Most simulations of galaxies and massive giant molecular clouds (GMCs) cannot explicitly resolve the formation (or predict the main-sequence masses) of individual stars. So they must use some prescription for the amount of feedback from an assumed population of massive stars (e.g. sampling the initial mass function [IMF]). We perform a methods study of simulations of a star-forming GMC with stellarfeedback from UV radiation, varying only the prescription for determining the luminosity of each stellar mass element formed (according to different IMF sampling schemes). We show that different prescriptions can lead to widely varying (factor of ∼ 3) starformation efficiencies (on GMC scales) even though the average mass- to-light ratios agree. Discreteness of sources is important: radiative feedback from fewer, more-luminous sources has a greater effect for a given total luminosity. These differences can dominate over other, more widely-recognized differences between similar literature GMC-scale studies (e.g. numerical methods, cloud initial conditions, presence of magnetic fields). Moreover the differences in these methods are not purely numerical: some make different implicit assumptions about where and how massive stars form, and this remains deeply uncertain in starformation theory.
The simulations presented in this paper of course have their limitations and there is scope for improvement, which may in ﬂ uence the ﬁ nal results. For example, it will be necessary to combine these feedback models with magne- tized ISM simulations. Additional feedback effects such as protostellar out ﬂ ows from forming stars, stellar winds, and radiation pressure will also need to be considered. The momentum input from these sources can be comparable to that from supernovae, although in the case of out ﬂ ows and winds, much of this momentum may be lost in oppositely directed ﬂ ows from a distributed population of stars. Also, initial test simulations including radiation pressure by J. Rosdahl et al. ( 2017, in preparation ) indicate that it plays `a relatively minor role on the scale of this kiloparsec patch. On smaller scales, radiation pressure has been studied by, e.g., Krumholz & Matzner ( 2009 ) , who ﬁ nd it can be important feedback in modifying H II region dynamics around massive star clusters. The contribution of relativistic cosmic rays, which are known to have a similar energy density to magnetic and thermal pressures in the local Galactic ISM, will also need to be included. To constrain these more complex models and their particular parameter choices against observed systems, we will also need to examine more detailed statistical metrics, e.g., the structural, kinematic, and chemical properties of the ISM, in addition to SFR activity. Improved resolution down to the core scale of individual starformation is also highly desirable, especially for the resolution of the early stages of ionizing feedback, which will then also necessitate a fully stochastic treatment of the initial stellar mass function of massive stars. On the other hand, the region we have simulated is relatively limited in spatial extent, which may lead to stochastic effects related to the particular GMCs that are included in the volume. Future work should also aim to expand the number of different realizations simulated, the size of the simulated volume, and also explore the effects of a range of galactic environments.
Given that starformation is one of the fundamental processes driving galaxy for- mation, it is crucial to understand what governs starformation, both on local and galactic scales. One of the open questions regarding starformation on galactic scales is whether it is coherent in space and/or time because of, e.g., gas accretion or environmental effects or highly stochastic because of, e.g., violent stellarfeedback. The relatively tight correlation found between the starformation rate (SFR) and stel- lar mass ( M ∗ ) of actively star-forming galaxies at a range of redshifts (Brinchmann et al., 2004; Noeske et al., 2007; Peng et al., 2010; Wuyts et al., 2011), commonly referred to as the starformation main sequence (MS), is sometimes taken as evi- dence of the former. In particular, some authors argue that galaxies evolve smoothly along the sequence (rather than cross it), as is typically the case in large-volume cosmological simulations (such as those of the Illustris and EAGLE projects; Vo- gelsberger et al. 2014; Schaye et al. 2015) that rely on sub-grid ISM models. In such simulations, galaxies maintain their positions relative to the locus of the MS for 100-Myr timescales (Sparre, Hayward, Springel, et al., 2015; Schaye et al., 2015). However, high-resolution cosmological zoom-in simulations that include explicit multi-channel stellarfeedback suggest that starformation is very bursty in some regimes (due to the clustered nature of starformation, violent stellarfeedback, galactic fountains, and stochastic gas inflow), including at high redshift. This bursti- ness causes galaxy-scale starformation to be a chaotic process in which galaxies cross the MS many times rather than evolve smoothly along it (Hopkins et al., 2014; Muratov et al., 2015; Sparre, Hayward, Feldmann, et al., 2017; Faucher-Giguère, 2018).
with the cluster for ∼ 20 − 50 Myr. These stars are unbound due to the rapid decrease of potential energy as the gas is removed on timescales shorter than a crossing time (e.g. ). Observations of the three young clusters also show excess light at large radii (Fig. 1, left panel), strongly suggesting that they are experiencing violent relaxation . Hence these clusters are not in dynamical equilibrium.
• We nd a feedback-driven instability developing in a typical isolated disk galaxy if the gaseous disk becomes too viscous. The hot bubbles created by SN explosions are too viscous to be sheared away by the dierential rotation. They merge with one another and eventually form large holes in the disk. The AV switch with a strong limiter is able to suppress the viscosity so that the disk remains stable. On the other hand, it could also be possible that it suppresses viscosity too much and cannot capture shocks properly. However, the Sedov explosion test is recovered equally well and provides credibility for our modied AV scheme. In the cases where both shocks and shear ows coexist, it is in fact dicult to determine how much viscosity is appropriate. Compromises have to be made between properly modeling shocks and avoiding articial shear viscosity. This seems to be a general issue for most SPH schemes that use AV for shock capturing (an exception might be 'Godunov SPH', e.g. Cha, Inutsuka & Nayakshin, 2010, Murante et al., 2011). Adopting the PE formulation and including AC alleviates the situation, which suggests that spurious surface tension also plays a role for the instability. The boundaries between the hot bubbles and the cold ISM are sustained by the spurious surface tension in the DE formulation. As such, they are dicult to destroy once being created.
There is evidence for the gas fraction being higher in the field galaxies than those in denser environments as previous accretion of galaxies into their current halos made them gas-stripped (Cortese & Hughes 2009; Fabello et al. 2012; Catinella et al. 2013; Boselli & Gavazzi 2014). It is also possible that the cold gas accretion from the surrounding LSS can more easily/efficiently/numerously penetrate isolated and satellite galaxies than centrals that are located in the densest regions of groups/clusters. van de Voort et al. (2017) simulations show that more massive centrals and satellites both have a higher gas accretion rate than less massive ones (also see Dekel et al. 2013). They also find that gas accretion rate is lower or fully suppressed (depending on the halo mass) in the center of halos. They also find a strong environmental dependence of accretion rate primarily for satellites. This might explain the strong quenching of low-mass satellites in denser regions seen in Figure 4. Therefore, a combination of stellar mass and halo-centric and environmental dependence of gas accretion rate, gas fraction, and mergers can potentially explain the observed trends. Further analyses including gas and age dependence of
Figure 9 shows a cumulative histogram of the final stellar sink masses for both the standard and photoionised simulations. It shows the total numbers of sinks formed by the end of each simulation as a function of their mass. It can be seen that fewer sinks have formed in the photoionised case compared to the stan- dard case but both follow a similar distribution. The most obvi- ous change in the sink mass distribution takes place at the high mass end where there are a lower number of sinks due to the action of PI feedback. The eﬀects of photoionisation are to re- duce the accretion rates and also the maximum mass attained by a sink during the evolution of the system. In the standard case sinks are allowed to grow freely, accreting gas without the e ﬀ ect of feedback and in this case we end up with several sinks with masses >1000 M and the highest sink mass of 1968 M . In the photoionised simulation the highest mass sink is 1154 M and fewer of the sinks grow in excess of 600 M . Here the eﬀects of feedback have prevented the sinks accreting gas as it becomes ionised before its accretion and eﬀectively reduces the source of gas available to the high mass clusters.
Frenk 1991; Hopkins et al. 2012). Feedback from super- massive black holes (SMBHs) is implemented to prevent excessive star-formation in high-mass halos (De Lucia & Blaizot 2007; Croton et al. 2006; Bower et al. 2006; Vogelsberger et al. 2014; Schaye et al. 2015). The phys- ical prescriptions differentiate between radiative-mode feedback and jet-mode feedback. Radiative-mode feed- back (quasar mode) is associated with the high accretion rate of the cold gas onto the SMBH and is related to the gas outflows (Shakura & Sunyaev 1973; Di Matteo et al. 2005). Jet-mode (radio mode) feedback is asso- ciated with a low accretion rate of hot (‘coronal’) gas onto the SMBH. The feedback loop is thought to ex- ist between the cooling of hot gas that feeds the SMBH (e.g., Blanton et al. 2001) to trigger an active galactic nuclei (AGN) phase that subsequently provides a heat- ing source, counter-acting cooling and preventing fur- ther growth in stellar mass.
In Figure 32 we illustrate the colours of our single- age starburst stellar models, overplotted on this SDSS galaxy data, as a function of stellar population age and metallicity. Given that in this case no correction has been made for dust or nebular emission, the broad agreement, in terms of range of colours probed by star forming galaxies and their trends, is good. The mod- els sweep out much of the colour-colour space occupied by the galaxy population during their lifetime. There is evidence, however, that the simple single-age stellar models are not sufficient. The gradient of the galaxy colour-colour relation in the u − g − r plane is steeper than that of the models, while there appears to be an offset of approximately 0.2 mag in r − i, the ef- fects of which become more apparent with increasing metallicity. There are likely several components adding to an explanation for these offsets: the role of nebu- lar emission in modifying the continuum flux, the role of dust extinction in individual galaxies and the role of complex starformation histories, as well as the over- representation of AGB stars in our single star models at late times. With increasing present-day metallicity the number of permutations of these factors rises. The well established mass-metallicity relation in the local Uni- verse (Tremonti et al., 2004) means that such galaxies are typically larger, older and thus have been through more interactions, starbursts and stages of galactic evo- lution than low-metallicity galaxies.
Massive stars play an important role in the ecology of galaxies, providing a major source of ionising UV radiation, mechanical energy and chemical enrichment (Smith 2005). However, seri- ous gaps in our understanding of massive stars exist through- out their lifecycle, in large part due to the rapid evolution – and hence rarity – of such objects. For example, the lack of accurate observational constraints on the metallicity depen- dant mass loss rates of such stars – particularly for short lived phases such as Luminous Blue Variables (LBVs) and Yellow Hypergiants (YHGs) – restricts our ability to follow their post- Main Sequence (MS) evolution. In particular, currently we can- not predict what path a star of given initial mass will follow as it evolves from the Main Sequence through the Wolf Rayet star (WR) phase to supernova (e.g., the “Conti” scenario; Conti 1976; Maeder & Conti 1994). Clearly, the most direct way to address such problems is to identify and study massive stars
- erarchical formation scenario. For this reason, modern models of galaxy formation and evolution are based on this latter scenario. They typically follow the collapse and merging of dark matter haloes of proto-galaxies, computed either analytically using the extended Press-Schechter formalism (e.g. Lacey & Cole ) or from N-body simulations. The for- mation of the galaxies themselves is then modeled by adopting a treatment of the baryons associated with a given halo where starformation is regulated by internal processes, such as the cooling rate of the gas and feedback by supernovae, as well as interactions between the dark matter haloes. The determination of the cosmological model and precise mea- surements of its parameters (e.g. Spergel et al. ) makes it possible to constrain the hierarchical scenario of galaxy formation using the observed properties of galaxies. As the starformation history of galaxies in a hierarchical merging model depends in part on the merging history of the dark matter haloes, early-type galaxies, which have evolved in a mostly passive way since their last significant episode of starformation at high redshift, appear ideally suited to test such models.
Feedback from stars is critical to galaxy evolution in order to slow down starformation and remove baryons from the galactic disc (e.g. Hopkins, Quataert & Murray 2012). Galactic winds are one particular mechanism thought to slow the formation of stars over cosmic time. They potentially do this by removing large amounts of molecular gas from the disc of star-forming galaxies, i.e., ejective feedback (e.g. Somerville & Dav´e 2015). Starburst driven winds are a result of feedback in the form of supernovae and stellar winds (e.g. Veilleux, Cecil & Bland-Hawthorn 2005; Li, Bryan & Ostriker 2017). In this paper we explore the nuclear starburst of the nearby galaxy NGC 253 using astrophysical H 2 O and CH 3 OH masers.
the early phases of mergers (Teyssier et al. 2010; Hopkins et al. 2013; Renaud et al. 2015). In this sense, integral field spec- troscopy (IFS) is a very promising technique, because it can provide relevant information to characterize the extent of starformation, and how/when it is produced. Extended star forma- tion has also been observationally reported in many early-stage mergers, mainly in the form of widespread star clusters, most of which are located at the intersections between progenitors and/or tidal structures (Wang et al. 2004; Elmegreen et al. 2006; Smith et al. 2016), and through stellar population analysis re- lying on IFS (Cortijo-Ferrero et al. 2017; hereafter CF17). In fact, the results of the two early-stage merger LIRGs reported in CF17, IC 1623 W and NGC 6090, are compared throughout the paper with the merger LIRG NGC 2623. The advantage of NGC 2623 is that it is a more advanced system, in the merger stage, where a triggering of the starformation is expected to oc- cur, but at the same time, it also keeps a fossil record in the stellar populations of previous starformation bursts (i.e., when it was at the early-stage merger phase). Therefore, NGC 2623 represents an interesting nearby LIRG to study the role that major mergers play in galaxy evolution using spatially resolved spectroscopy.
nario can be proposed. The galaxies can experience their growth through mergers. Several merging scenarios can be instigated, both major / minor and gas-poor / gas-rich mergers. Major merg- ers are rare events, but they are expected to be mostly gas rich at high-redshift as the gas fraction increases significantly (e.g. Tacconi et al. 2010). Notably, in the case of a major merger, gas can be e ffi ciently brought to the inner part of the galaxy (<1 kpc) and probably feeds the black hole and starformation simultane- ously, allowing a new episode of black hole growth. The trigger- ing event of the radio galaxy episode is still an open discussion, but recent studies suggest that major mergers can play an im- portant role (e.g. Ramos Almeida et al. 2013). In the case of minor mergers, gas-rich companions could form the stars and be accreted within the cosmic time. This scenario is supported by some observational evidence thanks to high-resolution imag- ing with HST (Miley et al. 2006; Seymour et al. 2012). This is also related to the size evolution of galaxies as well as the com- pactness of early-type galaxies at high-redshift (e.g. Daddi et al. 2005; van Dokkum et al. 2008; Delaye et al. 2014), the change in the mass function with cosmic time (Ilbert et al. 2013), the light profiles and elemental abundance ratios in the outer regions of massive ellipticals (Huang et al. 2013; Greene et al. 2013), and the fact that at constant co-moving density, the mass of mas- sive early-type galaxies grew by about a factor of 4 over ap- proximately the last 10 Gyr (e.g. van Dokkum et al. 2010; Ilbert et al. 2013). As HzRGs are found in dense environments, proba- bly in the centre of proto-clusters (e.g. Wylezalek et al. 2013a), they are likely to experience an important series of minor dry mergers, consistent with the size evolution scenario. Therefore,
Trujillo et al. 2006; Franx et al. 2008; Bell et al. 2012; van der Wel et al. 2014 ) . Knowledge of the stellar ages and metallicities is crucial to understand the starformation history, and stellar velocity dispersions are a crucial element in quantifying the dynamical scaling relations such as the Faber & Jackson ( 1976 ) relation, the Tully & Fisher ( 1977 ) relation, and the fundamental plane ( Djorgovski & Davis 1987; Dressler et al. 1987 ) , the evolution of which has proved to be key in constraining galaxy formation models ( e.g., Franx 1993; van Dokkum et al. 1998; Holden et al. 2005; Treu et al. 2005; van der Wel et al. 2005; Bezanson et al. 2013, 2015 ) . Furthermore, the underlying relationships between the stellar content of galaxies and their mass, ongoing starformation, internal structure, environment, and nuclear activity can be explored at large lookback time for the ﬁ rst time by the LEGA-C data set. The LEGA-C survey can play a crucial and unique role in the context of previous, current, and upcoming developments in the ﬁ eld of galaxy evolution. Wide-area surveys with the Hubble Space Telescope ( HST ) such as COSMOS ( Scoville et al. 2007 ) and CANDELS ( Grogin et al. 2011; Koekemoer et al. 2011 ) have been completed, producing a high-resolution imaging data set, essentially “ frozen ” until the James Webb Space Telescope ( JWST ) and Euclid start operation. After several years with a focus on deep imaging campaigns, the deep spectroscopic effort to probe the stellar light of distant galaxies has fallen behind. Furthermore, ALMA is measuring cold gas masses for increasingly large samples of galaxies ( e.g., Carilli & Walter 2013; Decarli et al. 2014; Scoville et al. 2015 ) . The interplay between stellar populations, ongoing starformation and the available reservoirs for future starformation make for a powerful combination to reconstruct and predict galaxy evolution. Finally, in 2019 the JWST should start operation, marking the beginning of a new era in which JWST and new, large ground-based telescopes will explore the physical properties of galaxies at z ∼ 2 and beyond. Because the vast majority of all stars formed between z ∼ 2 and the present, connecting the galaxy populations at z > 2 and the present is an intractable problem without the intermediate redshift benchmark sample that LEGA-C provides. Yet, JWST is lacking spectroscopic capabilities at λ< 1 μ m, leaving many established spectral diagnostics of stellar populations inaccessible; crucially LEGA-C ﬁ lls this gap.
median of the density field of the redshift slice the galaxy is in. We choose to use number densities instead of mass density esti- mates (e.g. Wolf et al. 2009) to avoid introducing any bias due to any underlying relation between stellar mass and density that may exist. For a more detailed description of the method, we re- fer the reader to Darvish et al. (2014) and Darvish et al. (2015b). We have computed the value of the overdensity for each galaxy by interpolating the density field to their angular position and spectroscopic redshift. We show in Figure 6 the distribution of our galaxies according to their overdensity and labelled by the region they are likely to belong to, as defined by the cosmic web measurements computed by Darvish et al. (2014, 2017). We note that when referring to galaxies within our spectroscopic sample in cluster regions we are mostly referring to either rich groups or the outskirts of massive clusters as our observational setup does not allow for a good sampling of densely populated regions due to slit collision problems.
We present hydrodynamic simulations of the evolution of self-gravitating dense gas on scales of 1 kpc down to parsec in a galactic disk, designed to study dense clump formation from giant molecular clouds (GMCs). These structures are expected to be the precursors to star clusters and this process may be the rate limiting step controlling starformation rates in galactic systems as described by the Kennicutt–Schmidt relation. We follow the thermal evolution of the gas down to ∼5 K using extinction-dependent heating and cooling functions. We do not yet include magnetic fields or localized stellarfeedback, so the evolution of the GMCs and clumps is determined solely by self-gravity balanced by thermal and turbulent pressure support and the large-scale galactic shear. While cloud structures and densities change significantly during the simulation, GMC virial parameters remain mostly above unity for timescales exceeding the free-fall time of GMCs indicating that energy from galactic shear and large-scale cloud motions continuously cascades down to and within the GMCs. We implement starformation at a slow, inefficient rate of 2% per local free-fall time, but even this yields global starformation rates that are about two orders of magnitude larger than the observed Kennicutt–Schmidt relation due to overproduction of dense gas clumps. We expect a combination of magnetic support and localized stellarfeedback is required to inhibit dense clump formation to ∼ 1% of the rate that results from the nonmagnetic, zero-feedback limit.
additional CGs as lower-limits in total group stellar mass in our figures. Note that for CGs embedded within larger structures, we only consider the properties of galaxies that make up the compact region. While this exclusion of the extended populations may seem in error, Palumbo et al. (1995) examined the extended populations of the Hickson (1982) sample and found that the compact cores and extended halos showed statistically di↵erent properties (e.g., spiral fraction) indicating that the compact groups are “disconnected” from their environments. Evidence of this distinction between CG galaxies and their surrounding environment can be seen in the work of Johnson et al. (2007), Walker et al. (2010), and Walker et al. (2012) who found a gap in the mid-IR color distribution of CG galaxies suggestive of accelerated evolution attributed to the CG environment. Further, the galaxies far from the compact cores are, in most cases, dwarf galaxies that do not add significant stellar mass to the group. Dozens of such galaxies would be required to significantly a↵ect our results. While the group members far outside the core may also add substantially to the total group starformation rate, these members are not yet impacted by ram-pressure stripping nor have they contributed much gas to the formation of the intragroup medium, therefore we exclude them in the discussion of the link between starformation and di↵use X-ray luminosity.
Figure 2.8 demonstrates the same point by showing the 3D radial profiles of the gas metallicity at the end of the simulation for all three models, compared to the initial metal- licity profile. In the no-BH model, the star-formation and subsequent stellarfeedback in the cold, dense disc and core raises the metallicity in the centre up to two to three times the solar value, but the constant inflow of gas from the outer parts of the galaxy, combined with the lack of any large-scale outflows, leads to a total depletion of metals in the CGM gas (with less than 10% solar metallicity outside of a radius of 20 kpc). On the other hand, with black-hole feedback (in the BH-W and BH-WR models), the central metallicity stays lower, close to the initial value, while the outer parts of the galactic gas become significantly enriched with metals out to a radius of about 30 kpc. There is little difference between the two models that include black-hole feedback, as the wind feedback is the relevant part to create large-scale outflows capable of enriching the CGM. Outside of a radius of ca. 30 kpc, the gas becomes depleted in metals even in the feedback-including models, as the AGN-driven outflows lose their momentum and stop progressing before they can reach further out. The two models with black-hole feedback also agree much better with the observations of O’Sullivan et al. (2007), at least out to ∼ 30 kpc, but considering the observational uncertainties, as well as the slope of the initial metallicity profile in the simulations (which is set to follow the stellar profile, and therefore not necessarily realistic), on should be cautious before reading to much into this agreement.
In this paper, we have attempted a systematic exploration of different qualitative physical mechanisms by which energy can be injected into massive haloes to quench galaxies and suppress cooling flows. We specifically considered models with radial momentum injection (e.g., “wind” or “radiation pressure” or “isotropic kinetic” models), thermal heating (e.g., “shocked wind” or “isotropic sound wave” or “photo/Compton-heating” or “blastwave” models), turbulent “stirring” (e.g., “con- vective/buoyant bubble” or “precessing jet” or “jet/bubble instability-driven” or “subhalo/merger/satellite wind-driven” models), and cosmic ray injection (e.g., CRs from compact or extended radio jets/lobes, shocked disk winds, or inflated bubbles). We vary the associated energetics and/or momentum fluxes, spatial coupling/driving scales, and halo mass scale from ∼ 10 12 − 10 14 M . These were studied in fully global but non-cosmological simulations including radiative heating and cooling, self-gravity, starformation, and stellarfeedback from supernovae, stellar mass-loss, and radiation, enabling a truly “live” response of starformation and the multi-phase ISM to cooling flows; we used a hierarchical super-Lagrangian refinement scheme to reach ∼ 10 4 M mass resolution, much higher than many previous global studies. Of the cases surveyed, only turbulent stirring within a radius of order the halo scale radius, or cosmic ray injection (with appropriate energetics) were able to maintain a stable, cool-core, low-SFR halo for extended periods of time, across all halo masses surveyed, without obviously violating observational constraints on halo