Stellar feedback and bulge formation in clumpy discs

We use numerical simulations of isolated galaxies to study the effects of stellar feedback on the formation and evolution of giant star-forming gas ‘clumps’ in high-redshift, gas-rich galaxies. Such galactic discs are unstable to the formation of bound gas-rich clumps whose properties initially depend only on global disc properties, not the microphysics of feedback. In simulations without stellar feedback, clumps turn an order-unity fraction of their mass into stars and sink to the centre, forming a large bulge and kicking most of the stars out into a much more extended stellar envelope. By contrast, strong radiative stellar feedback disrupts even the most massive clumps after they turn ∼10–20 per cent of their mass into stars, in a time-scale of ∼10–100 Myr, ejecting some material into a superwind and recycling the rest of the gas into the diffuse interstellar medium (ISM). This suppresses the bulge formation rate by direct ‘clump coalescence’ by a factor of several. However, the galactic discs do undergo significant internal evolution in the absence of mergers: clumps form and disrupt continuously and torque gas to the galactic centre. The resulting evolution is qualitatively similar to bar/spiral evolution in simulations with a more homogeneous ISM

Structure-acidity-IR spectra correlations for p-substituted N-phenylsulfonylbenzamides

The wavenumbers of the IR absorption bands of the C=O, S=O and N-H stretching vibrations for a series of p-substituted N-phenylsulfonylbenzamides were measured in trichloromethane. The bond orders, Mulliken charges, charge densities and heats of formation were calculated using the PM3 method. Fifty significant mutual mono parameter (MP) and six dual parameter (DP) correlations were found for the IR spectral, theoretical structural data, substituent constants and previously reported dissociation constants in five polar organic solvents. The transmission of the substituent effects has been discussed and the solvent effect on the slopes of some linear correlations was evaluated using different solvent parameters. The results showed that the factors describing the electronic structure and controlling the dissociation equilibrium and the IR spectra properties of p-substituted N-phenylsulfonylbenzamides must be the same

When Feedback Fails: The Scaling and Saturation of Star Formation Efficiency

We present a suite of 3D multi-physics MHD simulations following star formation in isolated turbulent molecular gas disks ranging from 5 to 500 parsecs in radius. These simulations are designed to survey the range of surface densities between those typical of Milky Way GMCs (\sim 10^2 M_\odot\,pc^{-2}}) and extreme ULIRG environments (\sim 10^2 M_\odot\,pc^{-2}}) so as to map out the scaling of the cloud-scale star formation efficiency (SFE) between these two regimes. The simulations include prescriptions for supernova, stellar wind, and radiative feedback, which we find to be essential in determining both the instantaneous per-freefall ($\epsilon_{ff}$) and integrated ($\epsilon_{int}$) star formation efficiencies. In all simulations, the gas disks form stars until a critical stellar surface density has been reached and the remaining gas is blown out by stellar feedback. We find that surface density is a good predictor of $\epsilon_{int}$, as suggested by analytic force balance arguments from previous works. SFE eventually saturates to $\sim 1$ at high surface density. We also find a proportional relationship between $\epsilon_{ff}$ and $\epsilon_{int}$, implying that star formation is feedback-moderated even over very short time-scales in isolated clouds. These results have implications for star formation in galactic disks, the nature and fate of nuclear starbursts, and the formation of bound star clusters. The scaling of $\epsilon_{ff}$ with surface density is not consistent with the notion that $\epsilon_{ff}$ is always $\sim 1\%$ on the scale of GMCs, but our predictions recover the $\sim 1\%$ value for GMC parameters similar to those found in sprial galaxies, including our own.Comment: 21 pages, 7 figures. Accepted to MNRA

Accretion does not drive the turbulence in galactic discs

Rapid accretion of cold intergalactic gas plays a crucial role in getting gas into galaxies. It has been suggested that this gas accretion proceeds along narrow streams that might also directly drive the turbulence in galactic gas, dynamical disturbances and bulge formation. In cosmological simulations, however, it is impossible to isolate and hence disentangle the effect of cold stream accretion from internal instabilities and mergers. Moreover, in most current cosmological simulations, the phase structure and turbulence in the interstellar medium (ISM) arising from stellar feedback are treated in an approximate (subgrid) manner, so that the feedback cannot generate turbulence in the ISM. In this paper we therefore test the effects of cold streams in extremely high-resolution simulations of otherwise isolated galaxy discs using detailed models for star formation and stellar feedback; we then include or exclude mock cold flows falling on to the galaxies, with mass accretion rates, velocities and flow geometry set to maximize their effect on the gaseous disc. We find (1) turbulent velocity dispersions in gas discs are identical with or without the presence of the cold flow; the energy injected by the flow is efficiently dissipated where it meets the disc. (2) In runs without stellar feedback, the presence of a cold flow has essentially no effect on runaway fragmentation (local collapse), resulting in star formation rates (SFRs) that are an order-of-magnitude too large. (3) Model discs in runs with both explicit feedback and cold flows have higher SFRs, but only insofar as they have more gas. (4) Because the flows are extended, relative to the size of the disc, they do not trigger strong resonant responses and so induce weak gross morphological perturbation (bulge formation via instabilities/fragmentation is not accelerated). (5) However, flows can thicken the disc by direct contribution of out-of-plane or misaligned star-forming streams/filaments. We conclude that while inflows are critical over cosmological time-scales to determine the supply and angular momentum of gas discs, they have weak instantaneous dynamical effects on galaxies

Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution and implications for the IMF

High-redshift submillimetre galaxies (SMGs) are some of the most rapidly star-forming galaxies in the Universe. Historically, galaxy formation models have had difficulty explaining the observed number counts of SMGs. We combine a semi-empirical model with 3D hydrodynamical simulations and 3D dust radiative transfer to predict the number counts of unlensed SMGs. Because the stellar mass functions, gas and dust masses, and sizes of our galaxies are constrained to match observations, we can isolate uncertainties related to the dynamical evolution of galaxy mergers and the dust radiative transfer. The number counts and redshift distributions predicted by our model agree well with observations. Isolated disc galaxies dominate the faint (S_(1.1) ≲ 1 or S_(850) ≲ 2 mJy) population. The brighter sources are a mix of merger-induced starbursts and galaxy-pair SMGs; the latter subpopulation accounts for ∼30–50 per cent of all SMGs at all S_(1.1) ≳ 0.5 mJy (S_(850) ≳ 1 mJy). The mean redshifts are ∼3.0–3.5, depending on the flux cut, and the brightest sources tend to be at higher redshifts. Because the galaxy-pair SMGs will be resolved into multiple fainter sources by the Atacama Large Millimeter/submillimeter Array (ALMA), the bright ALMA counts should be as much as two times less than those observed using single-dish telescopes. The agreement between our model, which uses a Kroupa initial mass function (IMF), and observations suggests that the IMF in high-redshift starbursts need not be top heavy; if the IMF were top heavy, our model would overpredict the number counts. We conclude that the difficulty some models have reproducing the observed SMG counts is likely indicative of more general problems – such as an underprediction of the abundance of massive galaxies or a star formation rate and stellar mass relation normalization lower than that observed – rather than a problem specific to the SMG population

Testing the Recovery of Intrinsic Galaxy Sizes and Masses of z~2 Massive Galaxies Using Cosmological Simulations

Accurate measurements of galaxy masses and sizes are key to tracing galaxy evolution over time. Cosmological zoom-in simulations provide an ideal test bed for assessing the recovery of galaxy properties from observations. Here, we utilize galaxies with $M_*\sim10^{10}-10^{11.5}M_{\odot}$ at z~1.7-2 from the MassiveFIRE cosmological simulation suite, part of the Feedback in Realistic Environments (FIRE) project. Using mock multi-band images, we compare intrinsic galaxy masses and sizes to observational estimates. We find that observations accurately recover stellar masses, with a slight average underestimate of ~0.06 dex and a ~0.15 dex scatter. Recovered half-light radii agree well with intrinsic half-mass radii when averaged over all viewing angles, with a systematic offset of ~0.1 dex (with the half-light radii being larger) and a scatter of ~0.2 dex. When using color gradients to account for mass-to-light variations, recovered half-mass radii also exceed the intrinsic half-mass radii by ~0.1 dex. However, if not properly accounted for, aperture effects can bias size estimates by ~0.1 dex. No differences are found between the mass and size offsets for star-forming and quiescent galaxies. Variations in viewing angle are responsible for ~25% of the scatter in the recovered masses and sizes. Our results thus suggest that the intrinsic scatter in the mass-size relation may have previously been overestimated by ~25%. Moreover, orientation-driven scatter causes the number density of very massive galaxies to be overestimated by ~0.5 dex at $M_*\sim10^{11.5}M_{\odot}$.Comment: Published in the Astrophysical Journal Letters (7 pages, 5 figures; updated to match published version