Author(s): Huang, L; Dey, A; Schlegel, DJ; Lang, D; Blum, R; Burleigh, K; Fan, X; Findlay, JR; Finkbeiner, D; Herrera, D; Juneau, S; Landriau, M; Levi, M; McGreer, I; Meisner, A; Myers, AD; Moustakas, J; Nugent, P; Patej, A; Schlafly, EF; Walker, AR; Valdes, F; Weaver, BA; Yeche, C; Zou, H; Zhou, X; Abareshi, B; Abbott, TMC; Abolfathi, B; Aguilera, C; Alam, S; Allen, L; Alvarez, A; Annis, J; Ansarinejad, B; Aubert, M; Beechert, J; Bell, EF; Benzvi, SY; Beutler, F; Bielby, RM; Bolton, AS; Briceno, C; Buckley-Geer, EJ; Butler, K; Calamida, A; Carlberg, RG; Carter, P; Casas, R; Castander, FJ; Choi, Y; Comparat, J; Cukanovaite, E; Delubac, T; Devries, K; Dey, S; Dhungana, G; Dickinson, M; Ding, Z; Donaldson, JB; Duan, Y; Duckworth, CJ; Eftekharzadeh, S; Eisenstein, DJ; Etourneau, T; Fagrelius, PA; Farihi, J; Fitzpatrick, M; Font-Ribera, A; Fulmer, L; Gansicke, BT; Gaztanaga, E; George, K; Gerdes, DW; A Gontcho, SG; Gorgoni, C; Green, G; Guy, J; Harmer, D; Hernandez, M; Honscheid, K; Huang, LW; James, DJ; Jannuzi, BT; Jiang, L | Abstract: © 2019. The American Astronomical Society. All rights reserved.. The DESI Legacy Imaging Surveys (http://legacysurvey.org/) are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image ≈14,000 deg2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12, and 22 μm) observed by the Wide-field Infrared Survey Explorer satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project.

Overview of the DESI Legacy Imaging Surveys

An order of magnitude more dwarf galaxies are expected to inhabit the Local Group, based on currently accepted galaxy formation models, than have been observed. This discrepancy has been noted in environments ranging from the field to rich clusters. However, no complete census of dwarf galaxies exist in any environment. The discovery of the smallest and faintest dwarfs is hampered by the limitations in detecting such faint and low surface brightness galaxies. An even greater difficulty is establishing distances to or group/cluster membership for such faint galaxies. The M81 group provides an almost unique opportunity for establishing membership for galaxies in a low density region complete to magnitudes as faint as M_{r'} = -10. With a distance modulus of 27.8, the tip of the red giant branch just resolves in ground-based surveys. We have surveyed a 65 square degree region around M81 with the CFHT/MegaCam. From these images we have detected 22 new dwarf galaxy candidates. Photometric, morphological, and structural properties are presented for the candidates. The group luminosity function has a faint end slope characterized by the parameter alpha = -1.27+/-0.06. We discuss implications of this dwarf galaxy population on cosmological models.

Discovery of New Dwarf Galaxies in the M81 Group

The M81 Group is one of the nearest groups of galaxies, but its properties are quite different from those of the Local Group. It has therefore provided a different environment for the evolution of its member galaxies. We have carried out a CCD survey of the M81 Group to search for analogs to Local Group dwarf elliptical (dE) galaxies. All the M81 dwarfs previously identified in photographic surveys were recovered, and we also discovered several new systems whose surface brightnesses fall within the range found for Local Group dE's. We have obtained Hubble Space Telescope Wide Field Planetary Camera 2 images through the F555W and F814W filters of two M81 Group dE's, BK5N and a new system, designated F8D1. The resulting color-magnitude diagrams show the upper 2 mag of the red giant branch. The I magnitudes of the red giant branch tip in both galaxies yield distances that are consistent with membership in the M81 Group. Surface brightness and total magnitude measurements indicate that BK5N and F8D1 have similar central surface brightnesses (24.5 and 25.4 mag arcsec-2 in V, respectively), but F8D1's larger length scale results in its being 3 mag more luminous than BK5N. BK5N lies on the relation between central surface brightness and absolute magnitude defined by Local Group dwarf elliptical galaxies, but F8D1 does not. F8D1 is more luminous for its central surface brightness than the relation predicts, similar to the large low surface brightness dwarf galaxies found in, for example, Virgo. The mean color of the giant branch is used to establish the mean abundance of each galaxy. F8D1, the more luminous galaxy, is significantly more metal-rich ([Fe/H] ≈ -1.0) than BK5N ([Fe/H] ≈ -1.7). Both BK5N and F8D1 lie on the relation between absolute magnitude and metal abundance defined by Local Group dwarf elliptical galaxies. However, as regards the relation between central surface brightness and metal abundance, BK5N again follows the Local Group dwarfs while F8D1 deviates significantly from this relation. This suggests that the total amount of luminous matter is more fundamental in controlling metal enrichment than the surface density of luminous matter. We have also used the color width of the giant branch compared with the photometric errors to establish abundance ranges in both galaxies, the sizes of which are comparable to those in Local Group dE's. From the numbers and luminosities of asymptotic giant branch stars more luminous than the red giant branch tip, we infer that, again like many of the Local Group dE's, both BK5N and F8D1 have had extended epochs of star formation. F8D1 contains stars as young as 3–4 Gyr, while in BK5N stars as young as ~8 Gyr are present. The fractions of intermediate-age population, at ~30%, are similar in both galaxies. Further, one globular cluster has been found in F8D1, but none are present in BK5N. These numbers of clusters are consistent with our expectations from Local Group dwarfs. Overall, we find that BK5N is similar in all respects to the Local Group dE's. Thus, despite the different environments of the M81 Group and the Local Group, dwarf galaxies of very similar properties are present in each. F8D1, on the other hand, has no Local Group counterpart and is indeed rare even in large clusters of galaxies. Our study of its stellar population is the first for a large, low surface brightness dwarf.

Dwarf Elliptical Galaxies in the M81 Group: The Structure and Stellar Populations of BK5N and F8D1

Fast radio bursts (FRBs) are flashes of unknown physical origin1. The majority of FRBs have been seen only once, although some are known to generate multiple flashes2,3. Many models invoke magnetically powered neutron stars (magnetars) as the source of the emission4,5. Recently, the discovery6 of another repeater (FRB 20200120E) was announced, in the direction of the nearby galaxy M81, with four potential counterparts at other wavelengths6. Here we report observations that localized the FRB to a globular cluster associated with M81, where it is 2 parsecs away from the optical centre of the cluster. Globular clusters host old stellar populations, challenging FRB models that invoke young magnetars formed in a core-collapse supernova. We propose instead that FRB 20200120E originates from a highly magnetized neutron star formed either through the accretion-induced collapse of a white dwarf, or the merger of compact stars in a binary system7. Compact binaries are efficiently formed inside globular clusters, so a model invoking them could also be responsible for the observed bursts. 

A repeating fast radio burst source in a globular cluster

Since the discovery of the first fast radio burst (FRB) in 2007, and their confirmation as an abundant extragalactic population in 2013, the study of these sources has expanded at an incredible rate. In our 2019 review on the subject we presented a growing, but still mysterious, population of FRBs -- 60 unique sources, 2 repeating FRBs, and only 1 identified host galaxy. However, in only a few short years new observations and discoveries have given us a wealth of information about these sources. The total FRB population now stands at over 600 published sources, 24 repeaters, and 19 host galaxies. Higher time resolution data, sustained monitoring, and precision localisations have given us insight into repeaters, host galaxies, burst morphology, source activity, progenitor models, and the use of FRBs as cosmological probes. The recent detection of a bright FRB-like burst from the Galactic magnetar SGR~1935+2154 provides an important link between FRBs and magnetars. There also continue to be surprising discoveries, like periodic modulation of activity from repeaters and the localisation of one FRB source to a relatively nearby globular cluster associated with the M81 galaxy. In this review, we summarise the exciting observational results from the past few years. We also highlight their impact on our understanding of the FRB population and proposed progenitor models. We build on the introduction to FRBs in our earlier review, update our readers on recent results, and discuss interesting avenues for exploration as the field enters a new regime where hundreds to thousands of new FRBs will be discovered and reported each year. 

/pdf/fast-radio-bursts-at-the-dawn-of-the-2020s-17y5ac4m.pdf

Fast radio bursts at the dawn of the 2020s

We report on the discovery of FRB 20200120E, a repeating fast radio burst (FRB) with a low dispersion measure (DM) detected by the Canadian Hydrogen Intensity Mapping Experiment FRB project. The source DM of 87.82 pc cm−3 is the lowest recorded from an FRB to date, yet it is significantly higher than the maximum expected from the Milky Way interstellar medium in this direction (∼50 pc cm−3). We have detected three bursts and one candidate burst from the source over the period 2020 January–November. The baseband voltage data for the event on 2020 January 20 enabled a sky localization of the source to within ≃14 arcmin2 (90% confidence). The FRB localization is close to M81, a spiral galaxy at a distance of 3.6 Mpc. The FRB appears on the outskirts of M81 (projected offset ∼20 kpc) but well inside its extended H i and thick disks. We empirically estimate the probability of a chance coincidence with M81 to be <10−2. However, we cannot reject a Milky Way halo origin for the FRB. Within the FRB localization region, we find several interesting cataloged M81 sources and a radio point source detected in the Very Large Array Sky Survey. We search for prompt X-ray counterparts in Swift Burst Alert Telescope and Fermi/GBM data, and, for two of the FRB 20200120E bursts, we rule out coincident SGR 1806−20-like X-ray bursts. Due to the proximity of FRB 20200120E, future follow-up for prompt multiwavelength counterparts and subarcsecond localization could be constraining of proposed FRB models.

M. Rafiei-Ravandi

Papers

A Nearby Repeating Fast Radio Burst in the Direction of M81