We present results from a pilot observation of nearby (∼20 Mpc) galaxies with masses similar to that of the Milky Way (MW) to address the missing satellite problem. This is the first paper from an ...ongoing project to address the problem with a statistical sample of galaxies outside of the Local Group (LG) without employing an assumption that the LG is a typical halo in the universe. Thanks to the close distances of our targets, dwarf galaxies around them can be identified as extended, diffuse galaxies. By applying a surface brightness cut together with a careful visual screening to remove artifacts and background contamination, we construct a sample of dwarf galaxies. The luminosity function (LF) of one of the targets is broadly consistent with that of the MW, but the other has a more abundant dwarf population. Numerical simulations by Okamoto seem to overpredict the number of dwarfs on average, while more recent predictions from Copernicus Complexio are in better agreement. In both observations and simulations, there is a large diversity in the LFs, demonstrating the importance of addressing the missing satellite problem with a statistically representative sample. We also characterize the projected spatial distributions of the satellites and do not observe strong evidence for alignments around the central galaxies. Based on this successful pilot observation, we are carrying out further observations to increase the sample of nearby galaxies, which we plan to report in our future paper.
ABSTRACT We report the discovery of a new ultra-faint dwarf satellite companion of the Milky Way (MW) based on the early survey data from the Hyper Suprime-Cam Subaru Strategic Program. This new ...satellite, Virgo I, which is located in the constellation of Virgo, has been identified as a statistically significant (5.5 ) spatial overdensity of star-like objects with a well-defined main sequence and red giant branch in the color-magnitude diagram. The significance of this overdensity increases to 10.8 when the relevant isochrone filter is adopted for the search. Based on the distribution of the stars around the likely main-sequence turnoff at r ∼ 24 mag, the distance to Virgo I is estimated as 87 kpc, and its most likely absolute magnitude calculated from a Monte Carlo analysis is MV = −0.8 0.9 mag. This stellar system has an extended spatial distribution with a half-light radius of pc, which clearly distinguishes it from a globular cluster with comparable luminosity. Thus, Virgo I is one of the faintest dwarf satellites known and is located beyond the reach of the Sloan Digital Sky Survey. This demonstrates the power of this survey program to identify very faint dwarf satellites. This discovery of Virgo I is based only on about 100 square degrees of data, thus a large number of faint dwarf satellites are likely to exist in the outer halo of the MW.
The Progenitor of the Peculiar Galaxy NGC 3077 Okamoto, Sakurako; Arimoto, Nobuo; Ferguson, Annette M. N. ...
The Astrophysical journal,
07/2023, Letnik:
952, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
We present a study of the structural properties and metallicity distribution of the nearby peculiar galaxy NGC 3077. Using data from our survey of the M81 Group with the Hyper Suprime-Cam on ...board the Subaru Telescope, we construct deep color–magnitude diagrams that are used to probe the old red giant branch (RGB) population of NGC 3077. We map these stars out to and beyond the nominal tidal radius, which allows us to derive the structural properties and stellar content of the peripheral regions. We show that NGC 3077 has an extended stellar halo and pronounced “S-shaped” tidal tails that diverge from the radial profile of the inner region. The average metallicity of the old population in NGC 3077 is estimated from individual RGBs to be M/H = −0.98 ± 0.26, which decreases with the distance from the galaxy center as M/H = −0.17 dex
R
h
−
1
. The metallicity of the S-shaped structure is similar to that of the regions lying at
r
∼ 4 ×
R
h
( ∼ 30 kpc), indicating that the stellar constituents of the tidal tails have come from the outer envelope of NGC 3077. These results suggest that this peculiar galaxy was probably a rather normal dwarf elliptical galaxy before the tidal interaction with M81 and M82. We also examine the evidence in our data set for the six recently reported ultrafaint dwarf candidates around NGC 3077. We recover a spatial overdensity of sources coinciding with only one of these.
Abstract
We present the results from a search for new Milky Way (MW) satellites from the first two years of data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) ∼300 deg2 and report ...the discovery of a highly compelling ultra-faint dwarf galaxy candidate in Cetus. This is the second ultra-faint dwarf we have discovered after Virgo I reported in our previous paper. This satellite, Cetus III, has been identified as a statistically significant (10.7 σ) spatial overdensity of star-like objects, which are selected from a relevant isochrone filter designed for a metal-poor and old stellar population. This stellar system is located at a heliocentric distance of 251$^{+24}_{-11}\:$kpc with a most likely absolute magnitude of MV = −2.4 ± 0.6 mag estimated from a Monte Carlo analysis. Cetus III is extended with a half-light radius of $r_h = 90^{+42}_{-17}\:$pc, suggesting that this is a faint dwarf satellite in the MW located beyond the detection limit of the Sloan Digital Sky Survey. Further spectroscopic studies are needed to assess the nature of this stellar system. We also revisit and update the parameters for Virgo I, finding $M_V = -0.33^{+0.75}_{-0.87}\:$mag and $r_h = 47^{+19}_{-13}\:$pc. Using simulations of Λ-dominated cold dark matter models, we predict that we should find one or two new MW satellites from ∼300 deg2 HSC-SSP data, in rough agreement with the discovery rate so far. The further survey and completion of HSC-SSP over ∼1400 deg2 will provide robust insights into the missing satellites problem.
Abstract
We report on the discovery of a new Milky Way (MW) satellite in Boötes based on data from the ongoing Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP). This satellite, named Boötes IV, ...is the third ultra-faint dwarf that we have discovered in the HSC-SSP. We have identified a statistically significant (32.3σ) overdensity of stars with characteristics of a metal-poor, old stellar population. The distance to this stellar system is $D_{\odot }=209^{+20}_{-18}\:$kpc with a V-band absolute magnitude of $M_V=-4.53^{+0.23}_{-0.21}\:$mag. Boötes IV has a half-light radius of $r_{\rm h}=462^{+98}_{-84}\:$pc and an ellipticity of $0.64^{+0.05}_{-0.05}$, which clearly suggests that this is a dwarf satellite galaxy. We also found another overdensity that appears to be a faint globular cluster with $M_V=-0.20^{+0.59}_{-0.83}\:$mag and $r_{\rm h}=5.9^{+1.5}_{-1.3}\:$pc located at $D_{\odot }=46^{+4}_{-4}\:$kpc. Adopting the recent prediction for the total population of satellites in a MW-sized halo by Newton et al. (2018, MNRAS, 479, 2853), which combined the characteristics of the satellites observed by the Sloan Digital Sky Survey and the Dark Energy Survey with the subhalos obtained in ΛCDM models, we estimate that there should be about two MW satellites at MV ≤ 0 in the ∼676 deg2 covered by HSC-SSP, whereas that area includes six satellites (Sextans, Leo IV, Pegasus III, Cetus III, Virgo I, and Boötes IV). Thus, the observed number of satellites is larger than the theoretical prediction. On the face of it, we have a problem of too many satellites, instead of the well-known missing satellites problem whereby the ΛCDM theory overpredicts the number of satellites in a MW-sized halo. This may imply that the models need more refinement for the assignment of subhalos to satellites, such as considering those found by the current deeper survey. More statistically robust constraints on this issue will be brought by further surveys of HSC-SSP over the planned ∼1400 deg2 area.
ABSTRACT
We present the discovery of a giant tidal tail of stars associated with F8D1, the closest known example of an ultra-diffuse galaxy (UDG). F8D1 sits in a region of the sky heavily ...contaminated by Galactic cirrus and has been poorly studied since its discovery two decades ago. The tidal feature was revealed in a deep map of resolved red giant branch stars constructed using data from our Subaru Hyper Suprime-Cam survey of the M81 Group. It has an average surface brightness of μg ∼ 32 mag arcsec−2 and can be traced for over a degree on the sky (60 kpc at the distance of F8D1) with our current imagery. We revisit the main body properties of F8D1 using deep multiband imagery acquired with MegaCam on CFHT and measure effective radii of 1.7–1.9 kpc, central surface brightnesses of 24.7–25.7 mag, and a stellar mass of ∼7 × 107M⊙. Assuming a symmetric feature on the other side of the galaxy, we calculate that 30–36 per cent of F8D1’s present-day luminosity is contained in the tail. We argue that the most likely origin of F8D1’s disruption is a recent close passage to M81, which would have stripped its gas and quenched its star formation. As the only UDG that has so far been studied to such faint surface brightness depths, the unveiling of F8D1’s tidal disruption is important. It leaves open the possibility that many other UDGs could be the result of similar processes, with the most telling signatures of this lurking below current detection limits.
Abstract
We report on the global structure of the Milky Way (MW) stellar halo up to its outer boundary based on the analysis of blue horizontal-branch stars (BHBs). These halo tracers are extracted ...from the (g, r, i, z)-band multi-photometry in the internal data release of the ongoing Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) surveyed over a ∼550 deg2 area. In order to select the most likely BHBs by removing blue straggler stars (BSs) and other contamination in a statistically significant manner, we have developed and applied an extensive Bayesian method, instead of the simple color cuts adopted in our previous work, where each of the template BHBs and non-BHBs obtained from the available catalogs are represented as a mixture of multiple Gaussian distributions in the color–color diagrams. We found from the candidate BHBs in the range of 18.5 < g < 23.5 mag that the radial density distribution over a Galactocentric radius of r = 36–360 kpc can be approximated as a single power-law profile with an index of $\alpha =3.74^{+0.21}_{-0.22}$ or a broken power-law profile with an index of $\alpha _{\rm in}=2.92^{+0.33}_{-0.33}$ at r below a broken radius of $r_{\rm b}=160^{+18}_{-19}\:$kpc and a very steep slope of $\alpha _{\rm out}=15.0^{+3.7}_{-4.5}$ at r > rb. The latter profile with a prolate shape having an axial ratio of $q=1.72^{+0.44}_{-0.28}$ is most likely and this halo may hold a rather sharp boundary at r ≃ 160 kpc. The slopes of the halo density profiles are compared with those from the suite of hydrodynamical simulations for the formation of stellar halos. This comparison suggests that the MW stellar halo may consist of the two overlapping components: the in situ inner halo as probed by RR Lyrae stars showing a relatively steep radial density profile and the ex situ outer halo with a shallow profile probed by BHBs here, which is made by accretion of small stellar systems.
Abstract We present the discovery of NGC253-SNFC-dw1, a new satellite galaxy in the remote stellar halo of the Sculptor Group spiral, NGC 253. The system was revealed using deep, resolved star ...photometry obtained as part of the Subaru Near-Field Cosmology Survey that uses the Hyper Suprime-Cam on the Subaru Telescope. Although rather luminous ( M V = −11.7 ± 0.2) and massive ( M * ∼ 1.25 × 10 7 M ⊙ ), the system is one of the most diffuse satellites yet known, with a half-light radius of R h = 3.37 ± 0.36 kpc and an average surface brightness of ∼30.1 mag arcmin −2 within the R h . The color–magnitude diagram shows a dominant, old (∼10 Gyr), and metal-poor (M/H = −1.5 ± 0.1 dex) stellar population, as well as several candidate thermally pulsing asymptotic giant branch stars. The distribution of red giant branch stars is asymmetrical and displays two elongated tidal extensions pointing toward NGC 253, suggestive of a highly disrupted system being observed at apocenter. NGC253-SNFC-dw1 has a size comparable to that of the puzzling Local Group dwarfs Andromeda XIX and Antlia 2 but is 2 magnitudes brighter. While unambiguous evidence of tidal disruption in these systems has not yet been demonstrated, the morphology of NGC253-SNFC-dw1 clearly shows that this is a natural path to produce such diffuse and extended galaxies. The surprising discovery of this system in a previously well-searched region of the sky emphasizes the importance of surface-brightness limiting depth in satellite searches.
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