Angular momentum, accretion and radial flows in chemodynamical models of spiral galaxies

Gas accretion and radial flows are key ingredients of the chemical evolution of spiral galaxies. They are also tightly linked to each other (accretion drives radial flows, due to angular momentum conservation) and should therefore be modelled simultaneously. We summarise an algorithm that can be used to consistently compute accretion profiles, radial flows and abundance gradients under quite general conditions and we describe illustrative applications to the Milky Way. We find that gas-phase abundance gradients strongly depend on the angular momentum of the accreting material and, in the outer regions, they are significantly affected by the choice of boundary conditions.Comment: 4 pages, 2 figures. Proceedings of the 592 WE-Heraeus Seminar. To appear in Astronomische Nachricthen, special issue "Reconstructing the Milky Way's history: spectroscopic surveys, asteroseismology and chemodynamical models", Guest Editors C. Chiappini, J. Montalban and M. Steffe

Implications of a spatially resolved main sequence for the size evolution of star-forming galaxies

Two currently debated problems in galaxy evolution, the fundamentally local or global nature of the main sequence of star formation and the evolution of the mass-size relation of star forming galaxies (SFGs), are shown to be intimately related to each other. As a preliminary step, a growth function $g$ is defined, which quantifies the differential change in half-mass radius per unit increase in stellar mass ($g = d \log R_{1/2}/d \log M_\star$) due to star formation. A general derivation shows that $g = K \Delta(sSFR)/sSFR$, meaning that $g$ is proportional to the relative difference in specific star formation rate between the outer and inner half of a galaxy, with $K$ a dimensionless structural factor for which handy expressions are provided. As an application, it is shown that galaxies obeying a fundamentally local main sequence also obey, to a good approximation, $g \simeq \gamma n$, where $\gamma$ is the slope of the normalized local main sequence ($sSFR \propto \Sigma_\star^{-\gamma}$) and $n$ the Sersic index. An exact expression is also provided. Quantitatively, a fundamentally local main sequence is consistent with SFGs growing along a stationary mass-size relation, but inconsistent with the continuation at $z=0$ of evolutionary laws derived at higher $z$. This demonstrates that either the main sequence is not fundamentally local, or the mass-size relation of SFGs has converged to an equilibrium state some finite time in the past, or both.Comment: Accepted for publication in MNRA

The density distribution of accreting cosmic filaments as shaped by Kelvin-Helmholtz instability

Cosmic filaments play a crucial role in galaxy evolution transporting gas from the intergalactic medium into galaxies. However, little is known about the efficiency of this process and whether the gas is accreted in a homogenous or clumpy way. Recent observations suggest the presence of broad gas density distributions in the circumgalactic medium which could be related to the accretion of filaments. By means of high-resolution hydrodynamical simulations, we explore here the evolution of cold accreting filaments flowing through the hot circumgalactic medium (CGM) of high-z galaxies. In particular, we examine the nonlinear effects of Kelvin-Helmholtz instability (KHI) on the development of broad gas density distributions and on the formation of cold, dense clumps. We explore a large parameter space in filament and perturbation properties, such as, filament Mach number, initial perturbation wavelength, and thickness of the interface between the filament and the halo. We find that the time averaged density distribution of the cold gas is qualitatively consistent with a skewed log-normal probability distribution function (PDF) plus an additional component in form of a high density tail for high Mach-numbers. Our results suggest a tight correlation between the accreting velocity and the maximum densities developing in the filament which is consistent with the variance-Mach number relation for turbulence. Therefore, cosmological accretion could be a viable mechanism to produce turbulence and broad gas density distributions within the CGM.Comment: 12 pages, 14 figures, submitted to MNRAS on April 3rd 201

A high baryon fraction in massive haloes at z~3

We investigate the baryon content of the circumgalactic medium (CGM) within the virial radius of $M_h \sim 10^{12} \; M_\odot$ haloes at z ~ 3, by modelling the surface brightness profile of the giant Ly$\alpha$ nebulae recently discovered by MUSE around bright QSOs at this redshift. We initially assume fluorescent emission from cold photo-ionized gas confined by the pressure of a hot halo. Acceptable CGM baryon fractions (equal or smaller than the cosmological value) require that the cold gas occupies $\lesssim$ 1% of the volume, but is about as massive as the hot gas. CGM baryon fractions as low as 30% of the cosmic value, as predicted by some strongly ejective feedback models at this redshift, are not easy to reconcile with observations, under our assumptions, unless both the QSO-hosting haloes at $z\sim3$ are more massive than recent BOSS estimates based on clustering and the photo-ionized gas is colder than expected in a standard QSO ionizing radiation field. We also consider the option that the emission is dominated by photons scattered from the QSO broad line region. In this scenario, a very stringent lower limit to the baryon fraction can be obtained under the extreme assumption of optically thin scattering. We infer in this case a baryon fraction of at least 70% of the cosmic value, for fiducial parameters. Lower values require halo masses or gas temperatures different than expected, or that some mechanism keeps the cold gas systematically over-pressured with respect to the ambient medium.Comment: Accepted for publication in MNRAS. 22 pages, 10 Figure

Angular Momentum Accretion onto Disc Galaxies

Throughout the Hubble time, gas makes its way from the intergalactic medium into galaxies fuelling their star formation and promoting their growth. One of the key properties of the accreting gas is its angular momentum, which has profound implications for the evolution of, in particular, disc galaxies. Here, we discuss how to infer the angular momentum of the accreting gas using observations of present-day galaxy discs. We first summarize evidence for ongoing inside-out growth of star forming discs. We then focus on the chemistry of the discs and show how the observed metallicity gradients can be explained if gas accretes onto a disc rotating with a velocity 20 - 30% lower than the local circular speed. We also show that these gradients are incompatible with accretion occurring at the edge of the discs and flowing radially inward. Finally, we investigate gas accretion from a hot corona with a cosmological angular momentum distribution and describe how simple models of rotating coronae guarantee the inside-out growth of disc galaxies

The angular momentum-mass relation: a fundamental law from dwarf irregulars to massive spirals

In a $\Lambda$CDM Universe, the specific stellar angular momentum ($j_\ast$) and stellar mass ($M_\ast$) of a galaxy are correlated as a consequence of the scaling existing for dark matter haloes ($j_{\rm h}\propto M_{\rm h}^{2/3}$). The shape of this law is crucial to test galaxy formation models, which are currently discrepant especially at the lowest masses, allowing to constrain fundamental parameters, e.g. the retained fraction of angular momentum. In this study, we accurately determine the empirical $j_\ast-M_\ast$ relation (Fall relation) for 92 nearby spiral galaxies (from S0 to Irr) selected from the Spitzer Photometry and Accurate Rotation Curves (SPARC) sample in the unprecedented mass range $7 \lesssim \log M_\ast/M_\odot \lesssim 11.5$. We significantly improve all previous estimates of the Fall relation by determining $j_\ast$ profiles homogeneously for all galaxies, using extended HI rotation curves, and selecting only galaxies for which a robust $j_\ast$ could be measured (converged $j_\ast(<R)$ radial profile). We find the relation to be well described by a single, unbroken power-law $j_\ast\propto M_\ast^\alpha$ over the entire mass range, with $\alpha=0.55\pm 0.02$ and orthogonal intrinsic scatter of $0.17\pm 0.01$ dex. We finally discuss some implications for galaxy formation models of this fundamental scaling law and, in particular, the fact that it excludes models in which discs of all masses retain the same fraction of the halo angular momentum.Comment: A&A Letters, accepte

"Zombie" or active? An alternative explanation to the properties of star-forming galaxies at high redshift

Star-forming galaxies at high redshift show anomalous values of infrared excess, which can be described only by extremizing the existing relations between the shape of their ultraviolet continuum emission and their infrared-to-ultraviolet luminosity ratio, or by constructing \textit{ad-hoc} models of star formation and dust distribution. We present an alternative explanation, based on unveiled AGN activity, to the existence of such galaxies. In fact, the presence of a weak AGN configures as a natural scenario in order to explain the observed spectral properties of such high-$z$ objects in terms of a continuum slope distribution rather than altered infrared excesses, due to the different shape of the AGN continuum emission with respect to quiescent galaxies. To this aim, we directly compare the infrared-to-ultraviolet properties of high-redshift galaxies to those of known categories of AGN (quasars and Seyferts). We also infer the characteristics of their possible X-ray emission. We find a strong similarity between the spectral shapes and luminosity ratios of AGN with the corresponding properties of such galaxies. In addition, we derive expected X-ray fluxes that are compatible with energetics from AGN activity. We conclude that a moderate AGN contribution to the UV emission of such high-$z$ objects is a valid alternative to explain their spectral properties. Even the presence of an active nucleus in each source would not violate the expected quasar statistics. Furthermore, we suggest that the observed similarities between anomalous star-forming galaxies and quasars may provide a benchmark for future theoretical and observational studies on the galaxy population in the early Universe.Comment: 13 pages, 7 figures, 4 tables, accepted for publication in A&

The instantaneous radial growth rate of stellar discs

We present a new and simple method to measure the instantaneous mass and radial growth rates of the stellar discs of spiral galaxies, based on their star formation rate surface density (SFRD) profiles. Under the hypothesis that discs are exponential with time-varying scalelengths, we derive a universal theoretical profile for the SFRD, with a linear dependence on two parameters: the specific mass growth rate $\nu_\textrm{M} \equiv \dot{M_\star}/M_\star$ and the specific radial growth rate $\nu_\textrm{R} \equiv \dot{R}_\star/R_\star$ of the disc. We test our theory on a sample of 35 nearby spiral galaxies, for which we derive a measurement of $\nu_\textrm{M}$ and $\nu_\textrm{R}$. 32/35 galaxies show the signature of ongoing inside-out growth ($\nu_\textrm{R} > 0$). The typical derived e-folding timescales for mass and radial growth in our sample are ~ 10 Gyr and ~ 30 Gyr, respectively, with some systematic uncertainties. More massive discs have a larger scatter in $\nu_\textrm{M}$ and $\nu_\textrm{R}$, biased towards a slower growth, both in mass and size. We find a linear relation between the two growth rates, indicating that our galaxy discs grow in size at ~ 0.35 times the rate at which they grow in mass; this ratio is largely unaffected by systematics. Our results are in very good agreement with theoretical expectations if known scaling relations of disc galaxies are not evolving with time.Comment: MNRAS, accepted. 14 pages, 4 figures, 3 tables. Additional material (Atlas.pdf) available at http://www.filippofraternali.com/downloads/index.htm

Most of the cool CGM of star-forming galaxies is not produced by supernova feedback

The characterization of the large amount of gas residing in the galaxy halos, the so called circumgalactic medium (CGM), is crucial to understand galaxy evolution across cosmic time. We focus here on the the cool ($T\sim10^4$ K) phase of this medium around star-forming galaxies in the local universe, whose properties and dynamics are poorly understood. We developed semi-analytical parametric models to describe the cool CGM as an outflow of gas clouds from the central galaxy, as a result of supernova explosions in the disc (galactic wind). The cloud motion is driven by the galaxy gravitational pull and by the interactions with the hot ($T\sim10^6$ K) coronal gas. Through a bayesian analysis, we compare the predictions of our models with the data of the COS-Halos and COS-GASS surveys, which provide accurate kinematic information of the cool CGM around more than 40 low-redshift star-forming galaxies, probing distances up to the galaxy virial radii. Our findings clearly show that a supernova-driven outflow model is not suitable to describe the dynamics of the cool circumgalactic gas. Indeed, to reproduce the data, we need extreme scenarios, with initial outflow velocities and mass loading factors that would lead to unphysically high energy coupling from the supernovae to the gas and with supernova efficiencies largely exceeding unity. This strongly suggests that, since the outflows cannot reproduce most of the cool gas absorbers, the latter are likely the result of cosmological inflow in the outer galaxy halos, in analogy to what we have previously found for early-type galaxies.Comment: Accepted for publication on MNRA