ABSTRACT
Matter distribution models of the Milky Way galaxy are usually stationary, although there are known to be wave-like perturbations in the disc at ∼10 per cent level of the total density. ...Modelling of the overall acceleration field by allowing non-equilibrium is a complicated task. We must learn to distinguish whether density enhancements are persistent or not by their nature. In this paper, we elaborate our orbital arc method to include the effects of massless perturbations and non-stationarities in the modelling. The method is tested by modelling of simulation data and shown to be valid. We apply the method to the Gaia Data Release 2 (DR2) data within a region of ∼0.5 kpc from the Sun and confirm that acceleration field in the solar neighbourhood has a perturbed nature – the phase-space density along the orbits of stars grow in the order of $h\lesssim 5$ per cent per Myr due to non-stationarity. This result is a temporally local value and can be used only within the time frame of a few Myr. An attempt to pinpoint the origin of the perturbation shows that the stars having larger absolute angular momentum are the main carriers of the local perturbation. As they are faster than the average thin disc star, they are either originating further away and are close in their pericentre or they are perturbed locally by a fast comoving perturber, such as gas disc inhomogeneities.
ABSTRACT
A galaxy moving through a background of dark matter particles induces an overdensity of these particles or a wake behind it. The back reaction of this wake on the galaxy is a force field ...that can be decomposed into an effective deceleration (called dynamical friction) and a tidal field. In this paper, we determine the tidal forces, thus generated on the galaxy, and the resulting observables, which are shown to be warps, lopsidedness, and/or kinematic-photometric position angle misalignments. We estimate the magnitude of the tidal-like effects needed to reproduce the observed warp and lopsidedness on the isolated galaxy IC 2487. Within a realistic range of dark matter distribution properties, the observed, warped, and lopsided kinematical properties of IC 2487 are possible to reproduce (the background medium of dark matter particles has a velocity dispersion of $\lesssim 80\, {\rm km\, s^{-1}}$ and the density $10^4{\!-\!}10^5\, {\rm M_\odot \, kpc^{-3}}$, more likely at the lower end). We conclude that the proposed mechanism can generate warps, lopsidedness, and misalignments observed in isolated galaxies or galaxies in loose groups. The method can be used also to constrain dark matter spatial and velocity distribution properties.
The radial velocities of 2768 fundamental-mode RR Lyrae stars (RRLs) toward the southern Galactic bulge are presented, spanning the southern bulge from −8° < l < + 8° and −3° < b < −6°. Distances ...derived from the pulsation properties of the RRLs are combined with Gaia proper motions to give constraints on the orbital motions of 1389 RRLs. The majority (∼75%) of the bulge RRLs have orbits consistent with these stars being permanently bound to <3.5 kpc from the Galactic Center, similar to the bar. However, unlike the bulge giants, the RRLs exhibit slower rotation and a higher velocity dispersion. The higher velocity dispersion arises almost exclusively from halo interlopers passing through the inner Galaxy. We present 82 stars with space velocities 500 km s−1 and find that the majority of these high-velocity stars are halo interlopers; it is unclear if a subsample of these stars with similar space velocities has a common origin. Once the 25% of the sample represented by halo interlopers is cleaned, we can clearly discern two populations of bulge RRLs in the inner Galaxy. The first population of RRLs is not as tightly bound to the Galaxy (but is still confined to the inner ∼3.5 kpc) and is both spatially and kinematically consistent with the barred bulge. The second population is more centrally concentrated and does not trace the bar. One possible interpretation is that this population was born prior to bar formation, as their spatial location, kinematics, and pulsation properties suggest, possibly from an accretion event at high redshift.
ABSTRACT
We measure the α/Fe abundances for 183 quiescent galaxies at z = 0.60−0.75 with stellar masses ranging 10.4 ≤ log10(M*/M⊙) ≤ 11.6 selected from the Large Early Galaxy Astrophysics Census ...survey. We estimate α/Fe from the ratio of the spectral indices Mgb (λ ∼ 5177 Å) and Fe4383, compared to predictions of simple stellar population models. We find that 91 per cent of quiescents in our sample have supersolar α/Fe, with an average value of α/Fe = +0.24 ± 0.01. We find no significant correlation between α/Fe and stellar metallicity, mass, velocity dispersion, and average formation time. Galaxies that formed the bulk of their stellar mass on time-scales shorter than 1 Gyr follow the same α/Fe distribution as those which formed on longer time-scales. In comparison to local early-type galaxies and to stacked spectra of quiescent galaxies at z = 0.38 and z = 0.07, we find that the average α/Fe has not changed between z = 0.75 and the present time. Our work shows that the vast majority of massive quiescent galaxies at z ∼ 0.7 are α-enhanced, and that no detectable evolution of the average α/Fe has taken place over the last ∼6.5 Gyr.
ABSTRACT
We explore the dependence of ultraviolet (UV) upturn colours in early-type cluster galaxies on the properties of their parent clusters (such as velocity dispersion and X-ray luminosity) and ...on the positions and kinematics of galaxies within them. We use a sample of 24 nearby clusters with highly complete spectroscopy and optical/infrared data to select a suitable sample of red-sequence galaxies, whose far-ultraviolet and NUV magnitudes we measure from archival GALEX data. Our results show that the UV upturn colour has no dependence on cluster properties and has the same range in all clusters. There is also no dependence on the projected position within clusters or on line-of-sight velocity. Therefore, our conclusion is that the UV upturn phenomenon is an intrinsic feature of cluster early-type galaxies, irrespective of their cluster environment.
We have carried out a joint photometric and structural analysis of red sequence galaxies in four clusters at a mean redshift of 〈z〉 ∼ 1.25 using optical and near-infrared Hubble Space Telescope ...imaging reaching to at least three magnitudes fainter than M*. As expected, the photometry and overall galaxy sizes imply purely passive evolution of stellar populations in red sequence cluster galaxies. However, the morphologies of red sequence cluster galaxies at these redshifts show significant differences to those of local counterparts. Apart from the most massive galaxies, the high-redshift red sequence galaxies are significantly discier than their low-redshift analogues. These galaxies also show significant colour gradients, again not present in their low-redshift equivalents, most straightforwardly explained by radial age gradients. A clear implication of these findings is that red sequence cluster galaxies originally arrive on the sequence as disc-dominated galaxies whose discs subsequently fade or evolve secularly to end up as high Sérsic index early-type galaxies (classical S0s or possibly ellipticals) at lower redshift. The apparent lack of growth seen in a comparison of high- and low-redshift red sequence galaxies implies that any evolution is internal and is unlikely to involve significant mergers. While significant star formation may have ended at high redshift, the cluster red sequence population continues to evolve (morphologically) for several gigayears thereafter.
ABSTRACT
We conduct a comparison of the merging galaxy populations detected by a sample of visual identification of tidal features around galaxies as well as spectroscopically detected close pairs of ...galaxies to determine whether our method of selecting merging galaxies biases our understanding of galaxy interactions. Our volume-limited parent sample consists of 852 galaxies from the Galaxy And Mass Assembly (GAMA) survey in the redshift range 0.04 ≤ z ≤ 0.20 and stellar mass range 9.50 ≤ log $_{10}(M_{\star }/\rm {M}_{\odot })\le ~11.0$. We conduct our comparison using images from the Ultradeep layer of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) to visually classify galaxies with tidal features and compare these to the galaxies in the GAMA spectroscopic close-pair sample. We identify 198 galaxies possessing tidal features, resulting in a tidal feature fraction ftidal = 0.23 ± 0.02. We also identify 80 galaxies involved in close pairs, resulting in a close pair fraction fpair = 0.09 ± 0.01. Upon comparison of our tidal feature and close pair samples we identify 42 galaxies that are present in both samples, yielding a fraction fboth = 0.05 ± 0.01. We find evidence to suggest that the sample of close pairs of galaxies is more likely to detect early stage mergers, where two separate galaxies are still visible, and the tidal feature sample detects later-stage mergers, where only one galaxy nucleus remains visible. The overlap of the close pair and tidal feature samples likely detect intermediate-stage mergers. Our results are in good agreement with the predictions of cosmological hydrodynamical simulations regarding the populations of merging galaxies detected by close pair and tidal feature samples.
Brightest cluster galaxies: the centre can(not?) hold De Propris, Roberto; West, Michael J; Andrade-Santos, Felipe ...
Monthly notices of the Royal Astronomical Society,
01/2021, Letnik:
500, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We explore the persistence of the alignment of brightest cluster galaxies (BCGs) with their local environment. We find that a significant fraction of BCGs do not coincide with the centroid ...of the X-ray gas distribution and/or show peculiar velocities (they are not at rest with respect to the cluster mean). Despite this, we find that BCGs are generally aligned with the cluster mass distribution even when they have significant offsets from the X-ray centre and significant peculiar velocities. The large offsets are not consistent with simple theoretical models. To account for these observations BCGs must undergo mergers preferentially along their major axis, the main infall direction. Such BCGs may be oscillating within the cluster potential after having been displaced by mergers or collisions, or the dark matter halo itself may not yet be relaxed.
Early-type galaxies (ETGs) exhibit a rise in their UV flux shortward of 3000Å, a phenomenon dubbed the UV upturn. It is believed that this UV flux in such old systems is driven by highly evolved hot ...horizontal branch (HB) stars, which are not expected to exist given standard cosmological timescales. However, observations of the evolution of the upturn with redshift suggests that the HB stars could potentially become UV-bright through a process of Helium enhancement, similar to what is observed in the multiple stellar populations of local globular clusters (GC). A highly sensitive UV detector such as CUBES can allow for the analysis of the CN and CH lines between
3000
-
4000
Å in local ETGs, as He-rich populations are also found to be heavily enhanced in Nitrogen and have a radial gradient in N/Fe (i.e. the Nitrogen is centrally concentrated) in GCs. If the same correlations are recovered in ETGs, it would suggest that the He-enhancement in both systems arose through a universal mechanism and thus have similar formation channels. Furthermore, at slightly higher redshifts (
0.2
<
z
<
0.6
), CUBES will allow us to probe directly the rest-frame near-UV indices of ETGs, particularly between
2200
-
3200
Å, which consist of the majority of spectral lines driven by the hot HB population and hence can be used to study its intrinsic properties (e.g. temperature, strength, etc). Current detectors require in excess of 20 hours of integration time to observe the brightest ETGs to a sufficient depth in this wavelength range, necessitating a highly efficient multiplexed UV sensitive spectrograph such as CUBES to make further progress, with the potential of improving exposure times by at least a factor of 2 and allowing for nearly all galaxies at or above
L
∗
in a given cluster to be simultaneously observed with multi-object spectroscopy.
We present new radial velocity measurements from the Bulge Radial Velocity Assay, a large-scale spectroscopic survey of M-type giants in the Galactic bulge/bar region. The sample of ~4500 new radial ...velocities, mostly in the region -10degrees < l < +10degrees and b approximate -6degrees, more than doubles the existent published data set. Our new data extend our rotation curve and velocity dispersion profile to +20degrees which is ~2.8 kpc from the Galactic center. The new data confirm the cylindrical rotation observed at -6degrees and -8degrees and are an excellent fit to the Shen et al. N-body bar model. We measure the strength of the TiOvarepsilon molecular band as a first step toward a metallicity ranking of the stellar sample, from which we confirm the presence of a vertical abundance gradient. Our survey finds no strong evidence of previously unknown kinematic streams. We also publish our complete catalog of radial velocities, photometry, TiO band strengths, and spectra, which is available at the Infrared Science Archive as well as at UCLA.
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