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Register a new userMassive Star Formation in Metal-Enriched Haloes at High Redshift
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John A. Regan
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The formation of supermassive stars has generally been studied under the assumption of rapid accretion of pristine metal-free gas. Recently it was found, however, that gas enriched to metallicities up to $Z sim 10^{-3}$ Z$_{odot}$ can also facilitate supermassive star formation, as long as the total mass infall rate onto the protostar remains sufficiently high. We extend the analysis further by examining how the abundance of supermassive star candidate haloes would be affected if all haloes with super-critical infall rates, regardless of metallicity were included. We investigate this scenario by identifying all atomic cooling haloes in the Renaissance simulations with central mass infall rates exceeding a fixed threshold. We find that among these haloes with central mass infall rates above 0.1 M$_{odot}$ yr$^{-1}$ approximately two-thirds of these haloes have metallicities of $Z > 10^{-3}$ Z$_{odot}$. If metal mixing within these haloes is inefficient early in their assembly and pockets of metal-poor gas can remain then the number of haloes hosting supermassive stars can be increased by at least a factor of four. Additionally the centres of these high infall-rate haloes provide ideal environments in which to grow pre-existing black holes. Further research into the (supermassive) star formation dynamics of rapidly collapsing haloes, with inhomogeneous metal distributions, is required to gain more insight into both supermassive star formation in early galaxies as well as early black hole growth.
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We present here a three-dimesional hydrodynamical simulation for star formation. Our aim is to explore the effect of the metal-line cooling on the thermodynamics of the star-formation process. We explore the effect of changing the metallicty of the gas from $Z/Z_{odot}=10^{-4}$ to $Z/Z_{odot}=10^{-2}$. Furthermore, we explore the implications of using the observational abundance pattern of a CEMP-no star, which have been considered to be the missing second-generation stars, the so-called Pop. III.2 stars. In order to pursue our aim, we modelled the microphysics by employing the public astrochemistry package KROME, using a chemical network which includes sixteen chemical species (H, H$^{+}$, H$^{-}$, He, He$^{+}$, He$^{++}$, e$^{-}$, H$_{2}$, H$_{2}^{+}$, C, C$^{+}$, O, O$^{+}$, Si, Si$^{+}$, and Si$^{++}$). We couple KROME with the fully three-dimensional Smoothed-particle hydrodynamics (SPH) code GRADSPH. With this framework we investigate the collapse of a metal-enhanced cloud, exploring the fragmentation process and the formation of stars. We found that the metallicity has a clear impact on the thermodynamics of the collapse, allowing the cloud to reach the CMB temperature floor for a metallicity $Z/Z_{odot}=10^{-2}$, which is in agreement with previous work. Moreover, we found that adopting the abundance pattern given by the star SMSS J031300.36-670839.3 the thermodynamics behavior is very similar to simulations with a metallicity of $Z/Z_{odot}=10^{-2}$, due to the high carbon abundance. As long as only metal line cooling is considered, our results support the metallicity threshold proposed by previous works, which will very likely regulate the first episode of fragmentation and potentially determine the masses of the resulting star clusters.
We present novel 3D multi-scale SPH simulations of gas-rich galaxy mergers between the most massive galaxies at $z sim 8 - 10$, designed to scrutinize the direct collapse formation scenario for massive black hole seeds proposed in citet{mayer+10}. The simulations achieve a resolution of 0.1 pc, and include both metallicity-dependent optically-thin cooling and a model for thermal balance at high optical depth. We consider different formulations of the SPH hydrodynamical equations, including thermal and metal diffusion. When the two merging galaxy cores collide, gas infall produces a compact, optically thick nuclear disk with densities exceeding $10^{-10}$ g cm$^3$. The disk rapidly accretes higher angular momentum gas from its surroundings reaching $sim 5$ pc and a mass of $gtrsim 10^9$ $M_{odot}$ in only a few $10^4$ yr. Outside $gtrsim 2$ pc it fragments into massive clumps. Instead, supersonic turbulence prevents fragmentation in the inner parsec region, which remains warm ($sim 3000-6000$ K) and develops strong non-axisymmetric modes that cause prominent radial gas inflows ($> 10^4$ $M_{odot}$ yr$^{-1}$), forming an ultra-dense massive disky core. Angular momentum transport by non-axisymmetric modes should continue below our spatial resolution limit, quickly turning the disky core into a supermassive protostar which can collapse directly into a massive black hole of mass $10^8-10^9$ $M_{odot}$ via the relativistic radial instability. Such a cold direct collapse explains naturally the early emergence of high-z QSOs. Its telltale signature would be a burst of gravitational waves in the frequency range $10^{-4} - 10^{-1}$ Hz, possibly detectable by the planned eLISA interferometer.
[Abridged] In order to understand which process (e.g. galactic winds, cold accretion) is responsible for the cool (T~10^4 K) halo gas around galaxies, we embarked on a program to study the star-formation properties of galaxies selected by their MgII absorption signature in quasar spectra. Specifically, we searched for the H-alpha line emission from galaxies near very strong z=2 MgII absorbers (with rest-frame equivalent width EW>2 AA) because these could be the sign-posts of outflows or inflows. Surprisingly, we detect H-alpha from only 4 hosts out of 20 sight-lines (and 2 out of the 19 HI-selected sight-lines), despite reaching a star-formation rate (SFR) sensitivity limit of 2.9 M/yr (5-sigma) for a Chabrier initial mass function. This low success rate is in contrast with our z=1 survey where we detected 66% (14/21) of the MgII hosts. Taking into account the difference in sensitivity between the two surveys, we should have been able to detect >11.4 of the 20 z=2 hosts whereas we found only 4 galaxies. Interestingly, all the z=2 detected hosts have observed SFR greater than 9 M/yr, well above our sensitivity limit, while at z=1 they all have SFR less than 9 M/yr, an evolution that is in good agreement with the evolution of the SFR main sequence. Moreover, we show that the z=2 undetected hosts are not hidden under the quasar continuum after stacking our data and that they also cannot be outside our surveyed area. Hence, strong MgII absorbers could trace star-formation driven winds in low-mass halos (Mhalo < 10^{10.6} Msun). Alternatively, our results imply that z=2 galaxies traced by strong MgII absorbers do not form stars at a rate expected (3--10 M/yr) for their (halo or stellar) masses, supporting the existence of a transition in accretion efficiency at Mhalo ~ 10^{11} Msun. This scenario can explain both the detections and the non-detections.
We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the response of cold dark matter (CDM) haloes to baryonic processes. Previous work has shown that the halo response is primarily a function of the ratio between galaxy stellar mass and total virial mass, and the density threshold above which gas is eligible to form stars, $n [{rm cm}^{-3}]$. At low $n$ all simulations in the literature agree that dwarf galaxy haloes are cuspy, but at high $nge 100$ there is no consensus. We trace halo contraction in dwarf galaxies with $nge 100$ reported in some previous simulations to insufficient spatial resolution. Provided the adopted star formation threshold is appropriate for the resolution of the simulation, we show that the halo response is remarkably stable for $nge 5$, up to the highest star formation threshold that we test, $n=500$. This free parameter can be calibrated using the observed clustering of young stars. Simulations with low thresholds $nle 1$ predict clustering that is too weak, while simulations with high star formation thresholds $nge 5$, are consistent with the observed clustering. Finally, we test the CDM predictions against the circular velocities of nearby dwarf galaxies. Low thresholds predict velocities that are too high, while simulations with $nsim 10$ provide a good match to the observations. We thus conclude that the CDM model provides a good description of the structure of galaxies on kpc scales provided the effects of baryons are properly captured.
We use the James Clerk Maxwell Telescopes SCUBA-2 camera to image a 400 arcmin^2 area surrounding the GOODS-N field. The 850 micron rms noise ranges from a value of 0.49 mJy in the central region to 3.5 mJy at the outside edge. From these data, we construct an 850 micron source catalog to 2 mJy containing 49 sources detected above the 4-sigma level. We use an ultradeep (11.5 uJy at 5-sigma) 1.4 GHz image obtained with the Karl G. Jansky Very Large Array together with observations made with the Submillimeter Array to identify counterparts to the submillimeter galaxies. For most cases of multiple radio counterparts, we can identify the correct counterpart from new and existing Submillimeter Array data. We have spectroscopic redshifts for 62% of the radio sources in the 9 arcmin radius highest sensitivity region (556/894) and 67% of the radio sources in the GOODS-N region (367/543). We supplement these with a modest number of additional photometric redshifts in the GOODS-N region (30). We measure millimetric redshifts from the radio to submillimeter flux ratios for the unidentified submillimeter sample, assuming an Arp 220 spectral energy distribution. We find a radio flux dependent K-z relation for the radio sources, which we use to estimate redshifts for the remaining radio sources. We determine the star formation rates (SFRs) of the submillimeter sources based on their radio powers and their submillimeter and find that they agree well. The radio data are deep enough to detect star-forming galaxies with SFRs >2000 solar masses per year to z~6. We find galaxies with SFRs up to ~6,000 solar masses per year over the redshift range z=1.5-6, but we see evidence for a turn-down in the SFR distribution function above 2000 solar masses per year.
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