arXiv subjects

F. Fraternali

Publications and source records attributed to F. Fraternali.

At least 37 records · Page 2Linked to original sources

arXiv2020

Massive disc galaxies in cosmological hydrodynamical simulations are too dark matter-dominated

We investigate the disc-halo connection in massive (Mstar/Msun>5e10) disc galaxies from the cosmological hydrodynamical simulations EAGLE and IllustrisTNG, and compare it with that inferred from the study of HI rotation curves in nearby massive spirals from the Spitzer Photometry and Accurate Rotation Curves (SPARC) dataset. We find that discrepancies between the the simulated and observed discs arise both on global and on local scales. Globally, the simulated discs inhabit halos that are a factor ~4 (in EAGLE) and ~2 (in IllustrisTNG) more massive than those derived from the rotation curve analysis of the observed dataset. We also use synthetic rotation curves of the simulated discs to demonstrate that the recovery of the halo masses from rotation curves are not systematically biased. We find that the simulations predict dark-matter dominated systems with stellar-to-total enclosed mass ratios that are a factor of 1.5-2 smaller than real galaxies at all radii. This is an alternative manifestation of the `failed feedback problem', since it indicates that simulated halos hosting massive discs have been too inefficient at converting their baryons into stars, possibly due to an overly efficient stellar and/or AGN feedback implementation.

astro-ph.GA↗

arXiv2019

Bending dominated response of layered mechanical metamaterials alternating pentamode lattices and confinement plates

A numerical study on the elastic response of single- and multi-layer systems formed by alternating pentamode lattices and stiffening plates is presented. Finite element simulations are conducted to analyze the dependence of the effective elastic moduli of such structures upon suitable aspect ratios, which characterize the geometry of the generic pentamode layer at the micro- and macro-scale, and the lamination scheme of the layered structure. The given numerical results highlight that the examined structures exhibit bending-dominated response, and are able to achieve low values of the effective shear modulus and, contemporarily, high values of the effective compression modulus. We are lead to conclude that confined pentamode lattices can be regarded as novel metamaterials that are well suited for seismic isolation and impact protection purposes. Their elastic response can be finely tuned by playing with several geometrical and mechanical design variables.

physics.app-ph↗

arXiv2019

Mechanical modeling of innovative metamaterials alternating pentamode lattices and confinement plates

This study examines the mechanical behavior of a novel class of mechanical metamaterials alternating pentamode lattices and stiffening plates. The unit cell of such lattices consists of a sub-lattice of the face cubic-centered unit cell typically analyzed in the current literature on pentamode materials. The studied systems exhibit only three soft deformation modes in the infinitesimal stretch-dominated regime, as opposed to the five zero-energy modes of unconfined pentamode lattices. We develop analytical formulae for the vertical and bending stiffness properties and study the dependence of such quantities on the main design parameters: the lattice constant, the solid volume fraction, the cross-section area of the rods, and the layer thickness. A noteworthy result is that the effective compression modulus of the analyzed structures is equal to two thirds of the Young modulus of the stiffest isotropic elastic networks currently available in the literature, being accompanied by zero-rigidity against infinitesimal shear and twisting mechanisms. The use of the proposed metamaterials as novel seismic-isolation devices and impact-protection equipment is discussed by drawing comparisons with the response of alternative devices already available or under development.

physics.app-ph↗

arXiv2019

The volumetric star formation law in the Milky Way

Several open questions on galaxy formation and evolution have their roots in the lack of a universal star formation law, that could univocally link the gas properties, e.g. its density, to the star formation rate (SFR) density. In a recent paper, we used a sample of nearby disc galaxies to infer the volumetric star formation (VSF) law, a tight correlation between the gas and the SFR volume densities derived under the assumption of hydrostatic equilibrium for the gas disc. However, due to the dearth of information about the vertical distribution of the SFR in these galaxies, we could not find a unique slope for the VSF law, but two alternative values. In this paper, we use the scale height of the SFR density distribution in our Galaxy adopting classical Cepheids (age$\lesssim 200$ Myr) as tracers of star formation. We show that this latter is fully compatible with the flaring scale height expected from gas in hydrostatic equilibrium. These scale heights allowed us to convert the observed surface densities of gas and SFR into the corresponding volume densities. Our results indicate that the VSF law $ρ_\mathrm{SFR} \propto ρ_\mathrm{gas}^α$ with $α\approx 2$ is valid in the Milky Way as well as in nearby disc galaxies.

astro-ph.GA↗

arXiv2019

HALOGAS: the properties of extraplanar HI in disc galaxies

We present a systematic study of the extraplanar gas (EPG) in a sample of 15 nearby late-type galaxies at intermediate inclinations using publicly available, deep interferometric HI data from the HALOGAS survey. For each system we mask the HI emission coming from the regularly rotating disc and use synthetic datacubes to model the leftover "anomalous" HI flux. Our model consists of a smooth, axisymmetric thick component described by 3 structural and 4 kinematical parameters, which are fit to the data via a Bayesian MCMC approach. We find that extraplanar HI is nearly ubiquitous in disc galaxies, as we fail to detect it in only two of the systems with the poorest spatial resolution. The EPG component encloses ~5-25% of the total HI mass, with a mean value of 14%, and has a typical thickness of a few kpc, incompatible with expectations based on hydrostatic equilibrium models. The EPG kinematics is remarkably similar throughout the sample, and consists of a lagging rotation with typical vertical gradients of about -10 km/s/kpc, a velocity dispersion of 15-30 km/s and, for most galaxies, a global inflow in both the vertical and radial directions with speeds of 20-30 km/s. The EPG HI masses are in excellent agreement with predictions from simple models of the galactic fountain powered by stellar feedback. The combined effect of photo-ionisation and interaction of the fountain material with the circumgalactic medium can qualitatively explain the kinematics of the EPG, but dynamical models of the galactic fountain are required to fully test this framework.

astro-ph.GA↗

arXiv2018

The angular momentum of disc galaxies at z=1

We investigate the relation between stellar mass and specific stellar angular momentum, or `Fall relation', for a sample of 17 isolated, regularly rotating disc galaxies at z=1. All galaxies have a) rotation curves determined from Halpha emission-line data; b) HST imaging in optical and infrared filters; c) robust determinations of their stellar masses. We use HST images in f814w and f160w filters, roughly corresponding to rest-frames B and I bands, to extract surface brightness profiles for our systems. We robustly bracket the specific angular momentum by assuming that rotation curves beyond the outermost Halpha rotation point stay either flat or follow a Keplerian fall-off. By comparing our measurements with those determined for disc galaxies in the local Universe, we find no evolution in the Fall relation in the redshift range 0<z<1, regardless of the band used and despite the uncertainties in the stellar rotation curves at large radii. This result holds unless stellar masses at z=1 are systematically underestimated by more than 50%. Our findings are compatible with expectations based on a LCDM cosmological framework and support a scenario where both the stellar Tully-Fisher and mass-size relations for spirals do not evolve significantly in this redshift range.

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arXiv2018

Starburst to quiescent from HST/ALMA: Stars and dust unveil minor mergers in submillimeter galaxies at $z \sim 4.5$

Dust-enshrouded, starbursting, submillimeter galaxies (SMGs) at $z \geq 3$ have been proposed as progenitors of $z \geq 2$ compact quiescent galaxies (cQGs). To test this connection, we present a detailed spatially resolved study of the stars, dust and stellar mass in a sample of six submillimeter-bright starburst galaxies at $z \sim 4.5$. The stellar UV emission probed by HST is extended, irregular and shows evidence of multiple components. Informed by HST, we deblend Spitzer/IRAC data at rest-frame optical finding that the systems are undergoing minor mergers, with a typical stellar mass ratio of 1:6.5. The FIR dust continuum emission traced by ALMA locates the bulk of star formation in extremely compact regions (median $r_{\rm{e}} = 0.70 \pm 0.29$ kpc) and it is in all cases associated with the most massive component of the mergers (median $\log (M_{*}/M_{\odot}) = 10.49 \pm 0.32$). We compare spatially resolved UV slope ($β$) maps with the FIR dust continuum to study the infrared excess ($\rm{IRX} = L_{\rm{IR}}/L_{\rm{UV}}$)-$β$ relation. The SMGs display systematically higher $\rm{IRX}$ values than expected from the nominal trend, demonstrating that the FIR and UV emissions are spatially disconnected. Finally, we show that the SMGs fall on the mass-size plane at smaller stellar masses and sizes than cQGs at $z = 2$. Taking into account the expected evolution in stellar mass and size between $z = 4.5$ and $z = 2$ due to the ongoing starburst and mergers with minor companions, this is in agreement with a direct evolutionary connection between the two populations.

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arXiv2018

ALMA view of a massive spheroid progenitor: a compact rotating core of molecular gas in an AGN host at z=2.226

We present ALMA observations at 107.291 GHz (band 3) and 214.532 GHz (band 6) of GMASS 0953, a star-forming galaxy at z=2.226 hosting an obscured AGN that has been proposed as a progenitor of compact quiescent galaxies (QG). We measure for the first time the size of the dust and molecular gas emission of GMASS 0953 that we find to be extremely compact ($\sim$1 kpc). This result, coupled with a very high ISM density (n$\sim$10$^{5.5}$ cm$^{-3}$), a low gas mass fraction ($\sim$0.2) and a short gas depletion timescale ($\sim$150 Myr) imply that GMASS 0953 is experiencing an episode of intense star-formation in its central region that will rapidly exhaust its gas reservoirs, likely aided by AGN-induced feedback, confirming its fate as a compact QG. Kinematic analysis of the CO(6-5) line shows evidence of rapidly-rotating gas ($V_{rot}$=320$^{+92}_{-53}$ km s$^{-1}$), as observed also in a handful of similar sources at the same redshift. On-going quenching mechanisms could either destroy the rotation or leave it intact leading the galaxy to evolve into a rotating QG.

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arXiv2018

Alone on a wide wide sea. The origin of SECCO 1, an isolated star-forming gas cloud in the Virgo cluster

SECCO1 is an extremely dark, low-mass (M_star=10^5 M_sun), star-forming stellar system lying in the Low Velocity Cloud (LVC) substructure of the Virgo cluster of galaxies, and hosting several HII regions. Here we review our knowledge of this remarkable system, and present the results of (a) additional analysis of our panoramic spectroscopic observations with MUSE, (b) the combined analysis of Hubble Space Telescope and MUSE data, and (c) new narrow-band observations obtained with OSIRIS@GTC to search for additional HII regions in the surroundings of the system. We provide new evidence supporting an age as young as 4 Myr for the stars that are currently ionising the gas in SECCO1. We identify only one new promising candidate HII region possibly associated with SECCO1, thus confirming the extreme isolation of the system. We also identify three additional candidate pressure-supported dark clouds in Virgo among the targets of the SECCO survey. Various possible hypotheses for the nature and origin of SECCO1 are considered and discussed, also with the help of dedicated hydrodynamical simulations showing that a hydrogen cloud with the characteristics of SECCO1 can likely survive for >1 Gyr while traveling within the LVC Intra Cluster Medium.

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arXiv2018

Molecular gas in AzTEC/C159: a star-forming disk galaxy 1.3Gyr after the Big Bang

We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at $z=4.567$. We secured $^{12}$CO molecular line detections for the $J=2\to1$ and $J=5\to4$ transitions using the Karl G. Jansky VLA and the NOEMA interferometer. The broad (FWHM$\sim750\,{\rm km\,s}^{-1}$) and tentative double-peaked profiles of both $^{12}$CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk as previously revealed from [CII] 158$μ$m line observations. Based on the $^{12}$CO(2$\to$1) emission line we derived $L'_{\rm{CO}}=(3.4\pm0.6)\times10^{10}{\rm \,K\,km\,s}^{-1}{\rm \,pc}^{2}$, that yields a molecular gas mass of $M_{\rm H_2 }(α_{\rm CO}/4.3)=(1.5\pm0.3)\times 10^{11}{\rm M}_\odot$ and unveils a gas-rich system with $μ_{\rm gas}(α_{\rm CO}/4.3)\equiv M_{\rm H_2}/M_\star=3.3\pm0.7$. The extreme star formation efficiency (SFE) of AzTEC/C159, parametrized by the ratio $L_{\rm{IR}}/L'_{\rm{CO}}=(216\pm80)\, {\rm L}_{\odot}{\rm \,(K\,km\,s}^{-1}{\rm \,pc}^{2})^{-1}$, is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies (ULIRGs) and SMGs. Likewise, the $^{12}$CO(5$\to$4)/CO(2$\to$1) line brightness temperature ratio of $r_{52}= 0.55\pm 0.15$ is consistent with high excitation conditions, similar to that observed in SMGs. We constrained the value for the $L'_{\text{CO}}-{\rm H}_2$ mass conversion factor in AzTEC/C159, i.e. $α_{\text{CO}}=3.9^{+2.7}_{-1.3}{\rm \,M}_{\odot}{\rm \,K}^{-1}{\rm \,km}^{-1}{\rm \,s\,pc}^{-2}$, that is consistent with a self-gravitating molecular gas distribution as observed in local star-forming disk galaxies. Cold gas streams from cosmological filaments might be fueling a gravitationally unstable gas-rich disk in AzTEC/C159, which breaks into giant clumps forming stars as efficiently as in merger-driven systems and generate high gas excitation.

astro-ph.GA↗

arXiv2018

Kinematics of the SN Refsdal host revealed by MUSE: a regularly rotating spiral galaxy at z~1.5

We use Multi Unit Spectroscopic Explorer (MUSE) observations of the galaxy cluster MACS J1149.5+2223 to explore the kinematics of the grand-design spiral galaxy Sp1149 hosting the SN Refsdal. Sp1149 lies at $z\simeq1.49$, has a stellar mass $M_*\simeq5\times10^9 \, \mathrm{M_\odot}$, a star-formation rate $\mathrm{SFR} \simeq1-6 \, \mathrm{M_\odot/yr}$ and represents a likely progenitor of a Milky-Way-like galaxy. All the four multiple images of Sp1149 in our data show strong OII-line emissions pointing to a clear rotation pattern. We take advantage of the gravitational lensing magnification effect ($\simeq 4 \times$) on the OII emission of the least distorted image to fit 3D kinematic models to the MUSE data-cube and derive the rotation curve and the velocity dispersion profile of Sp1149. We find that the rotation curve steeply rises, peaks at $R\simeq1$ kpc and then (initially) declines and flattens to an average $V_\mathrm{flat} = 128^{+29}_{-19}$ km/s. The shape of the rotation curve is well determined but the actual value of $V_\mathrm{flat}$ is quite uncertain because of the nearly face-on configuration of the galaxy. The intrinsic velocity dispersion due to gas turbulence is almost constant across the entire disc with an average of $27\pm5$ km/s. This value is consistent with $z=0$ measurements in the ionized gas component and a factor 2-4 lower than other estimates in different galaxies at similar redshifts. The average stellar-to-total mass fraction is of the order of one fifth. Our kinematic analysis returns the picture of a regular star-forming, mildly turbulent, rotation-dominated ($V / σ\simeq5$) spiral galaxy in a 4 Gyr old Universe.

astro-ph.GA↗

arXiv2018

Molecular outflow and feedback in the obscured Quasar XID2028 revealed by ALMA

We imaged with ALMA and ARGOS/LUCI the molecular gas and the dust and stellar continuum in XID2028, an obscured QSO at z=1.593, where the presence of a massive outflow in the ionized gas component traced by the [O III]5007 emission has been resolved up to 10 kpc. This target represents a unique test case to study QSO 'feedback in action' at the peak epoch of AGN-galaxy coevolution. The QSO has been detected in the CO(5-4) transition and in the 1.3mm continuum, at ~30 and ~20 σ significance respectively, with both emissions confined in the central (<4 kpc) radius area. Our analysis suggests the presence of a fast rotating molecular disc (v~400 km/s) on very compact scales, and well inside the galaxy extent seen in the rest-frame optical light (~10 kpc, as inferred from the LUCI data). Adding available measurements in additional two CO transitions, CO(2-1) and CO(3-2), we could derive a total gas mass of ~10$^{10}$ M$_\odot$, thanks to a critical assessment of CO excitation and the comparison with Rayleigh-Jeans continuum estimate. This translates into a very low gas fraction (<5%) and depletion time scales of 40-75 Myr, reinforcing the result of atypical gas consumption conditions in XID2028, possibly due to feedback effects on the host galaxy. Finally, we also detect at ~5σ the presence of high velocity CO gas, which we interpret as a signature of galaxy-scale molecular outflow, spatially coincident with the ionised gas outflow. XID2028 represents therefore a unique case where the measurement of total outflowing mass (~500-800 M$_\odot$/yr) including the molecular and atomic components, in both the ionised and neutral phases, has been attempted for a high-z QSO.

astro-ph.GA↗

arXiv2017

The first all-sky view of the Milky Way stellar halo with Gaia+2MASS RR Lyrae

We exploit the first \gaia data release to study the properties of the Galactic stellar halo as traced by RR Lyrae. We demonstrate that it is possible to select a pure sample of RR Lyrae using only photometric information available in the Gaia+2MASS catalogue. The final sample contains about 21600 RR Lyrae covering an unprecedented fraction ($\sim60\%$) of the volume of the Galactic inner halo ($\text{R}<28$ kpc). We study the morphology of the stellar halo by analysing the RR Lyrae distribution with parametric and non-parametric techniques. Taking advantage of the uniform all-sky coverage, we test halo models more sophisticated than usually considered in the literature, such as those with varying flattening, tilt and/or offset of the halo with respect to the Galactic disc. A consistent picture emerges: the inner halo is well reproduced by a smooth distribution of stars settled on triaxial ellipsoids. The minor axis is perpendicular to the Milky Ways disc, while the major axis is misaligned by $\sim20^{\circ}$ from the Galactic Y axis. The elongation along the major axis is mild ($\text{p}=1.27$), and the vertical flattening is shown to evolve from a squashed state with $\text{q}\approx0.57$ in the centre to a more spherical $\text{q}\approx0.75$ at the outer edge of our dataset. The density slope is well approximated by a single power-law with exponent $α=-2.96$. Within the range probed, we see no significant evidence for a change of the radial density slope, out of the plane tilt or an offset of the halo with respect to the Galaxy's centre.

astro-ph.GA↗

arXiv2017

An Overview of the MHONGOOSE Survey: Observing Nearby Galaxies with MeerKAT

MHONGOOSE is a deep survey of the neutral hydrogen distribution in a representative sample of 30 nearby disk and dwarf galaxies with HI masses from 10^6 to ~10^{11} M_sun, and luminosities from M_R ~ -12 to M_R ~ -22. The sample is selected to uniformly cover the available range in log(M_HI). Our extremely deep observations, down to HI column density limits of well below 10^{18} cm^{-2} - or a few hundred times fainter than the typical HI disks in galaxies - will directly detect the effects of cold accretion from the intergalactic medium and the links with the cosmic web. These observations will be the first ever to probe the very low-column density neutral gas in galaxies at these high resolutions. Combination with data at other wavelengths, most of it already available, will enable accurate modelling of the properties and evolution of the mass components in these galaxies and link these with the effects of environment, dark matter distribution, and other fundamental properties such as halo mass and angular momentum. MHONGOOSE can already start addressing some of the SKA-1 science goals and will provide a comprehensive inventory of the processes driving the transformation and evolution of galaxies in the nearby universe at high resolution and over 5 orders of magnitude in column density. It will be a Nearby Galaxies Legacy Survey that will be unsurpassed until the advent of the SKA, and can serve as a highly visible, lasting statement of MeerKAT's capabilities.

astro-ph.GA↗

arXiv2017

Structured star formation in the Magellanic inter-Cloud region

We use a new contiguous imaging survey conducted using the Dark Energy Camera to investigate the distribution and properties of young stellar populations in the Magellanic inter-Cloud region. These young stars are strongly spatially clustered, forming a narrow chain of low-mass associations that trace the densest HI gas in the Magellanic Bridge and extend, in projection, from the SMC to the outer disk of the LMC. The associations in our survey footprint have ages $\lesssim 30$ Myr, masses in the range $\sim 100-1200\,{\rm M}_\odot$, and very diffuse structures with half-light radii of up to $\sim 100$ pc. The two most populous are strongly elliptical, and aligned to $\approx 10^{\rm o}$ with the axis joining the centres of the LMC and SMC. These observations strongly suggest that the young inter-Cloud populations formed in situ, likely due to the compression of gas stripped during the most recent close LMC-SMC encounter. The associations lie at distances intermediate between the two Clouds, and we find no evidence for a substantial distance gradient across the imaged area. Finally, we identify a vast shell of young stars surrounding a central association, that is spatially coincident with a low column density bubble in the HI distribution. The properties of this structure are consistent with a scenario where stellar winds and supernova explosions from massive stars in the central cluster swept up the ambient gas into a shell, triggering a new burst of star formation. This is a prime location for studying stellar feedback in a relatively isolated environment.

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arXiv2017

Distribution and kinematics of atomic and molecular gas inside the Solar circle

The detailed distribution and kinematics of the atomic and the CO-bright molecular hydrogen in the disc of the Milky Way inside the Solar circle are derived under the assumptions of axisymmetry and pure circular motions. We divide the Galactic disc into a series of rings, and assume that the gas in each ring is described by four parameters: its rotation velocity, velocity dispersion, midplane density and its scale height. We fit these parameters to the Galactic HI and CO (J=1-0) data by producing artificial HI and CO line-profiles and comparing them with the observations. Our approach allows us to fit all parameters to the data simultaneously without assuming a-priori a radial profile for one of the parameters. We present the distribution and kinematics of the HI and H2 in both the approaching (QIV) and the receding (QI) regions of the Galaxy. Our best-fit models reproduces remarkably well the observed HI and CO longitude-velocity diagrams up to a few degrees of distance from the midplane. With the exception of the innermost 2.5 kpc, QI and QIV show very similar kinematics. The rotation curves traced by the HI and H2 follow closely each other, flattening beyond R=6.5 kpc. Both the HI and the H2 surface densities show a) a deep depression at 0.5<R<2.5 kpc, analogous to that shown by some nearby barred galaxies, b) local overdensities that can be interpreted in terms of spiral arms or ring-like features in the disk. The HI (H2) properties are fairly constant in the region outside the depression, with typical velocity dispersion of 8.9+/-1.1 (4.4+/-1.2) km/s, density of 0.43+/-0.11 (0.42+/-0.22) cm-3 and HWHM scale height of 202+/-28 (64+/-12) pc. We also show that the HI opacity in the LAB data can be accounted for by using an `effective' spin temperature of about 150 K: assuming an optically thin regime leads to underestimate the HI mass by about 30%.

astro-ph.GA↗

arXiv2017

The survival of gas clouds in the Circumgalactic Medium of Milky Way-like galaxies

Observational evidence shows that low-redshift galaxies are surrounded by extended haloes of multiphase gas, the so-called 'circumgalactic medium' (CGM). To study the survival of relatively cool gas (T < 10^5 K) in the CGM, we performed a set of hydrodynamical simulations of cold (T = 10^4 K) neutral gas clouds travelling through a hot (T = 2x10^6 K) and low-density (n = 10^-4 cm^-3) coronal medium, typical of Milky Way-like galaxies at large galactocentric distances (~ 50-150 kpc). We explored the effects of different initial values of relative velocity and radius of the clouds. Our simulations were performed on a two-dimensional grid with constant mesh size (2 pc) and they include radiative cooling, photoionization heating and thermal conduction. We found that for large clouds (radii larger than 250 pc) the cool gas survives for very long time (larger than 250 Myr): despite that they are partially destroyed and fragmented into smaller cloudlets during their trajectory, the total mass of cool gas decreases at very low rates. We found that thermal conduction plays a significant role: its effect is to hinder formation of hydrodynamical instabilities at the cloud-corona interface, keeping the cloud compact and therefore more difficult to destroy. The distribution of column densities extracted from our simulations are compatible with those observed for low-temperature ions (e.g. SiII and SiIII) and for high-temperature ions (OVI) once we take into account that OVI covers much more extended regions than the cool gas and, therefore, it is more likely to be detected along a generic line of sight.

astro-ph.GA↗

arXiv2017

The stellar mass-halo mass relation of isolated field dwarfs: a critical test of $Λ$CDM at the edge of galaxy formation

We fit the rotation curves of isolated dwarf galaxies to directly measure the stellar mass-halo mass relation ($M_*-M_{200}$) over the mass range $5 \times 10^5 < M_{*}/{\rm M}_\odot < 10^{8}$. By accounting for cusp-core transformations due to stellar feedback, we find a monotonic relation with little scatter. Such monotonicity implies that abundance matching should yield a similar $M_*-M_{200}$ if the cosmological model is correct. Using the 'field galaxy' stellar mass function from the Sloan Digital Sky Survey (SDSS) and the halo mass function from the $Λ$ Cold Dark Matter Bolshoi simulation, we find remarkable agreement between the two. This holds down to $M_{200} \sim 5 \times 10^9$M$_\odot$, and to $M_{200} \sim 5 \times 10^8$M$_\odot$ if we assume a power law extrapolation of the SDSS stellar mass function below $M_* \sim 10^7$M$_\odot$. However, if instead of SDSS we use the stellar mass function of nearby galaxy groups, then the agreement is poor. This occurs because the group stellar mass function is shallower than that of the field below $M_* \sim 10^9$M$_\odot$, recovering the familiar 'missing satellites' and 'too big to fail' problems. Our result demonstrates that both problems are confined to group environments and must, therefore, owe to 'galaxy formation physics' rather than exotic cosmology. Finally, we repeat our analysis for a $Λ$ Warm Dark Matter cosmology, finding that it fails at 68% confidence for a thermal relic mass of $m_{\rm WDM} < 1.25$keV, and $m_{\rm WDM} < 2$keV if we use the power law extrapolation of SDSS. We conclude by making a number of predictions for future surveys based on these results.

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