Observations Of External Galaxies Research Articles

Stellar populations and the origin of thick disks in AURIGA simulations

Context. Recent integral-field spectroscopy observations of edge-on galaxies have led to significant progress in our knowledge of the ages and chemical compositions of thick disks. However, the origin of thick disks and their evolutionary connection with thin disks is still a matter of debate. Aims. We provide new insights into this topic by connecting the stellar populations of thick disks at redshift z = 0 with their past formation and growth in 24 Milky Way-mass galaxies from the AURIGA zoom-in cosmological simulations. We assess the role played by mergers of satellite galaxies in the mass assembly of geometrically defined thick disks. Methods. We projected each galaxy edge on and decomposed it morphologically into two disk components in order to geometrically define the thin and thick disks, as is usually done in observations of external galaxies. We produced age, metallicity, and [Mg/Fe] edge-on maps of the 24 galaxies. We quantified the impact of satellite mergers by mapping the distribution of ex situ stars. Results. Thick disks are on average ∼3 Gyr older, ∼0.25 dex more metal poor, and ∼0.06 dex more [Mg/Fe]-enhanced than thin disks. Their average ages range from ∼6 to ∼9 Gyr, metallicities from ∼ − 0.15 to ∼0.1 dex, and [Mg/Fe] from ∼0.12 to ∼0.16 dex. These properties are the result of an early initial in situ formation, followed by a later growth driven by the combination of direct accretion of stars, some in situ star formation fueled by mergers, and dynamical heating of stars. The balance between these processes varies from galaxy to galaxy and impacts thick-disk ages and metallicities. The oldest thick disks (older than 8 Gyr) are hosted by galaxies with a low mass fraction of accreted stars (below 8%), while the youngest thick disks (younger than 7 Gyr) are found in galaxies with higher accreted fractions (larger than 25%). Mergers play a key role in the mass assembly of thick disks, contributing an average accreted mass fraction of ∼22% in the analyzed thick-disk-dominated regions. In two galaxies, about half of the geometric thick-disk mass was directly accreted. The mass fraction of accreted stars is lower than 10% only in four thick disks. While primordial thick disks form at high redshifts in all galaxies, young metal-rich thin disks, with much lower [Mg/Fe] abundances, start to form later but at different times (at higher or lower redshifts) depending on the galaxy. Conclusions. We conclude that thick disks, although mostly formed in situ, grow thanks to the significant contribution of satellite mergers, especially through the direct accretion of stars. They result from the interplay of external processes with the internal evolution of the galaxy.

The history of the Milky Way: The evolution of star formation, cosmic rays, metallicity, and stellar dynamics over cosmic time

Abstract We study the long-term evolution of the Milky Way (MW) over cosmic time by modeling the star formation, cosmic rays, metallicity, stellar dynamics, outflows, and inflows of the galactic system to obtain various insights into the galactic evolution. The mass accretion is modeled by the results of cosmological N-body simulations for the cold dark matter. We find that the star formation rate is about half the mass accretion rate of the disk, given the consistency between observed Galactic diffuse X-ray emissions (GDXEs) and possible conditions driving the Galactic wind.Our model simultaneously reproduces the quantities of star formation rate, cosmic rays, metals, and the rotation curve of the current MW. The most important predictions of the model are that there is an unidentified accretion flow with a possible number density of ∼10−2 cm−3 and that part of the GDXEs originates from a hot, diffuse plasma which is formed by consuming about $10\%$ of supernova explosion energy. The latter is the science case for future X-ray missions: XRISM, Athena, and so on. We also discuss further implications of our results for the planet formation and observations of external galaxies in terms of multi-messenger astronomy.

Signatures of the Galactic bar in high-order moments of proper motions measured by Gaia

Our location in the Milky Way provides an exceptional opportunity to gain insight on the galactic evolution processes, and complement the information inferred from observations of external galaxies. Since the Milky Way is a barred galaxy, the study of motions of individual stars in the bulge and disc is useful to understand the role of the bar. The Gaia mission enables such study by providing the most precise parallaxes and proper motions to date. In this theoretical work, we explore the effects of the bar on the distribution of higher-order moments – the skewness and kurtosis – of the proper motions by confronting two simulated galaxies, one with a bar and one nearly axisymmetric, with observations from the latest Gaia data release (Gaia DR2). We introduce the code ASGAIA to account for observational errors of Gaia in the kinematical structures predicted by the numerical models. As a result, we find clear imprints of the bar in the skewness distribution of the longitudinal proper motion μℓ in Gaia DR2, as well as other features predicted for the next Gaia data releases.

2D chemical evolution model: The impact of Galactic disc asymmetries on azimuthal chemical abundance variations

Context. Galactic disc chemical evolution models generally ignore azimuthal surface density variation that can introduce chemical abundance azimuthal gradients. Recent observations, however, have revealed chemical abundance changes with azimuth in the gas and stellar components of both the Milky Way and external galaxies.Aims. Our aim is to quantify the effects of spiral arm density fluctuations on the azimuthal variations of the oxygen and iron abundances in disc galaxies.Methods.We developed a new 2D Galactic disc chemical evolution model that is capable of following not just radial but also azimuthal inhomogeneities.Results. The density fluctuations resulting from a Milky Way-like N-body disc formation simulation produce azimuthal variations in the oxygen abundance gradients of the order of 0.1 dex. Moreover, the azimuthal variations are more evident in the outer Galactic regions, which is in agreement with the most recent observations in external galaxies. Using a simple analytical model, we show that the largest fluctuations with azimuth result near the spiral structure co-rotation resonance where the relative speed between the spiral and gaseous disc is the slowest.Conclusion. We provide a new 2D chemical evolution model capable of following azimuthal density variations. Density fluctuations extracted from a Milky Way-like dynamical model lead to a scatter in the azimuthal variations of the oxygen abundance gradient, which is in agreement with observations in external galaxies. We interpret the presence of azimuthal scatter at all radii by the presence of multiple spiral modes moving at different pattern speeds, as found in both observations and numerical simulations.

THE POWER SPECTRUM OF THE MILKY WAY: VELOCITY FLUCTUATIONS IN THE GALACTIC DISK

We investigate the kinematics of stars in the mid-plane of the Milky Way on scales between 25 pc and 10 kpc with data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), the Radial Velocity Experiment (RAVE), and the Geneva-Copenhagen Survey (GCS). Using red-clump stars in APOGEE, we determine the large-scale line-of-sight velocity field out to 5 kpc from the Sun in (0.75 kpc)^2 bins. The solar motion V_{sun-c} with respect to the circular velocity V_c is the largest contribution to the power on large scales after subtracting an axisymmetric rotation field; we determine the solar motion by minimizing the large-scale power to be V_{sun-c} = 24+/-1 (ran.)+/-2 (syst [V_c])+/-5 (syst. [large-scale]) km/s, where the systematic uncertainty is due to (a) a conservative 20 km/s uncertainty in V_c and (b) the estimated power on unobserved larger scales. Combining the APOGEE peculiar-velocity field with red-clump stars in RAVE out to 2 kpc from the Sun and with local GCS stars, we determine the power spectrum of residual velocity fluctuations in the Milky Way's disk on scales between 0.2/kpc < k < 40/kpc. Most of the power is contained in a broad peak between 0.2/kpc < k < 0.9/kpc. We investigate the expected power spectrum for various non-axisymmetric perturbations and demonstrate that the central bar with commonly used parameters but of relatively high mass can explain the bulk of velocity fluctuations in the plane of the Galactic disk near the Sun. Streaming motions ~10 km/s on >~3 kpc scales in the Milky Way are in good agreement with observations of external galaxies and directly explain why local determinations of the solar motion are inconsistent with global measurements.

The origin of physical variations in the star formation law

Observations of external galaxies and of local star-forming clouds in the Milky Way have suggested a variety of star formation laws, i.e., simple direct relations between the column density of star formation (Sigma_SFR: the amount of gas forming stars per unit area and time) and the column density of available gas (Sigma_gas). Extending previous studies, we show that these different, sometimes contradictory relations for Milky Way clouds, nearby galaxies, and high-redshift discs and starbursts can be combined in one universal star formation law in which Sigma_SFR is about 1% of the local gas collapse rate, Sigma_gas/t_ff, but a significant scatter remains in this relation. Using computer simulations and theoretical models, we find that the observed scatter may be primarily controlled by physical variations in the Mach number of the turbulence and by differences in the star formation efficiency. Secondary variations can be induced by changes in the virial parameter, turbulent driving and magnetic field. The predictions of our models are testable with observations that constrain both the Mach number and the star formation efficiency in Milky Way clouds, external disc and starburst galaxies at low and high redshift. We also find that reduced telescope resolution does not strongly affect such measurements when Sigma_SFR is plotted against Sigma_gas/t_ff.

A NEW MODEL OF THE GALACTIC MAGNETIC FIELD

A new, much improved model of the Galactic Magnetic Field (GMF) is presented. We use the WMAP7 Galactic Synchrotron Emission map and more than forty thousand extragalactic rotation measures to constrain the parameters of the GMF model, which is substantially generalized compared to earlier work to now include an out-of-plane component (as suggested by observations of external galaxies) and striated-random fields (motivated by theoretical considerations). The new model provides a greatly improved fit to observations. Consistent with our earlier analyses, the best-fit model has a disk field and an extended halo field. Our new analysis reveals the presence of a large, out-of-plane component of the GMF; as a result, the polarized synchrotron emission of our Galaxy seen by an edge-on observer is predicted to look intriguingly similar to what has been observed in external edge-on galaxies. We find evidence that the cosmic ray electron density is significantly larger than given by GALPROP, or else that there is a widespread striated component to the GMF.

Evidence from the HOT ISM from X-ray observations in external galaxies

The X-ray band provides a very powerful and unique way to explore the hot phase of the ISM in external galaxies. This phase has been known to exist in galaxies since the early Einstein observations, but only with recent detailed observations of objects in the local universe are we beginning to study it in detail and appreciate the importance of the hot ISM as a vehicle of chemical enrichment in different astrophysical environments. This knowledge can then be used to understand the properties of galaxies at the epoch of formation and their subsequent evolution, both in the field and in clusters.

Modelling the inner disc of the Milky Way with manifolds - I. A first step

We study the bar-driven dynamics in the inner part of the Milky Way by using invariant manifolds. This theory has been successfully applied to describe the morphology and kinematics of rings and spirals in external galaxies, and now, for the first time, we apply it to the Milky Way. We compute the orbits confined by the invariant manifolds of the unstable periodic orbits located at the ends of the bar. We discuss whether the COBE/DIRBE bar and the Long bar compose a single bar or two independent bars and perform a number of comparisons which, taken together, argue strongly in favour of the former. More specifically, we favour the possibility that the so-called COBE/DIRBE bar is the boxy/peanut bulge of a bar whose outer thin parts are the so-called Long bar. This possibility is in good agreement both with observations of external galaxies, with orbital structure theory and with simulations. We then analyse in detail the morphology and kinematics given by five representative Galactic potentials. Two have a Ferrers bar, two have a quadrupole bar and the last one a composite bar. We first consider only the COBE/DIRBE bar and then extend it to include the effect of the Long bar. We find that the large-scale structure given by the manifolds describes an inner ring, whose size is similar to the near and far 3-kpc arm, and an outer ring, whose properties resemble those of the Galactic Molecular Ring. We also analyse the kinematics of these two structures, under the different galactic potentials, and find they reproduce the main over-densities found in the galactic longitude-velocity CO diagram. Finally, we consider for what model parameters, the global morphology of the manifolds may reproduce the two outer spiral arms. We conclude that this would necessitate either more massive and more rapidly rotating bars, or including in the potential an extra component describing the spiral arms.

DEUTERATED SPECIES IN EXTRAGALACTIC STAR-FORMING REGIONS

We present a theoretical study of the deuterated species detectability in various types of extragalactic star-forming regions based on our predictions of chemical abundances. This work is motivated by the past and current attempts at observing deuterated species in external galaxies such as NGC~253, IC~342 and the LMC. Here, we investigate the influence of the density, the temperature, the FUV radiation field, the cosmic ray ionisation, and the metallicity on the fractional abundances and D/H abundance ratios of about 20 deuterated species. Without modelling any particular source, we determined how the deuterium chemistry behaves in different physical environments such as starburst, cosmic-rays enhanced environments, low metallicity and high redshift galaxies. In general, our predicted column densities seem in good agreement with those derived from the current limited dataset of observations in external galaxies. We provide, for the first time, a list of key deuterated species whose abundances are high enough to be possibly detectable by the Atacama Large Millimeter Array (ALMA) and Herschel, as a function of galactic nuclear activity and redshift.

Testing PDR models against ISO fine structure line data for extragalactic sources

AbstractStudies of our own Galaxy and observations of external galaxies have suggested that stellar ultraviolet radiation can ionize vast volumes of a galaxy and that far-ultraviolet radiation impinging on neutral cloud surfaces is responsible for a large fraction of the observed far-infrared (FIR) spectral line emission that cools the gas (Crawford &amp; al. (1985)). Fine structure (FS) emission lines can be used as tracers of nebular conditions such as density, excitation and ionization. By virtue of their different excitation potentials and critical densities, FS emission lines provide an insight into the energetics and chemical composition of the regions from which they originate. The far infrared [C ii]158 μm, [O i]145 μm and [O i]63 μm fine structure emission lines obtained with the Infrared Space Observatory (ISO) from 35 extragalactic sources are examined to investigate the chemical abundances and large scales physical properties of these sources. Line fluxes are compared with a grid of PDR models previously computed using the UCL_PDR code. We overplotted our model predictions against flux ratios from the [C ii]158 μm and [O i]63 μm and 145 μm ISO LWS fluxes. In this section we will only discuss the sensitivity of the ratios to changes in the input parameters. We find that the average radiation field G0 is 60–8 × 102 and the average density nH 104−9 × 104 cm−3. While ionised carbon, because of its ionisation potential, can be found in both neutral gas and ionised gas clouds, species such as ionised nitrogen [N ii], with ionisation potential of 14.53 eV, can arise only from H ii regions. The 11 sources that have detections of both [C ii] 158 μm and [N ii] 122 μm have mean and median [C ii]158/[N ii]122 flux ratios of 10.2 and 5.9 respectively. A H ii region [C ii]158/[N ii]122 ratio of 1.6 implies that H ii region contribute only 16% (mean case) and 27% (median case) of the overall [C ii] 158 μm flux that is observed. We used the above predicted H ii region [C ii]158/[N ii]122 ratio of 1.6 along with the observed [N ii] 122 μm fluxes, to correct the observed [C ii] 158 μm flux of these 11 sources for H ii region contributions. We estimate that 10-60% of the [C ii] is excited in ionised regions. When accounting for the contribution to the [C ii] 158 μm by H ii regions we found that our models fitted better the observations. We modeled the oxygen emission line profile emitted from an ensemble of PDRs and found a clear [O i] 63 μm self-absorbed profile. We estimate that approximately 20-70% of the [O i] 63 μm intensity may be suppressed through oxygen self-absorption depending on the physical parameters of the PDR regions. This work has been submitted for publication to MNRAS, Vasta et al. (2009).

Brightness at night

The average brightness of the night sky can be used to determine a general relation between the number density of stars and their average lifetime. Plausible values for stellar lifetimes then imply that a single-galaxy cosmology is inadequate, even without observations of external galaxies. This is the positive content of the old riddle of why the sky is dark at night. Although Kelvin gave the correct resolution to this riddle he did not realize that it requires a far larger universe than he had assumed.

Swing amplification of nonaxisymmetric perturbations in stars and gas in a sheared galactic disk

We present a study of the growth of local, nonaxisymmetric perturbations in gravitationally coupled stars and gas in a differentially rotating galactic disk. The stars and gas are treated as two isothermal fluids of different velocity dispersions, with the stellar velocity dispersion being greater than that for the gas. We examine the physical effects of inclusion of a low-velocity dispersion component (gas) on the growth of non-axisymmetric perturbations in both stars and gas, as done for the axisymmetric case by Jog & Solomon. The amplified perturbations in stars and gas constitute trailing, material, spiral features which may be identified with the local spiral features seen in all spiral galaxies. The formulation of the two-fluid equations closely follows the one-fluid treatment by Goldreich & Lynden-Bell. The local, linearized perturbation equations in the sheared frame are solved to obtain the results for a temporary growth via swing amplification. The problem is formulated in terms of five dimensionless parameters-namely, the Q-factors for stars and gas, respectively; the gas mass fraction; the shearing rate in the galactic disk; and the length scale of perturbation. By using the observed values of these parameters, we obtain the amplifications and the pitch angles for features in stars and gas for dynamically distinct cases, as applicable for different regions of spiral galaxies. A real galaxy consisting of stars and gas may display growth of nonaxisymmetric perturbations even when it is stable against axisymmetric perturbations and/or when either fluid by itself is stable against non-axisymmetric perturbations. Due to its lower velocity dispersion, the gas exhibits a higher amplification than do the stars, and the amplified gas features are slightly more tightly wound than the stellar features. When the gas contribution is high, the stellar amplification and the range of pitch angles over which it can occur are both increased, due to the gravitational coupling between the two fluids. Thus, the two-fluid scheme can explain the origin of the broad spiral arms in the underlying old stellar populations of galaxies, as observed by Schweizer and Elmegreen & Elmegreen. The arms are predicted to be broader in gas-rich galaxies, as is indeed seen for example in M33. In the linear regime studied here, the arm contrast is shown to increase with radius in the inner Galaxy, in agreement with observations of external galaxies by Schweizer. These results follow directly due to the inclusion of gas in the problem.

The Cosmic Infrared Background

AbstractCosmic infrared background radiation, expected to carry the radiative record of luminous objects since they first formed, has yet to be detected. The Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) satellite is designed to search for this primeval background over the spectral range from 1 to 300 μm. Initial examination of data from this experiment show that foreground radiations from the solar system and Galaxy dominate the diffuse infrared sky brightness, with relative minima near 3.5 μm and in the submillimeter wavelength range. DIRBE measurements do not confirm some previous data from rockets and IRAS. Preliminary upper limits on the cosmic infrared background are generally above theoretical expectations for pregalactic and protogalactic sources, and substantially above estimated lower limits based upon observations of external galaxies. Careful foreground modeling is required to reduce these limits or identify an isotropic residual.

The Evolution of Giant Molecular Clouds

We discuss the origin, lifetime, destruction, spatial distribution and relation to star formation of giant molecular clouds. A coagulation model including the effects of spiral density wave shocks is described. We explore implications for CO observations of external galaxies. The collective effects of OB star winds and supernova remnants in disrupting clouds are considered.