Gas Surface Density Profiles Research Articles

Metallicity gradients in local field star-forming galaxies: insights on inflows, outflows, and the coevolution of gas, stars and metals

We present metallicity gradients in 49 local field star-forming galaxies. We derive gas-phase oxygen abundances using two widely adopted metallicity calibrations based on the [O iii]/Hβ, [N ii]/Hα, and [N ii]/[O ii] line ratios. The two derived metallicity gradients are usually in good agreement within |$\pm 0.14\ {\rm dex} R_{25}^{-1}$| (R25 is the B-band iso-photoal radius), but the metallicity gradients can differ significantly when the ionization parameters change systematically with radius. We investigate the metallicity gradients as a function of stellar mass (⁠|$8 {<} \log (M_\ast /M_\odot) {<} 11$|⁠) and absolute B-band luminosity (−16 > MB > −22). When the metallicity gradients are expressed in dex kpc−1, we show that galaxies with lower mass and luminosity, on average, have steeper metallicity gradients. When the metallicity gradients are expressed in |${\rm dex} R_{25}^{-1}$|⁠, we find no correlation between the metallicity gradients, and stellar mass and luminosity. We provide a local benchmark metallicity gradient of field star-forming galaxies useful for comparison with studies at high redshifts. We investigate the origin of the local benchmark gradient using simple chemical evolution models and observed gas and stellar surface density profiles in nearby field spiral galaxies. Our models suggest that the local benchmark gradient is a direct result of the coevolution of gas and stellar disc under virtually closed-box chemical evolution when the stellar-to-gas mass ratio becomes high (≫0.3). These models imply low current mass accretion rates ( ≲ 0.3 × SFR), and low-mass outflow rates ( ≲ 3 × SFR) in local field star-forming galaxies.

Gas structure inside dust cavities of transition disks: Ophiuchus IRS 48 observed by ALMA

(Abridged) Transition disks are recognized by the absence of emission of small dust grains inside a radius of up to several 10s of AUs. Due to the lack of angular resolution and sensitivity, the gas content of such dust holes has not yet been determined, but is of importance to constrain the mechanism leading to the dust holes. Transition disks are thought to currently undergo the process of dispersal, setting an end to the giant planet formation process. We present new high-resolution observations with the Atacama Large Millimeter/ submillimeter Array (ALMA) of gas lines towards the transition disk Oph IRS 48 previously shown to host a large dust trap. ALMA has detected the $J=6-5$ line of $^{12}$CO and C$^{17}$O around 690 GHz (434 $\mu$m) at a resolution of $\sim$0.25$''$ corresponding to $\sim$30 AU (FWHM). The observed gas lines are used to set constraints on the gas surface density profile. New models of the physical-chemical structure of gas and dust in Oph IRS 48 are developed to reproduce the CO line emission together with the spectral energy distribution (SED) and the VLT-VISIR 18.7 $\mu$m dust continuum images. Integrated intensity cuts and the total spectrum from models having different trial gas surface density profiles are compared to observations. Using the derived surface density profiles, predictions for other CO isotopologues are made, which can be tested by future ALMA observations of the object. The derived gas surface density profile points to the clearing of the cavity by one or more massive planet/companion rather than just photoevaporation or grain-growth.

Balance among gravitational instability, star formation and accretion determines the structure and evolution of disc galaxies

Over the past 10 Gyr, star-forming galaxies have changed dramatically, from clumpy and gas rich, to rather quiescent stellar-dominated disks with specific star formation rates lower by factors of a few tens. We present a general theoretical model for how this transition occurs, and what physical processes drive it, making use of 1D axisymmetric thin disk simulations with an improved version of the Gravitational Instability-Dominated Galaxy Evolution Tool (GIDGET) code. We show that at every radius galaxies tend to be in a slowly evolving equilibrium state wherein new accretion is balanced by star formation, galactic winds, and radial transport of gas through the disk by gravitational instability (GI) -driven torques. The gas surface density profile is determined by which of these terms are in balance at a given radius - direct accretion is balanced by star formation and galactic winds near galactic centers, and by transport at larger radii. We predict that galaxies undergo a smooth transition from a violent disk instability phase to secular evolution. This model provides a natural explanation for the high velocity dispersions and large clumps in z~2 galaxies, the growth and subsequent quenching of bulges, and features of the neutral gas profiles of local spiral galaxies.

Growing dust grains in protoplanetary discs – I. Radial drift with toy growth models

In a series of papers, we present a comprehensive analytic study of the global motion of growing dust grains in protoplanetary discs, addressing both the radial drift and the vertical settling of the particles. Here we study how the radial drift of dust particles is affected by grain growth. In a first step, toy models in which grain growth can either be constant, accelerate or decelerate are introduced. The equations of motion are analytically integrable and therefore the grains dynamics is easy to understand. The radial motion of growing grains is governed by the relative efficiency of the growth and migration processes which is expressed by the dimensionless parameter Lambda, as well as the exponents for the gas surface density and temperature profiles, denoted p and q respectively. When Lambda is of order unity, growth and migration are strongly coupled, providing the most efficient radial drift. For the toy models considered, grains pile up when -p+q+1/2<0. Importantly, we show the existence of a second process which can help discs to retain their solid materials. For accelerating growth, grains end up their migration at a finite radius, thus avoiding being accreted onto the central star.

Explaining millimeter-sized particles in brown dwarf disks

Planets have been detected around a variety of stars, including low-mass objects, such as brown dwarfs. However, such extreme cases are challenging for planet formation models. Recent sub-millimeter observations of disks around brown dwarf measured low spectral indices of the continuum emission that suggest that dust grains grow to mm-sizes even in these very low mass environments. To understand the first steps of planet formation in scaled-down versions of T-Tauri disks, we investigate the physical conditions that can theoretically explain the growth from interstellar dust to millimeter-sized grains in disks around brown dwarf. We modeled the evolution of dust particles under conditions of low-mass disks around brown dwarfs. We used coagulation, fragmentation and disk-structure models to simulate the evolution of dust, with zero and non-zero radial drift. For the non-zero radial drift, we considered strong inhomogeneities in the gas surface density profile that mimic long-lived pressure bumps in the disk. We studied different scenarios that could lead to an agreement between theoretical models and the spectral slope found by millimeter observations. We find that fragmentation is less likely and rapid inward drift is more significant for particles in brown dwarf disks than in T-Tauri disks. We present different scenarios that can nevertheless explain millimeter-sized grains. As an example, a model that combines the following parameters can fit the millimeter fluxes measured for brown dwarf disks: strong pressure inhomogeneities of $\sim$ 40% of amplitude, a small radial extent $\sim$ 15 AU, a moderate turbulence strength $\alpha_{\mathrm{turb}}= 10^{-3}$, and average fragmentation velocities for ices $v_f = 10 m s^{-1}$.

THE SIZE-VIRIAL RADIUS RELATION OF GALAXIES

Sizes of galaxies are an important diagnostic for galaxy formation models. In this study I use the abundance matching ansatz, which has proven to be successful in reproducing galaxy clustering and other statistics, to derive estimates of the virial radius, R200, for galaxies of different morphological types and wide range of stellar mass. I show that over eight of orders of magnitude in stellar mass galaxies of all morphological types follow an approximately linear relation between 3D half-mass radius of their stellar distribution, rhalf and virial radius, rhalf~0.015R200 with a scatter of ~0.2 dex. Such scaling is in remarkable agreement with expectation of models which assume that galaxy sizes are controlled by halo angular momentum, which implies rhalf\propto lambda R200, where lambda is the spin of galaxy parent halo. The scatter about the relation is comparable with the scatter expected from the distribution of $\lambda$ and normalization of the relation agrees with that predicted by the model of Mo, Mao & White (1998), if galaxy sizes were set on average at z~1-2. Moreover, I show that when stellar and gas surface density profiles of galaxies of different morphological types are rescaled using radius r_n= 0.015 R200, the rescaled surface density profiles follow approximately universal exponential (for late types) and de Vaucouleurs (for early types) profiles with scatter of only 30-50% at R~1-3r_n. Remarkably, both late and early type galaxies have similar mean stellar surface density profiles at R>r_n. The main difference between their stellar distributions is thus at R<r_n. The results of this study imply that galaxy sizes and radial distribution of baryons are shaped primarily by properties of their parent halo and that sizes of both late type disks and early type spheroids are controlled by halo angular momentum.

The origins of active galactic nuclei obscuration: the ‘torus’ as a dynamical, unstable driver of accretion

Multi-scale simulations have made it possible to follow gas inflows onto massive black holes (BHs) from galactic scales to the accretion disk. When sufficient gas is driven towards the BH, gravitational instabilities generically form lopsided, eccentric disks that propagate inwards. The lopsided stellar disk exerts a strong torque on the gas disk, driving inflows that fuel rapid BH growth. Here, we investigate whether the same gas disk is the 'torus' invoked to explain obscured AGN. The disk is generically thick and has characteristic ~1-10 pc sizes and masses resembling those required of the torus. The scale heights and obscured fractions of the predicted torii are substantial even in the absence of strong stellar feedback providing the vertical support. Rather, they can be maintained by strong bending modes and warps excited by the inflow-generating instabilities. Other properties commonly attributed to feedback processes may be explained by dynamical effects: misalignment between torus and host galaxy, correlations between local SFR and turbulent gas velocities, and dependence of obscured fractions on AGN luminosity or SFR. We compare the predicted torus properties with observations of gas surface density profiles, kinematics, scale heights, and SFR densities in AGN nuclei, and find that they are consistent. We argue that it is not possible to reproduce these observations and the observed column density (N_H) distribution without a clumpy gas distribution, but allowing for clumping on small scales the predicted N_H distribution is in good agreement with observations from 10^20-27 cm^-2. We examine how N_H scales with galaxy and AGN properties, and find that AGN feedback may be necessary to explain some trends with luminosity and/or redshift. The torus is not merely a bystander or passive fuel source for accretion, but is itself the mechanism driving accretion.

THE OUTER DISKS OF DWARF IRREGULAR GALAXIES

To explore the properties of extreme outer stellar disks, we obtained ultra-deep V and GALEX UV images of 4 dwarf irregular galaxies and one Blue Compact Dwarf galaxy and ultra-deep B images of 3 of these. Our V-band surface photometry extends to 29.5 magnitudes arcsec^-2. We convert the FUV and V-band photometry, along with Halpha photometry, into radial star formation rate profiles that are sensitive to timescales from 10 Myrs to the lifetime of the galaxy. We also compare the stellar distributions, surface brightness profiles, and star formation rate profiles to HI-line emission maps, gas surface density profiles, and gas kinematics. Our data lead us to two general observations: First, the exponential disks in these irregular galaxies are extraordinarily regular. The stellar disks continue to decline exponentially as far as our measurements extend. In spite of lumpiness in the distribution of young stars and HI distributions and kinematics that have significant unordered motions, sporadic processes that have built the disks-star formation, radial movement of stars, and perhaps even perturbations from the outside-have, nevertheless, conspired to produce standard disk profiles. Second, there is a remarkable continuity of star formation throughout these disks over time. In four out of five of our galaxies the star formation rate in the outer disk measured from the FUV tracks that determined from the V-band, to within factors of 5, requiring star formation at a fairly steady rate over the galaxy's lifetime. Yet, the HI surface density profiles generally decline with radius more shallowly than the stellar light, and the gas is marginally gravitationally stable against collapse into clouds. Outer stellar disks are challenging our concepts of star formation and disk growth and provide a critical environment in which to understand processes that mold galaxy disks.

Consequences of the simultaneous formation of giant planets by the core accretion mechanism

<i>Context. <i/>The core accretion mechanism is presently the most widely accepted cause of the formation of giant planets. For simplicity, most models presently assume that the growth of planetary embryos occurs in isolation.<i>Aims. <i/>We explore how the simultaneous growth of two embryos at the present locations of Jupiter and Saturn affects the outcome of planetary formation.<i>Methods. <i/>We model planet formation on the basis of the core accretion scenario and include several key physical ingredients. We consider a protoplanetary gas disk that exponentially decays with time. For planetesimals, we allow for a distribution of sizes from 100 m to 100 km with most of the mass in the smaller objects. We include planetesimal migration as well as different profiles for the surface density <i>Σ<i/> of the disk. The core growth is computed in the framework of the oligarchic growth regime and includes the viscous enhancement of the planetesimal capture cross-section. Planet migration is ignored.<i>Results. <i/>By comparing calculations assuming formation of embryos in isolation to calculations with simultaneous embryo growth, we find that the growth of one embryo generally significantly affects the other. This occurs in spite of the feeding zones of each planet never overlapping. The results may be classified as a function of the gas surface density profile <i>Σ<i/>: if <i>Σ<i/> and the protoplanetary disk is rather massive, Jupiter's formation inhibits the growth of Saturn. If <i>Σ<i/> <i>r<i/><sup>-1<sup/> isolated and simultaneous formation lead to very similar outcomes; in the the case of <i>Σ<i/> Saturn grows faster and induces a density wave that later accelerates the formation of Jupiter.<i>Conclusions. <i/>Our results indicate that the simultaneous growth of several embryos impacts the final outcome and should be taken into account by planet formation models.

Morphology and star formation in nearby low surface brightness galaxies

We present observations (B, R, K, Hα and H i) of six nearby low surface brightness galaxies (LSBGs). They show an astonishing amount of variety; while some systems appear smooth and featureless, others resolve into loose assemblies of gas clouds. We have derived rotation curves, gas surface density profiles and star formation thresholds for three of the galaxies. The results have been used to test two ideas describing their star formation: one in which star formation depends solely on the H i gas surface density, and one that depends on differential rotation. We find that a critical H i surface density criterion in the range 2.6–12.6 × 10(20) cm(−2) (2.1–10.1 M⊙ pc(−2)) best describes the star-forming ability of these galaxies on local and global scales. A critical gas surface density based on the rotation of the gas is also able to describe the results on a global scale for two of the three galaxies for which we were able to derive rotation curves.

Disc galaxies at z = 0 and at high redshift: an explanation of the observed evolution of damped Lya absorption systems

The analysis of disk formation is based on the White & Rees (1978) picture, in which disk galaxies form by continuous cooling and accretion of gas within a merging hierarchy of dark matter halos. A simple Kennicutt law of star formation for disks, based on a single- fluid gravitational instability model is introduced. Since the gas supply in the disk is regulated by infall from the surrounding halo, the gas is always maintained at a critical threshold surface density. Chemical enrichment of the disks occurs when the surrounding hot halo gas is enriched with heavy elements ejected during surpernova explosions. This gas then cools onto the disk producing a new generation of metal-rich stars. I first show that models of this type can reproduce many of the observed properties of a typical spiral galaxy like the Milky Way, including its gas and stellar surface density profiles and the observed relationship between the ages and metallicities of solar neighbourhood stars. I then use the models to make inferences about the properties of disk galaxies at high redshift. The total neutral hydrogen density Omega(HI) increases at higher z. The predicted increase is mild, but is roughly consistent with the latest derivation of Omega(HI) as a function of z by Storrie-Lombardi & MacMahon (1995). The models are also able to account for some of the other trends other trends seen in the high-redshift data, including the increase in the number of high column-density systems at high redshift, as well as the metallicity distribution of damped Lyman-alpha systems at redshifts 2-3.

Formation and evolution of the disc in spiral galaxies: viscous models without infall

We study models for the evolution of the disc of spiral galaxies that include the effects of viscosity but assume no continuous infall of gas on to the disc. The time-scale for star formation is assumed to be of the same order as the time-scale for viscous redistribution of the mass and angular momentum in the disc. We confirmed that these models are able to reproduce the observed radial distribution of gas and stars for present-day spirals for a variety of model parameters and initial conditions. Furthermore, we generally found that the evolution of both the stellar and gas surface density profiles is approximately self-similar over at least 90 per cent of the history of the disc, and that the size of the initial gas disc is approximately equal to the present Holmberg radius for the galaxy. Implications of these results for the evolution of spiral structure and for the interpretation of damped Ly α lines in the spectra of high-redshift QSOs are considered. A discussion of the possible physical mechanisms for the viscosity and the associated time-scales is also presented.