The Next Generation Virgo Cluster Survey. XII. Stellar Populations and Kinematics of Compact, Low-Mass Early-Type Galaxies from Gemini GMOS-IFU Spectroscopy

We present Gemini GMOS-IFU data of eight compact low-mass early-type galaxies (ETGs) in the Virgo cluster. We analyse their stellar kinematics, stellar population, and present two-dimensional maps of these properties covering the central 5"x 7" region. We find a large variety of kinematics: from non- to highly-rotating objects, often associated with underlying disky isophotes revealed by deep images from the Next Generation Virgo Cluster Survey. In half of our objects, we find a centrally-concentrated younger and more metal-rich stellar population. We analyze the specific stellar angular momentum through the lambdaR parameter and find six fast-rotators and two slow-rotators, one having a thin counter-rotating disk. We compare the local galaxy density and stellar populations of our objects with those of 39 more extended low-mass Virgo ETGs from the SMAKCED survey and 260 massive ($M>10^{10}$\Msun) ETGs from the A3D sample. The compact low-mass ETGs in our sample are located in high density regions, often close to a massive galaxy and have, on average, older and more metal-rich stellar populations than less compact low-mass galaxies. We find that the stellar population parameters follow lines of constant velocity dispersion in the mass-size plane, smoothly extending the comparable trends found for massive ETGs. Our study supports a scenario where low-mass compact ETGs have experienced long-lived interactions with their environment, including ram-pressure stripping and gravitational tidal forces, that may be responsible for their compact nature.Comment: Accepted in ApJ, 19 pages, 10 figure

Advancements in multi-object integral-field spectroscopy

The motivation of this thesis is not only to unravel the development of multi-object integral-field spectroscopy in optical astronomy, but also to address its observational impact on current and pervading theories of galaxy evolution: in particular, star formation in the local Universe. Classically, spectroscopic observations for galaxy evolution studies in the context of optical astronomy had two modes: (1) multi-object spectroscopy: observe N galaxies with a single aperture on each galaxy, or (2) integral-field spectroscopy: observe a single galaxy with a monolithic 2D array of spatially distributed apertures. Although the first mode is quick to build statistically large samples of galaxies (millions), having a single aperture measurement of fixed size introduces biases when observing galaxies that overfill the aperture (either too close or too large). To reduce the effect of these biases pervading through galaxy evolution models, elaborate schemes have been developed to predict the galaxy properties outside of the aperture using other observational data (so called "aperture corrections"). However, inherent observational biases mean that understanding the errors on the assumptions that go into these aperture corrections is essential to the fidelity of galaxy evolution theory. The second mode of observation is designed to eliminate / minimise these biases by obtaining spatially resolved spectroscopy over the entire galaxy. However, the time and cost taken to build up statistically significant samples of galaxies with current instrumentation limits this mode to observing a few hundred in a survey lifetime. Enter the third mode of observation: multi-object integral-field spectroscopy. This mode takes the N available apertures, and divides them into multiple smaller 2D arrays to obtain integral-field spectroscopy of tens of galaxies at once. A prime instrument of this observational mode is the Sydney-AAO Multi-object Integral-field spectrograph (SAMI), developed at the University of Sydney and the Australian Astronomical Observatory with first light on the 3.9 m Anglo-Australian Telescope (AAT) in July 2011. The SAMI Galaxy Survey started in 2013, and now provides the largest integral-field spectroscopic sample of nearby galaxies across a broad range in stellar mass and environment. These data have for the first time been used to vigorously test the two most popular aperture corrections for calculating a galaxy’s star formation rate. The results of this quantitative and qualitative analysis reveal significant biases that arise in different populations of galaxies. It is then imperative for astronomers to carefully investigate any possible bias due to their target selections. A clear example of where a single aperture misses important information is shown in the SAMI observation of GAMA J141103.98-003242.3, where an intense off-centred star forming region was discovered. This dwarf-galaxy is remarkably similar to the Large Magellanic Cloud and the 30 Doradus system, differing only in its isolation. The detection of a substantial H I reservoir, a stable velocity field, and the H II complex being 0.2 dex lower in metallicity than the rest of the galaxy, strengthens the case of stochastic star formation. This type of process is analogous to more massive galaxies at higher redshifts that are undergoing a phase of extreme star formation activity ("clump-clusters"). Understanding the role of these dwarf galaxies, and tying them together with their higher redshift counterparts helps constrain the low-mass end of galaxy evolution models. The invaluable data from SAMI hasn’t been possible without innovation in astronomical instrumentation. In order to relax constraints on the survey target selection and observational strategy, a new guiding system was developed, utilising new imaging bundles made from polymer. These bundles are 1.5 mm in diameter with 7095 cores, and have the ability to translate a spatially coherent image. Although polymer has a much higher attenuation than silica, manufacturing constraints on the filling factor of silica imaging bundles results in polymer based imaging bundles being more efficient over short lengths (< 4 m). To validate their functionality as field acquisition and guide probes in SAMI, laboratory characterisation was undertaken that revealed their possible use in other multi-object astronomical applications such as: wavefront sensing, narrow-band imaging, aperture masking, and speckle imaging. Further advancement in the field of multi-object integral-field spectroscopy is already underway with other world leading telescopes taking up the charge, but also with sights to SAMI’s successor on the AAT. Improved data quality and two orders of magnitude more galaxies will open up the path of galaxy evolution by studying the effects of large scale environmental structure and accretion histories of individual galaxies within statistically significant populations

Evolution of gas-phase metallicity across cosmic time

Chemical enrichment of the interstellar medium (ISM) in galaxies by generations of star-formation is a crucial ingredient to tracing galaxy evolution. Local galaxies have been the rigorously studied owing to their proximity, whereas distant and relatively faint high-redshift (1 < z < 4) galaxies remain poorly understood. In order to trace chemical evolution in high-z galaxies, it is necessary to develop and test new UV nebular diagnostics because at high-z we primarily observe the rest-frame UV spectra. We compare new rest-frame UV and existing optical nebular emission line diagnostics, by applying both sets, for the first time, to the brightest known lensed galaxy at cosmic noon (z~2) RCS0327-E. We infer the metallicity (12+log(O/H)), ionisation parameter (log (q)), electron temperature (T_e), electron density (n_e) and ISM pressure (log(P/k)) via UV and optical emission line diagnostics. Moreover, we extend an existing Bayesian inference code (IZI) to 3D (IZIP), enabling it to infer 12+log(O/H), log (q) and log (P/k) simultaneously. We find that 12+log(O/H) is harder to infer based on UV emission lines alone while inferred values log (q) and T_e are broadly similar for UV and optical diagnostics. UV diagnostics yield a higher (~1.5 dex) pressure and density than optical, because the UV lines probe the inner, denser parts of nebula. We employ the new UV diagnostics to obtain metallicities for the full MEGaSaURA sample -- consisting of rest-frame UV spectra of 22 bright, star-forming, gravitationally lensed galaxies in redshift 1.6 < z < 3.6. We focus on the N3O3 ([N III] 1750/[O III] 1660,6) diagnostic because this ratio has the highest number of detections (N = 11) in our sample. Our sample expands the literature of lensed galaxy metallicities in the above redshift range by ~30%. We compare the N3O3 metallicities with those inferred from IZIP, as well as stellar and optical gas-phase metallicities from the literature. We find it difficult to draw strong conclusions regarding the redshift evolution of metallicity in the MEGaSaURA sample because of large scatter in metallicities -- both intrinsic to the sample and across different diagnostics. Probing distant galaxies not only requires us to study the rest-frame UV regime, but also leads to worsening spatial resolution of the observations. This impacts studies of the spatial distribution of metals, particularly the metallicity gradient { which is crucial to understand gas flow history of galaxies. We quantify the impact of spatial resolution and signal-to-noise ratio (SNR) on inferred metallicity gradients in integral field unit (IFU) datacubes by modeling a suite of synthetic IFU data cubes from a simulation of an isolated, Milky Way-type disk galaxy. We find that coarse resolution leads to artificially shallow inferred metallicity gradients, and that recovery of the true gradient with an accuracy of ~10% requires resolving the galaxy scale length by at least 4-5 resolution elements. Based on these numerical experiments, we propose a method to correct observed metallicity gradients for spatial resolution effects. Thereafter, we correct the observed metallicity gradients of the MaNGA, CALIFA and SAMI samples and study the mass-metallicity gradient relation. We find that while the correction has little effect on the overall average mass-metallicity gradient relation, individual galaxies can undergo significant correction depending on the resolution. Our results are qualitatively consistent across the three different IFU samples -- more massive galaxies exhibit steeper gradients, up to log (M_*/M_sun) < 10.5, above which the the gradients are shallower, likely due to an increased prevalence of major mergers leading to gas mixing. Our models and method of correcting for resolution effects, along with our cross-survey comparisons, will facilitate planning, analysing and comparing current and future large surveys

Formation and physical properties of central structures in barred galaxies

The evolution of disc galaxies is governed not only by violent and external processes, but also by slow and continuous, internal evolution. One of the main drivers of this secular evolution are bars: these strongly non-axisymmetric stellar structures are present in 2/3 of all massive disc galaxies in the local Universe and have a substantial impact on their evolution. Amongst many other effects, they efficiently facilitate the inflow of gas from the main galaxy disc to their central regions, where this gas settles and eventually new stellar structures, such as nuclear discs, nuclear rings, and inner bars, are built. In this thesis, we aim to better characterise the properties of these central stellar structures and constrain their formation and evolutionary history. To this end, we employ integral-field spectroscopic observations from MUSE, obtained as part of the TIMER survey. The sample consists of 21 barred disc galaxies in the local Universe exhibiting a variety of central structures, and about 100000 science-ready spectra are available for each galaxy. In order to facilitate the exploitation of this massive data set, we develop the sophisticated software framework GIST for the analysis and visualisation of spectroscopic data. Using this tool, we derive stellar kinematics and mean population properties in the central regions of these galaxies at an unprecedented spatial resolution. We find that nuclear discs are characterised by high rotational velocities, low velocity dispersions, and near-circular orbits, while consisting of stellar populations that are significantly younger, more metal-rich, and less [α/Fe] enhanced, as compared to their direct surroundings. These properties of nuclear discs are consistent with the bar-driven formation scenario. Based on our radial population profiles in the nuclear discs, in particular stellar ages decreasing from the galaxy centre to their outer edge, and a correlation between the radii of (gaseous) nuclear rings and the bar length, we propose a new inside-out formation scenario for nuclear discs: in this picture, (stellar) nuclear discs are formed from a series of star-forming nuclear rings that grow in radius as the bar evolves. Combining measurements of both kinematics and stellar population properties, we find only little evidence for the presence of large, classical bulges in these galaxies. Although the galaxy sample is biased towards barred galaxies, the absence of classical bulges in such massive galaxies is surprising. Investigating the stellar population content of the three inner bars in TIMER, we find that these structures are characterised by high metallicities and low [α/Fe] abundances, similar to main bars. Moreover, inner bars exhibit slightly younger stellar ages at their outer ends, an effect known from studies on main bars as orbital age separation. In addition, radial profiles of metallicities and [α/Fe] enhancements are flat along the inner bar major axis, but show significantly steeper slopes along their minor axis, again analogous to previous findings in the context of main bars. These results suggest that inner and main bars are dynamically similar structures that differ only in the spatial scale on which they exist

The Chemodynamics of the Stellar Populations in M31 from APOGEE Integrated Light Spectroscopy

We present analysis of nearly 1,000 near-infrared, integrated light spectra from APOGEE in the inner $\sim$7 kpc of M31. We utilize full spectrum fitting with A-LIST simple stellar population spectral templates that represent a population of stars with the same age, [M/H], and [$\alpha$/M]. With this, we determine the mean kinematics, metallicities, $\alpha$ abundances, and ages of the stellar populations of M31's bar, bulge, and inner disk ($\sim$4-7 kpc). We find a non-axisymmetric velocity field in M31 resulting from the presence of a bar. The bulge of M31 is metal-poor relative to the disk ([M/H] = $-0.149^{+0.067}_{-0.081}$ dex), features minima in metallicity on either side of the bar ([M/H] $\sim$ -0.2), and is enhanced in $\alpha$ abundance ([$\alpha$/M] = $0.281^{+0.035}_{-0.038}$). The disk of M31 within $\sim$7 kpc is enhanced in both metallicity ([M/H] = $-0.023^{+0.050}_{-0.052}$) and $\alpha$ abundance ([$\alpha$/M] = $0.274^{+0.020}_{-0.025}$). Both of these structural components are uniformly old at $\simeq$ 12 Gyr. We find the metallicity increases with distance from the center of M31, with the steepest gradient along the disk major axis ($0.043\pm0.021$ dex/kpc). This gradient is the result of changing light contributions from the metal-poor bulge and metal-rich disk. The chemodynamics of stellar populations encodes information about a galaxy's chemical enrichment, star formation history, and merger history, allowing us to discuss new constraints on M31's formation. Our results provide a stepping stone between our understanding of the Milky Way and other external galaxies

The role of metals from molecular clouds to galactic discs

From the time the first generation of stars formed (redshift ~ 30) to the present-day, elements that were not produced by the Big Bang (hereafter, metals) have witnessed the assembly of structure in the Universe in great detail. Although metals only form in stars and stellar remnants, they are ubiquitously present everywhere -- from planetary cores to the intergalactic medium. However, we still do not understand how metals are effectively dispersed throughout the Universe, and what role do they play in shaping galaxies as we know today. In this thesis, we use a multi-scale approach to study the role of metals in galaxy evolution, from pc-sized molecular clouds to kpc-sized galactic discs. On smaller scales, we focus on understanding the physical processes that shape up the initial mass function (IMF, with a particular emphasis on metal-free and metal-poor environments) that directly set the integrated yield of metals in the first and early galaxies. On larger scales, we focus on the physics of gas-phase metal distribution in diverse galaxies both in the local and the high-z Universe. We develop a large suite of chemo-magnetohydrodynamic (chemo-MHD) simulations to study the formation of the first stars in metal-free environments. We find that initially weak magnetic fields exponentially grow in a short duration of time and suppress fragmentation of primordial molecular clouds, thereby preventing the formation of low mass first stars that would otherwise have lived to the present day. Magnetic fields grow via dynamo amplification during collapse, and later on in the accretion discs around the first stars. The overall impact of dynamically strong magnetic fields is that the mass distribution of the first stars shifts towards higher masses as compared to chemo-hydrodynamic simulations. An important consequence of our results is that the IMF of the first stars was likely top-heavy, as against the bottom-heavy IMF of the metal-rich Milky Way. We further create analytic models to explore the transition in the IMF as a function of the metallicity of the interstellar medium. We find that the IMF transitioned from top- to bottom-heavy in normal star-forming galaxies when the metallicity increased to 1/10000-1/100 times the Solar metallicity. Our models also provide an explanation for the more bottom-heavy IMF observed in the centres of massive ellipticals. Thanks to integral field unit spectroscopy, spatially-resolved gas-phase metallicities have now been measured for several thousand galaxies. In particular, the last decade has seen a surge in the measurements of metallicity gradients in galaxies, wherein galaxy centres are typically more metal enriched as compared to the outskirts. We develop a new, first-principles model to understand the physics behind these metallicity gradients. In contrast to existing models, our model incorporates all key modes of galactic metal transport (such as metal advection and diffusion), and allows for differential metal enrichment of galactic outflows. Further, the model is inherently linked to a galaxy evolution model, which ensures that metals are treated self-consistently with the gas in galactic discs and the number of free parameters in the model is restricted. We use our model to provide the first joint explanation for the mass-metallicity relation and the mass-metallicity gradient relation in local galaxies. We show that galaxies naturally transition from an advection-dominated to an accretion-dominated regime as they increase in mass. We also show that low mass galaxies preferentially loose more metals in galactic winds. Finally, we use the model to explore the complex relationship between metallicity gradients and galaxy kinematics, and use it to explain the trends observed in high-z galaxies. By studying the role of metals from pc to kpc scales, this research has thus set the scene to study the first galaxies and explore metal-poor environments of nearby dwarf galaxies

Stellar halos of massive galaxies: morphology, kinematics, and cosmological origin

This thesis studies the kinematics, the photometry, and the intrinsic shapes of massive early type galaxies (ETGs) out to large radii, using both observations and cosmological simulations. The goal is the characterisation of the structural properties of these galaxies, their variation with radius, and their dependence on the merger history. The bright central regions (~1 Re) of ETGs have long been known to display a bimodal distribution of physical properties, so that they are distinguished in fast (FRs) and slow rotators (SRs). On the other hand, much less is known about the dynamical structure of ETGs at larger radii. Stellar kinematic measurements in the ETG faint outskirts are observationally challenging as they rely on absorption line spectroscopy, which is limited to the central ~2 Re. In this study, this issue is overcome by using planetary nebulae (PNe) as tracers of the stellar halo kinematics. As part of the ePN.S survey, I performed a kinematic analysis of 33 nearby ETG halos out to typically 6 Re. This work revealed that ETGs have a larger diversity of kinematic behaviors in the halos than they do in their central regions: a considerable fraction of the ePN.S FRs shows reduced rotational support at large radii, and almost half of the FR sample shows indications for a variation of their intrinsic shape, from oblate in the center to triaxial in the halo. SRs instead are found to have increased but still modest rotation at large radii. These results were compared and interpreted using simulated galaxies from the IllustrisTNG cosmological magneto-hydrodynamical simulations. Kinematics and intrinsic shapes are found to be deeply connected: transitions to lower rotational support in the halos of FRs are accompanied by changes from flattened and oblate to more spheroidal shapes, with a higher degree of triaxiality. SRs have more homogeneous structural properties with radius, with overall high triaxiality and modest rotational support. The properties of simulated ETG stellar halos are largely determined by the balance between the in-situ component and the stars accreted through mergers, which strongly depends on stellar mass. In low mass systems, the in-situ stars determine peaked rotation profiles and near-oblate shapes with flattening decreasing with radius. In higher mass systems, mergers modify both rotation and shape profiles, generating local correlations between rotational support, shapes, and ex-situ fractions, and dynamically couple the stellar component to the dark matter halo. These results suggest that the large variety of kinematic and photometric properties of stellar halos is the direct consequence of the evolution of ETGs in a cosmological context: at large radii the FR/SR dichotomy of the cores partially breaks and is substituted by a smooth continuity of halo properties. In this picture, ETG halos would represent the connection between the bimodal core regions and the accretion dominated stochastic regime of large scale structure formation