arXiv subjects

Sownak Bose

Publications and source records attributed to Sownak Bose.

At least 127 records · Page 7Linked to original sources

arXiv2020

Extensions to models of the galaxy-halo connection

We explore two widely used empirical models for the galaxy-halo connection, subhalo abundance matching (SHAM) and the halo occupation distribution (HOD) and compare their predictions with the hydrodynamical simulation IllustrisTNG (TNG) for a range of statistics that quantify the galaxy distribution at $n_{\rm gal}\approx1.3\times10^{-3}\,[{\rm Mpc}/h]^{-3}$. We observe that in their most straightforward implementations, both models fail to reproduce the two-point clustering measured in TNG. We find that SHAM models constructed using the relaxation velocity, $V_{\rm relax}$, and the peak velocity, $V_{\rm peak}$, perform best, and match the clustering reasonably well, although neither model captures adequately the one-halo clustering. Splitting the total sample into sub-populations, we discover that SHAM overpredicts the clustering of high-mass, blue, star-forming, and late-forming galaxies and uderpredicts that of low-mass, red, quiescent, and early-forming galaxies. We also study various baryonic effects, finding that subhalos in the dark-mater-only simulation have consistently higher values of their SHAM-proxy properties than their full-physics counterparts. We then consider a two-dimensional implementation of the HOD model augmented with a secondary parameter (environment, velocity anisotropy, $σ^2R_{\rm halfmass}$, and total potential) and tuned so as to match the two-point clustering of the IllustrisTNG galaxies on large scales. We analyze these galaxy populations adopting alternative statistical tools such as galaxy-galaxy lensing, void-galaxy cross-correlations and cumulants of the smoothed density field, finding that the hydrodynamical galaxy distribution disfavors $σ^2 R_{\rm halfmass}$ and the total potential as secondary parameters, while the environment and velocity anisotropy samples are consistent with full-physics across all statistical probes examined.

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arXiv2020

Limitations to the "basic'' HOD model and beyond

We make use of the IllustrisTNG cosmological, hydrodynamical simulations to test fundamental assumptions of the mass-based Halo Occupation Distribution (HOD) approach to modelling the galaxy-halo connection. By comparing the clustering of galaxies measured in the 300 Mpc TNG box (TNG300) with that predicted by the standard (``basic'') HOD model, we find that, on average, the ``basic'' HOD model underpredicts the real-space correlation function in the TNG300 box by $\sim$ 15\% on scales of $1 \ {\rm Mpc}/h < r < 20 \ {\rm Mpc}/h$, which is well beyond the target precision demanded of next-generation galaxy redshift surveys. We perform several tests to establish the robustness of our findings to systematic effects, including the effect of finite box size and the choice of halo finder. In our exploration of ``secondary'' parameters with which to augment the ``basic'' HOD, we find that the local environment of the halo, the velocity dispersion anisotropy, $β$, and the product of the half-mass radius and the velocity dispersion, $σ^2 R_{\rm halfmass}$, are the three most effective measures of assembly bias that help reconcile the ``basic'' HOD-predicted clustering with that in TNG300. In addition, we test other halo properties such as halo spin, formation epoch and halo concentration. We also find that at fixed halo mass, galaxies in one type of environment cluster differently from galaxies in another. We demonstrate that a more complete model of the galaxy-halo connection can be constructed if we combine both mass and local environment information about the halo.

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arXiv2020

The Accuracy of Weak Lensing Simulations

We investigate the accuracy of weak lensing simulations by comparing the results of five independently developed lensing simulation codes run on the same input $N$-body simulation. Our comparison focuses on the lensing convergence maps produced by the codes, and in particular on the corresponding PDFs, power spectra and peak counts. We find that the convergence power spectra of the lensing codes agree to $\lesssim 2\%$ out to scales $\ell \approx 4000$. For lensing peak counts, the agreement is better than $5\%$ for peaks with signal-to-noise $\lesssim 6$. We also discuss the systematic errors due to the Born approximation, line-of-sight discretization, particle noise and smoothing. The lensing codes tested deal in markedly different ways with these effects, but they nonetheless display a satisfactory level of agreement. Our results thus suggest that systematic errors due to the operation of existing lensing codes should be small. Moreover their impact on the convergence power spectra for a lensing simulation can be predicted given its numerical details, which may then serve as a validation test.

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arXiv2020

Rapid Reionization by the Oligarchs: The Case for Massive, UV-Bright, Star-Forming Galaxies with High Escape Fractions

The protagonists of cosmic reionization remain elusive. Faint star-forming galaxies are leading candidates because they are numerous and may have significant ionizing photon escape fractions ($f_{esc}$). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the drop in star-formation density at z>8). We generate an ionizing spectrum for each galaxy in our model and constrain $f_{esc}$ using latest measurements of the reionization timeline (e.g., Ly$α$ damping of quasars and galaxies at z>7). Assuming a constant $f_{esc}$, we find $M_{UV}$<-13.5 galaxies need $f_{esc}=0.21^{+0.06}_{-0.04}$ to complete reionization. The inferred IGM neutral fraction is [0.9, 0.5, 0.1] at z=[8.2, 6.8, 6.2]$\pm$0.2, i.e., the bulk of reionization transpires in 300 Myrs. Inspired by the emergent sample of Lyman Continuum (LyC) leakers that overwhelmingly displays higher-than-average star-formation surface density ($Σ$), we propose a model relating $f_{esc}$ to $Σ$ and find $f_{esc}\proptoΣ^{0.4\pm0.1}$. Since $Σ$ falls by ~2.5 dex between z=8 and z=0, our model explains the humble upper limits on $f_{esc}$ at lower redshifts and its required evolution to ~0.2 at z>6. Within this model, strikingly, <5% of galaxies with $M_{UV}$<-18 (the `oligarchs') account for >80% of the reionization budget. In fact, faint sources ($M_{UV}$>-16) must be relegated to a limited role to ensure high neutral fractions at z=7-8. Shallow faint-end slopes of the UV luminosity function ($α$>-2) and/or $f_{esc}$ distributions skewed toward bright galaxies produce the required late and rapid reionization. We predict LyC leakers like COLA1 (z=6.6, $f_{esc}$~30%, $M_{UV}$=-21.5) become increasingly common towards z~6 and that the drivers of reionization do not lie hidden across the faint-end of the luminosity function, but are already known to us. (abridged)

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arXiv2020

The BUFFALO HST Survey

The Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) is a 101 orbit + 101 parallel Cycle 25 Hubble Space Telescope Treasury program taking data from 2018-2020. BUFFALO will expand existing coverage of the Hubble Frontier Fields (HFF) in WFC3/IR F105W, F125W, and F160W and ACS/WFC F606W and F814W around each of the six HFF clusters and flanking fields. This additional area has not been observed by HST but is already covered by deep multi-wavelength datasets, including Spitzer and Chandra. As with the original HFF program, BUFFALO is designed to take advantage of gravitational lensing from massive clusters to simultaneously find high-redshift galaxies which would otherwise lie below HST detection limits and model foreground clusters to study properties of dark matter and galaxy assembly. The expanded area will provide a first opportunity to study both cosmic variance at high redshift and galaxy assembly in the outskirts of the large HFF clusters. Five additional orbits are reserved for transient followup. BUFFALO data including mosaics, value-added catalogs and cluster mass distribution models will be released via MAST on a regular basis, as the observations and analysis are completed for the six individual clusters.

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arXiv2019

Galaxy Formation with BECDM -- II. Cosmic Filaments and First Galaxies

Bose-Einstein Condensate Dark Matter (BECDM; also known as Fuzzy Dark Matter) is motivated by fundamental physics and has recently received significant attention as a serious alternative to the established Cold Dark Matter (CDM) model. We perform cosmological simulations of BECDM gravitationally coupled to baryons and investigate structure formation at high redshifts ($z \gtrsim 5$) for a boson mass $m=2.5\cdot 10^{-22}~{\rm eV}$, exploring the dynamical effects of its wavelike nature on the cosmic web and the formation of first galaxies. Our BECDM simulations are directly compared to CDM as well as to simulations where the dynamical quantum potential is ignored and only the initial suppression of the power spectrum is considered -- a Warm Dark Matter-like ("WDM") model often used as a proxy for BECDM. Our simulations confirm that "WDM" is a good approximation to BECDM on large cosmological scales even in the presence of the baryonic feedback. Similarities also exist on small scales, with primordial star formation happening both in isolated haloes and continuously along cosmic filaments; the latter effect is not present in CDM. Global star formation and metal enrichment in these first galaxies are delayed in BECDM/"WDM" compared to the CDM case: in BECDM/"WDM" first stars form at $z\sim 13$/$13.5$ while in CDM star formation starts at $z\sim 35$. The signature of BECDM interference, not present in "WDM", is seen in the evolved dark matter power spectrum: although the small scale structure is initially suppressed, power on kpc scales is added at lower redshifts. Our simulations lay the groundwork for realistic simulations of galaxy formation in BECDM.

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arXiv2019

First star-forming structures in fuzzy cosmic filaments

In hierarchical models of structure formation, the first galaxies form in low-mass dark matter potential wells, probing the behavior of dark matter on kiloparsec (kpc) scales. Even though these objects are below the detection threshold of current telescopes, future missions will open an observational window into this emergent world. In this Letter we investigate how the first galaxies are assembled in a `fuzzy' dark matter (FDM) cosmology where dark matter is an ultralight $\sim 10^{-22}$~eV boson and the primordial stars are expected to form along dense dark matter filaments. Using a first-of-its-kind cosmological hydrodynamical simulation, we explore the interplay between baryonic physics and unique wavelike features inherent to FDM. In our simulation, the dark matter filaments show coherent interference patterns on the boson de Broglie scale and develop cylindrical soliton-like cores which are unstable under gravity and collapse into kpc-scale spherical solitons. Features of the dark matter distribution are largely unaffected by the baryonic feedback. On the contrary, the distributions of gas and stars, which do form along the entire filament, exhibit central cores imprinted by dark matter -- a smoking gun signature of FDM.

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arXiv2019

The little things matter: relating the abundance of ultrafaint satellites to the hosts' assembly history

Ultrafaint dwarf galaxies ($M_\star\lesssim10^{5-6}\,{\rm M}_\odot$) are relics of an early phase of galaxy formation. They contain some of the oldest and most metal-poor stars in the Universe which likely formed before the epoch of hydrogen reionisation. These galaxies are so faint that they can only be detected as satellites of the Milky Way. They are so small that they are just barely resolved in current cosmological hydrodynamics simulations. Here we combine very high resolution cosmological $N$-body simulations with a semi-analytic model of galaxy formation to study the demographics and spatial distribution of ultrafaint satellites in Milky Way-mass haloes. We show that the abundance of these galaxies is correlated with the assembly history of the host halo: at fixed mass, haloes assembled earlier contain, on average, more ultrafaint satellites today than haloes assembled later. We identify simulated galactic haloes that experience an ancient Gaia-Enceladus-Sausage-like and a recent LMC-like accretion event and find that the former occurs in 33% of the sample and the latter in 9%. Only 3% experience both events and these are especially rich in ultrafaint satellites, most acquired during the ancient accretion event. Our models predict that the radial distribution of satellites is more centrally concentrated in early-forming haloes. Accounting for the depletion of satellites by tidal interactions with the central disc, we find a very good match to the observed radial distribution of satellites in the Milky Way over the entire radial range. This agreement is mainly due to the ability of our model to track 'orphan' galaxies after their subhaloes fall below the resolution limit of the simulation.

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arXiv2019

On the road to percent accuracy: nonlinear reaction of the matter power spectrum to dark energy and modified gravity

We present a general method to compute the nonlinear matter power spectrum for dark energy and modified gravity scenarios with percent-level accuracy. By adopting the halo model and nonlinear perturbation theory, we predict the reaction of a $Λ$CDM matter power spectrum to the physics of an extended cosmological parameter space. By comparing our predictions to $N$-body simulations we demonstrate that with no-free parameters we can recover the nonlinear matter power spectrum for a wide range of different $w_0$-$w_a$ dark energy models to better than 1% accuracy out to $k \approx 1 \, h \, {\rm Mpc}^{-1}$. We obtain a similar performance for both DGP and $f(R)$ gravity, with the nonlinear matter power spectrum predicted to better than 3% accuracy over the same range of scales. When including direct measurements of the halo mass function from the simulations, this accuracy improves to 1%. With a single suite of standard $Λ$CDM $N$-body simulations, our methodology provides a direct route to constrain a wide range of non-standard extensions to the concordance cosmology in the high signal-to-noise nonlinear regime.

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arXiv2019

No cores in dark matter-dominated dwarf galaxies with bursty star formation histories

Measurements of the rotation curves of dwarf galaxies are often interpreted as requiring a constant density core at the centre, at odds with the "cuspy" inner profiles predicted by $N$-body simulations of cold dark matter (CDM) haloes. It has been suggested that this conflict could be resolved by fluctuations in the inner gravitational potential caused by the periodic removal of gas following bursts of star formation. Earlier work has suggested that core formation requires a bursty and extended star formation history (SFH). Here we investigate the structure of CDM haloes of dwarf galaxies ($M_{\rm DM} \sim 10^9-5\times10^{10}\,{\rm M}_\odot$) formed in the APOSTLE ('A Project of Simulating the Local Environment') and AURIGA cosmological hydrodynamic simulations. Our simulations have comparable or better resolution than others that make cores ($M_{\rm gas} \sim 10^4\,{\rm M}_\odot$, gravitational softening $\sim 150$ pc). Yet, we do not find evidence of core formation at {\it any} mass or any correlation between the inner slope of the DM density profile and temporal variations in the SFH. APOSTLE and AURIGA dwarfs display a similar diversity in their cumulative SFHs to available data for Local Group dwarfs. Dwarfs in both simulations are DM-dominated on all resolved scales at all times, likely limiting the ability of gas outflows to alter significantly the central density profiles of their haloes. We conclude that recurrent bursts of star formation are not sufficient to cause the formation of cores, and that other conditions must also be met for baryons to be able to modify the central DM cusp.

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arXiv2019

ETHOS - an Effective Theory of Structure Formation: detecting dark matter interactions through the Lyman-$α$ forest

We perform a series of cosmological hydrodynamic simulations to investigate the effects of non-gravitational dark matter (DM) interactions on the intergalactic medium (IGM). In particular, we use the ETHOS framework (Cyr-Racine et al. 2016; Vogelsberger et al. 2016) to compare statistics of the Lyman-$α$ forest in cold dark matter (CDM) with an alternative model in which the DM couples strongly with a relativistic species in the early universe. These models are characterised by a cutoff in the linear power spectrum, followed by a series of 'dark acoustic oscillations' (DAOs) on sub-dwarf scales. While the primordial cutoff delays the formation of the first galaxies, structure builds-up more rapidly in the interacting DM model compared to CDM. We show that although DAOs are quickly washed away in the non-linear clustering of DM at $z\lesssim10$, their signature can be imprinted prominently in the Lyman-$α$ flux power spectrum at $z>5$. On scales larger than the cutoff ($k\sim0.08$ s/km for the specific model considered here), the relative difference to CDM is reminiscent of a warm dark matter (WDM) model with a similar initial cutoff; however, the redshift evolution on smaller scales is distinctly different. The appearance and disappearance of DAOs in the Lyman-$α$ flux spectrum provides a powerful way to distinguish interacting DM models from WDM and, indeed, variations in the thermal history of the IGM that may also induce a small-scale cutoff.

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arXiv2019

Revealing the galaxy-halo connection in IllustrisTNG

We use the IllustrisTNG (TNG) simulations to explore the galaxy-halo connection as inferred from state-of-the-art cosmological, magnetohydrodynamical simulations. With the high mass resolution and large volume achieved by combining the 100 Mpc (TNG100) and 300 Mpc (TNG300) volumes, we establish the mean occupancy of central and satellite galaxies and their dependence on the properties of the dark matter haloes hosting them. We derive best-fitting HOD parameters from TNG100 and TNG300 for target galaxy number densities of $\bar{n}_g = 0.032\,h^3$Mpc$^{-3}$ and $\bar{n}_g = 0.016\,h^3$Mpc$^{-3}$, respectively, corresponding to a minimum galaxy stellar mass of $M_\star\sim1.9\times10^9\,{\rm M}_\odot$ and $M_\star\sim3.5\times10^9\,{\rm M}_\odot$, respectively, in hosts more massive than $10^{11}\,{\rm M}_\odot$. Consistent with previous work, we find that haloes located in dense environments, with low concentrations, later formation times, and high angular momenta are richest in their satellite population. At low mass, highly-concentrated haloes and those located in overdense regions are more likely to contain a central galaxy. The degree of environmental dependence is sensitive to the definition adopted for the physical boundary of the host halo. We examine the extent to which correlations between galaxy occupancy and halo properties are independent and demonstrate that HODs predicted by halo mass and present-day concentration capture the qualitative dependence on the remaining halo properties. At fixed halo mass, concentration is a strong predictor of the stellar mass of the central galaxy, which may play a defining role in the fate of the satellite population. The radial distribution of satellite galaxies, which exhibits a universal form across a wide range of host halo mass, is described accurately by the best-fit NFW density profile of their host haloes.

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arXiv2019

Simulating the dark matter decay signal from the Perseus galaxy cluster

The nearby Perseus galaxy cluster is a key target for indirect detection searches for decaying dark matter. We use the C-EAGLE simulations of galaxy clusters to predict the flux, width and shape of a dark matter decay line, paying particular attention to the unexplained 3.55keV line detected in the spectra of some galaxies and clusters, and the upcoming XRISM X-ray observatory mission. We show that the line width in C-EAGLE clusters similar to Perseus is typically [600-800]$\mathrm{kms^{-1}}$, and therefore narrower than the amplitude of the velocity dispersion of galaxies in the cluster. Haloes that are significantly disturbed can, however, exhibit galaxy velocity dispersions higher than $1000\mathrm{kms^{-1}}$, and in this case will show a large difference between the line profiles of on- and off-center observations. We show that the line profile is likely to be slightly asymmetric, but still well approximated by a Gaussian at the 10% level, and that the halo asymmetry can lead to fluxes that vary by a factor of two. In summary, we predict that, if the previously reported 3.55keV line detections do originate from dark matter decay, the XRISM mission will detect a line with a roughly Gaussian profile at a rest frame energy of 3.55keV, with a width $>600\mathrm{kms^{-1}}$ and flux approximately in the range $[4-9]\times10^{-8}\mathrm{counts/sec/cm^{2}}$.

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arXiv2019

Dark Matter Science in the Era of LSST

Astrophysical observations currently provide the only robust, empirical measurements of dark matter. In the coming decade, astrophysical observations will guide other experimental efforts, while simultaneously probing unique regions of dark matter parameter space. This white paper summarizes astrophysical observations that can constrain the fundamental physics of dark matter in the era of LSST. We describe how astrophysical observations will inform our understanding of the fundamental properties of dark matter, such as particle mass, self-interaction strength, non-gravitational interactions with the Standard Model, and compact object abundances. Additionally, we highlight theoretical work and experimental/observational facilities that will complement LSST to strengthen our understanding of the fundamental characteristics of dark matter.

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arXiv2019

The Santiago-Harvard-Edinburgh-Durham void comparison II: unveiling the Vainshtein screening using weak lensing

We study cosmic voids in the normal-branch Dvali-Gabadadze-Porrati (nDGP) braneworld models, which are representative of a class of modified gravity theories where deviations from General Relativity are usually hidden by the Vainshtein screening in high-density environments. This screening is less efficient away from these environments, which makes voids ideally suited for testing this class of models. We use N-body simulations of $Λ$-cold dark matter ($Λ$CDM) and nGDP universes, where dark matter haloes are populated with mock galaxies that mimic the clustering and number densities of the BOSS CMASS galaxy sample. We measure the force, density and weak lensing profiles around voids identified with six different algorithms. Compared to $Λ$CDM, voids in nDGP are more under-dense due to the action of the fifth force that arises in these models, which leads to a faster evacuation of matter from voids. This leaves an imprint on the weak lensing tangential shear profile around nDGP voids, an effect that is particularly strong for 2D underdensities that are identified in the plane-of-the-sky. We make predictions for the feasibility of distinguishing between nDGP and $Λ$CDM using void lensing in upcoming large-scale surveys such as LSST and Euclid. We compare with the analysis of voids in chameleon gravity theories and find that the weak lensing signal for 3D voids is similar to nDGP, whereas for 2D voids the differences with $Λ$CDM are much stronger for the chameleon gravity case, a direct consequence of the different screening mechanisms operating in these theories.

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arXiv2018

A redshift-independent efficiency model: star formation and stellar masses in dark matter halos at z>4

We explore the connection between the UV luminosity functions (LFs) of high-$z$ galaxies and the distribution of stellar masses and star-formation histories (SFHs) in their host dark matter halos. We provide a baseline for a redshift-independent star-formation efficiency model to which observations and models can be compared. Our model assigns a star-formation rate (SFR) to each dark matter halo based on the growth rate of the halo and a redshift-independent star-formation efficiency. The dark matter halo accretion rate is obtained from a high-resolution $N$-body simulation in order to capture the stochasticity in accretion histories and to obtain spatial information for the distribution of galaxies. The halo mass dependence of the star-formation efficiency is calibrated at $z=4$ by requiring a match to the observed UV LF at this redshift. The model then correctly predicts the observed UV LF at $z=5-10$. We present predictions for the UV luminosity and stellar mass functions, \textit{JWST} number counts, and SFHs. In particular, we find a stellar-to-halo mass relation at $z=4-10$ that scales with halo mass at $M_{\rm h}<10^{11}~M_{\odot}$ as $M_{\star}\propto M_{\rm h}^2$, with a normalization that is higher than the relation inferred at $z=0$. The average SFRs increase as a function of time to $z=4$, although there is significant scatter around the average: about 6\% of the $z=4$ galaxies show no significant mass growth. Using these SFHs, we present redshift-dependent UV-to-SFR conversion factors, mass return fractions, and mass-to-light ratios for different intial mass functions and metallicities, finding that current estimates of the cosmic SFR density at $z\sim10$ may be overestimated by $\sim0.1-0.2~\mathrm{dex}$.

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arXiv2018

The signal of decaying dark matter with hydrodynamical simulations

Dark matter particles may decay, emitting photons. Drawing on the EAGLE family of hydrodynamic simulations of galaxy formation -- including the APOSTLE and C-EAGLE simulations -- we assess the systematic uncertainties and scatter on the decay flux from different galaxy classes, from Milky Way satellites to galaxy clusters, and compare our results to studies of the 3.55~keV line. We demonstrate that previous detections and non-detections of this line are consistent with a dark matter interpretation. For example, in our simulations the width of the the dark matter decay line for Perseus-analogue galaxy clusters lies in the range 1300-1700~\kms. Therefore, the non-detection of the 3.55~keV line in the centre of the Perseus cluster by the {\it Hitomi} collaboration is consistent with detections by other instruments. We also consider trends with stellar and halo mass and evaluate the scatter in the expected fluxes arising from the anisotropic halo mass distribution and from object-to-object variations. We provide specific predictions for observations with {\it XMM-Newton} and with the planned X-ray telescopes {\it XRISM} and {\it ATHENA}. If future detections of unexplained X-ray lines match our predictions, including line widths, we will have strong evidence that we have discovered the dark matter.

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arXiv2018

The imprint of cosmic reionisation on the luminosity function of galaxies

The (re)ionisation of hydrogen in the early universe has a profound effect on the formation of the first galaxies: by raising the gas temperature and pressure, it prevents gas from cooling into small haloes thus affecting the abundance of present-day small galaxies. Using the Galform semi-analytic model of galaxy formation, we show that two key aspects of the reionisation process -- when reionisation takes place and the characteristic scale below which it suppresses galaxy formation -- are imprinted in the luminosity function of dwarf galaxies. We focus on the luminosity function of satellites of galaxies like the Milky Way and the LMC, which is easier to measure than the luminosity function of the dwarf population as a whole. Our results show that the details of these two characteristic properties of reionisation determine the shape of the luminosity distribution of satellites in a unique way, and is largely independent of the other details of the galaxy formation model. Our models generically predict a bimodality in the distribution of satellites as a function of luminosity: a population of faint satellites and population of bright satellites separated by a 'valley' forged by reionisation. We show that this bimodal distribution is present at high statistical significance in the combined satellite luminosity function of the Milky Way and M31. We make predictions for the expected number of satellites around LMC-mass dwarfs where the bimodality may also be measurable in future observational programmes. Our preferred model predicts a total of $26 \pm 10$ (68 per cent confidence) satellites brighter than ${\rm M}_V=0$ in LMC-mass systems.

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