Clumpy Accretion Research Articles

The MAGPI survey: using kinematic asymmetries in stars and gas to dissect drivers of galaxy dynamical evolution

ABSTRACT We present a study of kinematic asymmetries from the integral field spectroscopic surveys MAGPI and SAMI. By comparing the asymmetries in the ionized gas and stars, we aim to disentangle the physical processes that contribute to kinematic disturbances. We normalize deviations from circular motion by S05, allowing us to study kinematic asymmetries in the stars and gas, regardless of kinematic temperature. We find a similar distribution of stellar asymmetries in galaxies where we do and do not detect ionized gas, suggesting that whatever is driving the stellar asymmetries does not always lead to gas removal. In both MAGPI and SAMI, we find an anticorrelation between stellar asymmetry and stellar mass, that is absent in the gas asymmetries. After stellar mass and mean-stellar-age matching distributions, we find that at all stellar masses, MAGPI galaxies display larger stellar asymmetry compared to SAMI galaxies. In both MAGPI and SAMI galaxies, we find that star-forming galaxies with old mean-stellar-ages typically have larger asymmetries in their gas compared to their stars, whereas galaxies with young mean-stellar-ages have larger asymmetries in their stars compared to their gas. We suggest that this results from continuous, clumpy accretion of gas.

Accretion Funnel Reconfiguration during an Outburst in a Young Stellar Object: EX Lupi

EX Lupi, a low-mass young stellar object, went into an accretion-driven outburst in 2022 March. The outburst caused a sudden phase change of ∼112° ± 5° in periodically oscillating multiband lightcurves. Our high-resolution spectra obtained with the High Resolution Spectrograph (HRS) on board the Southern African Large Telescope also revealed a consistent phase change in the periodically varying radial velocities (RVs), along with an increase in the RV amplitude of various emission lines. The phase change and increase in RV amplitude morphologically translates to a change in the azimuthal and latitudinal location of the accretion hotspot over the stellar surface, which indicates a reconfiguration of the accretion funnel geometry. Our three-dimensional magnetohydrodynamic simulations reproduce the phase change for EX Lupi. To explain the observations, we explored the possibility of forward shifting of the dipolar accretion funnel as well as the possibility of the emergence of a new accretion funnel. During the outburst, we also found evidence of the hotspot’s morphology extending azimuthally asymmetrically with a leading hot edge and cold tail along the stellar rotation. Further, our high-cadence photometry showed that the accretion flow has clumps. We also detected possible clumpy accretion events in the HRS spectra that showed episodically highly blueshifted wings in the Ca ii IR triplet and Balmer H lines.

Three-dimensional SPH Simulations of FU Orionis Star Flares in the Clumpy Accretion Model

One of the early hypotheses about the origin of FU Orionis star (FUORs) outbursts explains them based on the fall of gas clumps from the remnants of protostellar clouds onto protoplanetary disks surrounding young stars. To calculate the consequences of such an event, we make 3D hydrodynamic simulations using the smoothed particle hydrodynamics method. It is shown that the fall of the clump on the disk in the vicinity of the star actually causes a burst of the star’s accretion activity, resembling in its characteristics the flares of known FUORs. In the region of incidence, an inhomogeneous gas ring is formed, which is inclined relative to the outer disk. During several revolutions around the star, this ring combines with the inner disk and forms a tilted disk. In the process of evolution, the inner disk expands, and its inclination relative to the outer disk decreases. After 100 revolutions, the angle of inclination is a few degrees. This result is of interest in connection with the discovery in recent years of protoplanetary disks, the inner region of which is inclined relative to the outer one. Such structures are usually associated with the existence in the vicinity of a star of a massive body (planet or brown dwarf), whose orbit is inclined relative to the plane of the disk. The results of our modeling indicate the possibility of an alternative explanation for this phenomenon.

Strong and Rapid X-Ray Variability of the Super-Eddington Accreting Quasar SDSS J081456.10+532533.5

We report strong and rapid X-ray variability found from the super-Eddington accreting quasar SDSS J081456.10+532533.5 at z = 0.1197. It has a black hole mass of 2.7 × 107 M ⊙ and a dimensionless accretion rate of ≈4 measured from reverberation-mapping observations. It showed weak X-ray emission in the 2021 February Chandra observation, with the 2 keV flux density being times lower compared to an archival Swift observation. The 2 keV flux density is also times weaker compared to the expectation from its optical/UV emission. In a follow-up XMM-Newton observation 32 days later, the 2 keV flux density increased by a factor of , and the spectra are best described by a power law modified with partial-covering absorption; the absorption-corrected intrinsic continuum is at a nominal flux level. Nearly simultaneous optical spectra reveal no variability, and there is only mild long-term optical/infrared variability from archival data (with a maximum variability amplitude of ≈50%). We interpret the X-ray variability with an obscuration scenario, where the intrinsic X-ray continuum does not vary but the absorber has a variable column density and covering factor along the line of sight. The absorber is likely the small-scale clumpy accretion wind that has been proposed to be responsible for similar X-ray variability in other super-Eddington accreting quasars.

Deciphering the radio–star formation correlation on kpc scales

Given the multiple energy-loss mechanisms of cosmic-ray (CR) electrons in galaxies, the tightness of the infrared (IR)–radio continuum correlation is surprising. As the radio continuum emission at GHz frequencies is optically thin, this offers the opportunity to obtain unbiased star formation rates (SFRs) from radio-continuum flux-density measurements. The calorimeter theory can naturally explain the tightness of the far-infrared (FIR)–radio correlation but makes predictions that do not agree with observations. Noncalorimeter models often have to involve a conspiracy to maintain the tightness of the FIR–radio correlation. We extended a published analytical model of galactic disks by including a simplified prescription for the synchrotron emissivity. The galactic gas disks of local spiral galaxies, low-z starburst galaxies, high-z main sequence star-forming galaxies, and high-z starburst galaxies are treated as turbulent clumpy accretion disks. The magnetic field strength is determined by the equipartition between the turbulent kinetic and the magnetic energy densities. Our fiducial model, which includes neither galactic winds nor CR electron secondaries, reproduces the observed radio continuum spectral energy distributions of most (∼70%) of the galaxies. Except for the local spiral galaxies, fast galactic winds can potentially make the conflicting models agree with observations. The observed IR–radio correlations are reproduced by the model within 2σ of the joint uncertainty of model and data for all datasets. The model agrees with the observed SFR–radio correlations within ∼4σ. Energy equipartition between the CR particles and the magnetic field only approximately holds in our models of main sequence star-forming galaxies. If a CR electron calorimeter is assumed, the slope of the IR–radio correlation flattens significantly. Inverse Compton losses are not dominant in the starburst galaxies because in these galaxies not only the gas density but also the turbulent velocity dispersion is higher than in normal star-forming galaxies. Equipartition between the turbulent kinetic and magnetic field energy densities then leads to very high magnetic field strengths and very short synchrotron timescales. The exponents of our model SFR–radio correlations at 150 MHz and 1.4 GHz are very close to one.

The evolution of turbulent galactic discs: gravitational instability, feedback, and accretion

ABSTRACT We study the driving of turbulence in star-forming disc galaxies of different masses at different epochs, using an analytic ‘bathtub’ model. The disc of gas and stars is assumed to be in marginal Toomre instability. Turbulence is assumed to be sustained via an energy balance between its dissipation and three simultaneous energy sources. These are stellar feedback, inward transport due to disc instability and clumpy accretion via streams. The transport rate is computed with two different formalisms, with similar results. To achieve the energy balance, the disc self-regulates either the mass fraction in clumps or the turbulent viscous torque parameter. In this version of the model, the efficiency by which the stream kinetic energy is converted into turbulence is a free parameter, ξa. We find that the contributions of the three energy sources are in the same ball park, within a factor of ∼2 in all discs at all times. In haloes that evolve to a mass $\le 10^{12}\, {\rm M_{\odot }}$ by z = 0 ($\le 10^{11.5}\, {\rm M_{\odot }}$ at z ∼ 2), feedback is the main driver throughout their lifetimes. Above this mass, the main driver is either transport or accretion for very low or very high values of ξa, respectively. For an assumed ξa(t) that declines in time, galaxies in haloes with present-day mass >1012 M⊙ make a transition from accretion to transport dominance at intermediate redshifts, z ∼ 3, when their mass was ${\ge }10^{11.5}\, {\rm M_{\odot }}$. The predicted relation between star formation rate and gas velocity dispersion is consistent with observations.

Clumpy Accretion in Pre-main-sequence Stars as a Source of Perturbations in Circumstellar Disks

The development of perturbations in the circumstellar disks of pre-main-sequence stars caused by clumpy accretion was investigated. Here we perform 3D smoothed particle hydrodynamics simulations of disks perturbed by a recent clump accretion event. These simulations are further explored by radiative transfer calculations to quantify the observational appearance of such disks. It was shown that the density waves in the disks were formed at the fall of the clump. After several revolutions they can transform into spirals and ring structures. Their images in millimeter wavelengths are very similar to those observed with Atacama Large Millimeter/submillimeter Array in some protoplanetary disks. We assume that clumpy accretion may be the source of such structures.

Inception of a first quasar at cosmic dawn

ABSTRACT Earliest quasars at the cosmic dawn are powered by mass accretion on to supermassive black holes of a billion solar masses. Massive black hole (MBH) seeds forming through the direct collapse mechanism are considered the most promising candidates but how do they grow and coevolve with their host galaxies at early cosmic times remains unknown. We here present results from a cosmological radiation hydrodynamical simulation including self-consistent modelling of both Population III (Pop III) and Population II (Pop II) star formation, their radiative and supernova feedback in the host galaxy along with X-ray feedback from an accreting MBH of $\rm 10^5 \, M_{\odot }$ in a halo of $\rm 2 \times 10^9 \, M_{\odot }$ from z = 26 down to z = 16. Our results show that energy deposition from X-rays in the proximity of MBH suppresses Pop III star formation for about 12 Myr while at the same time these X-rays catalyse $\rm H_2$ formation that leads to the formation of a Pop III star cluster of 500 $\rm M_{\odot }$ in the close vicinity of the MBH. We find that mode of star formation for Pop III is episodic and bursty due to the clumpy accretion, while for Pop II it is continuous. The stellar mass of the host galaxy at z ∼ 16 is $\rm 2 \times 10^7 \, M_{\odot }$ with a star formation rate of ${\sim} 0.1\!-\!1 \, \mathrm{ M}_{\odot }\, \mathrm{ yr}^{-1}$. In total, the MBH accretes $\rm 1.5 \times 10^6\, M_{\odot }$ during 120 Myr with the mean accretion rate of ${\sim} 0.01\, \mathrm{ M}_{\odot }\, \mathrm{ yr}^{-1}$ corresponding to an average Eddington fraction of 50 per cent.

Disk evolution of the M87’s nucleus observed in 2008

We report the discovery of year-scale X-ray variation in the nuclear region of the M87 by reanalyze the eight Chandra observations from 2007 to 2008. The X-ray spectra are fitted and decomposed into disk and flaring components. This year-scale X-ray variability can be explained quite well by a simple clumpy accretion model. We conclude that the central super-massive black hole of M87 was accreting a cloud of ∼ 0.5 M⊙ at that time.

Clumpy wind accretion in Supergiant X-ray Binaries

AbstractSupergiant X-ray Binaries host a compact object, generally a neutron star, orbiting an evolved O/B star. Mass transfer proceeds through the intense radiatively-driven wind of the stellar donor, a fraction of which is captured by the gravitational field of the neutron star. The subsequent accretion process onto the neutron star is responsible for the abundant X-ray emission from those systems. They also display variations in time of the X-ray flux by a factor of a few 10, along with changes in the hardness ratios believed to be due to varying absorption along the line-of-sight. We used the most recent results on the inhomogeneities (aka clumps) in the non-stationary wind of massive hot stars to evaluate their impact on the time-variable accretion process. We ran three-dimensional simulations of the wind in the vicinity of the accretor to witness the formation of the bow shock and follow the inhomogeneous flow over several spatial orders of magnitude, down to the neutron star magnetosphere. In particular, we show that the impact of the clumps on the time-variability of the intrinsic mass accretion rate is severely damped by the crossing of the shock, compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We also account for the variable absorption due to clumps passing by the line-of-sight and estimate the final effective variability of the mass accretion rate for different orbital separations. These results are confronted to recent analysis of Vela X-1 observations with Chandra by Grinberg et al. (2017). It shows that clumps account well for time-variability at low luminosity but can not generate, per se, the high luminosity activity observed.

Multi-scale simulations of black hole accretion in barred galaxies

Due to the non-axisymmetric potential of the central bar, in addition to their characteristic arms and bar, barred spiral galaxies form a variety of structures within the thin gas disk, such as nuclear rings, inner spirals, and dust lanes. These structures in the inner kiloparsec are extremely important in order to explain and understand the rate of black hole feeding. The aim of this work is to investigate the influence of stellar bars in spiral galaxies on the thin self-gravitating gas disk. We focus on the accretion of gas onto the central supermassive black hole and its time-dependent evolution. We conducted multi-scale simulations simultaneously resolving the galactic disk and the accretion disk around the central black hole. In all the simulations we varied the initial gas disk mass. As an additional parameter we chose either the gas temperature for isothermal simulations or the cooling timescale for non-isothermal simulations. Accretion was either driven by a gravitationally unstable or clumpy accretion disk or by energy dissipation in strong shocks. Most of the simulations show a strong dependence of the accretion rate at the outer boundary of the central accretion disk (r< 300 pc) on the gas flow at kiloparsec scales. The final black hole masses reach up to ~109 M⊙after 1.6 Gyr. Our models show the expected influence of the Eddington limit and a decline in growth rate at the corresponding sub-Eddington limit.

Black hole growth and AGN feedback under clumpy accretion

High-resolution simulations of supermassive black holes in isolated galaxies have suggested the importance of short (~10 Myr) episodes of rapid accretion caused by interactions between the black hole and massive dense clouds within the host. Accretion of such clouds could potentially provide the dominant source for black hole growth in high-z galaxies, but it remains unresolved in cosmological simulations. Using a stochastic subgrid model calibrated by high-resolution isolated galaxy simulations, we investigate the impact that variability in black hole accretion rates has on black hole growth and the evolution of the host galaxy. We find this clumpy accretion to more efficiently fuel high-redshift black hole growth. This increased mass allows for more rapid accretion even in the absence of high-density clumps, compounding the effect and resulting in substantially faster overall black hole growth. This increased growth allows the black hole to efficiently evacuate gas from the central region of the galaxy, driving strong winds up to ~2500 km/s, producing outflows ~10x stronger than the smooth accretion case, suppressing the inflow of gas onto the host galaxy, and suppressing the star formation within the galaxy by as much as a factor of two. This suggests that the proper incorporation of variability is a key factor in the co-evolution between black holes and their hosts.

Cold, clumpy accretion onto an active supermassive black hole.

Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds--a departure from the 'hot mode' accretion model--although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities produce a rain of cold clouds that fall towards the galaxy's centre, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core. The observations show that these cold clouds also fuel black hole accretion, revealing 'shadows' cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.

The origin of UV‐optical variability in AGN and test of disc models: XMM‐Newton and ground‐based observations of NGC 4395

AbstractThe origin of short timescale (weeks/months) variability of AGN, whether due to intrinsic disc variations or reprocessing of X‐ray emission by a surrounding accretion disc, has been a puzzle for many years. However recently a number of observational programmes, particularly of NGC 5548 with Swift, have shown that the UV/optical variations lag behind the X‐ray variations in a manner strongly supportive of X‐ray reprocessing. Somewhat surprisingly, the implied size of the accretion disc is ∼3 times greater than expected from a standard, smooth, Shakura‐Sunyaev thin disc model. Although the difference may be explained by a clumpy accretion disc, it is not clear whether the difference will occur in all AGN or whether it may change as, eg, a function of black hole mass, accretion rate, or disc temperature. Measurements of interband lags for most AGN require long timescale monitoring, which is hard to arrange. However for low mass (<106 M⊙) AGN, the combination of XMM‐Newton EPIC (X‐rays) with the optical monitor in fast readout mode allows an X‐ray/UV‐optical lag to be measured within a single long observation. Here we summarise previous related observations and report on XMM‐Newton observations of NGC 4395 (mass 100 times lower, accretion rate ∼20 times lower than for NGC 5548). We find that the UVW1 lags the X‐rays by ∼470 s. Simultaneous observations at 6 different ground based observatories also allowed the g‐band lag (∼800 s) to be measured. These observations are in agreement with X‐ray reprocessing but initial analysis suggests that, for NGC 4395, they do not differ markedly from the predictions of the standard thin disc model. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The binary millisecond pulsar PSR J1023+0038 during its accretion state – I. Optical variability

We present time-resolved optical photometry of the binary millisecond `redback' pulsar PSR J1023+0038 (=AY Sex) during its low-mass X-ray binary phase. The light curves taken between 2014 January and April show an underlying sinusoidal modulation due to the irradiated secondary star and accretion disc. We also observe superimposed rapid flaring on time-scales as short as ~20 s with amplitudes of ~0.1-0.5 mag and additional large flare events on time-scales of ~5-60 min with amplitudes ~0.5-1.0 mag. The power density spectrum of the optical flare light curves is dominated by a red-noise component, typical of aperiodic activity in X-ray binaries. Simultaneous X-ray and UV observations by the Swift satellite reveal strong correlations that are consistent with X-ray reprocessing of the UV light, most likely in the outer regions of the accretion disc. On some nights we also observe sharp-edged, rectangular, flat-bottomed dips randomly distributed in orbital phase, with a median duration of ~250 s and a median ingress/egress time of ~20 s. These rectangular dips are similar to the mode-switching behaviour between disc `active' and `passive' luminosity states, observed in the X-ray light curves of other redback millisecond pulsars. This is the first time that the optical analogue of the X-ray mode-switching has been observed. The properties of the passive and active state light curves can be explained in terms of clumpy accretion from a trapped inner accretion disc near the corotation radius, resulting in rectangular, flat-bottomed optical and X-ray light curves.

Optical spectroscopy of EX Lupi during quiescence and outburst

We explore the accretion mechanisms in EX Lupi, prototype of EXor variables, during its quiescence and outburst phases. We analyse high-resolution optical spectra taken before, during, and after its 2008 outburst. In quiescence and outburst, the star presents many permitted emission lines, including typical CTTS lines and numerous neutral and ionized metallic lines. During the outburst, the number of emission lines increases to over a thousand, with narrow plus broad component structure (NC+BC). The BC profile is highly variable on short timescales (24-72h). An active chromosphere can explain the metallic lines in quiescence and the outburst NC. The dynamics of the BC line profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform accretion column that suffers velocity variations along the line-of-sight on timescales of days. Assuming Keplerian rotation, the emitting region would be located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity profiles of the lines reveal a combination of rotation and infall. Line ratios of ions and neutrals can be reproduced with a temperature of T~6500 K for electron densities of a few times 10$^{12}$cm$^{-3}$ in the line-emitting region. The data confirm that the 2008 outburst was an episode of increased accretion, albeit much stronger than previous EX Lupi and typical EXors outbursts. The line profiles are consistent with the infall/rotation of a non-axisymmetric structure that could be produced by clumpy accretion during the outburst phase. A strong inner disk wind appears in the epochs of higher accretion. The rapid recovery of the system after the outburst and the similarity between the pre-outburst and post-outburst states suggest that the accretion channels are similar during the whole period, and only the accretion rate varies, providing a superb environment for studying the accretion processes.

CLUMPY ACCRETION ONTO BLACK HOLES. I. CLUMPY-ADVECTION-DOMINATED ACCRETION FLOW STRUCTURE AND RADIATION

In this paper, we investigate the dynamics of clumps embedded in and confined by the advection-dominated accretion flows (ADAF), in which collisions among the clumps are neglected. We start from the collisionless Boltzmann equation and assume that interaction between the clumps and the ADAF is responsible for transporting angular momentum of clumps outward. The inner edge of the clumpy-ADAF is set to be the tidal radius of the clumps. We consider strong and weak coupling cases, in which the averaged properties of clumps follow the ADAF dynamics and mainly determined by the black hole potential, respectively. We get the analytical solution of the dynamics of clumps for the two cases. The velocity dispersion of clumps is one magnitude higher than the ADAF for the strong coupling case. For the weak coupling case, we find that the mean radial velocity of clumps is linearly proportional to the coefficient of the drag force. We show that the tidally disrupted clumps would lead to accumulation of the debris to form a debris disk in the Shakura-Sunyaev regime. The entire hot ADAF will be efficiently cooled down by photons from the debris disk, giving rise to collapse of the ADAF and quench the clumpy accretion. Subsequently, evaporation of the collapsed ADAF drives resuscitate of a new clumpy-ADAF, resulting in an oscillation of the global clumpy-ADAF. Applications of the present model are briefly discussed to X-ray binaries, ionization nuclear emission regions (LINERs) and BL Lac objects.

Gravitational quenching by clumpy accretion in cool-core clusters: convective dynamical response to overheating

Many galaxy clusters pose a "cooling-flow problem", where the observed X-ray emission from their cores is not accompanied by enough cold gas or star formation. A continuous energy source is required to balance the cooling rate over the whole core volume. We address the feasibility of a gravitational heating mechanism, utilizing the gravitational energy released by the gas that streams into the potential well of the cluster dark-matter halo. We focus here on a specific form of gravitational heating in which the energy is transferred to the medium thorough the drag exerted on inflowing gas clumps. Using spheri-symmetric hydro simulations with a subgrid representation of these clumps, we confirm our earlier estimates that in haloes >=10^13 solar masses the gravitational heating is more efficient than the cooling everywhere. The worry was that this could overheat the core and generate an instability that might push it away from equilibrium. However, we find that the overheating does not change the global halo properties, and that convection can stabilize the cluster by carrying energy away from the overheated core. In a typical rich cluster of 10^{14-15}solar masses, with ~5% of the accreted baryons in gas clumps of ~10^8 solar masses, we derive upper and lower limits for the temperature and entropy profiles and show that they are consistent with those observed in cool-core clusters. We predict the density and mass of cold gas and the level of turbulence driven by the clump accretion. We conclude that gravitational heating is a feasible mechanism for preventing cooling flows in clusters.

SUSTAINING STAR FORMATION RATES IN SPIRAL GALAXIES: SUPERNOVA-DRIVEN TURBULENT ACCRETION DISK MODELS APPLIED TO THINGS GALAXIES

Gas disks of spiral galaxies can be described as clumpy accretion disks without a coupling of viscosity to the actual thermal state of the gas. The model description of a turbulent disk consisting of emerging and spreading clumps (Vollmer & Beckert 2003) contains free parameters, which can be constrained by observations of molecular gas, atomic gas and the star formation rate for individual galaxies. Radial profiles of 18 nearby spiral galaxies from THINGS, HERACLES, SINGS, and GALEX data are used to compare the observed star formation efficiency, molecular fraction, and velocity dispersion to the model. The observed radially decreasing velocity dispersion can be reproduced by the model. In the framework of this model the decrease in the inner disk is due to the stellar mass distribution which dominates the gravitational potential. Introducing a radial break in the star formation efficiency into the model improves the fits significantly. This change in star formation regime is realized by replacing the free fall time in the prescription of the star formation rate with the molecule formation timescale. Depending on the star formation prescription, the break radius is located near the transition region between the molecular-gas-dominated and atomic-gas-dominated parts of the galactic disk or closer to the optical radius. It is found that only less massive galaxies (log (M (M_solar)) <~ 10) can balance gas loss via star formation by radial gas accretion within the disk. These galaxies can thus access their gas reservoirs with large angular momentum. On the other hand, the star formation of massive galaxies is determined by the external gas mass accretion rate from a putative spherical halo of ionized gas or from satellite accretion.

X-ray power law spectra in active galactic nuclei

X-ray spectra of active galactic nuclei (AGN) are usually described as power law spectra, characterized by the spectral slope $\alpha$ or photon index $\Gamma$. Here we discuss the X-ray spectral properties within the framework of clumpy accretion flows, and estimate the power law slope as a function of the source parameters. We expect harder spectra in massive objects than in less massive sources, and steeper spectra in higher accretion rate systems. The predicted values of the photon index cover the range of spectral slopes typically observed in Seyfert galaxies and quasars. The overall trends are consistent with observations, and may account for the positive correlation of the photon index with Eddington ratio (and the possible anticorrelation with black hole mass) observed in different AGN samples. Spectral properties are also closely related to variability properties. We obtain that shorter characteristic time scales are associated with steeper spectra. This agrees with the observed `spectral-timing' correlation.