arXiv subjects

Fergus Cullen

Publications and source records attributed to Fergus Cullen.

58 records · Page 4Linked to original sources

arXiv2019

Optimal neighbourhood to nurture giants: a fundamental link between star forming galaxies and direct collapse black holes

Massive $10^{4-5}\rm\ M_{\odot}$ black hole seeds resulting from the \textit{direct} collapse of pristine gas require a metal-free atomic cooling halo with extremely low H$_2$ fraction, allowing the gas to cool isothermally in the presence of atomic hydrogen. In order to achieve this chemo-thermodynamical state, the gas needs to be irradiated by both: Lyman-Werner (LW) photons in the energy range $11.2-13.6$ eV capable of photodissociating H$_2$, and $0.76$ eV photons capable of photodetaching H$^-$. Employing cosmological simulations capable of creating the first galaxies in high resolution, we explore if there exists a subset that favours DCBH formation in their vicinity. We find a fundamental relation between the maximum distance at which a galaxy can cause DCBH formation and its star formation rate (SFR), which automatically folds in the chemo-thermodynamical effects of both H$_2$ photo-dissociation and H$^-$ photo-detachment. This is in contrast to the scatter in the LW flux parameter seen at the maximum distance. It shows up to a 3 order of magnitude scatter, which can be interpreted as a scatter in `J$_{crit}$'. Thus, computing the rates and/or the LW flux from a galaxy is no longer necessary to identify neighbouring sites of DCBH formation, as our relation allows one to distinguish regions where DCBH formation could be triggered in the vicinity of a galaxy of a given SFR.

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arXiv2017

The Star Formation Main Sequence in the Hubble Space Telescope Frontier Fields

We investigate the relation between the star formation rate (SFR) and the stellar mass (M), i.e. the Main Sequence (MS) relation of star-forming galaxies, at 1.3<=z<6 in the first four HST Frontier Fields, based on rest-frame UV observations. Gravitational lensing combined with deep HST observations allows us to extend the analysis of the MS down to stellar masses as low as logM/Msun~7.5 at z<~4 and logM/Msun~8 at higher redshifts, a factor of ~10 below most previous results. We perform an accurate simulation to take into account the effect of observational uncertainties on the MS and correct for the Eddington bias. This step allows us to reliably measure the MS and in particular its slope. While the normalization increases with redshift, we fit an unevolving and approximately linear slope. We nicely extend to lower masses the results of brighter surveys. Thanks to the large dynamic range in mass and by making use of the simulation, we analyzed any possible mass dependence of the dispersion around the MS. We find tentative evidence that the scatter decreases with increasing stellar masses, suggesting a larger variety of star formation histories in low mass galaxies. This trend agrees with theoretical predictions, and is explained as either a consequence of the smaller number of progenitors of low mass galaxies in a hierarchical scenario and/or of the efficient but intermittent stellar feedback processes in low mass halos. Finally, we observe an increase in the SFR per unit stellar mass with redshift milder than predicted by theoretical models, implying a still incomplete understanding of the processes responsible for galaxy growth.

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arXiv2017

Effects of binary stellar populations on direct collapse black hole formation

The critical Lyman--Werner flux required for direct collapse blackholes (DCBH) formation, or J$_{crit}$, depends on the shape of the irradiating spectral energy distribution (SED). The SEDs employed thus far have been representative of {realistic} single stellar populations. We study the effect of binary stellar populations on the formation of DCBH, as a result of their contribution to the Lyman--Werner radiation field. Although binary populations with ages $>$ 10 Myr yield a larger LW photon output, we find that the corresponding values of J$_{crit}$ can be up to 100 times higher than single stellar populations. We attribute this to the shape of the binary SEDs as they produce a sub--critical rate of H$^-$ photodetaching 0.76 eV photons as compared to single stellar populations, reaffirming the role that H$^-$ plays in DCBH formation. This further corroborates the idea that DCBH formation is better understood in terms of a critical region in the H$_2$--H$^-$ photo--destruction rate parameter space, rather than a single value of LW flux.

astro-ph.GA↗

arXiv2014

Determining the stellar masses of submillimetre galaxies: the critical importance of star formation histories

Submillimetre (submm) galaxies are among the most rapidly star-forming and most massive high-redshift galaxies; thus, their properties provide important constraints on galaxy evolution models. However, there is still a debate about their stellar masses and their nature in the context of the general galaxy population. To test the reliability of their stellar mass determinations, we used a sample of simulated submm galaxies for which we derived stellar masses via spectral energy distribution (SED) modelling (with Grasil, Magphys, Hyperz and LePhare) adopting various star formation histories (SFHs). We found that the assumption of SFHs with two independent components leads to the most accurate stellar masses. Exponentially declining SFHs (tau) lead to lower masses (albeit still consistent with the true values), while the assumption of single-burst SFHs results in a significant mass underestimation. Thus, we conclude that studies based on the higher masses inferred from fitting the SEDs of real submm galaxies with double SFHs are most likely to be correct, implying that submm galaxies lie on the high-mass end of the main sequence of star-forming galaxies. This conclusion appears robust to assumptions of whether or not submm galaxies are driven by major mergers, since the suite of simulated galaxies modelled here contains examples of both merging and isolated galaxies. We identified discrepancies between the true and inferred stellar ages (rather than the dust attenuation) as the primary determinant of the success/failure of the mass recovery. Regardless of the choice of SFH, the SED-derived stellar masses exhibit a factor of ~2 scatter around the true value; this scatter is an inherent limitation of the SED modelling due to simplified assumptions. Finally, we found that the contribution of active galactic nuclei does not have any significant impact on the derived stellar masses.

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