90 research outputs found
The SAMI Galaxy Survey: Revisiting Galaxy Classification through High-order Stellar Kinematics
Recent cosmological hydrodynamical simulations suggest that integral field spectroscopy can connect the high-order stellar kinematic moments h_3 (~skewness) and h_4 (~kurtosis) in galaxies to their cosmological assembly history. Here, we assess these results by measuring the stellar kinematics on a sample of 315 galaxies, without a morphological selection, using two-dimensional integral field data from the SAMI Galaxy Survey. Proxies for the spin parameter (λ_(R_e)) and ellipticity (ε_e) are used to separate fast and slow rotators; there exists a good correspondence to regular and non-regular rotators, respectively, as also seen in earlier studies. We confirm that regular rotators show a strong h_3 versus V/σ anti-correlation, whereas quasi-regular and non-regular rotators show a more vertical relation in h_3 and V/σ. Motivated by recent cosmological simulations, we develop an alternative approach to kinematically classify galaxies from their individual h_3 versus V/σ signatures. Within the SAMI Galaxy Survey, we identify five classes of high-order stellar kinematic signatures using Gaussian mixture models. Class 1 corresponds to slow rotators, whereas Classes 2–5 correspond to fast rotators. We find that galaxies with similar λ_(R_e) - ε_e values can show distinctly different h_3 - V/σ signatures. Class 5 objects are previously unidentified fast rotators that show a weak h_3 versus V/σ anti-correlation. From simulations, these objects are predicted to be disk-less galaxies formed by gas-poor mergers. From morphological examination, however, there is evidence for large stellar disks. Instead, Class 5 objects are more likely disturbed galaxies, have counter-rotating bulges, or bars in edge-on galaxies. Finally, we interpret the strong anti-correlation in h_3 versus V/σ as evidence for disks in most fast rotators, suggesting a dearth of gas-poor mergers among fast rotators
The connection between mass, environment and slow rotation in simulated galaxies
Recent observations from integral field spectroscopy (IFS) indicate that the
fraction of galaxies that are slow rotators, , depends primarily on
stellar mass, with no significant dependence on environment. We investigate
these trends and the formation paths of slow rotators (SRs) using the EAGLE and
Hydrangea hydro-dynamical simulations. EAGLE consists of several cosmological
boxes of volumes up to , while Hydrangea consists of
cosmological simulations of galaxy clusters and their environment. Together
they provide a statistically significant sample in the stellar mass range
, of galaxies. We
construct IFS-like cubes and measure stellar spin parameters, , and ellipticities, allowing us to classify galaxies into slow/fast
rotators as in observations. The simulations display a primary dependence of
on stellar mass, with a weak dependence on environment. At fixed
stellar mass, satellite galaxies are more likely to be SRs than centrals.
shows a dependence on halo mass at fixed stellar mass for central
galaxies, while no such trend is seen for satellites. We find that % of SRs at have experienced at least one merger with mass ratio , with dry mergers being at least twice more common than wet mergers.
Individual dry mergers tend to decrease , while wet mergers
mostly increase it. However, % of SRs at have not experienced
mergers, and those inhabit halos with median spins twice smaller than the halos
hosting the rest of the SRs. Thus, although the formation paths of SRs can be
varied, dry mergers and/or halos with small spins dominate.Comment: Accepted for publications in MNRAS (20 pages, 17 main body, 1.5
appendix). Changes include analysis of the orbital angular momentum effect on
lambdaR and slightly different ellipticity calculatio
The Milky Way in Context: Building an integral-field spectrograph data cube of the Galaxy
The Milky Way (MW) is by far the best-studied galaxy and has been regarded as
an ideal laboratory for understanding galaxy evolution. However, direct
comparisons of Galactic and extra-galactic observations are marred by many
challenges, including selection effects and differences in observations and
methodology. In this study, we present a novel code GalCraft to address these
challenges by generating mock integral-field spectrograph data cubes of the MW
using simple stellar population models and a mock stellar catalog of the Galaxy
derived from E-Galaxia. The data products are in the same format as external
galaxies, allowing for direct comparisons. We investigate the ability of pPXF
to recover kinematics and stellar population properties for an edge-on mock
observation of the MW. We confirm that pPXF can distinguish kinematic and
stellar population differences between thin and thick disks. However, pPXF
struggles to recover star formation history, where the SFR is overestimated in
the ranges between 2-4 and 12-14 Gyr compared to the expected values. This is
likely due to the template age spacing, pPXF regularization algorithm, and
spectral similarities in old population templates. Furthermore, we find
systematic offsets in the recovered kinematics, potentially due to insufficient
spectral resolution and the variation of line-of-sight velocity with [M/H] and
age through a line-of-sight. With future higher resolution and
multi-[/Fe] SSP templates, GalCraft will be useful to identify key
signatures such as [/Fe]-[M/H] distribution at different and
and potentially measure radial migration and kinematic heating efficiency to
study detailed chemodynamical evolution of MW-like galaxies.Comment: 27 pages, 27 figures (8 figs in appendix), submitted to MNRAS.
Comments are welcom
The evolution in the stellar mass of Brightest Cluster Galaxies over the past 10 billion years
Using a sample of 98 galaxy clusters recently imaged in the near infra-red
with the ESO NTT, WIYN and WHT telescopes, supplemented with 33 clusters from
the ESO archive, we measure how the stellar mass of the most massive galaxies
in the universe, namely Brightest Cluster Galaxies (BCG), increases with time.
Most of the BCGs in this new sample lie in the redshift range ,
which has been noted in recent works to mark an epoch over which the growth in
the stellar mass of BCGs stalls. From this sample of 132 clusters, we create a
subsample of 102 systems that includes only those clusters that have estimates
of the cluster mass. We combine the BCGs in this subsample with BCGs from the
literature, and find that the growth in stellar mass of BCGs from 10 billion
years ago to the present epoch is broadly consistent with recent semi-analytic
and semi-empirical models. As in other recent studies, tentative evidence
indicates that the stellar mass growth rate of BCGs may be slowing in the past
3.5 billion years. Further work in collecting larger samples, and in better
comparing observations with theory using mock images is required if a more
detailed comparison between the models and the data is to be made.Comment: 15 pages, 8 tables, 7 figures - Accepted for publication in MNRA
Star formation concentration as a tracer of environmental quenching in action: a study of the Eagle and C-Eagle simulations
We study environmental quenching in the Eagle}/C-Eagle cosmological
hydrodynamic simulations over the last 11 Gyr (i.e. ). The simulations
are compared with observations from the SAMI Galaxy Survey at . We focus
on satellite galaxies in galaxy groups and clusters (
< ). A star-formation
concentration index [-index ] is defined, which measures how concentrated star
formation is relative to the stellar distribution. Both Eagle/C-Eagle and SAMI
show a higher fraction of galaxies with low -index in denser environments at
. Low -index galaxies are found below the SFR- main
sequence (MS), and display a declining specific star formation rate (sSFR) with
increasing radii, consistent with ``outside-in'' environmental quenching.
Additionally, we show that -index can be used as a proxy for how long
galaxies have been satellites. These trends become weaker at increasing
redshift and are absent by . We define a quenching timescale as how long it takes satellites to transition from the MS to the
quenched population. We find that simulated galaxies experiencing
``outside-in'' environmental quenching at low redshift () have a
long quenching timescale (median > 2 Gyr). The simulated
galaxies at higher redshift () experience faster quenching (median
< 2Gyr). At galaxies undergoing environmental
quenching have decreased sSFR across the entire galaxy with no ``outside-in''
quenching signatures and a narrow range of -index, showing that on average
environmental quenching acts differently than at .Comment: 21 pages, 17 figures
SAMI-HI: the connection between global asymmetry in the ionised and neutral atomic hydrogen gas in galaxies
Observations of the neutral atomic hydrogen (HI) gas in galaxies are
predominantly spatially unresolved, in the form of a global HI spectral line.
There has been substantial work on quantifying asymmetry in global HI spectra
(`global HI asymmetry'), but due to being spatially unresolved, it remains
unknown what physical regions of galaxies the asymmetry traces, and whether the
other gas phases are affected. Using optical integral field spectrograph (IFS)
observations from the Sydney AAO Multi-object IFS (SAMI) survey for which
global HI spectra are also available (SAMI-HI), we study the connection between
asymmetry in galaxies' ionised and neutral gas reservoirs to test if and how
they can help us better understand the origin of global HI asymmetry. We
reconstruct the global H spectral line from the IFS observations and
find that, while some global H asymmetries can arise from disturbed
ionised gas kinematics, the majority of asymmetric cases are driven by the
distribution of H-emitting gas. When compared to the HI, we find no
evidence for a relationship between the global H and HI asymmetry.
Further, a visual inspection reveals that cases where galaxies have
qualitatively similar H and HI spectral profiles can be spurious, with
the similarity originating from an irregular 2D H flux distribution.
Our results highlight that comparisons between global H and HI
asymmetry are not straightforward, and that many global HI asymmetries trace
disturbances that do not significantly impact the central regions of galaxies.Comment: 11 pages, 6 figures, 1 appendix, accepted for publication in MNRA
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