of analogous objects being detected just outside the Local Group.

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arXiv2023

Dwarf Galaxy Discoveries from the KMTNet Supernova Program III. the Milky-Way Analog NGC~2997 Group

We present the discovery of 48 new and the analysis of 55, including 7 previously discovered, dwarf galaxy candidates around the giant spiral galaxy NGC~2997 using deep $BVI$ images from the KMTNet Supernova Program. Their $V$-band central surface brightness and total absolute magnitudes are in the range of 20.3--26.7 mag arcsec$^{-2}$ and --(8.02--17.69) mag, respectively, while the $I$-band effective radii are 0.14--2.97 kpc. We obtain $α$ $\simeq$ --1.43 $\pm$ 0.02 for the faint-end slope of their luminosity function, comparable to previously measured values but shallower than theoretical predictions based on $Λ$CDM models. The distance-independent distributions of their mass and color suggest that the group could have recently accreted new massive members from the surrounding fields. The systematically bluer colors of the brighter members indicate younger stellar population and higher star formation activities in them, which appears to be consistent with similar findings from the SAGA or ELVES survey. We suggest that the massive and bluer dwarf galaxies in the group have experienced less environmental quenching due to their recent accretion, while environmental quenching is more effective for the low-mass members. The interpretation of NGC~2997 being populationally young with recent accretion of massive members is also consistent with the overall morphological distribution of the dwarf galaxies showing a lack of morphologically evolved candidates but a plethora of irregularly shaped ones. Our detection rate of dwarf galaxy candidates in the NGC~2997 group and their inferred star formation activities are comparable to those found in Milky Way analog systems from recent surveys within the magnitude limit M$_{V}$ $\lesssim$ --13 mag.

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arXiv2023

The Quenched Satellite Population Around Milky Way Analogs

We study the relative fractions of quenched and star-forming satellite galaxies in the Satellites Around Galactic Analogs (SAGA) survey and Exploration of Local VolumE Satellites (ELVES) program, two nearby and complementary samples of Milky Way-like galaxies that take different approaches to identify faint satellite galaxy populations. We cross-check and validate sample cuts and selection criteria, as well as explore the effects of different star-formation definitions when determining the quenched satellite fraction of Milky Way analogs. We find the mean ELVES quenched fraction ($\langle QF\rangle$), derived using a specific star formation rate (sSFR) threshold, decreases from $\sim$50% to $\sim$27% after applying a cut in absolute magnitude to match that of the SAGA survey ($\langle QF\rangle_{SAGA}\sim$9%). We show these results are consistent for alternative star-formation definitions. Furthermore, these quenched fractions remain virtually unchanged after applying an additional cut in surface brightness. Using a consistently-derived sSFR and absolute magnitude limit for both samples, we show that the quenched fraction and the cumulative number of satellites in the ELVES and SAGA samples broadly agree. We briefly explore radial trends in the ELVES and SAGA samples, finding general agreement in the number of star-forming satellites per host as a function of radius. Despite the broad agreement between the ELVES and SAGA samples, some tension remains with these quenched fractions in comparison to the Local Group and simulations of Milky Way analogs.

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arXiv2023

Distant Echoes of the Milky Way's Last Major Merger

The majority of the Milky Way's stellar halo consists of debris from our Galaxy's last major merger, the Gaia-Sausage-Enceladus (GSE). In the past few years, stars from GSE have been kinematically and chemically studied in the inner $30$ kpc of our Galaxy. However, simulations predict that accreted debris could lie at greater distances, forming substructures in the outer halo. Here we derive metallicities and distances using Gaia DR3 XP spectra for an all-sky sample of luminous red giant stars, and map the outer halo with kinematics and metallicities out to $100$ kpc. We obtain follow-up spectra of stars in two strong overdensities - including the previously identified Outer Virgo Overdensity - and find them to be relatively metal-rich and on predominantly retrograde orbits, matching predictions from simulations of the GSE merger. We argue that these are apocentric shells of GSE debris, forming $60-90$ kpc counterparts to the $15-20$ kpc shells that are known to dominate the inner stellar halo. Extending our search across the sky with literature radial velocities, we find evidence for a coherent stream of retrograde stars encircling the Milky Way from $50-100$ kpc, in the same plane as the Sagittarius stream but moving in the opposite direction. These are the first discoveries of distant and structured imprints from the GSE merger, cementing the picture of an inclined and retrograde collision that built up our Galaxy's stellar halo.

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arXiv2023

Discovery of the Magellanic Stellar Stream Out to 100 Kiloparsecs

The Magellanic Stream (MS) - an enormous ribbon of gas spanning $140^\circ$ of the southern sky trailing the Magellanic Clouds - has been exquisitely mapped in the five decades since its discovery. However, despite concerted efforts, no stellar counterpart to the MS has been conclusively identified. This stellar stream would reveal the distance and 6D kinematics of the MS, constraining its formation and the past orbital history of the Clouds. We have been conducting a spectroscopic survey of the most distant and luminous red giant stars in the Galactic outskirts. From this dataset, we have discovered a prominent population of 13 stars matching the extreme angular momentum of the Clouds, spanning up to $100^\circ$ along the MS at distances of $60-120$ kpc. Furthermore, these kinemetically-selected stars lie along a [$α$/Fe]-deficient track in chemical space from $-2.5 < \mathrm{[Fe/H]} < -0.5$, consistent with their formation in the Clouds themselves. We identify these stars as high-confidence members of the Magellanic Stellar Stream. Half of these stars are metal-rich and closely follow the gaseous MS, whereas the other half are more scattered and metal-poor. We argue that the metal-rich stream is the recently-formed tidal counterpart to the MS, and speculate that the metal-poor population was thrown out of the SMC outskirts during an earlier interaction between the Clouds. The Magellanic Stellar Stream provides a strong set of constraints - distances, 6D kinematics, and birth locations - that will guide future simulations towards unveiling the detailed history of the Clouds.

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arXiv2023

Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes). V. The Complete SMUDGes Catalog and the Nature of Ultra-Diffuse Galaxies

We present the completed catalog of ultra-diffuse galaxy (UDG) candidates (7070 objects) from our search of the DR9 Legacy Survey images, including distance and total mass estimates for 1529 and 1436 galaxies, respectively, that we provide and describe in detail. From the sample with estimated distances, we obtain a sample of 585 UDGs ($μ_{0,g} \ge 24$ mag arcsec$^{-2}$ and $r_e \ge 1.5$ kpc) over 20,000 sq. deg of sky in various environments. We conclude that UDGs in our sample are limited to $10^{10} \lesssim$ M$_h$/M$_\odot \lesssim 10^{11.5}$ and are on average a factor of 1.5 to 7 deficient in stars relative to the general population of galaxies of the same total mass. That factor increases with increasing galaxy size and mass up to a factor of $\sim$10 when the total mass of the UDG increases beyond M$_h = 10^{11}$ M$_\odot$. We do not find evidence that this factor has a dependence on the UDG's large-scale environment.

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arXiv2023

An Enigmatic 380 kpc Long Linear Collimated Galactic Tail

We present an intriguing, serendipitously-detected system consisting of an S0/a galaxy, which we refer to as the "Kite", and a highly-collimated tail of gas and stars that extends over 380 kpc and contains pockets of star formation. In its length, narrowness, and linearity the Kite's tail is an extreme example relative to known tails. The Kite (PGC 1000273) has a companion galaxy, Mrk 0926 (PGC 070409), which together comprise a binary galaxy system in which both galaxies host active galactic nuclei. Despite this systems being previously searched for signs of tidal interactions, the tail had not been discovered prior to our identification as part of the validation process of the SMUDGes survey for low surface brightness galaxies. We confirm the kinematic association between various H$α$ knots along the tail, a small galaxy, and the Kite galaxy using optical spectroscopy obtained with the Magellan telescope and measure a velocity gradient along the tail. The Kite shares characteristics common to those formed via ram pressure stripping ("jellyfish" galaxies) and formed via tidal interactions. However, both scenarios face significant challenges that we discuss, leaving open the question of how such an extreme tail formed. We propose that the tail resulted from a three-body interaction from which the lowest-mass galaxy was ejected at high velocity.

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arXiv2023

The Local Cluster Survey II: Disk-Dominated Cluster Galaxies with Suppressed Star Formation

We investigate the role of dense environments in suppressing star formation by studying $\rm \log_{10}(M_\star/M_\odot) > 9.7$ star-forming galaxies in nine clusters from the Local Cluster Survey ($0.0137 < z < 0.0433$) and a large comparison field sample drawn from the Sloan Digital Sky Survey. We compare the star-formation rate (SFR) versus stellar mass relation as a function of environment and morphology. After carefully controlling for mass, we find that in all environments, the degree of SFR suppression increases with increasing bulge-to-total (B/T) ratio. In addition, the SFRs of cluster and infall galaxies at a fixed mass are more suppressed than their field counterparts at all values of B/T. These results suggest a quenching mechanism that is linked to bulge growth that operates in all environments and an additional mechanism that further reduces the SFRs of galaxies in dense environments. We limit the sample to $B/T < 0.3$ galaxies to control for the trends with morphology and find that the excess population of cluster galaxies with suppressed SFRs persists. We model the timescale associated with the decline of SFRs in dense environments and find that the observed SFRs of the cluster core galaxies are consistent with a range of models including: a mechanism that acts slowly and continuously over a long (2-5 Gyr) timescale, and a more rapid ($<1$ Gyr) quenching event that occurs after a delay period of 1-6 Gyr. Quenching may therefore start immediately after galaxies enter clusters.

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arXiv2023

Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes). IV. Ultra-Diffuse Satellites of Milky Way Analogs

To better understand the formation of large, low surface brightness galaxies, we measure the correlation function between ultra-diffuse galaxy (UDG) candidates and Milky Way analogs (MWAs). We find that (1) the projected radial distribution of UDG satellites (projected surface density $\propto r^{-0.84\pm0.06}$) is consistent with that of normal satellite galaxies, (2) the number of UDG satellites per MWA ($S_{\rm UDG}$) is $\sim 0.5\pm0.1$ over projected radii from 20 to 250 kpc and $-17< M_r < -13.5$, (3) $S_{\rm UDG}$ is consistent with a linear extrapolation of the relationship between the number of UDGs per halo vs. halo mass obtained over galaxy group and cluster scales, (4) red UDG satellites dominate the population of UDG satellites ($\sim80$%), (5) over the range of satellite magnitudes studied, UDG satellites comprise $\sim$ 10% of the satellite galaxy population of MWAs, (6) a significant fraction of these ($\sim$13%) have estimated total masses $>$ 10$^{10.9}$ M$_\odot$ or, equivalently, at least half the halo mass of the LMC, and populate a large fraction ($\sim$ 18%) of the expected subhalos down to these masses. All of these results suggest a close association between the overall low mass galaxy population and UDGs, which we interpret as favoring models where UDG formation principally occurs within the general context of low mass galaxy formation over models invoking more exotic physical processes specifically invoked to form UDGs.

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arXiv2022

Extending Ultra-Diffuse Galaxy Abundances to Milky Way Analogs

We extend the Ultra-Diffuse Galaxy (UDG) abundance relation, $N_{UDG}-M_{200}$, to lower halo mass hosts $(M_{200}\sim10^{11.6-12.2}M_{\odot})$. We select UDG satellites from published catalogs of dwarf satellite galaxies around Milky Way analogs, namely the Exploration of Local Volume Satellites (ELVES) survey, Satellite Around Galactic Analogs (SAGA) survey, and a survey of Milky Way-like systems conducted using the Hyper-Suprime Cam. Of the 516 satellites around a total of 75 Milky Way-like hosts, we find 41 satellites around 33 hosts satisfy the UDG criteria. The distributions of host halo masses peak around $M_{200}\sim10^{12}M_{\odot}$ independent of whether the host has a UDG satellite or not. We use literature UDG abundances and those derived here to trace the $N_{UDG}-M_{200}$ relation over three orders of magnitude down to $M_{200}=10^{11.6}M_{\odot}$ and find a best-fit linear relation of $N_{UDG} = (38\pm5)\cdot(\frac{M_{200}}{10^{14}})^{0.89\pm0.04}$. This sub-linear slope is consistent with earlier studies of UDG abundances as well as abundance relations for brighter dwarf galaxies, excluding UDG formation mechanisms that require high-density environments. However, we highlight the need for further homogeneous characterization of UDGs across a wide range of environments to properly understand the $N_{UDG}-M_{200}$ relation.

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arXiv2022

Photometric Mass Estimation and the Stellar Mass-Halo Mass Relation for Low Mass Galaxies

We present a photometric halo mass estimation technique for local galaxies that enables us to establish the stellar mass-halo mass (SMHM) relation down to stellar masses of 10$^5$ M$_\odot$. We find no detectable differences among the SMHM relations of four local galaxy clusters or between the cluster and field relations and we find agreement with extrapolations of previous SMHM relations derived using abundance matching approaches. We fit a power law to our empirical SMHM relation and find that for adopted NFW dark matter profiles and for M$_* < 10^9$ M$_\odot$, the halo mass is M$_h = 10^{10.35\pm0.02}({\rm M}_*/10^8 {\rm M}_\odot)^{0.63\pm0.02}$. The normalisation of this relation is susceptible to systematic modelling errors that depend on the adopted dark matter potential and the quoted uncertainties refer to the uncertainties in the median relation. For galaxies with M$_* < 10^{9}$ M$_\odot$ that satisfy our selection criteria, the scatter about the fit in $M_h$, including uncertainties arising from our methodology, is 0.3 dex. Finally, we place lower luminosity Local Group galaxies on the SMHM relationship using the same technique, extending it to M$_* \sim 10^3$ M$_\odot$ and suggest that some of these galaxies show evidence for additional mass interior to the effective radius beyond that provided by the standard dark matter profile. If this mass is in the form of a central black hole, the black hole masses are in the range of intermediate mass black holes, $10^{(5.7\pm0.6)}$ M$_\odot$, which corresponds to masses of a few percent of M$_h$, well above values extrapolated from the relationships describing more massive galaxies.

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arXiv2022

A Ghost in Boötes: The Least Luminous Disrupted Dwarf Galaxy

We report the discovery of Specter, a disrupted ultrafaint dwarf galaxy revealed by the H3 Spectroscopic Survey. We detected this structure via a pair of comoving metal-poor stars at a distance of 12.5 kpc, and further characterized it with Gaia astrometry and follow-up spectroscopy. Specter is a $25^\circ \times 1^\circ$ stream of stars that is entirely invisible until strict kinematic cuts are applied to remove the Galactic foreground. The spectroscopic members suggest a stellar age $τ\gtrsim 12$ Gyr and a mean metallicity $\langle\text{[Fe/H]}\rangle = -1.84_{-0.18}^{+0.16}$, with a significant intrinsic metallicity dispersion $σ_{ \text{[Fe/H]}} = 0.37_{-0.13}^{+0.21}$. We therefore argue that Specter is the disrupted remnant of an ancient dwarf galaxy. With an integrated luminosity $M_{\text{V}} \approx -2.6$, Specter is by far the least-luminous dwarf galaxy stream known. We estimate that dozens of similar streams are lurking below the detection threshold of current search techniques, and conclude that spectroscopic surveys offer a novel means to identify extremely low surface brightness structures.

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