Galactic Scales Research Articles

Evidence for a large off-centered galactic outflow and its connection to the extraplanar diffuse ionized gas in IC 1553

Aims. We analyze a MUSE optical integral field spectrum of the star-forming edge-on galaxy IC 1553 in order to study its extraplanar diffuse ionized gas (eDIG) and the processes shaping its disk-halo interface. Methods. We extracted the optical emission line properties from the integral field spectrum and generated the commonly used emission line diagnostic diagrams in order to analyze the ionization conditions and the distribution of the eDIG. Furthermore, we performed gravitational potential fitting to investigate the kinematics of a suspected galactic outflow. Results. We find that the eDIG scale height has a maximum value of approximately 1.0 kpc and decreases roughly linearly with the radial distance from the galactic center in projection. The ionization state of the eDIG is not consistent with a pure photoionization scenario and instead requires a significant contribution from shock ionization. This, in addition to the gas kinematics, strongly suggests the presence of a galactic scale outflow, the origin of which lies at least 1.4 kpc away from the galactic center. The inferred shock velocity in the eDIG of approximately 225 km s−1 is comparable to the escape velocity estimated from our potential modelling. The asymmetric distribution of currently star-forming clusters produces a range of different ionization conditions in the eDIG. As a result, the vertical emission line profiles vary quantitatively and qualitatively along the major axis of the galaxy. This analysis illustrates that it is crucial in studies of the eDIG to use observations that take the spatial and kinematical distributions into account, such as those done with integral field units, to form an accurate picture of the relevant physical properties.

A new method for spatially resolving the turbulence-driving mixture in the ISM with application to the Small Magellanic Cloud

ABSTRACT Turbulence plays a crucial role in shaping the structure of the interstellar medium. The ratio of the three-dimensional density contrast ($\sigma _{\rho /\rho _0}$) to the turbulent sonic Mach number ($\mathcal {M}$) of an isothermal, compressible gas describes the ratio of solenoidal to compressive modes in the turbulent acceleration field of the gas, and is parameterized by the turbulence driving parameter: $b=\sigma _{\rho /\rho _0}/\mathcal {M}$. The turbulence driving parameter ranges from b = 1/3 (purely solenoidal) to b = 1 (purely compressive), with b = 0.38 characterizing the natural mixture (1/3 compressive, 2/3 solenoidal) of the two driving modes. Here, we present a new method for recovering $\sigma _{\rho /\rho _0}$, $\mathcal {M}$, and b, from observations on galactic scales, using a roving kernel to produce maps of these quantities from column density and centroid velocity maps. We apply our method to high-resolution ${\rm H}\,\rm{\small I}$ emission observations of the Small Magellanic Cloud (SMC) from the GASKAP-HI survey. We find that the turbulence driving parameter varies between b ∼ 0.3 and 1.0 within the main body of the SMC, but the median value converges to b ∼ 0.51, suggesting that the turbulence is overall driven more compressively (b > 0.38). We observe no correlation between the b parameter and ${\rm H}\,\rm{\small I}$ or H α intensity, indicating that compressive driving of ${\rm H}\,\rm{\small I}$ turbulence cannot be determined solely by observing ${\rm H}\,\rm{\small I}$ or H α emission density, and that velocity information must also be considered. Further investigation is required to link our findings to potential driving mechanisms such as star-formation feedback, gravitational collapse, or cloud–cloud collisions.

Superfluid dark matter in tension with weak gravitational lensing data

Superfluid dark matter (SFDM) is a model that promises to reproduce the successes of both particle dark matter on cosmological scales and those of Modified Newtonian Dynamics (MOND) on galactic scales. SFDM reproduces MOND only up to a certain distance from the galactic center, and only for kinematic observables: it does not affect trajectories of light. We test whether this is consistent with a recent analysis of weak gravitational lensing that has probed accelerations around galaxies to unprecedentedly large radii. This analysis found the data to be close to the prediction of MOND, suggesting they might be difficult to fit with SFDM. To investigate this matter, we solved the equations of motion of the model and compared the result to observational data. Our results show that the SFDM model is incompatible with the weak-lensing observations, at least in its current form.

Dark Matter with DAMA/LIBRA and its perspectives

The existence of Dark Matter particles as a significant portion of the Universe’s total mass is suggested by theoretical reasoning at the galactic and larger scales as well as experimental data. This motivates the DAMA experiment’s efforts to investigate the presence of the Dark Matter particles in the galactic halo by utilizing a model-independent signature with an extremely radio-pure set-up underground. The long-standing, model-independent annual modulation effect at Gran Sasso seen by DAMA with several experimental set-ups is summarized in this paper, along with its perspectives. DAMA/LIBRA-phase2 set-up, ≃ 250 kg highly radio-pure NaI(Tl) further confirms the evidence of a signal that meets all the requirements of the model independent Dark Matter annual modulation signature at high C.L.; the combined exposure of DAMA/NaI and DAMA/LIBRA is 2.86 ton × yr over 22 independent annual cycles. The experiment is currently collecting data in the DAMA/LIBRA–phase2 empowered configuration with an even lower software energy threshold.

Dark Universe phenomenology from Yukawa potential?

We argue that the effect of cold dark matter in the cosmological setup can be explained by the coupling between the baryonic matter particles in terms of the long-range force having a graviton mass mg via the Yukawa gravitational potential. Such a quantum-corrected Yukawa-like gravitational potential is characterized by the coupling parameter α, the wavelength parameter λ, which is related to the graviton mass via mg=ħ/(λc), that determines the range of the force and, finally, a Planck length quantity l0 that makes the potential regular at the centre. The modified Friedmann equations are obtained using Verlinde’s entropic force interpretation of gravity based on the holographic scenario and the equipartition law of energy. The parameter α modifies the Newton’s constant as Geff=G(1+α). Interestingly, we find an equation that relates the dark matter density, dark energy density, and baryonic matter density. It is given by ΩD=2ΩBΩΛ(1+z)3. We argue that dark matter is an apparent effect as no dark matter particle exists in this picture. Furthermore, the dark energy is also related to graviton mass and α; in particular, we point out that the cosmological constant can be viewed as a self-interaction effect between gravitons. We further show a precise correspondence with Verlinde’s emergent gravity theory, and due to the long-range force, the theory can be viewed as a non-local gravity theory. To this end, we performed the phase space analyses and estimated λ≃103 [Mpc] and α≃0.04, respectively. Finally, from these values, for the graviton mass, we get mg≃10−68 kg, and cosmological constant Λ≃10−52m−2. Further, we argue how this theory reproduces the MOND phenomenology on galactic scales via the acceleration of Milgrom a0≃10−10m/s2.

Aether scalar tensor theory confronted with weak lensing data at small accelerations

Context.The recently proposed aether scalar tensor (AeST) model reproduces both the successes of particle dark matter on cosmological scales and those of modified Newtonian dynamics (MOND) on galactic scales. But the AeST model reproduces MOND only up to a certain maximum galactocentric radius. Since MOND is known to fit very well to observations at these scales, this raises the question of whether the AeST model comes into tension with data.Aims.We tested whether or not the AeST model is in conflict with observations using a recent analysis of data for weak gravitational lensing.Methods.We solved the equations of motion of the AeST model, analyzed the solutions’ behavior, and compared the results to observational data.Results.The AeST model shows some deviations from MOND at the radii probed by weak gravitational lensing. The data show no clear indication of these predicted deviations.

Discovery of spectacular quasar-driven superbubbles in red quasars.

Quasar-driven outflows on galactic scales are a routinely invoked ingredient for galaxy formation models. We report the discovery of ionized gas nebulae surrounding three luminous red quasars at z ~ 0.4 from Gemini integral field unit observations. All these nebulae feature unprecedented pairs of "superbubbles" extending ~20 kpc in diameter, and the line-of-sight velocity difference between the red- and blueshifted bubbles reaches up to ~1200 km/s. Their spectacular dual-bubble morphology (in analogy to the galactic "Fermi bubbles") and their kinematics provide unambiguous evidence for galaxy-wide quasar-driven outflows, in parallel with the quasi-spherical outflows similar in size from luminous type 1 and type 2 quasars at concordant redshift. These bubble pairs manifest themselves as a signpost of the short-lived superbubble "break-out" phase, when the quasar wind drives the bubbles to escape the confinement from the dense environment and plunge into the galactic halo with a high-velocity expansion.

The Density Parity Model for the Evolution of the Subhalo Inner Spin Alignments with the Cosmic Web

We develop a new model within which the radius-dependent transition of the subhalo inner spins with respect to the cosmic web and the variation of the transition threshold radius (r th) with subhalo mass (M vir), smoothing scale (r f ), and redshift (z) can be coherently explained. The key tenet of this model is that the competition between the pressure effect of the inner mass and the compression effect of the local tidal field determines which principal direction of the tidal field the inner spins are aligned with. If the former predominates, then only the tidal torques turn on, resulting in the alignments of the inner spins with the intermediate principal axes of the tidal field. Otherwise, the subhalo spins acquire a tendency to be aligned with the shortest axes of the subhalo shapes, which is in the major principal directions of the tidal field. Quantifying the two effects in terms of the densities, we make a purely analytical prediction for r th (M vir, z, r f ). Testing this model against the numerical results from a high-resolution dark-matter-only N-body simulation in the redshift range of 0 ≤ z ≤ 3 on the galactic mass scale of for two different cases of r f /(h -1 Mpc) = 0.5 and 1, we find excellent agreements of the model predictions with the numerical results. It is also shown that this model naturally predicts the alignments between the inner spins of the present subhalos with the principal axes of the high-z tidal field at the progenitors’ locations.

Consistent cosmological structure formation on all scales in relativistic extensions of MOND

General relativity manifests very similar equations in different regimes, notably in large scale cosmological perturbation theory, non-linear cosmological structure formation, and in weak field galactic dynamics. The same is not necessarily true in alternative gravity theories, in particular those that possess MONDian behaviour (“relativistic extensions” of MOND). In these theories different regimes are typically studied quite separately, sometimes even with the freedom in the theories chosen differently in different regimes. If we wish to properly and fully test complete cosmologies containing MOND against the ΛCDM paradigm then we need to understand cosmological structure formation on all scales, and do so in a coherent and consistent manner. We propose a method for doing so and apply it to generalised Einstein-Aether theories as a case study. We derive the equations that govern cosmological structure formation on all scales in these theories and show that the same free function (which may contain both Newtonian and MONDian branches) appears in the cosmological background, linear perturbations, and non-linear cosmological structure formation. We show that MONDian behaviour on galactic scales does not necessarily result in MONDian behaviour on cosmological scales, and for MONDian behaviour to arise cosmologically, there will be no modification to the Friedmann equations governing the evolution of the homogeneous cosmological background. We comment on how existing N-body simulations relate to complete and consistent generalised Einstein-Aether cosmologies. The equations derived in this work allow consistent cosmological N-body simulations to be run in these theories whether or not MONDian behaviour manifests on cosmological scales.

Dark Matter in Fractional Gravity. I. Astrophysical Tests on Galactic Scales

We explore the possibility that the dark matter (DM) component in galaxies may originate fractional gravity. In such a framework, the standard law of inertia continues to hold, but the gravitational potential associated with a given DM density distribution is determined by a modified Poisson equation including fractional derivatives (i.e., derivatives of noninteger type) that are meant to describe nonlocal effects. We analytically derive the expression of the potential that in fractional gravity corresponds to various spherically symmetric density profiles, including the Navarro–Frenk–White (NFW) distribution that is usually exploited to describe virialized halos of collisionless DM as extracted from N-body cosmological simulations. We show that in fractional gravity, the dynamics of a test particle moving in a cuspy NFW density distribution is substantially altered with respect to the Newtonian case, mirroring what in Newtonian gravity would instead be sourced by a density profile with an inner core. We test the fractional gravity framework on galactic scales, showing that (i) it can provide accurate fits to the stacked rotation curves of spiral galaxies with different properties, including dwarfs; (ii) it can reproduce to reasonable accuracy the observed shape and scatter of the radial acceleration relation over an extended range of galaxy accelerations; and (iii) it can properly account for the universal surface density and the core radius versus disk scale length scaling relations. Finally, we discuss the possible origin of the fractional gravity behavior as a fundamental or emerging property of the elusive DM component.

Sensitivity of the Cherenkov Telescope Array to TeV photon emission from the Large Magellanic Cloud

ABSTRACT A deep survey of the Large Magellanic Cloud at ∼0.1–100 TeV photon energies with the Cherenkov Telescope Array is planned. We assess the detection prospects based on a model for the emission of the galaxy, comprising the four known TeV emitters, mock populations of sources, and interstellar emission on galactic scales. We also assess the detectability of 30 Doradus and SN 1987A, and the constraints that can be derived on the nature of dark matter. The survey will allow for fine spectral studies of N 157B, N 132D, LMC P3, and 30 Doradus C, and half a dozen other sources should be revealed, mainly pulsar-powered objects. The remnant from SN 1987A could be detected if it produces cosmic-ray nuclei with a flat power-law spectrum at high energies, or with a steeper index 2.3–2.4 pending a flux increase by a factor of >3–4 over ∼2015–2035. Large-scale interstellar emission remains mostly out of reach of the survey if its >10 GeV spectrum has a soft photon index ∼2.7, but degree-scale 0.1–10 TeV pion-decay emission could be detected if the cosmic-ray spectrum hardens above >100 GeV. The 30 Doradus star-forming region is detectable if acceleration efficiency is on the order of 1−10 per cent of the mechanical luminosity and diffusion is suppressed by two orders of magnitude within <100 pc. Finally, the survey could probe the canonical velocity-averaged cross-section for self-annihilation of weakly interacting massive particles for cuspy Navarro–Frenk–White profiles.

Galactic rotation dynamics in a new f({mathcal {R}}) gravity model

We propose to test the viability of the recently introduced f(mathcal {R}) gravity model in the galactic scales. For this purpose we consider test particles moving in stable circular orbits around the galactic center. We study the Palatini approach of f(mathcal {R}) gravity via Weyl transformation, which is the frame transformation from the Jordan frame to the Einstein frame. We derive the expression of rotational velocities of test particles in the new f(mathcal {R}) gravity model. For the observational data of samples of high surface brightness and low surface brightness galaxies, we show that the predicted rotation curves are well fitted with observations, thus implying that this model can explain flat rotation curves of galaxies. We also study an ultra diffuse galaxy, AGC 242019 which has been claimed in literature to be a dark matter dominated galaxy similar to low surface brightness galaxies with a slowly rising rotation curve. The rotation curve of this galaxy also fits well with the model prediction in our study. Furthermore, we studied the Tully–Fisher relation for the entire sample of galaxies and found that the model prediction shows the consistency with the data.

A test of invariance of dark matter halo surface density using multiwavelength mock galaxy catalogues

ABSTRACT A large number of observations have shown that the dark matter halo surface density, given by the product of halo core radius and core density, is nearly constant for a diverse suite of galaxies. Although this invariance of the halo surface density is violated at galaxy cluster and group scales, it is still an open question on whether the aforementioned constancy on galactic scales can be explained within Lambda cold dark matter (ΛCDM). For this purpose, we probe the variation of halo surface density as a function of mass using multiwavelength mock galaxy catalogues from ΛCDM simulations, where the adiabatic contraction of dark matter haloes in the presence of baryons has been taken into account. We find that these baryonified ΛCDM haloes were best fitted with a generalized Navarro–Frenk–White profile, and the halo surface density from these haloes has a degeneracy with respect to both the halo mass and the virial concentration. We find that the correlation with mass when averaged over concentration is consistent with a constant halo surface density. However, a power-law dependence as a function of halo mass also cannot be ruled out.

Compact Steep Spectrum Radio Sources with Enhanced Star Formation Are Smaller Than 10 kpc

Compact steep spectrum (CSS) radio sources are active galactic nuclei (AGN) that have radio jets propagating only on galactic scales, defined as having projected linear size (LS) of up to 20 kpc. CSS sources are generally hosted by massive early-type galaxies with little ongoing star formation; however, a small fraction are known to have enhanced star formation. Using archival data from the Faint Images of the Radio Sky at Twenty cm survey, the Very Large Array Sky Survey, and the Sloan Digital Sky Survey, we identify a volume-limited sample of 166 CSS sources at z < 0.2 with L 1.4 GHz > 1024 W Hz−1. Comparing the star formation rates and linear sizes of these CSS sources, we find that the ≈14% of CSS sources with specific star formation rates above 0.01 Gyr−1 all have LS < 10 kpc. We discuss the possible mechanisms driving this result, concluding that it is likely the excess star formation in these sources occurred in multiple bursts and ceased prior to the AGN jet being triggered.

Gas Accretion Can Drive Turbulence in Galaxies

The driving of turbulence in galaxies is deeply connected with the physics of feedback, star formation, outflows, accretion, and radial transport in disks. The velocity dispersion of gas in galaxies therefore offers a promising observational window into these processes. However, the relative importance of each of these mechanisms remains controversial. In this work we revisit the possibility that turbulence on galactic scales is driven by the direct impact of accreting gaseous material on the disk. We measure this effect in a disk-like star-forming galaxy in IllustrisTNG, using the high-resolution cosmological magnetohydrodynamical simulation TNG50. We employ Lagrangian tracer particles with a high time cadence of only a few million years to identify accretion and other events. The energies of particles are measured by stacking the events in bins of time around the event. The average effect of each event is measured by fitting explicit models for the kinetic and turbulent energies as a function of time. These measurements are corroborated by cross-correlating the turbulent energy with other time series and searching for signals of causality, i.e., asymmetries across zero time lag. We find that accretion contributes to the large-scale turbulent kinetic energy even if it does not dominate in this ∼5 × 109 M ⊙ stellar mass galaxy. Extrapolating this finding to a range of galaxy masses, we find that there are regimes where energy from direct accretion may dominate the turbulent energy budget, particularly in disk outskirts, galaxies less massive than the Milky Way, and at redshift ∼2.

Towards testing a dark matter candidate that emerges from the scalar ether theory

According to a scalar theory of gravity with a preferred frame, electromagnetism in the presence of a gravitational field implies that there is an additional energy tensor, which might contribute to dark matter. The expression of this tensor is determined by a mere scalar p, that depends on the EM field and (for a weak field) on the Newtonian gravitational field. We briefly recall why this tensor arises and how the EM field in a galaxy can be calculated. The data fields that enter the PDE for the scalar field p oscillate very quickly in space and time, as does the EM field. This prevents integration of that PDE at the relevant galactic scale. Therefore, a homogenization of that PDE has to be operated. We discuss in some detail three possible ways of applying the homogenization theory to that PDE: time, space, or spacetime homogenization. The second and third ways may lead to feasible, albeit heavy calculations.

A sensitive APEX and ALMA CO(1–0), CO(2–1), CO(3–2), and [CI](1–0) spectral survey of 40 local (ultra-)luminous infrared galaxies

We present a high sensitivity, ground-based spectral line survey of low-J carbon monoxide (CO(Jup → Jup − 1) with Jup = 1, 2, 3) and neutral carbon [CI] 3P1−3P0 ([CI](1–0)) in 36 local ultra-luminous infrared galaxies (ULIRGs) and 4 additional LIRGs, all of which have previous Herschel OH 119 μm observations. The study is based on new single-dish observations conducted with the Atacama Pathfinder Experiment (APEX) and complemented with archival APEX and Atacama Large Millimeter Array (ALMA and ACA) data. Our methods are optimized for a multi-tracer study of the total molecular line emission from these ULIRGs, including any extended low-surface-brightness components. We find a tight correlation between the CO and [CI] line luminosities, which suggests that the emission from CO(1–0) (and CO(2–1)) arises from similar regions as the [CI](1–0), at least when averaged over galactic scales. By using [CI] to compute molecular gas masses, we estimate a median CO-to-H2 conversion factor of ⟨αCO⟩ = 1.7 ± 0.5 M⊙ (K km s−1pc2)−1 for ULIRGs. We derive median galaxy-integrated CO line ratios of 〈r21〉 = LCO(2-1)′/LCO(1-0)′ = 1.09, 〈r31〉 = LCO(3-2)′/LCO(1-0)′ = 0.76, and 〈r32〉 = LCO(3-2)′/LCO(2-1)′ = 0.76, significantly higher than normal star-forming galaxies, confirming the exceptional molecular gas properties of ULIRGs. We find that the r21 and r32 ratios are poor tracers of CO excitation in ULIRGs, while r31 shows a positive trend with LIR and star formation rates and a negative trend with the H2 gas depletion timescales (τdep). Our investigation of CO line ratios as a function of gas kinematics shows no clear trends, except for a positive relation between r21 and σv, which can be explained by CO opacity effects. These ULIRGs are also characterized by high L[CI](1-0)′/LCO(1-0)′ ratios, with a measured median value of ⟨rCICO⟩ = 0.18, higher than values from previous interferometric studies that were affected by missing [CI] line flux. The rCICO values do not show a significant correlation with any of the galaxy properties investigated, including OH outflow velocities and equivalent widths. We find that the widths of [CI](1–0) lines are ∼10% smaller than those of CO lines, and that this discrepancy becomes more significant in ULIRGs with broad lines (σv &gt; 150 km s−1) and when considering the high-v wings of the lines. This suggests that the low optical depth of [CI] can challenge its detection in diffuse, low-surface-brightness outflows and, therefore, its use as a tracer of CO-dark H2 gas in these components. Finally, we find that higher LAGN are associated with longer τdep, consistent with the hypothesis that active galactic nucleus feedback may reduce the efficiency of star formation. Our study highlights the need for sensitive single-dish multi-tracer H2 surveys of ULIRGs that are able to recover the flux that is missed by interferometers, especially in the high-frequency lines such as [CI]. The Atacama Large Aperture Submillimeter Telescope (AtLAST) will be transformational for this field.

Quantum Modified Gravity at Low Energy in the Ricci Flow of Quantum Spacetime

Quantum treatment of physical reference frame leads to the Ricci flow of quantum spacetime, which is a quite rigid framework to quantum and renormalization effect of gravity. The theory has a low characteristic energy scale described by a unique constant: the critical density of the universe. At low energy long distance (cosmic or galactic) scale, the theory modifies Einstein's gravity which naturally gives rise to a cosmological constant as a counter term of the Ricci flow at leading order and an effective scale dependent Einstein-Hilbert action. In the weak and static gravity limit, the framework gives rise to a transition trend away from Newtonian gravity and similar to the MOdified Newtonian Dynamics (MOND) around the characteristic scale. When local curvature is large, Newtonian gravity is recovered. When local curvature is low enough to be comparable with the asymptotic background curvature corresponding to the characteristic energy scale, the transition trend produces the baryonic Tully-Fisher relation. For intermediate general curvature around the background curvature, the interpolating Lagrangian function yields a similar transition trend to the observed radial acceleration relation of galaxies. When the baryonic matter density is much lower than the critical density at the outskirt of a galaxy, there may be a universal "acceleration floor" corresponding to the acceleration expansion of the universe, which differs from MOND at its deep-MOND limit. The critical acceleration constant $a_0$ introduced in MOND is related to the low characteristic energy scale of the theory. The cosmological constant gives a universal leading order contribution to it and the flow effect gives the next order scale dependent contribution, which equivalently induces the "cold dark matter" to the theory. $a_0$ is consistent with galaxian data when the "dark matter" is about 5 times the baryonic matter.

Star cluster formation and survival in the first galaxies

ABSTRACT Using radiation-hydrodynamic cosmological simulations, we present a detailed (0.1 pc resolution), physically motivated portrait of a typical-mass dwarf galaxy before the epoch of reionization, resolving the formation, and evolution of star clusters into individual 10 M⊙ star particles. In the rest-frame ultraviolet, the galaxy has an irregular morphology with no bulge or disc, dominated by light emitted from numerous, compact, and gravitationally-bound star clusters. This is especially interesting in light of recent James Webb Space Telescope observations that − aided by the magnifying power of gravitational lenses – have imaged, at parsec-scale resolution, individual young star clusters forming in similar galaxies at z&amp;gt; 6. Because of their low metallicities and high temperatures, star-forming gas clouds in this galaxy have densities ∼100 times higher than typical giant molecular clouds; hence, their expected star formation efficiencies (SFEs) are high enough (around 10 − 70 per cent) to produce a sizeable population of potential globular cluster progenitors, but typically smaller (a few 100 − 2 ×104M⊙, half-mass radii of up to 3 pc) and of lower metallicities (10−3.5– 10−2.5 Z⊙). The initial mass function of the star-forming clouds is log-normal, whereas the bound star cluster mass function is a power-law with a slope that depends mainly on SFE but also on the temporal proximity to a major starburst. We find slopes between −0.5 and −2.5 depending on the assumed sub-grid SFE. Star formation is self-regulated on galactic scales; however, the multimodal metallicity distribution of the star clusters and the fraction of stars locked into surviving bound star clusters depends on SFE.

A simple sub-grid model for cosmic ray effects on galactic scales

ABSTRACT Many recent numerical studies have argued that cosmic rays (CRs) from supernovae (SNe) or active galactic nuclei (AGNs) could play a crucial role in galaxy formation, in particular by establishing a CR-pressure-dominated circumgalactic medium (CGM). But explicit CR-magnetohydrodynamics (CR-MHD) remains computationally expensive, and it is not clear whether those results can be applied to simulations that do not explicitly treat magnetic fields or resolved interstellar medium phase structure. We therefore present an intentionally extremely simplified ‘sub-grid’ model for CRs, which attempts to capture the key qualitative behaviors of greatest interest for those interested in simulations or semi-analytical models including some approximate CR effects on galactic (≳ kpc) scales, while imposing negligible computational overhead. The model is numerically akin to some recently developed sub-grid models for radiative feedback, and allows for a simple constant parametrization of the CR diffusivity and/or streaming speed; it allows for an arbitrary distribution of sources (proportional to black hole accretion rates or star–particle SNe rates or gas/galaxy star formation rates), and interpolates between the limits where CRs escape the galaxies with negligible losses and those where CRs lose most of their energy catastrophically before escape (relevant in e.g. starburst galaxies). The numerical equations are solved trivially alongside gravity in most codes. We compare this to explicit CR-MHD simulations and discuss where the (many) sub-grid approximations break down, and what drives the major sources of uncertainty.