arXiv subjects

B. D. Oppenheimer

Publications and source records attributed to B. D. Oppenheimer.

17 recordsLinked to original sources

arXiv2026

The impact of strong feedback on galaxy group scaling relations

Feedback from active supermassive black holes alters the distribution of matter in the Universe by injecting energy in the neighbouring hot gaseous medium, which leads to ejection of gas from the halos of galaxy groups and massive galaxies. Recent cosmological simulations such as FLAMINGO calibrate their feedback model on the baryon fractions of galaxy groups to tune the efficiency of gas ejection. However, recent observational constraints from optically selected groups and the kinetic Sunyaev-Zel'dovich effect yield lower baryon fractions than previous studies, which indicates that feedback may be more ejective than previously thought. Here we show that models involving highly ejective feedback are inconsistent with the scaling relations of local galaxy groups in the mass range $10^{13}-10^{14}M_\odot$. We study the X-ray luminosity-temperature relation in a sample of 44 galaxy groups with high-quality XMM-Newton observations. We show that highly ejective models under-predict the luminosity of galaxy groups at fixed mass at high significance ($5.7σ$). This conclusion is robust against selection effects and is obtained from directly measurable and minimally correlated quantities. We point out that turning observable quantities into gas fraction estimates is challenging, especially in the context of stacking large samples of heterogeneous systems. We argue that calibrating feedback models on baryon fractions is prone to systematic uncertainties and that observable scaling relations are better suited for this task.

astro-ph.CO↗

arXiv2026

Bound or blown: the fate of hot gas in galaxy groups

The impact of AGN feedback on the hot gas content of galaxy groups remains a key uncertainty in galaxy formation and its connection to the large scale structure of the Universe. We aim to compare the XMM-Newton Group AGN Project (X-GAP) sample to the hydrodynamical FLAMINGO simulations, which span a wide range of AGN feedback prescriptions. We construct X-GAP analogues by forward-modelling the full selection function, including detection and observational systematics, and generate end-to-end XMM-Newton mock observations analysed consistently with the data. We study multiple observables, including the L--T and Mgas--T relations, number of groups, mean temperature, and velocity dispersion, accounting for their covariance. The forward model accurately recovers input luminosities, gas masses, and core-excised temperatures for regular systems, enabling direct comparison in observable space. The normalisation of the scaling relations is the best discriminator between feedback models, while cosmic variance introduces > 20% fluctuations in the number of detected systems, making counts alone a weak discriminator. Models with intermediate feedback strength provide the best agreement with X-GAP, with the fgas-2sigma model yielding the lowest tension of only 0.8sigma, while the most extreme feedback scenario (fgas-8sigma) is ruled out at > 4sigma. Our results indicate that the thermodynamic properties of galaxy groups favour feedback stronger than the fiducial FLAMINGO calibration, but disfavour the most ejective models. This highlights the importance of combining forward modelling and multi-observable constraints to probe the fate of hot baryons in low-mass haloes.

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arXiv2025

Extreme AGN feedback in the fossil galaxy group SDSSTG 4436

Supermassive black hole feedback is the currently favoured mechanism to regulate the star formation rate of galaxies and prevent the formation of ultra-massive galaxies ($M_\star>10^{12}M_\odot$). However, the mechanism through which the outflowing energy is transferred to the surrounding medium strongly varies from one galaxy evolution model to another, such that a unified model for AGN feedback does not currently exist. The hot atmospheres of galaxy groups are highly sensitive laboratories of the feedback process, as the injected black hole energy is comparable to the binding energy of halo gas particles. Here we report multi-wavelength observations of the fossil galaxy group SDSSTG 4436. The hot atmosphere of this system exhibits a highly relaxed morphology centred on the giant elliptical galaxy NGC~3298. The X-ray emission from the system features a compact core ($<$10 kpc) and a steep increase in the entropy and cooling time of the gas, with the cooling time reaching the age of the Universe $\sim15$ kpc from the centre of the galaxy. The observed entropy profile implies a total injected energy of $\sim1.5\times10^{61}$ ergs, which given the high level of relaxation could not have been injected by a recent merging event. Star formation in the central galaxy NGC~3298 is strongly quenched and its stellar population is very old ($\sim$10.6 Gyr). The currently detected radio jets have low power and are confined within the central compact core. All the available evidence implies that this system was affected by giant AGN outbursts which excessively heated the neighbouring gas and prevented the formation of a self-regulated feedback cycle. Our findings imply that AGN outbursts can be energetic enough to unbind gas particles and lead to the disruption of cool cores.

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arXiv2024

Equilibrium States of Galactic Atmospheres I: The Flip Side of Mass Loading

This paper presents a new framework for understanding the relationship between a galaxy and its circumgalactic medium (CGM). It focuses on how imbalances between heating and cooling cause either expansion or contraction of the CGM. It does this by tracking \textit{all} of the mass and energy associated with a halo's baryons, including their gravitational potential energy, even if feedback has pushed some of those baryons beyond the halo's virial radius. We show how a star-forming galaxy's equilibrium state can be algebraically derived within the context of this framework, and we analyze how the equilibrium star formation rate depends on supernova feedback. We consider the consequences of varying the mass loading parameter etaM = Mdot_wind / Mdot_* relating a galaxy's gas mass outflow rate (Mdot_wind) to its star formation rate (Mdot_*) and obtain results that challenge common assumptions. In particular, we find that equilibrium star formation rates in low-mass galaxies are generally insensitive to mass loading, and when mass loading does matter, increasing it actually results in \textit{more} star formation because more supernova energy is needed to resist atmospheric contraction.

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arXiv2024

Equilibrium States of Galactic Atmospheres II: Interpretation and Implications

The scaling of galaxy properties with halo mass suggests that feedback loops regulate star formation, but there is no consensus yet about how those feedback loops work. To help clarify discussions of galaxy-scale feedback, Paper I presented a very simple model for supernova feedback that it called the minimalist regulator model. This followup paper interprets that model and discusses its implications. The model itself is an accounting system that tracks all of the mass and energy associated with a halo's circumgalactic baryons--the central galaxy's atmosphere. Algebraic solutions for the equilibrium states of that model reveal that star formation in low-mass halos self-regulates primarily by expanding the atmospheres of those halos, ultimately resulting in stellar masses that are insensitive to the mass-loading properties of galactic winds. What matters most is the proportion of supernova energy that couples with circumgalactic gas. However, supernova feedback alone fails to expand galactic atmospheres in higher-mass halos. According to the minimalist regulator model, an atmospheric contraction crisis ensues, which may be what triggers strong black-hole feedback. The model also predicts that circumgalactic medium properties emerging from cosmological simulations should depend largely on the specific energy of the outflows they produce, and we interpret the qualitative properties of several numerical simulations in light of that prediction.

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arXiv2024

Properties of the Line-of-Sight Velocity Field in the Hot and X-ray Emitting Circumgalactic Medium of Nearby Simulated Disk Galaxies

The hot, X-ray-emitting phase of the circumgalactic medium of massive galaxies is believed to be the reservoir of baryons from which gas flows onto the central galaxy and into which feedback from AGN and stars inject mass, momentum, energy, and metals. These effects shape the velocity fields of the hot gas, which can be observed via the Doppler shifting and broadening of emission lines by X-ray IFUs. In this work, we analyze the gas kinematics of the hot circumgalactic medium of Milky Way-mass disk galaxies from the TNG50 simulation with synthetic observations to determine how future instruments can probe this velocity structure. We find that the hot phase is often characterized by outflows from the disk driven by feedback processes, radial inflows near the galactic plane, and rotation, though in some systems the velocity field is more disorganized and turbulent. With a spectral resolution of $\sim$1 eV, fast and hot outflows ($\sim$200-500 km s$^{-1}$) can be measured, depending on the orientation of the galaxy on the sky. The rotation velocity of the hot phase ($\sim$100-200 km s$^{-1}$) can be measured using line shifts in edge-on galaxies, and is slower than that of colder gas phases but similar to stellar rotation velocities. By contrast, the slow inflows ($\sim$50-100 km s$^{-1}$) are difficult to measure in projection with these other components, but may be detected in multi-component spectral fits. We find that the velocity measured is sensitive to which emission lines are used. Measuring these flows will constrain theories of how the gas in these galaxies evolves.

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arXiv2023

Black Hole Growth, Baryon Lifting, Star Formation, and IllustrisTNG

Quenching of star formation in the central galaxies of cosmological halos is thought to result from energy released as gas accretes onto a supermassive black hole. The same energy source also appears to lower the central density and raise the cooling time of baryonic atmospheres in massive halos, thereby limiting both star formation and black hole growth, by lifting the baryons in those halos to greater altitudes. One predicted signature of that feedback mechanism is a nearly linear relationship between the central black hole's mass (MBH) and the original binding energy of the halo's baryons. We present the increasingly strong observational evidence supporting a such a relationship, showing that it extends up to halos of mass Mhalo ~10^14 MSun. We then compare current observational constraints on the MBH--Mhalo relation with numerical simulations, finding that black hole masses in IllustrisTNG appear to exceed those constraints at Mhalo < 10^13 MSun and that black hole masses in EAGLE fall short of observations at Mhalo ~ 10^14 MSun. A closer look at IllustrisTNG shows that quenching of star formation and suppression of black hole growth do indeed coincide with black hole energy input that lifts the halo's baryons. However, IllustrisTNG does not reproduce the observed MBH--Mhalo relation because its black holes gain mass primarily through accretion that does not contribute to baryon lifting. We suggest adjustments to some of the parameters in the IllustrisTNG feedback algorithm that may allow the resulting black hole masses to reflect the inherent links between black hole growth, baryon lifting, and star formation among the massive galaxies in those simulations.

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arXiv2019

Tentative Detection of the Circumgalactic Medium of the Isolated Low-Mass Dwarf Galaxy WLM

We report a tentative detection of the circumgalactic medium (CGM) of WLM, an isolated, low-mass (log$M_*/M_\odot\approx7.6$), dwarf irregular galaxy in the Local Group (LG). We analyze an HST/COS archival spectrum of a quasar sightline (PHL2525), which is 45 kpc (0.5 virial radius) from WLM and close to the Magellanic Stream (MS). Along this sightline, two ion absorbers are detected in Si II, Si III, Si IV, C II, and C IV at velocities of $\sim$-220 km s$^{-1}$ (Component v-220) and $\sim$-150 km s$^{-1}$ (Component v-150). To identify their origins, we study the position-velocity alignment of the components with WLM and the nearby MS. Near the Magellanic longitude of PHL2525, the MS-related neutral and ionized gas moves at $\lesssim-190$ km s$^{-1}$, suggesting an MS origin for Component v-220, but not for Component v-150. Because PHL2525 passes near WLM and Component v-150 is close to WLM's systemic velocity ($\sim$-132 km s$^{-1}$), it is likely that Component v-150 arises from the galaxy's CGM. This results in a total Si mass in WLM's CGM of $M_{\rm Si}^{\rm CGM}\sim(0.2-1.0)\times10^5~M_\odot$ using assumption from other COS dwarf studies. Comparing $M_{\rm Si}^{\rm CGM}$ to the total Si mass synthesized in WLM over its lifetime ($\sim$1.3$\times10^5~M_\odot$), we find $\sim$3% is locked in stars, $\sim$6% in the ISM, $\sim$15%-77% in the CGM, and the rest ($\sim$14%-76%) is likely lost beyond the virial radius. Our finding resonates with other COS dwarf galaxy studies and theoretical predictions that low-mass galaxies can easily lose metals into their CGM due to stellar feedback and shallow gravitational potential.

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arXiv2016

The Minimum Halo Mass for Star Formation at z = 6 - 8

Recent analysis of strongly-lensed sources in the Hubble Frontier Fields indicates that the rest-frame UV luminosity function of galaxies at $z=$6--8 rises as a power law down to $M_\mathrm{UV}=-15$, and possibly as faint as -12.5. We use predictions from a cosmological radiation hydrodynamic simulation to map these luminosities onto physical space, constraining the minimum dark matter halo mass and stellar mass that the Frontier Fields probe. While previously-published theoretical studies have suggested or assumed that early star formation was suppressed in halos less massive than $10^9$--$10^{11} M_\odot$, we find that recent observations demand vigorous star formation in halos at least as massive as (3.1, 5.6, 10.5)$\times10^9 M_\odot$ at $z=(6,7,8)$. Likewise, we find that Frontier Fields observations probe down to stellar masses of (8.1, 18, 32)$\times10^6 M_\odot$; that is, they are observing the likely progenitors of analogues to Local Group dwarfs such as Pegasus and M32. Our simulations yield somewhat different constraints than two complementary models that have been invoked in similar analyses, emphasizing the need for further observational constraints on the galaxy-halo connection.

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arXiv2016

The Soft, Fluctuating UVB at $z\sim6$ as Traced by C IV, SiIV, and CII

The sources that drove cosmological reionization left clues regarding their identity in the slope and inhomogeneity of the ultraviolet ionizing background (UVB): Bright quasars (QSOs) generate a hard UVB with predominantly large-scale fluctuations while Population II stars generate a softer one with smaller-scale fluctuations. Metal absorbers probe the UVB's slope because different ions are sensitive to different energies. Likewise, they probe spatial fluctuations because they originate in regions where a galaxy-driven UVB is harder and more intense. We take a first step towards studying the reionization-epoch UVB's slope and inhomogeneity by comparing observations of 12 metal absorbers at $z\sim6$ versus predictions from a cosmological hydrodynamic simulation using three different UVBs: a soft, spatially-inhomogeneous "galaxies+QSOs" UVB; a homogeneous "galaxies+QSOs" UVB (Haardt & Madau 2012); and a QSOs-only model. All UVBs reproduce the observed column density distributions of CII, SiIV, and CIV reasonably well although high-column, high-ionization absorbers are underproduced, reflecting numerical limitations. With upper limits treated as detections, only a soft, fluctuating UVB reproduces both the observed SiIV/CIV and CII/CIV distributions. The QSOs-only UVB overpredicts both CIV/CII and CIV/SiIV, indicating that it is too hard. The Haardt & Madau (2012) UVB underpredicts CIV/SiIV, suggesting that it lacks amplifications near galaxies. Hence current observations prefer a soft, fluctuating UVB as expected from a predominantly Population II background although they cannot rule out a harder one. Future observations probing a factor of two deeper in metal column density will distinguish between the soft, fluctuating and QSOs-only UVBs.

astro-ph.CO↗

arXiv2015

The Reionization of Carbon

Observations suggest that CII was more abundant than CIV in the intergalactic medium towards the end of the hydrogen reionization epoch. This transition provides a unique opportunity to study the enrichment history of intergalactic gas and the growth of the ionizing background (UVB) at early times. We study how carbon absorption evolves from z=10-5 using a cosmological hydrodynamic simulation that includes a self-consistent multifrequency UVB as well as a well-constrained model for galactic outflows to disperse metals. Our predicted UVB is within 2-4 times that of Haardt & Madau (2012), which is fair agreement given the uncertainties. Nonetheless, we use a calibration in post-processing to account for Lyman-alpha forest measurements while preserving the predicted spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way that it always exceeds the volume average in regions where metals are found. This implies both that a spatially-uniform UVB is a poor approximation and that metal absorption is not sensitive to the epoch when HII regions overlap globally even at column densites of 10^{12} cm^{-2}. We find, consistent with observations, that the CII mass fraction drops to low redshift while CIV rises owing the combined effects of a growing UVB and continued addition of carbon in low-density regions. This is mimicked in absorption statistics, which broadly agree with observations at z=6-3 while predicting that the absorber column density distributions rise steeply to the lowest observable columns. Our model reproduces the large observed scatter in the number of low-ionization absorbers per sightline, implying that the scatter does not indicate a partially-neutral Universe at z=6.

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arXiv2013

The Host Halos of OI Absorbers in the Reionization Epoch

We use a radiation hydrodynamic simulation of the hydrogen reionization epoch to study OI absorbers at z~6. The intergalactic medium (IGM) is reionized before it is enriched, hence OI absorption originates within dark matter halos. The predicted abundance of OI absorbers is in reasonable agreement with observations. At z=10, roughly 70% of sightlines through atomically-cooled halos encounter a visible (N_OI > 10^14 cm^-2) column. Reionization ionizes and removes gas from halos less massive than 10^8.4 M_0, but 20% of sightlines through more massive halos encounter visible columns even at z=5. The mass scale of absorber host halos is 10-100 times smaller than the halos of Lyman break galaxies and Lyman-alpha emitters, hence absorption probes the dominant ionizing sources more directly. OI absorbers have neutral hydrogen columns of 10^19-10^21 cm^-2, suggesting a close resemblance between objects selected in OI and HI absorption. Finally, the absorption in the foreground of the z=7.085 quasar ULASJ1120+0641 cannot originate in a dark matter halo because halo gas at the observed HI column density is enriched enough to violate the upper limits on the OI column. By contrast, gas at less than one third the cosmic mean density satisfies the constraints. Hence the foreground absorption likely originates in the IGM.

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arXiv2011

Quenching Massive Galaxies with On-the-fly Feedback in Cosmological Hydrodynamic Simulations

Massive galaxies today typically are not forming stars despite being surrounded by hot gaseous halos with short central cooling times. This likely owes to some form of "quenching feedback" such as merger-driven quasar activity or radio jets emerging from central black holes. Here we implement heuristic prescriptions for these phenomena on-the-fly within cosmological hydrodynamic simulations. We constrain them by comparing to observed luminosity functions and color-magnitude diagrams from SDSS. We find that quenching from mergers alone does not produce a realistic red sequence, because 1 - 2 Gyr after a merger the remnant accretes new fuel and star formation reignites. In contrast, quenching by continuously adding thermal energy to hot gaseous halos quantitatively matches the red galaxy luminosity function and produces a reasonable red sequence. Small discrepancies remain - a shallow red sequence slope suggests that our models underestimate metal production or retention in massive red galaxies, while a deficit of massive blue galaxies may reflect the fact that observed heating is intermittent rather than continuous. Overall, injection of energy into hot halo gas appears to be a necessary and sufficient condition to broadly produce red and dead massive galaxies as observed.

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arXiv2010

How is Star Formation Quenched in Massive Galaxies?

The bimodality in observed present-day galaxy colours has long been a challenge for hierarchical galaxy formation models, as it requires some physical process to quench (and keep quenched) star formation in massive galaxies. Here we examine phenomenological models of quenching by post-processing the star formation histories of galaxies from cosmological hydrodynamic simulations that reproduce observations of star-forming galaxies reasonably well. We consider recipes for quenching based on major mergers, halo mass thresholds, gas temperature thresholds, and variants thereof. We compare the resulting simulated star formation histories to observed g-r colour-magnitude diagrams and red and blue luminosity functions from SDSS. The merger and halo mass quenching scenarios each yield a distinct red sequence and blue cloud of galaxies that are in broad agreement with data, albeit only under rather extreme assumptions. In detail, however, the simulated red sequence slope and amplitude in each scenario is somewhat discrepant, perhaps traceable to low metallicities in simulated galaxies. Merger quenching produces more massive blue galaxies, earlier quenching, and more frosting of young stars; comparing to relevant data tends to favor merger over halo mass quenching. Although physically-motivated quenching models can produce a red sequence, interesting generic discrepancies remain that indicate that additional physics is required to reproduce the star formation and enrichment histories of red and dead galaxies.

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arXiv2009

The Late Reionization of Filaments

We study the topology of reionization using accurate three-dimensional radiative transfer calculations post-processed on outputs from cosmological hydrodynamic simulations. In our simulations, reionization begins in overdense regions and then "leaks" directly into voids, with filaments reionizing last owing to their combination of high recombination rate and low emissivity. This result depends on the uniquely-biased emissivity field predicted by our prescriptions for star formation and feedback, which have previously been shown to account for a wide array of measurements of the post-reionization Universe. It is qualitatively robust to our choice of simulation volume, ionizing escape fraction, and spatial resolution (in fact it grows stronger at higher spatial resolution) even though the exact overlap redshift is sensitive to each of these. However, it weakens slightly as the escape fraction is increased owing to the reduced density contrast at higher redshift. We also explore whether our results are sensitive to commonly-employed approximations such as using optically-thin Eddington tensors or substantially altering the speed of light. Such approximations do not qualitatively change the topology of reionization. However, they can systematically shift the overlap redshift by up to $Δz\sim 0.5$, indicating that accurate radiative transfer is essential for computing reionization. Our model cannot simultaneously reproduce the observed optical depth to Thomson scattering and ionization rate per hydrogen atom at $z=6$, which could owe to numerical effects and/or missing early sources of ionization.

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arXiv2005

The IMF in Extreme Star-Forming Environments: Searching for Variations vs. Initial Conditions

Any predictive theory of star formation must explain observed variations (or lack thereof) in the initial mass function. Recent work suggests that we might expect quantitative variations in the IMF as a function of metallicity (Larson 2005) or magnetic field strength (Shu et al. 2004). We summarize results from several on-going studies attempting to constrain the ratio of high to low mass stars, as well as stars to sub- stellar objects, in a variety of different environments, all containing high mass stars. First, we examine the ratio of stars to sub--stellar objects in the nearby Mon R2 region utilizing NICMOS/HST data. We compare our results to the IMF by Kroupa (2002)]} and to the observed ratios for IC 348 and Orion. Second, we present preliminary results for the ratio of high to low mass stars in W51, the most luminous HII region in the galaxy. Based on ground--based multi--colour images of the cluster obtained with the MMT adaptive optics system, we derive a lower limit to the ratio of high-mass to low-mass stars and compare it to the ratios for nearby clusters. Finally, we present the derived IMF for the R136 region in the LMC where the metallicity is 1/4 solar using HST/NICMOS data. We find that the IMF is consistent with that characterizing the field (Chabrier 2003), as well as nearby star--forming regions, down to 1.0 Msun outside 2 pc. Whereas the results for both Mon R2 and R136 are consistent with the nearby clusters, the ratio of high to low mass stars in W51 tentatively indicates a lack of low--mass objects.

astro-ph↗

arXiv2004

High Resolution Mid - Infrared Imaging of the AGB Star RV Boo with the Steward Observatory Adaptive Optics System

We present high resolution (~0.1"), very high Strehl ratio (0.97+-0.03) mid-infrared (IR) adaptive optics (AO) images of the AGB star RV Boo utilizing the MMT adaptive secondary AO system. RV Boo was observed at a number of wavelengths over two epochs (9.8 um in May 2003, 8.8, 9.8 and 11.7 um in February 2004) and appeared slightly extended at all wavelengths. While the extension is very slight at 8.8 and 11.7 um data, the extension is somewhat more pronounced at 9.8 um. With such high Strehls we can achieve super-resolutions of 0.1" by deconvolving RV Boo with a point-spread function (PSF) derived from an unresolved star. We tentatively resolve RV Boo into a 0.16" FWHM extension at a position angle of 120 degrees. At a distance of 390(+250)(-100) pc, this corresponds to a FWHM of 60(+40)(-15) AU. We measure a total flux at 9.8 um of 145+-24 Jy for the disk and star. Based on a dust thermal emission model for the observed IR spectral energy distribution and the 9.8 um AO image, we derive a disk dust mass of 1.6x10^-6 Msun and an inclination of 30 to 45 degrees from edge-on. We discuss whether the dust disk observed around RV Boo is an example of the early stages in the formation of asymmetric structure in planetary nebula.

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