Young Galaxies Research Articles

A young galaxy cluster in the old Universe

ABSTRACT Galaxies evolve from a blue star-forming phase into a red quiescent one by quenching their star formation activity. In high-density environments, this galaxy evolution proceeds earlier and more efficiently. Therefore, local galaxy clusters are dominated by well-evolved red elliptical galaxies. The fraction of blue galaxies in clusters monotonically declines with decreasing redshift, i.e. the Butcher–Oemler effect. In the local Universe, observed blue fractions of massive clusters are as small as ≲0.2. Here we report a discovery of a ‘blue cluster’ that is a local galaxy cluster with an unprecedentedly high fraction of blue star-forming galaxies yet hosted by a massive dark matter halo. The blue fraction is 0.57, which is 4.0σ higher than those of the other comparison clusters under the same selection and identification criteria. The velocity dispersion of the member galaxies is 510 km s−1, which corresponds to a dark matter halo mass of 2.0$^{+1.9}_{-1.0}\times 10^{14}$ M⊙. The blue fraction of the cluster is more than 4.7σ beyond the standard theoretical predictions including semi-analytic models of galaxy formation. The probability to find such a high blue fraction in an individual cluster is only 0.003 per cent, which challenges the current standard frameworks of the galaxy formation and evolution in the ΛCDM universe. The spatial distribution of galaxies around the blue cluster suggests that filamentary cold gas streams can exist in massive haloes even in the local Universe. However these cold streams have already disappeared in the theoretically simulated local universes.

Thirsty galaxies thriving on gas streams

Young galaxies are most effective at converting gas into stars. Intense accretion of fuel is required to keep galaxies growing, but these gas streams have largely eluded observations. New instruments at optical telescopes are now uncovering clues of their existence.

Spectral energy distribution of the first galaxies: contribution from pre-main-sequence stars

ABSTRACT One of the major goals of next-generation space-borne and ground-based telescopes is to detect and characterize the first galaxies that were in place in the first few hundred million years after the big bang. We study the spectral energy distribution (SED) of the first galaxies and discuss the prospects for detection and identification. We consider very young star-forming galaxies at z = 15 and incorporate the contribution from pre-main-sequence (PMS) stars. Unlike in the present-day galaxies, primordial protostars are not embedded in dusty gas clouds, and hence the light from them can be visible at a wide range of wavelengths. We use mesa to follow the PMS evolution and use the BT-Settl model to calculate the SED of individual PMS stars. We show that PMS stars contribute to boost the flux in the mid-infrared, and that the galaxy SED at very early evolutionary phases is overall redder than at later phases. The infrared flux contribution is comparable to that caused by emission lines powered by massive stars. We argue that the contribution from PMS stars is important for characterizing young galaxies in the early Universe and also for target selection with future deep galaxy surveys.

Mapping the structure and source of outflows from star-forming galaxies at z = 2 – 3

AbstractAs we enter the era of JWST our need to characterise the rest-frame UV spectra of star-forming galaxies becomes essential. By combining the NIR capabilities of JWST with our understanding of UV wavelength science, we have the opportunity to explore fundamental properties of the gas, such as its metallicity and density, as well as the extent, velocity, and magnitude of their outflowing gas, in galaxies out to z ∼ 6. Galaxy outflows in particular play a fundamental role in the evolution of young galaxies at high redshifts, but their properties remain largely unknown as it is difficult to spatially resolve the outflowing gas. To-date, only two attempts to resolve outflows at redshift ∼ 2 have been made using lensing magnification, producing contradictory results on the origin of the outflows. In this talk I will present results from one such groundbreaking study where we combine gravitational lensing with VLT-MUSE to perform one of the first spatially resolved absorption line studies of a galaxy at z = 2 – 3. I will discuss how the the distinct kinematical structure and uniform column densities obtained from the outflowing gas maps reveal ‘global’ rather than ‘locally’ sourced outflows. I will also present preliminary results from our latest attempt to accurately constrain the structure and source of outflows in star-forming galaxies by observing the brightest galaxy-scale lens known with KCWI. I will conclude with the benefits and limitations of spatially resolved observations in this wavelength range, and possible implications on NIRSpec observations of the high-z Universe.

From Primordial Seed Magnetic Fields to the Galactic Dynamo

The origin and maintenance of coherent magnetic fields in the Universe is reviewed with an emphasis on the possible challenges that arise in their theoretical understanding. We begin with the interesting possibility that magnetic fields originated at some level from the early universe. This could be during inflation, the electroweak, or the quark-hadron phase transitions. These mechanisms can give rise to fields which could be strong, but often with much smaller coherence scales than galactic scales. Their subsequent turbulent decay decreases their strength but increases their coherence. We then turn to astrophysical batteries which can generate seed magnetic fields. Here the coherence scale can be large, but the field strength is generally very small. These seed fields need to be further amplified and maintained by a dynamo to explain observed magnetic fields in galaxies. Basic ideas behind both small and large-scale turbulent dynamos are outlined. The small-scale dynamo may help to understand the first magnetization of young galaxies, while the large-scale dynamo is important for the generation of fields with scales larger than the stirring scale, as observed in nearby disk galaxies. The current theoretical challenges that turbulent dynamos encounter and their possible resolution are discussed.

Formation of massive black holes in rapidly growing pre-galactic gas clouds

The origin of the supermassive black holes that inhabit the centres of massive galaxies remains unclear1,2. Direct-collapse black holes-remnants of supermassive stars, with masses around 10,000 times that of the Sun-are ideal seed candidates3-6. However, their very existence and their formation environment in the early Universe are still under debate, and their supposed rarity makes modelling their formation difficult7,8. Models have shown that rapid collapse of pre-galactic gas (with a mass infall rate above some critical value) in metal-free haloes is a requirement for the formation of a protostellar core that will then form a supermassive star9,10. Here we report a radiation hydrodynamics simulation of early galaxy formation11,12 that produces metal-free haloes massive enough and with sufficiently high mass infall rates to form supermassive stars. We find that pre-galactic haloes and their associated gas clouds that are exposed to a Lyman-Werner intensity roughly three times the intensity of the background radiation and that undergo at least one period of rapid mass growth early in their evolution are ideal environments for the formation of supermassive stars. The rapid growth induces substantial dynamical heating13,14, amplifying the Lyman-Werner suppression that originates from a group of young galaxies 20 kiloparsecs away. Our results strongly indicate that the dynamics of structure formation, rather than a critical Lyman-Werner flux, is the main driver of the formation of massive black holes in the early Universe. We find that the seeds of massive black holes may be much more common than previously considered in overdense regions of the early Universe, with a co-moving number density up to 10-3 per cubic megaparsec.

J1234+3901: an extremely metal-deficient compact star-forming dwarf galaxy at redshift 0.133

We have obtained optical spectroscopy of one of the most metal-poor dwarf star-forming galaxies (SFG) in the local Universe, J1234+3901, with the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS). This blue compact dwarf (BCD) galaxy with a redshift z=0.133 was selected from the Data Release 14 (DR14) of the Sloan Digital Sky Survey (SDSS). Its properties are extreme in many ways. Its oxygen abundance 12 + log O/H = 7.035+/-0.026 is among the lowest ever observed for a SFG. Its absolute magnitude Mg = -17.35 mag makes it the brightest galaxy among the known BCDs with 12 + log O/H < 7.3. With its low metallicity, low stellar mass M* = 10^7.13 Msun and very low mass-to-light ratio M*/Lg ~ 0.01 (in solar units), it deviates strongly from the mass-metallicity and luminosity-metallicity relations defined by the bulk of the SFGs in SDSS DR14. J1234+3901 has a very high specific star-formation rate sSFR ~ 100 Gyr^-1, indicating very active ongoing star-formation. Its spectrum shows a strong HeII 4686 emission line, with a flux ~ 2.4 per cent that of the Hbeta emission line. The most probable source of ionizing radiation for producing such a strong line is fast radiative shocks. J1234+3901 has a ratio O32 = [OIII]5007/[OII]3727 ~ 15, the highest among the lowest-metallicity SFGs, and is thus likely leaking Lyman continuum radiation. It is a good candidate for being a young dwarf galaxy, with a large fraction of its stars formed recently. As such, it is probably one of the best local counterparts of dwarf primeval galaxies responsible for the reionization of the early Universe.

Two Thresholds for Globular Cluster Formation and the Common Occurrence of Massive Clusters in the Early Universe

Abstract Young massive clusters (YMCs) are usually accompanied by lower-mass clusters and unbound stars with a total mass equal to several tens times the mass of the YMC. If this was also true when globular clusters (GCs) formed, then their cosmic density implies that most star formation before redshift ∼2 made a GC that lasted until today. Star-forming regions had to change after this time for the modern universe to be making very few YMCs. Here we consider the conditions needed for the formation of a ∼106 M ⊙ cluster. These include a star formation rate (SFR) inside each independent region that exceeds ∼1 M ⊙ yr−1 to sample the cluster mass function up to such a high mass, and an SFR per unit area of ΣSFR ∼ 1 M ⊙ kpc−2 yr−1 to get the required high gas surface density from the Kennicutt–Schmidt relation, and therefore the required high pressure from the weight of the gas. High pressures are implied by the virial theorem at cluster densities. The ratio of these two quantities gives the area of a GC-forming region, ∼1 kpc2, and the young stellar mass converted to a cloud mass gives a typical gas surface density of 500–1000 M ⊙ pc−2. Observations of star-forming clumps in young galaxies are consistent with these numbers, suggesting that they formed today’s GCs. Observations of the cluster cutoff mass in local galaxies agree with the maximum mass calculated from ΣSFR. Metal-poor stellar populations in local dwarf irregular galaxies confirm the dominant role of GC formation in building their young disks.

Refuelled and shielded – the early evolution of tidal dwarf galaxies

We present, for the first time, numerical simulations of young tidal dwarf galaxies (TDGs), including a self-consistent treatment of the tidal arm in which they are embedded. Thereby, we do not rely on idealised initial conditions, as the initial data of the presented simulation emerge from a galaxy interaction simulation. By comparing models which are either embedded in or isolated form the tidal arm, we demonstrate its importance on the evolution of TDGs, as additional source of gas which can be accreted and is available for subsequent conversion into stars. During the initial collapse of the proto-TDG, with a duration of a few 100 Myr, the evolution of the embedded and isolated TDGs are indistinguishable. Significant differences appear after the collapse has halted and the further evolution is dominated by the possible accretion of material form the surroundings of the TDGs. The inclusion of the tidal arm in the simulation of TDGs results in roughly a doubling of the gas mass ($M_\mathrm{gas}$) and gas fraction ($f_\mathrm{gas}$), an increase in stellar mass by a factor of 1.5 and a $\sim3$ times higher star formation rate (SFR) compared to the isolated case. Moreover, we perform a parametric study on the influence of different environmental effects, i.e. the tidal field and ram pressure. Due to the orbit of the chosen initial conditions, no clear impact of the environmental effects on the evolution of TDG candidates can be found.

Warm FIRE: simulating galaxy formation with resonant sterile neutrino dark matter

We study the impact of a warm dark matter (WDM) cosmology on dwarf galaxy formation through a suite of cosmological hydrodynamical zoom-in simulations of $M_{\rm halo} \approx10^{10}\,M_{\odot}$ dark matter halos as part of the Feedback in Realistic Environments (FIRE) project. A main focus of this paper is to evaluate the combined effects of dark matter physics and stellar feedback on the well-known small-scale issues found in cold dark matter (CDM) models. We find that the $z=0$ stellar mass of a galaxy is strongly correlated with the central density of its host dark matter halo at the time of formation, $z_{\rm f}$, in both CDM and WDM models. WDM halos follow the same $M_{\star}(z=0)-V_{\rm max}(z_{\rm f})$ relation as in CDM, but they form later, are less centrally dense, and therefore contain galaxies that are less massive than their CDM counterparts. As a result, the impact of baryonic effects on the central gravitational potential is typically diminished relative to CDM. However, the combination of delayed formation in WDM and energy input from stellar feedback results in dark matter profiles with lower overall densities. The WDM galaxies studied here have a wider diversity of star formation histories (SFHs) than the same systems simulated in CDM, and the two lowest $M_{\star}$ WDM galaxies form all of their stars at late times. The discovery of young ultra-faint dwarf galaxies with no ancient star formation -- which do not exist in our CDM simulations -- would therefore provide evidence in support of WDM.

Super-Eddington accretion of dusty gas onto seed black holes: metallicity-dependent efficiency of mass growth

The super-Eddington accretion onto intermediate seed BHs is a potential formation mode of supermassive black holes exceeding $10^9~M_\odot$ in the early universe. We here investigate how such rapid accretion may occur with finite amounts of heavy elements contained in the gas and dust. In our 1D radiation-hydrodynamics simulations, the radiative transfer is solved for both the direct UV lights emitted by an accretion disk and the diffuse IR lights thermally emitted by dust grains. Our results show that the radiative force by the IR lights causes a strong feedback to regulate the mass accretion. The resulting mean accretion rate is lower with the higher metallicity, and there is the critical metallicity $Z \sim 10^{-2}~Z_\odot$, above which the super-Eddington accretion is prevented by the radiation pressure of the IR lights. With this taken into account, we examine if the dusty super-Eddington accretion occurs in young galaxies using a simple model. We show that a sufficient number of galaxies at $z \gtrsim 10$ can be such potential sites if BHs accrete the cold dense gas with $T \sim 10^2$ K, approximately the thermal equilibrium value at $Z = 10^{-2}~Z_\odot$. We argue that the efficiency of the BH growth via the rapid accretion depends on the metallicity, and that the metallicity slightly lower than $10^{-2}~Z_\odot$ provides a chance for the most efficient growth.

Stellar Population Diagnostics of the Massive Star Binary Fraction

Abstract Populations of massive stars are directly reflective of the physics of stellar evolution. Counts of subtypes of massive stars and ratios of massive stars in different evolutionary states have been used ubiquitously as diagnostics of age and metallicity effects. While the binary fraction of massive stars is significant, inferences are often based upon models incorporating only single-star evolution. In this work, we utilize custom synthetic stellar populations from the Binary Population and Stellar Synthesis code to determine the effect of stellar binaries on number count ratios of different evolutionary stages in both young massive clusters and galaxies with massive stellar populations. We find that many ratios are degenerate in metallicity, age, and/or binary fraction. We develop diagnostic plots using these stellar count ratios to help break this degeneracy, and use these plots to compare our predictions to observed data in the Milky Way and the Local Group. These data suggest a possible correlation between the massive star binary fraction and metallicity. We also examine the robustness of our predictions in samples with varying levels of completeness. We find including binaries and imposing a completeness limit can both introduce ≳0.1 dex changes in inferred ages. Our results highlight the impact that binary evolution channels can have on the massive star population.

Galaxy and Mass Assembly (GAMA): Accurate number densities and environments of massive ultra-compact galaxies at 0.02 &lt; z &lt; 0.3

Context. Massive ultra-compact galaxies (MUGs) are common at z = 2−3, but very rare in the nearby Universe. Simulations predict that the few surviving MUGs should reside in galaxy clusters, whose large relative velocities prevent them from merging, thus maintaining their original properties (namely stellar populations, masses, sizes and dynamical state). Aims. Our goal is to obtain a complete census of the MUG population at 0.02 &lt; z &lt; 0.3, determining the number density, population properties and environment. Methods. We have taken advantage of the high-completeness, large-area spectroscopic GAMA survey, complementing it with deeper imaging from the KiDS and VIKING surveys. We find a set of 22 bona-fide MUGs, defined as having high stellar mass (&gt; 8 × 1010 M⊙) and compact size (Re &lt; 2 kpc). An additional set of seven lower-mass objects (6 × 1010 &lt; M⋆/M⊙ &lt; 8 × 1010) are also potential candidates according to typical mass uncertainties. Results. The comoving number density of MUGs at low redshift (z &lt; 0.3) is constrained at (1.0 ± 0.4)×10−6 Mpc−3, consistent with galaxy evolution models. However, we find a mixed distribution of old and young galaxies, with a quarter of the sample representing (old) relics. MUGs have a predominantly early or swollen disk morphology (Sérsic index 1 &lt; n &lt; 2.5) with high stellar surface densities (⟨Σe⟩∼1010 M⊙ Kpc−2). Interestingly, a large fraction feature close companions – at least in projection – suggesting that many (but not all) reside in the central regions of groups. Halo masses show these galaxies inhabit average-mass groups. Conclusions. As MUGs are found to be almost equally distributed among environments of different masses, their relative fraction is higher in more massive overdensities, matching the expectations that some of these galaxies fell in these regions at early times. However, there must be another channel leading some of these galaxies to an abnormally low merger history because our sample shows a number of objects that do not inhabit particularly dense environments.

PKS B1740$\mathbf {-}$517: An ALMA view of the cold gas feeding a distant interacting young radio galaxy

Cold neutral gas is a key ingredient for growing the stellar and central black hole mass in galaxies throughout cosmic history. We have used the Atacama Large Millimetre Array (ALMA) to detect a rare example of redshifted $^{12}$CO(2-1) absorption in PKS B1740-517, a young ($t \sim 1.6 \times 10^{3}$ yr) and luminous ($L_{\rm 5 GHz} \sim 6.6 \times 10^{43}$ erg s$^{-1}$ ) radio galaxy at $z = 0.44$ that is undergoing a tidal interaction with at least one lower-mass companion. The coincident HI 21-cm and molecular absorption have very similar line profiles and reveal a reservoir of cold gas ($M_{\rm gas} \sim 10^{7} - 10^{8}$ M$_{\odot}$), likely distributed in a disc or ring within a few kiloparsecs of the nucleus. A separate HI component is kinematically distinct and has a very narrow line width ($\Delta{v}_{\rm FWHM} \lesssim 5$ km s$^{-1}$), consistent with a single diffuse cloud of cold ($T_{\rm k} \sim 100$ K) atomic gas. The $^{12}$CO(2-1) absorption is not associated with this component, which suggests that the cloud is either much smaller than 100 pc along our sight-line and/or located in low-metallicity gas that was possibly tidally stripped from the companion. We argue that the gas reservoir in PKS B1740-517 may have accreted onto the host galaxy $\sim$50 Myr before the young radio AGN was triggered, but has only recently reached the nucleus. This is consistent with the paradigm that powerful luminous radio galaxies are triggered by minor mergers and interactions with low-mass satellites and represent a brief, possibly recurrent, active phase in the life cycle of massive early type galaxies.

The Active Assembly of the Virgo Cluster: Indications for Recent Group Infall From Early-type Dwarf Galaxies

Abstract Virgo is a dynamically young galaxy cluster with substructure in its spatial and kinematic distribution. Here we simultaneously study the phase-space distribution and the main characteristics of Virgo’s galaxies, particularly its most abundant galaxy population—the early-type dwarfs—to understand their environmental transformation histories. Aside from known correlations with morphological types—like the larger average clustercentric distance of late-type galaxies—we find an intriguing behavior of early types with magnitudes −17 ≥ M r ≥ −18. They show a large velocity spread and an asymmetric phase-space distribution, similar to the late-type galaxies and different from the early types just 1 mag brighter/fainter. Furthermore, we find a close phase-space aggregation of early-type dwarfs at large clustercentric distance and high relative velocity. Nearly all of them show signatures of disk components, and their colors imply stellar ages that are younger than the population average. They are not located closely together but spread azimuthally around the cluster center. We show that this is expected from simulations of an infalling galaxy group that slowly gets dispersed after its first pericentric passage. We thus conclude that these galaxies are recent arrivals and that the peculiar phase-space distribution of early-type dwarfs is evidence for the ongoing growth of this galaxy population. Studying galaxies based on their phase-space correlations is a unique way to compare the properties of recent and older cluster members and to understand which environment most influenced their present-day characteristics.

Observational signatures of massive black hole formation in the early Universe

Space telescope observations of massive black holes during their formation may be key to understanding the origin of supermassive black holes and high-redshift quasars. To create diagnostics for their detection and confirmation, we study a simulation of a nascent massive, so-called direct-collapse, black hole that induces a wave of nearby massive metal-free star formation, unique to this seeding scenario and to very high redshifts. Here we describe a series of distinct colors and emission line strengths, dependent on the relative strength of star formation and black hole accretion. We predict that the forthcoming James Webb Space Telescope might be able to detect and distinguish a young galaxy that hosts a direct-collapse black hole in this configuration at redshift 15 with as little as a 20,000-second total exposure time across four filters, critical for constraining supermassive black hole seeding mechanisms and early growth rates. We also find that a massive seed black hole produces strong, H$_2$-dissociating Lyman-Werner radiation.

ALMA Observations toward the Starburst Dwarf Galaxy NGC 5253. I. Molecular Cloud Properties and Scaling Relations

Abstract We present high-spatial-resolution (∼0.″2, or ∼3 pc) CO(2–1) observations of the nearest young starburst dwarf galaxy, NGC 5253, taken with the Atacama Large Millimeter/submillimeter Array. We have identified 118 molecular clouds with average values of 4.3 pc in radius and 2.2 km s−1 in velocity dispersion, which comprise the molecular cloud complexes observed previously with ∼100 pc resolution. We derive for the first time in this galaxy the I(CO)–N(H2) conversion factor, cm−2(K km s−1)−1, based on the virial method. The line width and mass-to-size relations of the resolved molecular clouds present an offset on average toward higher line widths and masses with respect to quiescent regions in other nearby spiral galaxies and our Galaxy. The offset in the scaling relation reaches its maximum in regions close to the central starburst, where velocity dispersions are ∼0.5 dex higher and gas mass surface densities are as high as . These central clouds are gravitationally bound despite the high internal pressure. A spatial comparison with star clusters found in the literature enables us to identify six clouds that are associated with young star clusters. Furthermore, the star formation efficiencies (SFEs) of some of these clouds exceed those found in star-cluster-forming clouds within our Galaxy. We conclude that once a super star cluster is formed, the parent molecular clouds are rapidly dispersed by the destructive stellar feedback, which results in such a high SFE in the central starburst of NGC 5253.

Exploring 3D data of Dwarf Galaxies

AbstractDwarf irregular galaxies are places of ongoing star-formation in the low-redshift Universe. Low metallicity dwarfs were originally thought to be the youngest galaxies in the local Universe, however, there is now evidence that they consist of matter which has previously undergone evolution and is enriched by star-formation. Here we present a sample of seven nearby metal-poor dwarf galaxies with a young stellar populations selected from the SDSS which we study using integral field unit (IFU) data from the VIMOS instrument, covering the spectral range between the He II 4686 line and the [SII] 6718/6733 Å doublet. We present property maps across the galaxies and compare different galaxies and different HII regions within the same galaxy. We find that the properties within one galaxy are not uniform and they also differ between different galaxies concerning star-formation, kinematics and metallicity and morphology. The observed differences across individual galaxies together with disturbed kinematics and morphologies can be interpreted as possible signs of recent interactions and/or mergers. Additionally, we present a comparison of different metallicity calibrations and search for systematic differences obtained using different methods.

Diffuse post-starburst galaxies: a smoking gun of formation of ultra-diffuse galaxies via ram pressure stripping

AbstractUltra-diffuse galaxies (UDGs) have sizes comparable to the Milky Way and stellar masses of about 1/1000 of it. They attracted a lot of attention as possible “dark galaxies” heavily dominated by dark matter, however, no reliable dynamical mass estimates were done because of their extremely low surface brightness. We have recently found 13 gas free diffuse young (300–500 Myr) post-starburst galaxies (PSGs) without ongoing star formation in Coma and Abell 2147, which, should they continue to evolve passively, will become UDGs in 5 Gyr. We obtained deep spectroscopic observations for 11 diffuse PSGs and derived their internal kinematics and stellar population properties. All of them possess disk-like kinematics (substantial rotation, low stellar velocity dispersion) and likely experienced starburst episodes prior to the star formation quenching by ram pressure stripping. Our results suggest that at least some UDGs were “normal” intermediate to large-sized disk galaxies in the past, which were later quenched by dense environment.

AGB stars and the cosmic dust cycle

AbstractTheoretical and observational studies of dust condensed in outflows of AGB stars have substantially advanced the understanding of dust mixture from individual stars. This detailed information incorporated in models of the lifecycle of interstellar grains provides a flexible tool to study the contribution of AGB stars to the galactic dust budget. The role of these stars in dust production depends on the morphological type and age of galaxy. While AGB stars are sub-dominant dust sources in evolved systems as the Milky Way, the observed relation between the dust-to-gas ratio and metallicity suggests that the dust input in young dwarf galaxies with 7≲12+log(O/H)≲8 can be dominated by the AGB stars. In application to post-starburst and early-type galaxies, the models for stardust evolution in combination with modern infrared observations give insights in the origin of their high dust content and its implications for their evolutionary scenarios.