Much of our knowledge of galaxies comes from analysing the radiation emitted by their stars, which depends on the present number of each type of star in the galaxy. The present number depends on the ...stellar initial mass function (IMF), which describes the distribution of stellar masses when the population formed, and knowledge of it is critical to almost every aspect of galaxy evolution. More than 50 years after the first IMF determination, no consensus has emerged on whether it is universal among different types of galaxies. Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot both be universal, but they could not convincingly discriminate between the two possibilities. Only recently were indications found that massive elliptical galaxies may not have the same IMF as the Milky Way. Here we report a study of the two-dimensional stellar kinematics for the large representative ATLAS(3D) sample of nearby early-type galaxies spanning two orders of magnitude in stellar mass, using detailed dynamical models. We find a strong systematic variation in IMF in early-type galaxies as a function of their stellar mass-to-light ratios, producing differences of a factor of up to three in galactic stellar mass. This implies that a galaxy's IMF depends intimately on the galaxy's formation history.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
ABSTRACT
The challenge of consistent identification of internal structure in galaxies – in particular disc galaxy components like spiral arms, bars, and bulges – has hindered our ability to study the ...physical impact of such structure across large samples. In this paper we present Galaxy Zoo: 3D (GZ:3D) a crowdsourcing project built on the Zooniverse platform that we used to create spatial pixel (spaxel) maps that identify galaxy centres, foreground stars, galactic bars, and spiral arms for 29 831 galaxies that were potential targets of the MaNGA survey (Mapping Nearby Galaxies at Apache Point Observatory, part of the fourth phase of the Sloan Digital Sky Surveys or SDSS-IV), including nearly all of the 10 010 galaxies ultimately observed. Our crowdsourced visual identification of asymmetric internal structures provides valuable insight on the evolutionary role of non-axisymmetric processes that is otherwise lost when MaNGA data cubes are azimuthally averaged. We present the publicly available GZ:3D catalogue alongside validation tests and example use cases. These data may in the future provide a useful training set for automated identification of spiral arm features. As an illustration, we use the spiral masks in a sample of 825 galaxies to measure the enhancement of star formation spatially linked to spiral arms, which we measure to be a factor of three over the background disc, and how this enhancement increases with radius.
ABSTRACT
We present an analytical model to identify thin discs in galaxies, and apply this model to a sample of SDSS MaNGA galaxies. This model fits the velocity and velocity dispersion fields of ...galaxies with regular kinematics. By introducing two parameters ζ related to the comparison of the model’s asymmetric drift correction to the observed gas kinematics and η related to the dominant component of a galaxy, we classify the galaxies in the sample as ‘disc-dominated, ‘non-disc-dominated’, or ‘disc-free’ indicating galaxies with a dominating thin disc, a non-dominating thin disc, or no thin disc detection with our method, respectively. The dynamical mass resulting from our model correlates with stellar mass, and we investigate discrepancies by including gas mass and variation of the initial mass function. As expected, most spiral galaxies in the sample are disc-dominated, while ellipticals are predominantly disc-free. Lenticular galaxies show a dichotomy in their kinematic classification, which is related to their different star formation rates and gas fractions. We propose two possible scenarios to explain these results. In the first scenario, disc-free lenticulars formed in more violent processes than disc-dominated ones, while in the second scenario, the quenching processes in lenticulars lead to a change in their kinematic structures as disc-dominated lenticulars evolve to disc-free ones.
ABSTRACT
We present an orbit-based method of combining stellar and cold gas kinematics to constrain the dark matter profile of early-type galaxies. We apply this method to early-type galaxy NGC 2974, ...using Pan-STARRS imaging and SAURON stellar kinematics to model the stellar orbits, and introducing H i kinematics from VLA observation as a tracer of the gravitational potential. The introduction of the cold gas kinematics shows a significant effect on the confidence limits of especially the dark halo properties: we exclude more than $95{{\ \rm per\ cent}}$ of models within the 1σ confidence level of Schwarzschild modelling with only stellar kinematics, and reduce the relative uncertainty of the dark matter fraction significantly to $10{{\ \rm per\ cent}}$ within 5Re. Adopting a generalized Navarro–Frenk–White (NFW) dark matter profile, we measure a shallow cuspy inner slope of $0.6^{+0.2}_{-0.3}$ when including the cold gas kinematics in our model. We cannot constrain the inner slope with the stellar kinematics alone.
We use interferometric 12CO(1-0) observations to compare and contrast the extent, surface brightness profiles and kinematics of the molecular gas in CO-rich ATLAS3D early-type galaxies (ETGs) and ...spiral galaxies. We find that the molecular gas extent is smaller in absolute terms in ETGs than in late-type galaxies, but that the size distributions are similar once scaled by the galaxies optical/stellar characteristic scalelengths. Amongst ETGs, we find that the extent of the gas is independent of its kinematic misalignment (with respect to the stars), but does depend on the environment, with Virgo cluster ETGs having less extended molecular gas reservoirs, further emphasizing that cluster ETGs follow different evolutionary pathways from those in the field. Approximately half of ETGs have molecular gas surface brightness profiles that follow the stellar light profile. These systems often have relaxed gas out to large radii, suggesting they are unlikely to have had recent merger/accretion events. A third of the sample galaxies show molecular gas surface brightness profiles that fall off slower than the light, and sometimes show a truncation. These galaxies often have a low mass, and either have disturbed molecular gas or are in the Virgo cluster, suggesting that recent mergers, ram pressure stripping and/or the presence of hot gas can compress/truncate the gas. The remaining galaxies have rings, or composite profiles, that we argue can be caused by the effects of bars. We investigated the kinematics of the molecular gas using position-velocity diagrams, and compared the observed kinematics with dynamical model predictions, and the observed stellar and ionized gas velocities. We confirm that the molecular gas reaches beyond the turnover of the circular velocity curve in 70 per cent of our CO-rich ATLAS3D ETGs, validating previous work on the CO Tully-Fisher relation. In general we find that in most galaxies the molecular gas is dynamically cold, and the observed CO rotation matches well model predictions of the circular velocity. In the galaxies with the largest molecular masses, dust obscuration and/or population gradients can cause model predictions of the circular velocity to disagree with observations of the molecular gas rotation; however, these effects are confined to the most star forming systems. Bars and non-equilibrium conditions can also make the gas deviate from circular orbits. In both these cases, one expects the model circular velocity to be higher than the observed CO velocity, in agreement with our observations. Molecular gas is a better direct tracer of the circular velocity than the ionized gas, justifying its use as a kinematic tracer for Tully-Fisher and similar analyses.
We present H i observations of the elliptical galaxy NGC 2974, obtained with the Very Large Array. These observations reveal that the previously detected H i disc in this galaxy is in fact a ring. By ...studying the harmonic expansion of the velocity field along the ring, we constrain the elongation of the halo and find that the underlying gravitational potential is consistent with an axisymmetric shape. We construct mass models of NGC 2974 by combining the H i rotation curve with the central kinematics of the ionized gas, obtained with the integral-field spectrograph SAURON. We introduce a new way of correcting the observed velocities of the ionized gas for asymmetric drift, and hereby disentangle the random motions of the gas caused by gravitational interaction from those caused by turbulence. To reproduce the observed flat rotation curve of the H i gas, we need to include a dark halo in our mass models. A pseudo-isothermal sphere provides the best model to fit our data, but we also tested an NFW halo and modified Newtonian dynamics, which fit the data marginally worse. The mass-to-light ratio M/LI increases in NGC 2974 from 4.3 M⊙/L⊙,I at one effective radius to 8.5 M⊙/L⊙,I at 5 Re. This increase of M/L already suggests the presence of dark matter: we find that within 5 Re at least 55 per cent of the total mass is dark.
ABSTRACT
We investigate the accuracy in the recovery of the stellar dynamics of barred galaxies when using axisymmetric dynamical models. We do this by trying to recover the mass‐to‐light ratio (M/L) ...and the anisotropy of realistic galaxy simulations using the Jeans Anisotropic Multi‐Gaussian Expansion (JAM) modelling method. However, given that the biases we find are mostly due to an application of an axisymmetric modelling algorithm to a non‐axisymmetric system and in particular to inaccuracies in the deprojected mass model, our results are relevant for general axisymmetric modelling methods. We run N‐body collisionless simulations to build a library with various luminosity distribution, constructed to mimic real individual galaxies, with realistic anisotropy. The final result of our evolved library of simulations contains both barred and unbarred galaxies. The JAM method assumes an axisymmetric mass distribution, and we adopt a spatially constant M/L and anisotropy distributions. The models are fitted to two‐dimensional maps of the second velocity moments of the simulations for various viewing angles position angle (PA) of the bar and inclination of the galaxy. We find that the inclination is generally well recovered by the JAM models, for both barred and unbarred simulations. For unbarred simulations the M/L is also accurately recovered, with negligible median bias and with a maximum one of just Δ(M/L) < 1.5 per cent when the galaxy is not too close to face on. At very low inclinations () the M/L can be significantly overestimated (9 per cent in our tests, but errors can be larger for very face‐on views). This is in agreement with previous studies. For barred simulations the M/L is on average (when PA = 45°) essentially unbiased, but we measure an over/underestimation of up to Δ(M/L) = 15 per cent in our tests. The sign of the M/L bias depends on the PA of the bar as expected: overestimation occurs when the bar is closer to end‐on, due to the increased stellar motion along the line‐of‐sight, and underestimation otherwise. For unbarred simulations, the JAM models are able to recover the mean value of the anisotropy with bias , within the region constrained by the kinematics. However when a bar is present, or for nearly face‐on models, the recovered anisotropy varies wildly, with biases up to Δβz≈ 0.3.
We observed the lenticular galaxy NGC 3998 with the Mitchell Integral-Field Spectrograph and extracted line-of-sight velocity distributions out to three half-light radii. We constructed collisionless ...orbit models in order to constrain NGC 3998's dark and visible structure, using kinematics from both the Mitchell and SAURON instruments. We find NGC 3998 to be almost axisymmetric, seen nearly face-on with a flattened intrinsic shape – i.e. a face-on fast rotator. We find an I-band mass-to-light ratio of
$4.7_{-0.45}^{+0.32}$
in good agreement with previous spectral fitting results for this galaxy. Our best-fitting orbit model shows a both a bulge and a disc component, with a non-negligible counter-rotating component also evident. We find that relatively little dark matter is needed to model this galaxy, with an inferred dark mass fraction of just
$(7.1^{+8.1}_{-7.1}){\rm per cent}$
within one half-light radius.
ABSTRACT The nature of warm, ionized gas outside of galaxies may illuminate several key galaxy evolutionary processes. A serendipitous observation by the MaNGA survey has revealed a large, asymmetric ...H complex with no optical counterpart that extends 8″ ( 6.3 kpc) beyond the effective radius of a dusty, starbursting galaxy. This H extension is approximately three times the effective radius of the host galaxy and displays a tail-like morphology. We analyze its gas-phase metallicities, gaseous kinematics, and emission-line ratios and discuss whether this H extension could be diffuse ionized gas, a gas accretion event, or something else. We find that this warm, ionized gas structure is most consistent with gas accretion through recycled wind material, which could be an important process that regulates the low-mass end of the galaxy stellar mass function.
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