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We present the 2020 version of the Siena Galaxy Atlas (SGA-2020), a multiwavelength optical and infrared imaging atlas of 383,620 nearby galaxies. The SGA-2020 uses opticalgrzimaging over ≈20,000 deg²from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys Data Release 9 and infrared imaging in four bands (spanning 3.4–22μm) from the 6 year unWISE coadds; it is more than 95% complete for galaxies larger thanR(26) ≈ 25″ andr< 18 measured at the 26 mag arcsec⁻²isophote in therband. The atlas delivers precise coordinates, multiwavelength mosaics, azimuthally averaged optical surface-brightness profiles, model images and photometry, and additional ancillary metadata for the full sample. Coupled with existing and forthcoming optical spectroscopy from the DESI, the SGA-2020 will facilitate new detailed studies of the star formation and mass assembly histories of nearby galaxies; enable precise measurements of the local velocity field via the Tully–Fisher and fundamental plane relations; serve as a reference sample of lasting legacy value for time-domain and multimessenger astronomical events; and more.

We present the first comprehensive halo occupation distribution (HOD) analysis of the Dark Energy Spectroscopic Instrument (DESI) One-Percent Survey luminous red galaxy (LRG) and Quasi Stellar Object (QSO) samples. We constrain the HOD of each sample and test possible HOD extensions by fitting the redshift-space galaxy 2-point correlation functions in 0.15 < r < 32 h−1 Mpc in a set of fiducial redshift bins. We use AbacusSummit cubic boxes at Planck 2018 cosmology as model templates and forward model galaxy clustering with the AbacusHOD package. We achieve good fits with a standard HOD model with velocity bias, and we find no evidence for galaxy assembly bias or satellite profile modulation at the current level of statistical uncertainty. For LRGs in 0.4 < z < 0.6, we infer a satellite fraction of $f_\mathrm{sat} = 11\pm 1~{y{\ \mathrm{per\,cent}}}$, a mean halo mass of $\log _{10}\overline{M}_h/M_\odot =13.40^{+0.02}_{-0.02}$, and a linear bias of $b_\mathrm{lin} = 1.93_{-0.04}^{+0.06}$. For LRGs in 0.6 < z < 0.8, we find $f_\mathrm{sat}=14\pm 1~{{\ \mathrm{per\,cent}}}$, $\log _{10}\overline{M}_h/M_\odot =13.24^{+0.02}_{-0.02}$, and $b_\mathrm{lin}=2.08_{-0.03}^{+0.03}$. For QSOs, we infer $f_\mathrm{sat}=3^{+8}_{-2}\mathrm{per\,cent}$, $\log _{10}\overline{M}_h/M_\odot = 12.65^{+0.09}_{-0.04}$, and $b_\mathrm{lin} = 2.63_{-0.26}^{+0.37}$ in redshift range 0.8 < z < 2.1. Using these fits, we generate a large suite of high fidelity galaxy mocks, forming the basis of systematic tests for DESI Y1 cosmological analyses. We also study the redshift-evolution of the DESI LRG sample from z = 0.4 up to z = 1.1, revealling significant and interesting trends in mean halo mass, linear bias, and satellite fraction.

Mapping out the populations of thick disk and halo brown dwarfs is important for understanding the metallicity dependence of low-temperature atmospheres and the substellar mass function. Recently, a new population of cold and metal-poor brown dwarfs has been discovered, withT_eff≲ 1400 K and metallicity ≲−1 dex. This population includes what may be the first known “extreme T-type subdwarfs” and possibly the first Y-type subdwarf, WISEA J153429.75−104303.3. We have conducted a GeminiYJHK/Ksphotometric follow-up campaign targeting potentially metal-poor T and Y dwarfs, utilizing the GNIRS and Flamingos-2 instruments. We present 14 near-infrared photometric detections of eight unique targets: six T subdwarf candidates, one moderately metal-poor Y dwarf candidate, and one Y subdwarf candidate. We have obtained the first-ever ground-based detection of the highly anomalous object WISEA J153429.75−104303.3. The F110W −Jcolor of WISEA J153429.75−104303.3 is significantly bluer than that of other late T and Y dwarfs, indicating that WISEA J153429.75−104303.3 has an unusual spectrum in the 0.9–1.4μm wavelength range which encompasses theJ-band peak. OurJ-band detection of WISEA J153429.75−104303.3 and corresponding model comparisons suggest a subsolar metallicity and temperature of 400–550 K for this object. JWST spectroscopic follow-up at near-infrared and mid-infrared wavelengths would allow us to better understand the spectral peculiarities of WISEA J153429.75−104303.3, assess its physical properties, and conclusively determine whether or not it is the first Y-type subdwarf.

Using a sample of 361 nearby brown dwarfs, we have searched for 4.6μm variability, indicative of large-scale rotational modulations or large-scale, long-term changes on timescales of over 10 yr. Our findings show no statistically significant variability in Spitzer’s Infrared Array Camera (IRAC) channel 2 (ch2) or Wide-field Infrared Survey Explorer W2 photometry. For Spitzer the ch2 1σlimits are ∼8 mmag for objects at 11.5 mag and ∼22 mmag for objects at 16 mag. This corresponds to no variability above 4.5% at 11.5 mag and 12.5% at 16 mag. We conclude that highly variable brown dwarfs, at least two previously published examples of which have been shown to have 4.6μm variability above 80 mmag, are very rare. While analyzing the data, we also developed a new technique for identifying brown dwarf binary candidates in Spitzer data. We find that known binaries have IRAC ch2 point response function (PRF) flux measurements that are consistently dimmer than aperture flux measurements. We have identified 59 objects that exhibit such PRF versus aperture flux differences and are thus excellent binary brown dwarf candidates.

While the vast majority of tidal disruption events (TDEs) have been identified by wide-field sky surveys in the optical and X-ray bands, recent studies indicate that a considerable fraction of TDEs may be dust obscured and thus preferentially detected in the infrared (IR) wave bands. In this Letter, we present the discovery of a luminous mid-IR nuclear flare (termed WTP14adbjsh), identified in a systematic transient search of archival images from the NEOWISE mid-IR survey. The source reached a peak luminosity ofL≃ 10⁴³erg s⁻¹at 4.6μm in 2015 before fading in the IR with a TDE-likeF∝t^−5/3decline, radiating a total of more than 3 × 10⁵¹erg in the last 7 yr. The transient event took place in the nearby galaxy NGC 7392, at a distance of around 42 Mpc; yet, no optical or X-ray flare is detected. We interpret the transient as the nearest TDE candidate detected in the last decade, which was missed at other wavelengths due to dust obscuration, hinting at the existence of TDEs that have been historically overlooked. Unlike most previously detected TDEs, the transient was discovered in a star-forming galaxy, corroborating earlier suggestions that dust obscuration suppresses significantly the detection of TDEs in these environments. Our results demonstrate that the study of IR-detected TDEs is critical in order to obtain a complete understanding of the physics of TDEs and to conclude whether TDEs occur preferentially in a particular class of galaxies.

We explore the galaxy-halo connection information that is available in low-redshift samples from the early data release of the Dark Energy Spectroscopic Instrument (DESI). We model the halo occupation distribution (HOD) fromz= 0.1 to 0.3 using Survey Validation 3 (SV3; a.k.a., the One-Percent Survey) data of the DESI Bright Galaxy Survey. In addition to more commonly used metrics, we incorporate counts-in-cylinders (CiC) measurements, which drastically tighten HOD constraints. Our analysis is aided by the Python package,galtab, which enables the rapid, precise prediction of CiC for any HOD model available inhalotools. This methodology allows our Markov chains to converge with much fewer trial points, and enables even more drastic speedups due to its GPU portability. Our HOD fits constrain characteristic halo masses tightly and provide statistical evidence for assembly bias, especially at lower luminosity thresholds: the HOD of central galaxies inz∼ 0.15 samples with limiting absolute magnitudeM_r< −20.0 andM_r< −20.5 samples is positively correlated with halo concentration with a significance of 99.9% and 99.5%, respectively. Our models also favor positive central assembly bias for the brighterM_r< −21.0 sample atz∼ 0.25 (94.8% significance), but there is no significant evidence for assembly bias with the same luminosity threshold atz∼ 0.15. We provide our constraints for each threshold sample’s characteristic halo masses, assembly bias, and other HOD parameters. These constraints are expected to be significantly tightened with future DESI data, which will span an area 100 times larger than that of SV3.

WISEA J153429.75−104303.3 (a.k.a. “The Accident”) is a brown dwarf with unique observational properties unlike those of any other known (sub)stellar object. It is thought to be a very cold, metal-poor brown dwarf, and potentially the first identified Y-type subdwarf. Although WISEA J153429.75−104303.3 is extremely faint atλ ∼ 1.25 μm, low-temperature atmosphere models suggest thatY-band flux could increase dramatically relative toJ-band as metallicity decreases from solar. We therefore obtained deepY-band follow-up imaging of WISEA J153429.75−104303.3 with the Dark Energy Camera, and report a resultingY-band limit ofY > 21.79 mag (Vega, 5σ).

Y dwarfs, the coolest known spectral class of brown dwarfs, overlap in mass and temperature with giant exoplanets, providing unique laboratories for studying low-temperature atmospheres. However, only a fraction of Y dwarf candidates have been spectroscopically confirmed. We present Keck/NIRES near-infrared spectroscopy of the nearby (d≈ 6–8 pc) brown dwarf CWISE J105512.11+544328.3. Although its near-infrared spectrum aligns best with the Y0 standard in theJband, no standard matches well across the fullYJHKwavelength range. The CWISE J105512.11+544328.3 NH₃-H= 0.427 ± 0.0012 and CH₄-J= 0.0385 ± 0.0007 absorption indices and absolute Spitzer [4.5] magnitude of 15.18 ± 0.22 are also indicative of an early-Y dwarf rather than a late-T dwarf. CWISE J105512.11+544328.3 additionally exhibits the bluest Spitzer [3.6]−[4.5] color among all spectroscopically confirmed Y dwarfs. Despite this anomalously blue Spitzer color given its low luminosity, CWISE J105512.11+544328.3 does not show other clear kinematic or spectral indications of low metallicity. Atmospheric model comparisons yield a log(g) ≤ 4.5 andT_eff≈ 500 ± 150 K for this source. We classify CWISE J105512.11+544328.3 as a Y0 (pec) dwarf, adding to the remarkable diversity of the Y-type population. JWST spectroscopy would be crucial to understanding the origin of this Y dwarf’s unusual preference for low-gravity models and blue 3–5μm color.

We present the analysis of two unusually red L dwarfs, CWISE J075554.14−325956.3 (W0755−3259) and CWISE J165909.91−351108.5 (W1659−3511), confirmed by their newly obtained near-infrared spectra collected with the TripleSpec4 spectrograph on the Southern Astrophysical Research Telescope. We classify W0755−3259 as an L7 very low-gravity dwarf, exhibiting extreme redness with a characteristic peakedH-band and spectral indices typical of low-gravity late-type L dwarfs. We classify W1659-3511 as a red L7 field-gravity dwarf, with a more roundedH-band peak and spectral indices that support a normal gravity designation. W1659−3511 is noticeably fainter than W0755−3259, and the roundedH-band of W1659−3511 may be evidence of CH₄absorption.

We present spectroscopic confirmation of a nearby L dwarf pair, CWISE J061741.79+194512.8AB. Keck/NIRES near-infrared spectroscopy shows that the pair is composed of an L2 dwarf primary and an L4 dwarf secondary. High resolution spectroscopy of the combined light system with Keck/NIRSPEC yields a radial velocity of 29.2 ± 0.3 km s⁻¹and a projected rotational velocity$v\sin i$=${41.6}_{-2.6}^{+2.7}$km s⁻¹. Our spectrophotometric distance estimate places the system at 28.2 ± 5.7 pc, significantly more distant than originally estimated in Kirkpatrick et al. The angular separation of the components is 1.″31 ± 0.″14, corresponding to a projected physical separation of 37 ± 8 au.