Marginal Gravitational Stability Research Articles

Metal enrichment due to embedded stars in AGN discs

ABSTRACT We separately assess elemental abundances in active galactic nuclei's (AGNs) broad and narrow emission line regions (BLR and NLR), based on a critical assessment of published results together with new photoionization models. We find (1) He/H enhancements in some AGN, exceeding what can be explained by normal chemical evolution and confirm, (2) super-solar α abundance, though to a lesser degree than previously reported. We also reaffirm, (3) an N/O ratio consistent with secondary production, (4) solar or slightly sub-solar Fe abundance, and (5) red-shift independent metallicity, in contrast with galactic chemical evolution. We interpret (6) the larger metallicity in the BLR than NRL in terms of an in situ stellar evolution and pollution in AGN discs (SEPAD) model. We attribute (a) the redshift independence to the heavy element pollutants being disposed into the disc and accreted onto the central supermassive black hole (SMBH), (b) the limited He excess to the accretion–wind metabolism of a top-heavy population of evolving massive main sequence stars, (c) the super-solar CNO enrichment to the nuclear synthesis during their post-main-sequence evolution, (d) the large N/O to the byproduct of multiple stellar generations, and (e) the Mg, Si, and Fe to the ejecta of type II supernovae in the disc. These results provide supporting evidence for (f) ongoing self-regulated star formation, (g) adequate stellar luminosity to maintain marginal gravitational stability, (h) prolific production of seeds, and (i) dense coexistence of subsequently grown residual black hole populations in AGN discs.

HI content in galactic disks: The role of gravitational instability

We examine the dependence between hydrogen total mass $M_{HI}$ and rotation speed $V_{rot}$, optical size $D_{25}$ or disc radial scale $R_0$ for two samples of late-type galaxies: a) isolated galaxy (AMIGA sample), and b) the edge-on galaxies (flat galaxies of Karachentsev et al. 1999). Estimates of $M_{HI}$, given in the HYPERLEDA database for flat galaxies appear to be on average higher at $\sim $0.2 dex, than for isolated galaxies with similar $V_{rot}$ or $D_{25}$ values, most probably, due to the overvaluation of self-absorption in the HI line. We confirm that the hydrogen mass for both samples closely correlates with galactic disc integral specific angular momentum $J$, which is proportional to $V_{rot}D_{25}$ or $V_{rot}R_0$, with low surface brightness galaxies lie along a common $V_{rot}R_0$ sequence. This relationship can be explained, assuming that gas mass in the disc is regulated by marginal gravitational stability condition of gas layer. A comparison of the observed and theoretically expected dependences leads to a conclusion that either gravitational stability corresponds to higher values of Toomre parameter than is usually assumed, or the threshold stability condition for most galaxies took place only in the past, when gas mass in discs was 2-4 times higher than at present (with the exception of galaxies with abnormally high $HI$ content). The last condition requires that the gas accretion was not compensated by gas consumption during the evolution of most of galaxies.

Gravitational stability and mass estimation of stellar disks

AbstractWe estimate masses of galaxy disks using the marginal gravitational stability criterion and compare them with the photometric disk‐mass evaluations. The comparison reveals that stellar disks of most spiral galaxies we have considered cannot be substantially overheated (at least within several radial scale lengths) and are therefore unlikely to have experienced a significant merging event in their history. However, for a substantial part of S0‐type galaxies, their stellar velocity dispersion is well in excess of the gravitational stability threshold suggesting a major merger event in the past. For four low‐surface‐brightness galaxies, we find the disk masses corresponding to the marginal stability condition to be significantly higher than one may expect from their brightness. Either their disks are dynamically overheated, or they contain a large amount of non‐luminous matter. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Testing Different Models of Dark Matter Distribution in Galaxies

Different numerical and analytical models of dark matter (DM) give divergent predictions about the behavior of the density of DM in the centers of galaxies. In the present work a comparison of the three model distribution of DM (NFW, Moore and Burkert) with varying degrees of singularity in the center, or without it, is carried out for four low surface brightness galaxies. Contrary to the claims in the literature, the results of fitting the observed rotation curves do not allow one to speak of the unique preference for any one of the models. For two of the four galaxies the highest levels of significance has the NFW model, and for the other two – the Burkert model. Simulation is carried out also taking into account the criteria of marginal gravitational stability of galactic disks. A comparison with the photometric data shows that the disks of galaxies under consideration, are perhaps overheated, because modeling without this criterion gives better agreement with the photometric data and better values of the chi-square test for rotation curves

Do low surface brightness galaxies have dense disks?

The disk masses of four low-surface-brightness (LSB) galaxies are estimated using the criterion of marginal gravitational stability. The constructed mass models are close to those for a maximum disk. The results show that the disks of LSB galaxies may be significantly more massive than is usually believed based on their brightnesses. In this case, their surface densities and masses are fairly typical for higher-surface-brightness spirals. Alternatively, it may be that LSB disks are dynamically overheated.

PROPERTIES OF GRAVITOTURBULENT ACCRETION DISKS

We explore the properties of cold gravitoturbulent accretion disks?non-fragmenting disks hovering on the verge of gravitational instability (GI)?using a realistic prescription for the effective viscosity caused by gravitational torques. This prescription is based on a direct relationship between the angular momentum transport in a thin accretion disk and the disk cooling in a steady state. Assuming that opacity is dominated by dust we are able to self-consistently derive disk properties for a given assuming marginal gravitational stability. We also allow external irradiation of the disk and account for a non-zero background viscosity, which can be due to the magneto-rotational instability. Spatial transitions between different co-existing disk states (e.g., between irradiated and self-luminous or between gravitoturbulent and viscous) are described and the location of the boundary at which the disk must fragment is determined in a variety of situations. We demonstrate in particular that at low enough external irradiation stabilizes the gravitoturbulent disk against fragmentation to very large distances thus providing means of steady mass transport to the central object. Implications of our results for the possibility of planet formation by GI in protoplanetary disks and star formation in the Galactic center and for the problem of feeding supermassive black holes in galactic nuclei are discussed.

Spectral energy distributions of marginally self-gravitating quasi-stellar object discs

We calculate spectral energy distributions (SEDs) of steady accretion discs at high accretion rates, as appropriate for bright quasi-stellar objects (QSOs), under the assumption that the outer pans are heated sufficientlyto maintain marginal gravitational stability, presumably by massive stars formed within the disc. The SED is independent of the nature of these auxiliary sources if their inputs are completely thermalized. Standard assumptions are made for angular momentum transport, with an alpha parameter less than unity. With these prescriptions, the luminosity of the disc is sensitive to its opacity, in contrast to standard discs powered by the release of orbital energy alone. Compared with the latter, our discs have a broader SED, with a second peak in the near-infrared that is energetically comparable with the blue bump. The energy in the second peak increases with the outer radius of the disc, provided that the accretion rate is constant with radius. By comparing our computed SEDs with observed ones, we limit the outer radius of the disc to be less than 10 5 Schwarzschild radii (Rs), or about 1 pc, in a typical QSO. We also discuss some properties of our minimum-Q discs in the regions where auxiliary heating is dominant (10 3 -10 5 R S ).

The Relationship between Gas Content and Star Formation in Molecule‐rich Spiral Galaxies

We investigate the relationship between H I, H2, and the star formation rate (SFR) using azimuthally averaged data for seven CO-bright spiral galaxies. Contrary to some earlier studies based on global fluxes, we find that ΣSFR exhibits a much stronger correlation with Σ than with Σ, as Σ saturates at a value of ~ 10 M☉ pc-2 or even declines for large ΣSFR. Hence, the good correlation between ΣSFR and the total (H I+H2) gas surface density Σgas is driven by the molecular component in these galaxies, with the observed relation between ΣSFR and Σ following a Schmidt-type of index nmol ≈ 0.8 if a uniform extinction correction is applied or nmol ≈ 1.4 for a radially varying correction dependent on gas density. The corresponding Schmidt indices for Σgas versus ΣSFR are 1.1 and 1.7 for the two extinction models, in rough agreement with previous studies of the disk-averaged star formation law. An alternative to the Schmidt law, in which the gas depletion timescale is proportional to the orbital timescale, is also consistent with the data if radially varying extinction corrections are applied. We find no clear evidence for a link between the gravitational instability parameter for the gas disk (Qg) and the SFR, and we suggest that Qg be considered a measure of the gas fraction. This implies that for a state of marginal gravitational stability to exist in galaxies with low gas fractions, it must be enforced by the stellar component. In regions where we have both H I and CO measurements, the ratio of H I to H2 surface density scales with radius as roughly R1.5, and we suggest that the balance between H I and H2 is determined primarily by the midplane interstellar pressure. These results favor a law of star formation in quiescent disks in which the ambient pressure and metallicity control the formation of molecular clouds from H I, with star formation then occurring at a roughly constant rate per unit H2 mass.