We study the stellar halo colour properties of six nearby massive highly inclined disc galaxies using Hubble space telescope Advanced Camera for Surveys and Wide Field Camera 3 observations in both ...F606W and F814W filters from the GHOSTS (Galaxy Halos, Outer disks, Substructure, Thick disks, and Star clusters) survey. The observed fields probe the stellar outskirts out to projected distances of ∼50–70 kpc from their galactic centre along the minor axis. The 50 per cent completeness levels of the colour–magnitude diagrams are typically at 2 mag below the tip of the red giant branch (RGB). We find that all galaxies have extended stellar haloes out to ∼50 kpc and two out to ∼70 kpc. We determined the halo colour distribution and colour profile for each galaxy using the median colours of stars in the RGB. Within each galaxy, we find variations in the median colours as a function of radius which likely indicates population variations, reflecting that their outskirts were built from several small accreted objects. We find that half of the galaxies (NGC 0891, NGC 4565, and NGC 7814) present a clear negative colour gradient in their haloes, reflecting a declining metallicity; the other have no significant colour or population gradient. In addition, notwithstanding the modest sample size of galaxies, there is no strong correlation between their halo colour/metallicity or gradient with galaxy's properties such as rotational velocity or stellar mass. The diversity in halo colour profiles observed in the GHOSTS galaxies qualitatively supports the predicted galaxy-to-galaxy scatter in halo stellar properties, a consequence of the stochasticity inherent in the assembling history of galaxies.
Recent work has shown that Milky Way-mass galaxies display an incredible range of stellar halo properties, yet the origin of this diversity is unclear. The nearby galaxy M81-currently interacting ...with M82 and NGC 3077-sheds unique light on this problem. We present a Subaru Hyper Suprime-Cam survey of the resolved stellar populations around M81, revealing M81's stellar halo in never-before-seen detail. We resolve the halo to unprecedented V-band equivalent surface brightnesses of 33 mag arcsec −2 and produce the first-ever global stellar mass density map for a Milky Way-mass stellar halo outside of the Local Group. Using the minor axis, we confirm M81's halo as one of the lowest mass and metal poorest known (M 1.16 × 109M , Fe/H −1.2)-indicating a relatively quiet prior accretion history. Yet, our global halo census finds that tidally unbound material from M82 and NGC 3077 provides a substantial infusion of metal-rich material (M 5.4 × 108 M , Fe/H −0.9). We further show that, following the accretion of its massive satellite M82 (and the LMC-like NGC 3077), M81 will host one of the most massive and metal-rich stellar halos in the nearby universe. Thus, the saga of M81: following a passive history, M81's merger with M82 will completely transform its halo from a low-mass, anemic halo rivaling the Milky Way, to a metal-rich behemoth rivaled only by systems such as M31. This dramatic transformation indicates that the observed diversity in stellar halo properties is primarily driven by diversity in the largest mergers these galaxies have experienced.
We have examined the resolved stellar populations at large galactocentric distances along the minor axis (from 10 kpc up to between 40 and 75 kpc), with limited major axis coverage, of six nearby ...highly inclined Milky Way (MW) mass disc galaxies using Hubble Space Telescope data from the Galaxy haloes, Outer discs, Substructure, Thick discs, and Star clusters (GHOSTS) survey. We select red giant branch stars to derive stellar halo density profiles. The projected minor axis density profiles can be approximated by power laws with projected slopes of -2 to -3.7 and a diversity of stellar halo masses of 1-6 x 10 super( 9) M..., or 2-14 per cent of the total galaxy stellar masses. The typical intrinsic scatter around a smooth power-law fit is 0.05-0.1 dex owing to substructure. By comparing the minor and major axis profiles, we infer projected axis ratios c/a at ~25 kpc between 0.4 and 0.75. The GHOSTS stellar haloes are diverse, lying between the extremes charted out by the (rather atypical) haloes of the MW and M31. We find a strong correlation between the stellar halo metallicities and the stellar halo masses. We compare our results with cosmological models, finding good agreement between our observations and accretion-only models where the stellar haloes are formed by the disruption of dwarf satellites. In particular, the strong observed correlation between stellar halo metallicity and mass is naturally reproduced. Low-resolution hydrodynamical models have unrealistically high stellar halo masses. Current high-resolution hydrodynamical models appear to predict stellar halo masses somewhat higher than observed but with reasonable metallicities, metallicity gradients, and density profiles. (ProQuest: ... denotes formulae/symbols omitted.)
Galaxies with Milky Way-like stellar masses have a wide range of bulge and black hole masses; in turn, these correlate with other properties such as star formation history. While many processes may ...drive bulge formation, major and minor mergers are expected to play a crucial role. Stellar halos offer a novel and robust measurement of galactic merger history; cosmologically motivated models predict that mergers with larger satellites produce more massive, higher-metallicity stellar halos, reproducing the recently observed stellar halo metallicity-mass relation. We quantify the relationship between stellar halo mass and bulge or black hole prominence using a sample of 18 Milky Way-mass galaxies with newly available measurements of (or limits on) stellar halo properties. There is an order of magnitude range in bulge mass, and two orders of magnitude in black hole mass, at a given stellar halo mass (or, equivalently, merger history). Galaxies with low-mass bulges show a wide range of quiet merger histories, implying formation mechanisms that do not require intense merging activity. Galaxies with massive "classical" bulges and central black holes also show a wide range of merger histories. While three of these galaxies have massive stellar halos consistent with a merger origin, two do not-merging appears to have had little impact on making these two massive "classical" bulges. Such galaxies may be ideal laboratories to study massive bulge formation through pathways such as early gas-rich accretion, violent disk instabilities, or misaligned infall of gas throughout cosmic time.
New globular cluster candidates in the M81 group Pan, Jiaming; Bell, Eric F; Smercina, Adam ...
Monthly notices of the Royal Astronomical Society,
07/2022, Letnik:
515, Številka:
1
Journal Article
Recenzirano
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ABSTRACT
The study of outer halo globular cluster (GC) populations can give insight into galaxy merging, GC accretion, and the origin of GCs. We use archival Subaru Hyper Suprime-Cam (HSC) data in ...concert with space-based GALEX, IRAC, and Gaia EDR3 data to select candidate GCs in the outer halo of the M81 group for confirmation and future study. We use a small sample of previously discovered GCs to tune our selection criteria, finding that bright already-known GCs in the M81 group have sizes that are typically slightly larger than the Subaru PSF in our fields. In the optical bands, GCs appear to have colours that are only slightly different from stars. The inclusion of archival IRAC data yields dramatic improvements in colour separation, as the long wavelength baseline aids somewhat in the separation from stars and clearly separates GCs from many compact background galaxies. We show that some previously spectroscopically identified GCs in the M81 group are instead foreground stars or background galaxies. GCs close to M82 have radial velocities, suggesting that they fell into the M81 group along with M82. The overall M81 GC luminosity function is similar to the Milky Way and M31. M81’s outer halo GCs are similar to the Milky Way in their metallicities and numbers, and much less numerous than M31’s more metal-rich outer halo GC population. These properties reflect differences in the three galaxies’ merger histories, highlighting the possibility of using outer halo GCs to trace merger history in larger samples of galaxies.
ABSTRACT
Chemical abundances are an essential tool in untangling the Milky Way’s enrichment history. However, the evolution of the interstellar medium abundance gradient with cosmic time is lost as a ...result of radial mixing processes. For the first time, we quantify the evolution of many observational abundances across the Galactic disc as a function of lookback time and birth radius, $\rm \text{R}_\text{birth}$. Using an empirical approach, we derive $\rm \text{R}_\text{birth}$ estimates for 145 447 APOGEE DR17 red giant disc stars, based solely on their ages and $\rm Fe/H$. We explore the detailed evolution of six abundances Mg, Ca (α), Mn (iron-peak), Al, C (light), Ce (s-process) across the Milky Way disc using 87 426 APOGEE DR17 red giant stars. We discover that the interstellar medium had three fluctuations in the metallicity gradient ∼9, ∼6, and ∼4 Gyr ago. The first coincides with the end of high-α sequence formation around the time of the Gaia–Sausage–Enceladus disruption, while the others are likely related to passages of the Sagittarius dwarf galaxy. A clear distinction is found between present-day observed radial gradients with age and the evolution with lookback time for both X/Fe and X/H, resulting from the significant flattening and inversion in old populations due to radial migration. We find the $\rm Fe/H$–$\rm \alpha /Fe$ bimodality is also seen as a separation in the $\rm \text{R}_\text{birth}$–$\rm X/Fe$ plane for the light and α-elements. Our results recover the chemical enrichment of the Galactic disc over the past 12 Gyr, providing tight constraints on Galactic disc chemical evolution models.
Galaxy masses Courteau, Stéphane; Cappellari, Michele; de Jong, Roelof S. ...
Reviews of modern physics,
01/2014, Letnik:
86, Številka:
1
Journal Article
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Information on the variety of galaxy masses is essential to understand the structure formation in the early Universe and the processes which contributed. Masses of galaxies (and their constituents ...such as stars, gas, and dark matter) are key properties for their evolution. This review discusses the various mass estimators by giving overviews on how to identify the contribution from stellar masses by utilizing the total light output, how to determine the total dynamical masses for gas-rich and gas-poor galaxies, how to utilize weak and strong gravitational lensing, and presents a detailed analysis of the Milky Way as well. Galaxy masses play a fundamental role in our understanding of structure formation models. This review addresses the variety and reliability of mass estimators that pertain to stars, gas, and dark matter. The different sections on masses from stellar populations, dynamical masses of gas-rich and gas-poor galaxies, with some attention paid to our Milky Way, and masses from weak and strong lensing methods all provide review material on galaxy masses in a self-consistent manner.
Abstract
M64, often called the “Evil Eye” galaxy, is unique among local galaxies. Beyond its dramatic, dusty nucleus, it also hosts an outer gas disk that counter-rotates relative to its stars. The ...mass of this outer disk is comparable to the gas content of the Small Magellanic Cloud (SMC), prompting the idea that it was likely accreted in a recent minor merger. Yet, detailed follow-up studies of M64's outer disk have shown no evidence of such an event, leading to other interpretations, such as a “flyby” interaction with the distant diffuse satellite Coma P. We present Subaru Hyper Suprime-Cam observations of M64's stellar halo, which resolve its stellar populations and reveal a spectacular radial shell feature, oriented ∼30° relative to the major axis and along the rotation axis of the outer gas disk. The shell is ∼45 kpc southeast of M64, while a similar but more diffuse plume to the northwest extends to >100 kpc. We estimate a stellar mass and metallicity for the southern shell of
M
⋆
= 1.80 ± 0.54 × 10
8
M
⊙
and M/H = −1.0, respectively, and a similar mass of 1.42 ± 0.71 × 10
8
M
⊙
for the northern plume. Taking into account the accreted material in M64's inner disk, we estimate a total stellar mass for the progenitor satellite of
M
⋆,prog
≃ 5 × 10
8
M
⊙
. These results suggest that M64 is in the final stages of a minor merger with a gas-rich satellite strikingly similar to the SMC, in which M64's accreted counter-rotating gas originated, and which is responsible for the formation of its dusty inner star-forming disk.
Abstract
It is not yet settled how the combination of secular processes and merging gives rise to the bulges and pseudobulges of galaxies. The nearby (
D
∼ 4.2 Mpc) disk galaxy M94 (NGC 4736) has the ...largest pseudobulge in the local universe, and offers a unique opportunity for investigating the role of merging in the formation of its pseudobulge. We present a first ever look at M94's stellar halo, which we expect to contain a fossil record of M94's past mergers. Using Subaru's Hyper Suprime-Cam, we resolve and identify red giant branch (RGB) stars in M94's halo, finding two distinct populations. After correcting for completeness through artificial star tests, we can measure the radial profile of each RGB population. The metal-rich RGB stars show an unbroken exponential profile to a radius of 30 kpc that is a clear continuation of M94's outer disk. M94's metal-poor stellar halo is detectable over a wider area and clearly separates from its metal-rich disk. By integrating the halo density profile, we infer a total accreted stellar mass of ∼2.8 × 10
8
M
⊙
, with a median metallicity of M/H = −1.4. This indicates that M94's most-massive past merger was with a galaxy similar to, or less massive than, the Small Magellanic Cloud. Few nearby galaxies have had such a low-mass dominant merger; therefore we suggest that M94's pseudobulge was not significantly impacted by merging.
Measuring the integrated stellar halo light around galaxies is very challenging. The surface brightness of these haloes is expected to be many magnitudes below dark sky and the central brightness of ...the galaxy. Here, I show that in some of the recent literature the effect of very extended Point Spread Function (PSF) tails on the measurements of halo light has been underestimated; especially in the case of edge-on disc galaxies. The detection of a halo along the minor axis of an edge-on galaxy in the Hubble Ultra Deep Field can largely be explained by scattered galaxy light. Similarly, depending on filter and the shape one assumes for the uncertain extended PSF, 20–80 per cent of the halo light found along the minor axis of scaled and stacked Sloan Digital Sky Survey (SDSS) edge-on galaxy images can be explained by scattered galaxy light. Scattered light also significantly contributes to the anomalous halo colours of stacked SDSS images. The scattered light fraction decreases when looking in the quadrants away from the minor axis. The remaining excess light is well modelled with a Sérsic profile halo with shape parameters based on star count halo detections of nearby galaxies. Even though, the contribution from PSF scattered light does not fully remove the need for extended components around these edge-on galaxies, it will be very challenging to make accurate halo light shape and colour measurements from integrated light without very careful PSF measurements and scattered light modelling.
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