The Milky Way’s middle-aged inner ring Wylie, Shola M.; Clarke, Jonathan P.; Gerhard, Ortwin E.
Astronomy and astrophysics (Berlin),
03/2022, Volume:
659
Journal Article
Peer reviewed
Open access
Aims.
We investigate the metallicity, age, and orbital anatomy of the inner Milky Way, specifically focussing on the outer bar region.
Methods.
We integrated a sample of
APOGEE
DR16 inner Galaxy ...stars in a state of the art bar-bulge potential with a slow pattern speed and investigated the link between the resulting orbits and their Fe/H and ages. By superimposing the orbits, we built density, Fe/H, and age maps of the inner Milky Way, which we divided further using the orbital parameters eccentricity, |
X
max
|, and |
Z
max
|.
Results.
We find that at low heights from the Galactic plane, the Galactic bar gradually transitions into a radially thick, vertically thin, elongated inner ring with average solar Fe/H. This inner ring is mainly composed of stars with AstroNN ages between 4 and 9 Gyr with a peak in age between 6 and 8 Gyr, making the average age of the ring ∼6 Gyr. The vertical thickness of the ring decreases markedly towards younger ages. We also find very large L4 Lagrange orbits that have average solar to super-solar metallicities and intermediate ages. Lastly, we confirm a clear X-shape in the Fe/H and density distributions at large Galactic heights.
Conclusions.
The orbital structure obtained for the
APOGEE
stars reveals that the Milky Way hosts an inner ring-like structure between the planar bar and corotation. This structure is on average metal rich, intermediately aged, and enhances the horizontal metallicity gradient along the bar’s major axis.
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Abstract
In our companion paper (Brought to Light I: Michea et al.), we reveal spectacular spiral-galaxy-like features in deep optical imaging of nine Virgo early-type dwarf galaxies, hidden beneath ...a dominating smooth stellar disk. Using a new combination of approaches, we find that bar- and spiral-like features contribute 2.2%–6.4% of the total flux within 2
R
eff
. In this study, we conduct high-resolution simulations of cluster harassment of passive dwarf galaxies. Following close pericenter passages of the cluster core, tidal triggering generates features in our model disks that bear a striking resemblance to the observed features. However, we find the disks must be highly rotationally supported (
V
peak
/
σ
0
∼ 3), much higher than typically observed. We propose that some early-type dwarfs may contain a few percent of their mass in a cold, thin disk that is buried in the light of a hot, diffuse disk and only revealed when they undergo tidal triggering. The red optical colors of our sample do not indicate any recent significant star formation, and our simulations show that very plunging pericenter passages (
r
peri
< 0.25
r
vir
) are required for tidal triggering. Thus, many cluster early-type dwarfs with less-plunging orbits may host a yet-undetected cold stellar disk component. We discuss possible origin scenarios and consider why similar-mass star-forming galaxies in the field are significantly more thin-disk dominated than in our cluster sample.
Abstract
Rejuvenating galaxies are unusual galaxies that fully quench and then subsequently experience a “rejuvenation” event to become star-forming once more. Rejuvenation rates vary substantially ...in models of galaxy formation: 10%–70% of massive galaxies are expected to experience rejuvenation by
z
= 0. Measuring the rate of rejuvenation is therefore important for calibrating the strength of star-formation feedback mechanisms. However, these observations are challenging because rejuvenating systems blend in with normal star-forming galaxies in broadband photometry. In this paper, we use the galaxy spectral energy distribution-fitting code Prospector to search for observational markers that distinguish normal star-forming galaxies from rejuvenating galaxies. We find that rejuvenating galaxies have smaller Balmer absorption line equivalent widths (EWs) than normal star-forming galaxies. This is analogous to the well-known “K+A” or post-starburst galaxies, which have strong Balmer absorption due to A-stars dominating the light: in this case, rejuvenating systems have a lack of A-stars, instead resembling “O—A” systems. We find star-forming galaxies that have H
β
, H
γ
, and/or H
δ
absorption EWs ≲3 Å corresponds to a highly pure selection of rejuvenating systems. Interestingly, while this technique is highly effective at identifying mild rejuvenation, “strongly” rejuvenating systems remain nearly indistinguishable from star-forming galaxies due to the well-known stellar outshining effect. We conclude that measuring Balmer absorption line EWs in star-forming galaxy populations is an efficient method to identify rejuvenating populations, and discuss several techniques to either remove or resolve the nebular emission which typically lies on top of these absorption lines.
Context. The rotation curve, the total mass and the gravitational potential of the Galaxy are sensitive measurements of the dark matter halo profile. Aims. Cuspy and cored DM halo profiles are ...analysed with respect to recent astronomical constraints in order to constrain the shape of the Galactic DM halo and the local DM density. Methods. All Galactic density components (luminous matter and DM) are parametrized. Then the total density distribution is constrained by astronomical observations: 1) the total mass of the Galaxy, 2) the total matter density at the position of the Sun, 3) the surface density of the visible matter, 4) the surface density of the total matter in the vicinity of the Sun, 5) the rotation speed of the Sun and 6) the shape of the velocity distribution within and above the Galactic disc. The mass model of the Galaxy is mainly constrained by the local matter density (Oort limit), the rotation speed of the Sun and the total mass of the Galaxy from tracer stars in the halo. Results. We showed from a statistical $\chi^2$ fit to all data that the local DM density is strongly positively (negatively) correlated with the scale length of the DM halo (baryonic disc). Since these scale lengths are poorly constrained the local DM density can vary from 0.2 to 0.4 GeV cm-3 ($0.005{-}0.01~M_\odot$ pc-3) for a spherical DM halo profile and allowing total Galaxy masses up to $2 \times 10^{\mathrm{12}}~M_\odot$. For oblate DM haloes and dark matter discs, as predicted in recent N-body simulations, the local DM density can be increased significantly.
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Abstract
We study the mass–size relation of quiescent galaxies across various environments, with a particular focus on its environmental dependence at the low-mass part of
log
(
M
star
/
M
⊙
)
≲
10.0
.... Our sample consists of 13,667 quiescent galaxies with
log
(
M
star
/
M
⊙
)
≥
9.4
and 0.01 <
z
< 0.04 from the Sloan Digital Sky Survey. We discover that the mass–size relation of low-mass quiescent galaxies (LQGs) with
log
(
M
star
/
M
⊙
)
≲
10.0
depends on their environment, with LQGs in the highest-density environments exhibiting an average size ∼70% larger than those in isolated environments. Moreover, the slope of the mass–size relation for LQGs in high-density environments is significantly shallower than that of their counterparts in isolated environments. This is in contrast with high-mass quiescent galaxies with
log
(
M
star
/
M
⊙
)
≳
10.5
that show a nearly identical mass–size relation across all environments. Combined with additional discoveries that the mass–size relation slopes of LQGs and star-forming galaxies are similar to each other in high-density environments, and that LQGs in higher-density environments exhibit more disk-like structures, our results support the idea that LQGs in high-density environments have evolved from star-forming galaxies through environmental effects, which are capable of causing their quenching and transformation into quiescent galaxies. With the aid of an analysis of merger rates for simulated galaxies from a cosmological galaxy formation simulation, we suggest that the steep slope and low normalization of the mass–size relation of LQGs in the lowest-density environments may originate from recent gas-rich mergers, which occur over 10–30 times more frequently in the progenitors of LQGs in the lowest-density environments than in their counterparts in high-density environments at low redshifts.
Abstract
To break the degeneracy among galactic stellar components, we extract kinematic structures using the framework that was described in Du et al. For example, the concept of stellar halos is ...generalized to weakly rotating structures that are composed of loosely bound stars, which can hence be associated to both disk and elliptical type morphologies. By applying this method to central galaxies with stellar mass 10
10−11.5
M
⊙
from the TNG50 simulation, we identify three broadly-defined types of galaxies: galaxies dominated by disk, by bulge, or by stellar halo structures. We then use the simulation to infer the underlying connection between the growth of structures and physical processes over cosmic time. By tracing galaxies back in time, we recognize three fundamental regimes: an early phase of evolution (
z
≳ 2), and internal and external (mainly mergers) processes that act at later times. We find that disk- and bulge-dominated galaxies are not significantly affected by mergers since
z
∼ 2. The difference in their present-day structures originates from two distinct evolutionary pathways—extended versus compact—that are likely to be determined by their parent dark matter halos (i.e., nature). In contrast, slow rotator elliptical galaxies are typically halo-dominated, forming by external processes (e.g., mergers) in the later phase (i.e., nurture). This picture challenges the general idea that elliptical galaxies are the same objects as classical bulges. In observations, both bulge- and halo-dominated galaxies are likely to be classified as early-type galaxies with compact morphology and quiescent star formation. However, here we find them to have very different evolutionary histories.
In the first paper of this series (Paper I) we presented a new approach for studying the chemodynamical evolution in disk galaxies, focusing on the Milky Way. While in Paper I we studied extensively ...the Solar vicinity, here we extend these results to different distances from the Galactic center, looking for variations of observables that can be related to on-going and future spectroscopic surveys. By separating the effects of kinematic heating and radial migration, we show that migration is much more important, even for the oldest and hottest stellar population. The distributions of stellar birth guiding radii and final guiding radii (signifying contamination from migration and heating, respectively) widen with increasing distance from the Galactic center. We show that radial migration cannot compete with the inside-out formation of the disk, exposed by the more centrally concentrated older disk populations, and consistent with recent observations.
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Growing interest in the chemical feature of r-process elements among nearby disk stars represented by the Eu/Fe versus Fe/H diagram has sprouted because it can assess the origin of r-process elements ...through comparison with theoretical models, including a test of whether neutron star mergers can be major sites of r-process nucleosynthesis. On the other hand, recent studies have revealed that local chemistry is strongly coupled with the dynamics of the Galactic disk, which predicts that stars radially move on the disk where the observed elemental feature is different at various Galactocentric distances. Here, we show that radial migration of stars across the Galactic disk plays a crucial role in shaping the r-process abundance feature in the solar vicinity. In this proposed scenario, we highlight the importance of migration from the outer disk where r-process/Fe of some old stars is predicted to be enhanced to a level beyond the expectation of the observed Galactic Fe and Eu radial gradient, which results in a large span of r-process/Fe among nearby disk stars. The variation in the r-process/Fe ratio seen across the Galactic disk as well as in dwarf galaxies may be an outcome of different stellar initial mass functions, which change the occurrence frequency between supernovae, leaving behind neutron stars and ones ending with black holes. Here we propose that enhancement in Eu/Fe is attributed to the initial mass function lacking high-mass stars such as 25 M in the scheme for which neutron star mergers are a major source of r-process elements.
We provide a coherent, uniform measurement of the evolution of the logarithmic star formation rate (SFR)-stellar mass (M*) relation, called the main sequence (MS) of star-forming galaxies , for ...star-forming and all galaxies out to . We measure the MS using mean stacks of 3 GHz radio-continuum images to derive average SFRs for ∼ 200,000 mass-selected galaxies at z > 0.3 in the COSMOS field. We describe the MS relation by adopting a new model that incorporates a linear relation at low stellar mass (log(M*/M ) < 10) and a flattening at high stellar mass that becomes more prominent at low redshift (z < 1.5). We find that the SFR density peaks at 1.5 < z < 2, and at each epoch there is a characteristic stellar mass (M* = 1-4 × 1010M ) that contributes the most to the overall SFR density. This characteristic mass increases with redshift, at least to z ∼ 2.5. We find no significant evidence for variations in the MS relation for galaxies in different environments traced by the galaxy number density at 0.3 < z < 3, nor for galaxies in X-ray groups at z ∼ 0.75. We confirm that massive bulge-dominated galaxies have lower SFRs than disk-dominated galaxies at a fixed stellar mass at z < 1.2. As a consequence, the increase in bulge-dominated galaxies in the local star-forming population leads to a flattening of the MS at high stellar masses. This indicates that "mass quenching" is linked with changes in the morphological composition of galaxies at a fixed stellar mass.
Abstract
Deep rest-optical observations are required to accurately constrain the stellar populations of
z
∼ 8 galaxies. Due to significant limitations in the availability of such data for ...statistically complete samples, observational results have been limited to modest numbers of bright or lensed sources. To revolutionize the present characterization of
z
∼ 8 galaxies, we exploit the ultradeep (∼27 mag, 3
σ
) Spitzer/Infrared Array Camera (IRAC) 3.6 and 4.5
μ
m data, probing the rest-frame optical at
z
∼ 8, over ∼200 arcmin
2
of the GOODS fields from the recently completed GOODS Re-ionization Era wide-Area Treasury from Spitzer program (GREATS), combined with observations in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS)/Ultra Deep Survey (UDS) and CANDELS/Cosmic Evolution Survey (COSMOS) fields. We stacked ≳100
z
∼ 8 Lyman-break galaxies in four bins of UV luminosity (
M
UV
∼ −20.7 to −18.4 mag) and study their
H
160
− 3.6 and 3.6–4.5 colors. We find young ages (≲100 Myr) for the three faintest stacks, inferred from their blue
H
160
− 3.6 ∼ 0 mag colors, consistent with a negative Balmer break. Meanwhile, the redder
H
160
− 3.6 color seen in the brightest stack is suggestive of slightly older ages. We explored the existence of a correlation between the UV luminosity and age, and find either no trend or fainter galaxies being younger. The stacked SEDs also exhibit very red 3.6–4.5 ∼ 0.5 mag colors, indicative of intense O
iii
+H
β
nebular emission and star formation rate (SFR). The correspondingly high specific SFRs, sSFR ≳10 Gyr
−1
, are consistent with recent determinations at similar redshifts and higher luminosities, and support the coevolution between the sSFR and the specific halo mass accretion rate.
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