Observed Star Formation Histories Research Articles

HALOGAS: Strong constraints on the neutral gas reservoir and accretion rate in nearby spiral galaxies

Context. Galaxies in the local Universe are thought to require ongoing replenishment of their gas reservoir in order to maintain the observed star formation rates. Cosmological simulations predict that this accretion can occur in both a dynamically hot and cold mode, depending on the redshift, halo mass, and the details of the included feedback processes. However, until now, observational evidence of the accretion required to match the observed star formation histories is lacking. Aims. Within the framework of the Hydrogen Accretion in LOcal GalaxieS (HALOGAS) survey, this paper attempts to determine whether galaxies in the local Universe possess a significant reservoir of cold neutral gas and the possible accretion rates these reservoirs could provide. Additionally, with this moderately sized sample, we can start to investigate whether the observed accretion is connected to intrinsic properties of the galaxies such as Hubble type, star formation rate, or environment. Methods. We searched the vicinity of 22 nearby galaxies in a systematic and automated manner for isolated H I clouds or distinct streams that are not yet connected to the galaxy disk. The HALOGAS observations were carried out with the Westerbork Synthesis Radio Telescope and represent one of the most sensitive and detailed H I surveys to date. These observations typically reach column density sensitivities of ∼1019 cm−2 over a 20 km s−1 line width. Results. We find 14 secure H I cloud candidates without an observed optical counterpart in the entire HALOGAS sample. These cloud candidates appear to be analogs to the most massive clouds detected in the extensive cloud distributions around the Milky Way and M 31. However, their numbers seem significantly reduced on average compared to the numbers in these galaxies. Within the framework of cold accretion, we constrain upper limits for H I accretion in the local Universe. The average H I mass currently observed in a state suggestive of accretion onto the galaxies amounts to a rate of 0.05 M⊙ yr−1 with a stringent upper limit of 0.22 M⊙ yr−1, confirming previous estimates. This is much lower than the average star formation rate in this sample. Our best estimate, based on the Green Bank Telescope detection limits of several galaxies in the sample, suggests that another 0.04 M⊙ yr−1 of neutral hydrogen at most could be accreted from clouds and streams that remain undetected. Conclusions. These results show that in nearby galaxies, neutral hydrogen is not being accreted at the same rate as stars are currently being formed. Our study cannot exclude that other forms of gas accretion are at work, such as those provided by direct infall of ionized intergalactic gas or the condensation of coronal gas, triggered by galactic fountain activities. However, these observations do not reveal extended neutral gas reservoirs around most nearby spiral galaxies either.

Massive quiescent galaxies at z ∼ 3: A comparison of selection, stellar population, and structural properties with simulation predictions

ABSTRACT We study stellar population and structural properties of massive log (M⋆/M⊙) > 11 galaxies at z ≈ 2.7 in the Magneticum and IllustrisTNG hydrodynamical simulations and GAEA semi-analytic model. We find stellar mass functions broadly consistent with observations, with no scarcity of massive, quiescent galaxies at z ≈ 2.7, but with a higher quiescent galaxy fraction at high masses in IllustrisTNG. Average ages of simulated quiescent galaxies are between ≈0.8 and ${1.0\, \textrm {Gyr}}$, older by a factor ≈2 than observed in spectroscopically confirmed quiescent galaxies at similar redshift. Besides being potentially indicative of limitations of simulations in reproducing observed star formation histories, this discrepancy may also reflect limitations in the estimation of observed ages. We investigate the purity of simulated UVJ rest-frame colour-selected massive quiescent samples with photometric uncertainties typical of deep surveys (e.g. COSMOS). We find evidence for significant contamination (up to ${60\, \rm {per\, cent}}$) by dusty star-forming galaxies in the UVJ region that is typically populated by older quiescent sources. Furthermore, the completeness of UVJ-selected quiescent samples at this redshift may be reduced by $\approx {30\, \rm {per\, cent}}$ due to a high fraction of young quiescent galaxies not entering the UVJ quiescent region. Massive, quiescent galaxies in simulations have on average lower angular momenta and higher projected axis ratios and concentrations than star-forming counterparts. Average sizes of simulated quiescent galaxies are broadly consistent with observations within the uncertainties. The average size ratio of quiescent and star-forming galaxies in the probed mass range is formally consistent with observations, although this result is partly affected by poor statistics.

The DIANOGA simulations of galaxy clusters: characterising star formation in protoclusters

Aims. We studied the star formation rate (SFR) in cosmological hydrodynamical simulations of galaxy (proto-)clusters in the redshift range 0 < z < 4, comparing them to recent observational studies; we also investigated the effect of varying the parameters of the star formation model on galaxy properties such as SFR, star-formation efficiency, and gas fraction. Methods. We analyse a set of zoom-in cosmological hydrodynamical simulations centred on 12 clusters. The simulations are carried out with the GADGET-3 Tree-PM smoothed-particle hydro-dynamics code which includes various subgrid models to treat unresolved baryonic physics, including AGN feedback. Results. Simulations do not reproduce the high values of SFR observed within protocluster cores, where the values of SFR are underpredicted by a factor ≳4 both at z ∼ 2 and z ∼ 4. The difference arises as simulations are unable to reproduce the observed starburst population and is greater at z ∼ 2 because simulations underpredict the normalisation of the main sequence (MS) of star forming galaxies (i.e. the correlation between stellar mass and SFR) by a factor of ∼3. As the low normalisation of the MS seems to be driven by an underestimated gas fraction, it remains unclear whether numerical simulations miss starburst galaxies due to overly underpredicted gas fractions or overly low star formation efficiencies. Our results are stable against varying several parameters of the star formation subgrid model and do not depend on the details of AGN feedback. Conclusions. The subgrid model for star formation, introduced to reproduce the self-regulated evolution of quiescent galaxies, is not suitable to describe violent events like high-redshift starbursts. We find that this conclusion holds, independently of the parameter choice for the star formation and AGN models. The increasing number of multi-wavelength high-redshift observations will help to improve the current star formation model, which is needed to fully recover the observed star formation history of galaxy clusters.

Evolution of Black Hole and Galaxy Growth in a Semi-numerical Galaxy Formation Model

Abstract We present a simple semi-numerical model designed to explore black hole growth and galaxy evolution. This method builds on a previous model for black hole accretion that uses a semi-numerical galaxy formation model and universal Eddington ratio distribution to describe the full active galactic nucleus (AGN) population by independently connecting galaxy and AGN growth to the evolution of the host dark matter halos. We fit observed X-ray luminosity functions up to a redshift of z ∼ 4, as well as investigate the evolution of the Eddington ratio distributions. We find that the Eddington ratio distribution evolves with redshift such that the slope of the low-Eddington accretion rate distribution increases with cosmic time, consistent with the behavior predicted in hydrodynamical simulations for galaxies with different gas fractions. We also find that the evolution of our average Eddington ratio is correlated with observed star formation histories, supporting a picture in which black holes and galaxies evolve together in a global sense. We further confirm the impact of luminosity limits on observed galaxy and halo properties by applying selection criteria to our fiducial model and comparing to surveys across a wide range of redshifts.

The suppression of star formation on the smallest scales: what role does environment play?

The predominantly ancient stellar populations observed in the lowest-mass galaxies (i.e. ultra-faint dwarfs) suggest that their star formation was suppressed by reionization. Most of the well-studied ultra-faint dwarfs, however, are within the central half of the Milky Way dark matter halo, such that they are consistent with a population that was accreted at early times and thus potentially quenched via environmental processes. To study the potential role of environment in suppressing star formation on the smallest scales, we utilize the Exploring the Local Volume in Simulations (ELVIS) suite of $N$-body simulations to constrain the distribution of infall times for low-mass subhalos likely to host the ultra-faint population. For the ultra-faint satellites of the Milky Way with star-formation histories inferred from $Hubble~Space~Telescope$ imaging, we find that environment is highly unlikely to play a dominant role in quenching their star formation. Even when including the potential effects of pre-processing, there is a $\lesssim 0.1\%$ probability that environmental processes quenched all of the known ultra-faint dwarfs early enough to explain their observed star-formation histories. Instead, we argue for a mass floor in the effectiveness of satellite quenching at roughly $M_{\star} \sim 10^{5}~M_{\odot}$, below which star formation in surviving galaxies is globally suppressed by reionization. We predict a large population of quenched ultra-faint dwarfs in the Local Field ($1 < R/R_{\rm{vir}} < 2$), with as many as $\sim250$ to be discovered by future wide-field imaging surveys.

EVOLUTION OF DWARF SPHEROIDAL SATELLITES IN THE COMMON SURFACE-DENSITY DARK HALOS

ABSTRACT We investigate the growth histories of dark matter halos associated with dwarf satellites in Local Group galaxies and the resultant evolution of the baryonic component. Our model is based on the recently proposed property that the mean surface density of a dark halo inside a radius at maximum circular velocity is universal over a large range of . Given that a surface density of 20 M ⊙ pc−2 well explains dwarf satellites in the Milky Way and Andromeda, we find that the evolution of the dark halo in this common surface-density scale is characterized by the rapid increase of the halo mass assembled by the redshift of the tidal truncation by its host halo, at early epochs of or km s−1. This mass growth of the halo is slow at lower or larger . Taking into account the baryon content in this dark halo evolution, under the influence of the ionizing background radiation, we find that the dwarf satellites are divided into roughly two families: those with km s−1 having high star formation efficiency and those with larger having less efficient star formation. This semianalytical model is in agreement with the high-resolution numerical simulation for galaxy formation and with the observed star formation histories for Fornax and Leo II. This suggests that the evolution of a dark halo may play a key role in understanding star formation histories in dwarf satellites.

A NEW CHEMICAL EVOLUTION MODEL FOR DWARF SPHEROIDAL GALAXIES BASED ON OBSERVED LONG STAR FORMATION HISTORIES

We present a new chemical evolution model for dwarf spheroidal galaxies (dSphs) in the Local Universe. Our main aim is to explain both their observed star formation histories and metallicity distribution functions simultaneously. Applying our new model for the four local dSphs, that is, Fornax, Sculptor, Leo II, and Sextans, we find that our new model reproduces the observed chemical properties of the dSphs consistently. Our results show that the dSphs have evolved with both a low star formation efficiency and a large gas outflow efficiency compared with the Milky Way, as suggested by previous works. Comparing the observed [alpha/Fe]--[Fe/H] relation of the dSphs with the model predictions, we find that our model favors a longer onset time of Type Ia supernovae (i.e., 0.5 Gyr) than that suggested in previous studies (i.e., 0.1 Gyr). We discuss the origin of this discrepancy in detail.

A PARAMETRIC STUDY OF POSSIBLE SOLUTIONS TO THE HIGH-REDSHIFT OVERPRODUCTION OF STARS IN MODELED DWARF GALAXIES

Both numerical hydrodynamic and semi-analytic cosmological models of galaxy formation struggle to match observed star formation histories of galaxies in low mass halos (M$_{\rm{H}} \lesssim 10^{11} M_\odot$), predicting more star formation at high redshift and less star formation at low redshift than observed. The fundamental problem is that galaxies' gas accretion and star formation rates are too closely coupled in the models: the accretion rate largely drives the star formation rate. Observations point to gas accretion rates that outpace star formation at high redshift, resulting in a buildup of gas and a delay in star formation until lower redshifts. We present three empirical adjustments of standard recipes in a semi-analytic model motivated by three physical scenarios that could cause this decoupling: 1) the mass-loading factors of outflows driven by stellar feedback may have a steeper dependence on halo mass at earlier times, 2) the efficiency of star formation may be lower in low mass halos at high redshift, and 3) gas may not be able to accrete efficiently onto the disk in low mass halos at high redshift. These new recipes, once tuned, better reproduce the evolution of $f_\star \equiv M_\star/M_{\rm{H}}$ as a function of halo mass as derived from abundance matching over redshifts $z=0$ to $3$, though they have different effects on cold gas fractions, star formation rates, and metallicities. Changes to gas accretion and stellar-driven winds are promising, while direct modification of the star formation timescale requires drastic measures that are not physically well-motivated.

The role of cold flows and reservoirs in galaxy formation with strong feedback

We examine gas accretion and subsequent star formation in representative galaxies from the McMaster Unbiased Galaxy Simulations (Stinson et al. 2010). Accreted gas is bimodal with a natural temperature division at $10^5$ K, near the peak of the cooling curve. Cold-mode accretion dominates inflows at early times, creating a peak in total accretion at redshift z=2-4 and declining exponentially below z$\sim$2. Hot-mode accretion peaks near z=1-2 and declines gradually. Hot-mode exceeds cold-mode accretion at z$\sim$1.8 for all four galaxies rather than when the galaxy reaches a characteristic mass. Cold-mode accretion can fuel immediate star formation, while hot-mode accretion preferentially builds a large, hot gas reservoir in the halo. Late-time star formation relies on reservoir gas accreted 2-8 Gyr prior. Thus, the reservoir allows the star formation rate to surpass the current overall gas accretion rate. Stellar feedback cycles gas from the interstellar medium back into the hot reservoir. Stronger feedback results in more gas cycling, gas removal in a galactic outflow and less star formation overall, enabling simulations to match the observed star formation history. For lower mass galaxies in particular, strong feedback can delay the star formation peak to z=1-2 from the accretion peak at z=2-4.

Gas accretion as a dominant formation mode in massive galaxies from the GOODS NICMOS Survey

The ability to resolve all processes which drive galaxy formation is one of the most fundamental goals in extragalactic astronomy. While star formation rates and the merger history are now measured with increasingly high certainty, the role of gas accretion from the intergalactic medium in supplying gas for star formation still remains largely unknown. We present in this paper indirect evidence for the accretion of gas into massive galaxies with initial stellar masses M_*>10^{11} M_sol and following the same merger adjusted co-moving number density at lower redshifts during the epoch 1.5 < z < 3, using results from the GOODS NICMOS Survey (GNS). We show that the measured gas mass fractions of these massive galaxies are inconsistent with the observed star formation history for the same galaxy population. We further demonstrate that this additional gas mass cannot be accounted for by cold gas delivered through minor and major mergers. We also consider the effects of gas outflows and gas recycling due to stellar evolution in these calculations. We argue that to sustain star formation at the observed rates there must be additional methods for increasing the cold gas mass, and that the likeliest method for establishing this supply of gas is by accretion from the intergalactic medium. We calculate that the average gas mass accretion rate into these massive galaxies between 1.5 < z < 3.0, is \dot{M} = 96+/-19 M_sol/yr after accounting for outflowing gas. We show that during this epoch, and for these very massive galaxies, 49+/-20% of baryonic mass assembly is a result of gas accretion and unresolved mergers. However, 66+/-20% of all star formation in this epoch is the result of gas accretion. This reveals that for the most massive galaxies at 1.5< z< 3 gas accretion is the dominant method for instigating new stellar mass assembly.

METALLICITY-DEPENDENT QUENCHING OF STAR FORMATION AT HIGH REDSHIFT IN SMALL GALAXIES

[abridged] The star formation rates (SFR) of low-metallicity galaxies depend sensitively on the gas metallicity, because metals are crucial to mediating the transition from intermediate-temperature atomic gas to cold molecular gas, a necessary precursor to star formation. We study the impact of this effect on the star formation history of galaxies. We incorporate metallicity-dependent star formation and metal enrichment in a simple model that follows the evolution of a halo main progenitor. Our model shows that including the effect of metallicity leads to suppression of star formation at redshift z>2 in dark halos with masses <~ 10^11 Msun, with the suppression becoming near total for halos below ~10^9.5-10 Msun. We find that at high redshift the SFR cannot catch up with the gas inflow rate (IR), because the SFR is limited by the free-fall time, and because it is suppressed further by a lack of metals. As a result, in each galaxy the SFR is growing in time faster than the IR, and the integrated cosmic SFR density is rising with time. The suppressed in situ SFR at high z makes the growth of stellar mass dominated by ex situ SFR which implies that the specific SFR (sSFR) remains constant with time. The intensely accreted gas at high z is accumulating as an atomic gas reservoir. This provides additional fuel for star formation in 10^10 - 10^12 Msun halos at z ~ 1-3, which allows the SFR to exceed the instantaneous IR, and may enable an even higher outflow rate. At z<1, following the natural decline in IR with time due to the universal expansion, the SFR and sSFR are expected to drop. We specify the expected dependence of sSFR and metallicity on stellar mass and redshift. At a given z, and below a critical mass, these relations are predicted to be flat and rising respectively. Our model predictions qualitatively match some of the puzzling features in the observed star formation history.

Reconciliation of damped Lyman α and H ii region metallicities

We present a model of chemical evolution in disc galaxies, which extends our semi-analytic model for galaxy evolution. This semi-analytic framework has already been shown to reproduce the observed star formation histories. We take an open box approach which tracks the metallicity of two phases, the hot coronal gas and the cold gas making up the disc. Gaseous infall at primordial metallicities is constrained by analytic fits to N-body simulations and the model includes supernova and active galactic nuclei feedback.We address the differences between observedmetallicities obtained from H II data and fromdamped Lyman alpha absorbers and show that they may be explained by observational selection effects resulting from radial metallicity gradients, without having to resort to a separate population of galaxies. Within the framework describing these selection effects the cold disc gas of our chemical evolution model is shown to reproduce the metallicities of both the H II regions and the damped Lyman alpha absorbers. DLA metallicities are shown to be largely determined by dwarf galaxies especially at high redshifts.

RADIO SOURCE FEEDBACK IN GALAXY EVOLUTION

We present a galaxy evolution model which incorporates a physically motivated implementation of active galactic nucleus feedback. Intermittent jets inflate cocoons of radio plasma which then expand supersonically, shock heating the ambient gas. The model reproduces observed star formation histories to the highest redshifts for which reliable data exist, as well as the observed galaxy color bimodality. Intermittent radio source feedback also naturally provides a way of keeping the black hole and spheroid growth in step. We find possible evidence for a top-heavy initial mass function for z > 2, consistent with observations of element abundances, and submillimeter and Lyman break galaxy counts.

Delving Deeper into the Tumultuous Lives of Galactic Dwarfs: Modeling Star Formation Histories

The paucity of observed dwarf galaxies in the Local Group relative to the abundance of predicted dark matter halos remains one of the greatest puzzles of the ΛCDM paradigm. Solving this puzzle now requires not only matching the numbers of objects but also understanding the details of their star formation histories. We present a summary of such histories derived from the HST data using the color-magnitude diagram fitting method. To reduce observational uncertainties, we condense the data into five cumulative parameters: the fractions of stellar mass formed in the last 1, 2, 5, and 10 Gyr, and the mean stellar age. We interpret the new data with a phenomenological model based on the mass assembly histories of dark matter halos and the Schmidt law of star formation. The model correctly predicts the radial distribution of the dwarfs and the fractions of stars formed in the last 5 and 10 Gyr. However, in order to be consistent with the observations, the model requires a significant amount of recent star formation in the last 2 Gyr. Within the framework of our model, this prolonged star formation can be achieved by adding a stochastic variation in the density threshold of the star formation law. The model results are not sensitive to late gas accretion, the slope of the Schmidt law, or the details of cosmic reionization. A few discrepancies still remain: our model typically predicts too large stellar masses, only a modest population of ultrafaint dwarfs, and a small number of dwarfs with anomalously young stellar populations. Nevertheless, the observed star formation histories of Local Group dwarfs are generally consistent the expected star formation in cold dark matter halos.

Numerical simulations of the cosmic star formation history

The cosmic star formation history in Cold Dark Matter dominated cosmological scenarios is studied by means of hydrodynamical numerical simulations. In particular, we explore a low density model with a Λ-term and two high density models with different power spectra, all of them being spatially flat. Our simulations employ a fully nonlinear N-body and Eulerian hydrodynamics algorithm with a model for star formation and supernovae feedback that depends on two phenomenological parameters determined in agreement with previous papers. We find a nearly constant star formation rate beyond , and we discuss which facts may determine the decrease in the SFR from to the present epoch. The Λ-term cosmology with realistic parameters for star formation and feedback best reproduces the observed star formation history.

Evolution of Interstellar Clouds in Local Group Dwarf Spheroidal Galaxies in the Context of Their Star Formation Histories

We consider evolution of interstellar clouds in Local Group dwarf spheroidal galaxies (dSph's) in the context of their observed star formation histories. The Local Group dSph's generally experienced initial bursts of star formation in their formation epochs (~15 Gyr ago), when hot gas originating from the supernovae could make the cold interstellar clouds evaporate. We find that the maximum size of evaporating clouds is 10 pc. Thus, clouds larger than 10 pc can survive during the initial star formation. These surviving clouds can contribute to the second star formation to produce intermediate-age (~3-10 Gyr ago) stellar populations. Assuming that collisions between clouds induce star formation and that the timescale of the second star formation is a few Gyr, we estimate the total mass of the clouds. The total mass is about 104 M☉, which is 1-3 orders of magnitude smaller than the typical stellar mass of a present dSph. This implies that the initial star formation is dominant over the second star formation, which is broadly consistent with the observed star formation histories. However, the variety of the dSph's in their star formation histories suggests that the effects of environments on the dSph's may be important.

The Diffuse Gamma‐Ray Background from Supernovae

The Cosmic Gamma-ray Background (CGB) in the MeV region is believed to be due to photons from radioactivity produced in SNe throughout the history of galaxies in the universe. In particular, gamma-ray line emission from the decay chain 56Ni-> 56Co->56Fe provides the dominant photon source. Although iron synthesis occurs in all types of SNe, the contribution to the CGB is dominated by SNIa events due to their higher photon escape probabilities. Estimates of the star formation history in the universe suggest a rapid increase by a factor \~ 10 from the present to a redshift z_p ~ 1.5, beyond which it either remains constant or decreases slowly. We integrate the observed star formation history to determine the CGB from the corresponding SN rate history. In addition to gamma-rays from short-lived radioactivity in SNIa and SNII/Ibc we also calculate the minor contributions from long-lived radioactivities (26Al, 44Ti, 60Co, and electron-positron pair annihilation). Although progenitor evolution for SNIa is not yet fully understood, various arguments suggest delays of order 1-2 Gy between star formation and the production of SNIa's. The effect of this delay on the CGB is discussed. We emphasize the value of gamma-ray observations of the CGB in the MeV range as an independent tool for studies of the cosmic star formation history. If the delay between star formation and SNIa activity exceeds 1 Gy substantially, and/or the peak of the cosmic star formation rate occurs at a redshift much larger than unity, the gamma-ray production of SNIa would be insufficient to explain the observed CGB. Alternatively, the cosmic star formation rate would have to be higher (by a factor 2-3) than commonly assumed, which is in accord with several upward revisions reported in the recent literature.

Evolutionary Histories of Dwarf Galaxies in the Local Group

The star formation histories of Local Group (LG) dwarf galaxies and more distant potential LG members are reviewed. Problems in defining the spatial extent of the LG and membership are briefly discussed. The morphological types found in the LG are presented, and it is suggested that we see continuous evolution from low-mass dwarf irregulars (dIrrs) to dwarf spheroidal galaxies (dSphs) in the LG.Star formation histories for LG dwarfs and nearby LG candidates are compiled using population boxes. No two dwarfs, irrespective of morphological type, show the same evolutionary history, and all vary widely in ages of their subpopulations and in their enrichment history. The lack of gas in dSphs and certain dwarf ellipticals (dEs) is puzzling, both with respect to their star formation histories and the expected mass loss from red giants, but a new photoionization scenario may reconcile these contradictions. Old populations, often spatially very extended, may be a common property of dwarf galaxies, though their fractions can be very small. Almost all types of dwarf galaxies studied in detail so far show spatial variations in ages and abundances such as radial age/metallicity gradients. The observed star formation histories impose constraints on merger and accretion scenarios.Properties of the Milky Way dwarf spheroidals are compared to the M31 dSphs and discussed in the framework of the ram pressure/tidal stripping scenario. It is demonstrated that the newly discovered LG dwarfs follow the same relationship for central surface brightness, mean metallicity, and absolute magnitude as the other LG dwarfs.