Star Formation Surface Density Research Articles

The Physical Drivers of the Luminosity-weighted Dust Temperatures in High-redshift Galaxies

Abstract The underlying distribution of galaxies’ dust spectral energy distributions (SEDs) (i.e., their spectra reradiated by dust from rest-frame ∼3 μm to 3 mm) remains relatively unconstrained owing to a dearth of far-IR/(sub)millimeter data for large samples of galaxies. It has been claimed in the literature that a galaxy’s dust temperature—observed as the wavelength where the dust SED peaks (λ peak)—is traced most closely by its specific star formation rate (sSFR) or parameterized “distance” to the SFR–M ⋆ relation (the galaxy “main sequence”). We present 024 resolved 870 μm ALMA dust continuum observations of seven z = 1.4–4.6 dusty star-forming galaxies chosen to have a large range of well-constrained luminosity-weighted dust temperatures. We also draw on similar-resolution dust continuum maps from a sample of ALESS submillimeter galaxies from Hodge et al (2016). We constrain the physical scales over which the dust radiates and compare those measurements to characteristics of the integrated SED. We confirm significant correlations of λ peak with both L IR (or SFR) and ΣIR (∝SFR surface density). We investigate the correlation between log10(λ peak) and log10(ΣIR) and find the relation to hold as would be expected from the Stefan–Boltzmann law, or the effective size of an equivalent blackbody. The correlations of λ peak with sSFR and distance from the SFR–M ⋆ relation are less significant than those for ΣIR or L IR; therefore, we conclude that the more fundamental tracer of galaxies’ luminosity-weighted integrated dust temperatures are indeed their star formation surface densities in line with local universe results, which relate closely to the underlying geometry of dust in the interstellar medium.

The case for thermalization as a contributor to the [C ii] deficit

ABSTRACT The [C ii] deficit, which describes the observed decrease in the ratio of [C ii] 158 μm emission to continuum infrared emission in galaxies with high star formation surface densities, places a significant challenge to the interpretation of [C ii] detections from across the observable universe. In an attempt to further decode the cause of the [C ii] deficit, the [C ii] and dust continuum emission from 18 Local Volume galaxies has been split based on conditions within the interstellar medium where it originated. This is completed using the Key Insights in Nearby Galaxies: a Far-Infrared Survey with Herschel (KINGFISH) and Beyond the Peak (BtP) surveys and the wide-range of wavelength information, from UV to far-infrared emission lines, available for a selection of star-forming regions within these samples. By comparing these subdivided [C ii] emissions to isolated infrared emission and other properties, we find that the thermalization (collisional de-excitation) of the [C ii] line in H ii regions plays a significant role in the deficit observed in our sample.

Cosmic rays across the star-forming galaxy sequence – I. Cosmic ray pressures and calorimetry

ABSTRACT In the Milky Way (MW), cosmic rays (CRs) are dynamically important in the interstellar medium (ISM), contribute to hydrostatic balance, and may help regulate star formation. However, we know far less about the importance of CRs in galaxies whose gas content or star formation rate (SFR) differ significantly from those of the MW. Here, we construct self-consistent models for hadronic CR transport, losses, and contribution to pressure balance as a function of galaxy properties, covering a broad range of parameters from dwarfs to extreme starbursts. While the CR energy density increases from ∼1 eV cm−3 to ∼1 keV cm−3 over the range from sub-MW dwarfs to bright starbursts, strong hadronic losses render CRs increasingly unimportant dynamically as the SFR surface density increases. In MW-like systems, CR pressure is typically comparable to turbulent gas and magnetic pressure at the galactic mid-plane, but the ratio of CR to gas pressure drops to ∼10−3 in dense starbursts. Galaxies also become increasingly CR calorimetric and gamma-ray bright in this limit. The degree of calorimetry at fixed galaxy properties is sensitive to the assumed model for CR transport, and in particular to the time CRs spend interacting with neutral ISM, where they undergo strong streaming losses. We also find that in some regimes of parameter space hydrostatic equilibrium discs cannot exist, and in Paper II of this series we use this result to derive a critical surface in the plane of star formation surface density and gas surface density beyond which CRs may drive large-scale galactic winds.

Exploring the dust content of galactic haloes with Herschel III. NGC 891

ABSTRACT We present deep far-infrared observations of the nearby edge-on galaxy NGC 891 obtained with the Herschel Space Observatory and the Spitzer Space Telescope. The maps confirm the detection of thermal emission from the inner circumgalactic medium (halo) and spatially resolve a dusty superbubble and a dust spur (filament). The dust temperature of the halo component is lower than that of the disc but increases across a region of diameter ≈8.0 kpc extending at least 7.7 kpc vertically from one side of the disc, a region we call a superbubble because of its association with thermal X-ray emission and a minimum in the synchrotron scale height. This outflow is breaking through the thick disc and developing into a galactic wind, which is of particular interest because NGC 891 is not considered a starburst galaxy; the star formation rate surface density, 0.03 M⊙ yr−1 kpc−2, and gas fraction, just $10{{\ \rm per\ cent}}$ in the inner disc, indicate the threshold for wind formation is lower than previous work has suggested. We conclude that the star formation surface density is sufficient for superbubble blowout into the halo, but the cosmic ray electrons may play a critical role in determining whether this outflow develops into a fountain or escapes from the gravitational potential. The high dust-to-gas ratio in the dust spur suggests the material was pulled out of NGC 891 through the collision of a minihalo with the disc of NGC 891. We conclude that NGC 891 offers an example of both feedback and satellite interactions transporting dust into the halo of a typical galaxy.

Disentangling the multiphase circumgalactic medium shared between a dwarf and a massive star-forming galaxy at z∼0.4

ABSTRACT The multiphase circumgalactic medium (CGM) arises within the complex environment around a galaxy, or collection of galaxies, and possibly originates from a wide range of physical mechanisms. In this paper, we attempt to disentangle the origins of these multiphase structures and present a detailed analysis of the quasar field Q0122−003 field using Keck/KCWI galaxy observations and HST/COS spectra probing the CGM. Our re-analysis of this field shows that there are two galaxies associated with the absorption. We have discovered a dwarf galaxy, G_27kpc (M⋆ = 108.7 M⊙), at z = 0.39863 that is 27 kpc from the quasar sightline. G_27kpc is only +21 km s−1 from a more massive (M⋆ = 1010.5 M⊙) star-forming galaxy, G_163kpc, at an impact parameter of 163 kpc. While G_163kpc is actively forming stars (SFR = 6.9 M⊙ yr−1), G_27kpc has a low star-formation rate (SFR = 0.08 ± 0.03 M⊙ yr−1) and star formation surface density (ΣSFR = 0.006 M⊙ kpc−2 yr−1), implying no active outflows. By comparing galaxy SFRs, kinematics, masses, and distances from the quasar sightline to the absorption kinematics, column densities, and metallicities, we have inferred the following: (1) Part of the low-ionization phase has a metallicity and kinematics consistent with being accreted on to G_27kpc. (2) The remainder of the low ionization phase has metallicities and kinematics consistent with being intragroup gas being transferred from G_27kpc to G_163kpc. (3) The high ionization phase is consistent with being produced solely by outflows originating from the massive halo of G_163kpc. Our results demonstrate the complex nature of the multiphase CGM, especially around galaxy groups, and that detailed case-by-case studies are critical for disentangling its origins.

GOODS-ALMA: The slow downfall of star formation in z = 2–3 massive galaxies

We investigate the properties of a sample of 35 galaxies, detected with the Atacama Large Millimeter/Submillimeter Array (ALMA) at 1.1 mm in the GOODS-ALMA field (area of 69 arcmin2, resolution = 0.60″, rms ≃ 0.18 mJy beam−1). Using the ultraviolet-to-radio deep multiwavelength coverage of the GOODS–South field, we fit the spectral energy distributions of these galaxies to derive their key physical properties. The galaxies detected by ALMA are among the most massive at z = 2−4 (M⋆, med = 8.5 × 1010 M⊙) and they are either starburst or located in the upper part of the galaxy star-forming main sequence. A significant portion of our galaxy population (∼40%), located at z ∼ 2.5 − 3, exhibits abnormally low gas fractions. The sizes of these galaxies, measured with ALMA, are compatible with the trend between the rest-frame 5000 Å size and stellar mass observed for z ∼ 2 elliptical galaxies, suggesting that they are building compact bulges. We show that there is a strong link between star formation surface density (at 1.1 mm) and gas depletion time: The more compact a galaxy’s star-forming region is, the shorter its lifetime will be (without gas replenishment). The identified compact sources associated with relatively short depletion timescales (∼100 Myr) are the ideal candidates to be the progenitors of compact elliptical galaxies at z ∼ 2.

On the origin of low escape fractions of ionizing radiation from massive star-forming galaxies at high redshift

ABSTRACT The physical origin of low escape fractions of ionizing radiation derived from massive star-forming galaxies at z ∼ 3–4 is not well understood. We perform idealized disc galaxy simulations to understand how galactic properties such as metallicity and gas mass affect the escape of Lyman continuum (LyC) photons using radiation-hydrodynamic simulations with strong stellar feedback. We find that the luminosity-weighted escape fraction from a metal-poor (Z = 0.002) galaxy embedded in a halo of mass $M_{\rm h}\simeq 10^{11}\, \mathrm{M}_\odot$ is $\left\langle {f_{\rm esc}^{\rm 3D}}\right\rangle \simeq 10\, {{\ \rm per\ cent}}$. Roughly half of the LyC photons are absorbed within scales of 100 pc, and the other half is absorbed in the ISM ($\lesssim 2\, {\rm kpc}$). When the metallicity of the gas is increased to Z = 0.02, the escape fraction is significantly reduced to $\left\langle {f_{\rm esc}^{\rm 3D}}\right\rangle \simeq 1{{\ \rm per\ cent}}$ because young stars are enshrouded by their birth clouds for a longer time. In contrast, increasing the gas mass by a factor of 5 leads to $\left\langle {f_{\rm esc}^{\rm 3D}}\right\rangle \simeq 5\, {{\ \rm per\ cent}}$ because LyC photons are only moderately absorbed by the thicker disc. Our experiments suggest that high metallicity is likely more responsible for the low escape fractions observed in massive star-forming galaxies, supporting the scenario in which the escape fraction is decreasing with increasing halo mass. Finally, negligible correlation is observed between the escape fraction and surface density of star formation or galactic outflow rates.

The Sizes of z ∼ 9−10 Galaxies Identified in the Brightest of Reionizing Galaxies (BoRG) Survey

Abstract Redshift z = 9–10 object selection is the effective limit of Hubble Space Telescope (HST) imaging capability, even when confirmed with Spitzer. If only a few photometry data points are available, it becomes attractive to add criteria based on their morphology in these J- and H-band images. One could do so through visual inspection, a size criterion, or alternate morphometrics. We explore a vetted sample of Brightest of Reionizing Galaxies (BoRG) z ∼ 9 and z ∼ 10 candidate galaxies and the object rejected by Morishita+ to explore the utility of a size criterion in z = 9–10 candidate selection. A stringent, point-spread function (PSF)-corrected effective radius criterion ( ) would result in the rejection of 65%–70% of the interlopers visually rejected by Morishita et al. It may also remove up to ∼20% of bona fide brightest ( ) z = 9 or 10 candidates from a BoRG selected sample based on the Mason et al. luminosity functions, assuming the Holwerda et al. size–luminosity relation. We argue that including a size constraint in lieu of a visual inspection may serve in wide-field searches for these objects in, e.g., Euclid or HST archival imaging with the understanding that some brightest ( ) candidates may be missed. The sizes of the candidates found by Morishita et al. follow the expected size distribution of z ∼ 9 for bright galaxies, consistent with the log normal in Shibuya et al. and single objects. Two candidates show high star formation surface density ( ) and all merit further investigation and follow-up observations.

How Do Supernovae Impact the Circumgalactic Medium? I. Large-scale Fountains around a Milky Way–like Galaxy

Abstract Feedback is indispensable in galaxy formation. However, lacking resolutions, cosmological simulations often use ad hoc feedback parameters. Conversely, small-box simulations, while they better resolve the feedback, cannot capture gas evolution beyond the simulation domain. We aim to bridge the gap by implementing small-box results of supernovae-driven outflows into dark matter halo-scale simulations and studying their impact on large scales. Galactic outflows are multiphase, but small-box simulations show that the hot phase (T ≈ 106–7 K) carries the majority of energy and metals. We implement hot outflows in idealized simulations of the Milky Way halo, and examine how they impact the circumgalactic medium. In this paper, we discuss the case when the star formation surface density is low and therefore the emerging hot outflows are gravitationally bound by the halo. We find that outflows form a large-scale, metal-enriched atmosphere with fountain motions. As hot gas accumulates, the inner atmosphere becomes “saturated.” Cool gas condenses, with a rate balancing the injection of the hot outflows. This balance leads to a universal density profile of the hot atmosphere, independent of mass outflow rate. The atmosphere has a radially decreasing temperature, naturally producing the observed X-ray luminosity and column densities of O vi, O vii, and O viii. The self-regulated atmosphere has a baryon and a metal mass of (0.5–1.2) × 1010 M ⊙ and (0.6–1.4) × 108 M ⊙, respectively, small compared to the “missing” baryons and metals from the halo. We conjecture that the missing materials reside at even larger radii, ejected by more powerful outflows in the past.

The Local Star Formation Rate Surface Density and Metallicity Relation for Star-forming Galaxies

Abstract We study the relations between gas-phase metallicity (Z), local stellar mass surface density (Σ*), and the local star formation surface density (ΣSFR) in a sample of 1120 star-forming galaxies from the MaNGA survey. At fixed Σ* the local metallicity increases as ΣSFR decreases or vice versa for metallicity calibrators of N2 and O3N2. Alternatively, at fixed ΣSFR metallicity increases as Σ* increases, but in the high mass region, the trend is flatter. However, the dependence of metallicity on ΣSFR nearly disappears for N2O2 and N2S2 calibrators. We investigate the local metallicity against ΣSFR with different metallicity calibrators and find negative/positive correlations depending on the choice of the calibrator. We demonstrate that the O32 ratio (or ionization parameter) is probably dependent on star formation rate at fixed local stellar mass surface density. Additionally, the shape of Σ*–Z–ΣSFR (fundamental metallicity relation; FMR) depends on metallicity calibrator and stellar mass range. Due to the large discrepancy between the empirical fitting-based (N2, O3N2) electronic temperature metallicity and the photoionization model-dependent (N2O2, N2S2) metallicity calibrations, we conclude that the selection of metallicity calibration affects the existence of FMR on ΣSFR.

A radiative transfer model for the spiral galaxy M33★

ABSTRACT We present the first radiative transfer (RT) model of a non-edge-on disc galaxy in which the large-scale geometry of stars and dust is self-consistently derived through the fitting of multiwavelength imaging observations from the ultraviolet to the submm. To this end, we used the axisymmetric RT model of Popescu et al. and a new methodology for deriving geometrical parameters, and applied this to decode the spectral energy distribution (SED) of M33. We successfully account for both the spatial and spectral energy distribution, with residuals typically within $7{{\ \rm per\ cent}}$ in the profiles of surface brightness and within $8{{\ \rm per\ cent}}$ in the spatially integrated SED. We predict well the energy balance between absorption and re-emission by dust, with no need to invoke modified grain properties, and we find no submm emission that is in excess of our model predictions. We calculate that $80\pm 8{{\ \rm per\ cent}}$ of the dust heating is powered by the young stellar populations. We identify several morphological components in M33, a nuclear, an inner, a main and an outer disc, showing a monotonic trend in decreasing star formation surface density (ΣSFR) from the nuclear to the outer disc. In relation to surface density of stellar mass, the ΣSFR of these components defines a steeper relation than the ‘main sequence’ of star-forming galaxies, which we call a ‘structurally resolved main sequence’. Either environmental or stellar feedback mechanisms could explain the slope of the newly defined sequence. We find the star formation rate to be ${\rm SFR}=0.28^{+0.02}_{-0.01}{\rm M}_{\odot }{\rm yr}^{-1}$.

Rapid Reionization by the Oligarchs: The Case for Massive, UV-bright, Star-forming Galaxies with High Escape Fractions

Abstract The protagonists of the last great phase transition of the universe—cosmic reionization—remain elusive. Faint star-forming galaxies are leading candidates because they are found to be numerous and may have significant ionizing photon escape fractions ( ). Here we update this picture via an empirical model that successfully predicts latest observations (e.g., the rapid drop in star-formation density ( at ). We generate an ionizing spectrum for each galaxy in our model and constrain by leveraging latest measurements of the reionization timeline (e.g., Lyα damping of quasars and galaxies at z > 7). Assuming a constant across all sources at z > 6, we find < −13.5 galaxies need = to complete reionization. The inferred Intergalactic Medium neutral fraction is [0.9, 0.5, 0.1] at —that is, the bulk of reionization transpires rapidly in 300 Myr, driven by the z > 8 ρ SFR and favored by high neutral fractions (∼60%–90%) measured at z ∼ 7–8. Inspired by the emergent sample of Lyman Continuum (LyC) leakers spanning z ∼ 0–6.6 that overwhelmingly displays higher-than-average star-formation surface density ( ), we propose a physically motivated model relating to and find . Since falls by ∼2.5 dex between z = 8 and z = 0, our model explains the humble upper limits on at lower redshifts and its required evolution to ∼ 0.2 at z > 6. Within this model, strikingly, <5% of galaxies with < −18 and log(M ⋆/M ⊙) > 8 (the “oligarchs”) account for ≳80% of the reionization budget—a stark departure from the canonical “democratic” reionization led by copious faint sources. In fact, faint sources ( > −16) must be relegated to a limited role in order to ensure high neutral fractions at z = 7–8. Shallow faint-end slopes of the UV luminosity function ( > −2) and/or distributions skewed toward massive galaxies produce the required late and rapid reionization. We predict that LyC leakers like COLA1 (z = 6.6, ∼ 30%, = −21.5) will become increasingly common toward z ∼ 6 and that the drivers of reionization do not lie hidden across the faint end of the luminosity function but are already known to us.

Simple Yet Powerful: Hot Galactic Outflows Driven by Supernovae

Abstract Supernovae (SNe) drive multiphase galactic outflows, impacting galaxy formation; however, cosmological simulations mostly use ad hoc feedback models for outflows, making outflow-related predictions from first principles problematic. Recent small-box simulations resolve individual SNe remnants in the interstellar medium (ISM), naturally driving outflows and permitting a determination of the wind loading factors of energy η E , mass , and metals . In this Letter, we compile small-box results, and find consensus that the hot outflows are much more powerful than the cool outflows: (i) hot outflows generally dominate the energy flux, and (ii) their specific energy e s,h is 10–1000 times higher than cool outflows. Moreover, the properties of hot outflows are remarkably simple: is almost invariant over four orders of magnitude of star formation surface density. Also, we find tentatively that 0.5. If corroborated by more simulation data, these correlations reduce the three hot phase loading factors into one. Finally, this one parameter is closely related to whether the ISM has a “breakout” condition. The narrow range of indicates that hot outflows cannot escape dark matter halos with log . This mass is also where the galaxy mass–metallicity relation reaches its plateau, implying a deep connection between hot outflows and galaxy formation. We argue that hot outflows should be included explicitly in cosmological simulations and (semi-)analytic modeling of galaxy formation.

Radio haloes of star-forming galaxies

ABSTRACT We study the synchrotron radio emission from extra-planar regions of star-forming galaxies. We use ideal magnetohydrodynamic simulations of a rotating Milky Way-type disc galaxy with distributed star formation sites for three star formation rates (0.3, 3, 30 M⊙ yr−1). From our simulations, we see emergence of galactic-scale magnetized outflows, carrying gas from the disc. We compare the morphology of the outflowing gas with hydrodynamic simulations. We look at the spatial distribution of magnetic field in the outflows. Assuming that a certain fraction of gas energy density is converted into cosmic ray energy density, and using information about the magnetic field, we obtain synchrotron emissivity throughout the simulation domain. We generate the surface brightness maps at 1.4 GHz. The outflows are more extended in the vertical direction than radial and hence have an oblate shape. We further find that the matter right behind the outer shock shines brighter in these maps than that above or below. To understand whether this feature can be observed, we produce vertical intensity profiles. We convolve the vertical intensity profile with the typical beam sizes of radio telescopes, for a galaxy located at 10 Mpc to estimate the radio scale height and compare with observations. The radio scale height is ∼300–1200 pc, depending on the resolution of the telescope. We relate the advection speed of the outer shock with the surface density of star formation as ${\rm v}_{\rm adv} \propto \Sigma _{\rm SFR}^{0.3}$, which is consistent with earlier observations and analytical estimates.

From peculiar morphologies to Hubble-type spirals: the relation between galaxy dynamics and morphology in star-forming galaxies at z ∼ 1.5

ABSTRACT We present an analysis of the gas dynamics of star-forming galaxies at z ∼ 1.5 using data from the KMOS Galaxy Evolution Survey. We quantify the morphology of the galaxies using HSTcandels imaging parametrically and non-parametrically. We combine the H α dynamics from KMOS with the high-resolution imaging to derive the relation between stellar mass (M*) and stellar specific angular momentum (j*). We show that high-redshift star-forming galaxies at z ∼ 1.5 follow a power-law trend in specific stellar angular momentum with stellar mass similar to that of local late-type galaxies of the form j* ∝ M$_*^{0.53\, \pm \, 0.10}$. The highest specific angular momentum galaxies are mostly disc-like, although generally both peculiar morphologies and disc-like systems are found across the sequence of specific angular momentum at a fixed stellar mass. We explore the scatter within the j* – M* plane and its correlation with both the integrated dynamical properties of a galaxy (e.g. velocity dispersion, Toomre Qg, H α star formation rate surface density ΣSFR) and its parametrized rest-frame UV / optical morphology (e.g. Sérsic index, bulge to total ratio, clumpiness, asymmetry, and concentration). We establish that the position in the j* – M* plane is strongly correlated with the star-formation surface density and the clumpiness of the stellar light distribution. Galaxies with peculiar rest-frame UV / optical morphologies have comparable specific angular momentum to disc- dominated galaxies of the same stellar mass, but are clumpier and have higher star formation rate surface densities. We propose that the peculiar morphologies in high-redshift systems are driven by higher star formation rate surface densities and higher gas fractions leading to a more clumpy interstellar medium.

2D-Galactic chemical evolution: the role of the spiral density wave

ABSTRACT We present a 2D chemical evolution code applied to a Milky Way type Galaxy, incorporating the role of spiral arms in shaping azimuthal abundance variations, and confront the predicted behaviour with recent observations taken with integral field units. To the usual radial distribution of mass, we add the surface density of the spiral wave and study its effect on star formation and elemental abundances. We compute five different models: one with azimuthal symmetry which depends only on radius, while the other four are subjected to the effect of a spiral density wave. At early times, the imprint of the spiral density wave is carried by both the stellar and star formation surface densities; conversely, the elemental abundance pattern is less affected. At later epochs, however, differences among the models are diluted, becoming almost indistinguishable given current observational uncertainties. At the present time, the largest differences appear in the star formation rate and/or in the outer disc (R ≥ 18 kpc). The predicted azimuthal oxygen abundance patterns for t ≤ 2 Gyr are in reasonable agreement with recent observations obtained with VLT/MUSE for NGC 6754.

Star formation activity and the spatial distribution and mass segregation of dense cores in the early phases of star formation

We examine the spatial distribution and mass segregation of dense molecular cloud cores in a number of nearby star forming regions (the region L1495 in Taurus, Aquila, Corona Australis, and W43) that span about four orders of magnitude in star formation activity. We used an approach based on the calculation of the minimum spanning tree, and for each region, we calculated the structure parameter 𝒬 and the mass segregation ratio ΛMSR measured for various numbers of the most massive cores. Our results indicate that the distribution of dense cores in young star forming regions is very substructured and that it is very likely that this substructure will be imprinted onto the nascent clusters that will emerge out of these clouds. With the exception of Taurus in which there is nearly no mass segregation, we observe mild-to-significant levels of mass segregation for the ensemble of the 6, 10, and 14 most massive cores in Aquila, Corona Australis, and W43, respectively. Our results suggest that the clouds’ star formation activity are linked to their structure, as traced by their population of dense cores. We also find that the fraction of massive cores that are the most mass segregated in each region correlates with the surface density of star formation in the clouds. The Taurus region with low star forming activity is associated with a highly hierarchical spatial distribution of the cores (low 𝒬 value) and the cores show no sign of being mass segregated. On the other extreme, the mini-starburst region W43-MM1 has a higher 𝒬 that is suggestive of a more centrally condensed structure. Additionally, it possesses a higher fraction of massive cores that are segregated by mass. While some limited evolutionary effects might be present, we largely attribute the correlation between the star formation activity of the clouds and their structure to a dependence on the physical conditions that have been imprinted on them by the large scale environment at the time they started to assemble.

The MOSDEF Survey: Sulfur Emission-line Ratios Provide New Insights into Evolving Interstellar Medium Conditions at High Redshift∗

Abstract We present results on the emission-line properties of 1.3 ≤ z ≤ 2.7 galaxies drawn from the complete the MOSFIRE Deep Evolution Field (MOSDEF) survey. Specifically, we use observations of the emission-line diagnostic diagram of [O iii]λ 5007/Hβ versus [S ii]λλ6717,6731/Hα, i.e., the “[S ii] BPT diagram,” to gain insight into the physical properties of high-redshift star-forming regions. High-redshift MOSDEF galaxies are offset toward lower [S ii]λλ6717,6731/Hα at fixed [O iii]λ5007/Hβ, relative to local galaxies from the Sloan Digital Sky Survey (SDSS). Furthermore, at fixed [O iii]λ5007/Hβ, local SDSS galaxies follow a trend of decreasing [S ii]λλ6717,6731/Hα as the surface density of star formation (ΣSFR) increases. We explain this trend in terms of the decreasing fractional contribution from diffuse ionized gas (f DIG) as ΣSFR increases in galaxies, which causes galaxy-integrated line ratios to shift toward the locus of pure H ii-region emission. The z ∼ 0 relationship between f DIG and ΣSFR implies that high-redshift galaxies have lower f DIG values than typical local systems, given their significantly higher typical ΣSFR. When an appropriate low-redshift benchmark with zero or minimal f DIG is used, high-redshift MOSDEF galaxies appear offset toward higher [S ii]λλ6717,6731/Hα and/or [O iii]λ 5007/Hβ. The joint shifts of high-redshift galaxies in the [S ii] and [N ii] BPT diagrams are best explained in terms of the harder spectra ionizing their star-forming regions at fixed nebular oxygen abundance (expected for chemically young galaxies), as opposed to large variations in N/O ratios or higher ionization parameters. The evolving mixture of H ii regions and diffuse ionized gas is an essential ingredient of our description of the interstellar medium over cosmic time.

SDSS-IV MaNGA: environmental dependence of gas metallicity gradients in local star-forming galaxies

ABSTRACT Within the standard model of hierarchical galaxy formation in a Λ cold dark matter universe, the environment of galaxies is expected to play a key role in driving galaxy formation and evolution. In this paper, we investigate whether and how the gas metallicity and the star formation surface density (ΣSFR) depend on galaxy environment. To this end, we analyse a sample of 1162 local, star-forming galaxies from the galaxy survey Mapping Nearby Galaxies at APO (MaNGA). Generally, both parameters do not show any significant dependence on environment. However, in agreement with previous studies, we find that low-mass satellite galaxies are an exception to this rule. The gas metallicity in these objects increases while their ΣSFR decreases slightly with environmental density. The present analysis of MaNGA data allows us to extend this to spatially resolved properties. Our study reveals that the gas metallicity gradients of low-mass satellites flatten and their ΣSFR gradients steepen with increasing environmental density. By extensively exploring a chemical evolution model, we identify two scenarios that are able to explain this pattern: metal-enriched gas accretion or pristine gas inflow with varying accretion time-scales. The latter scenario better matches the observed ΣSFR gradients, and is therefore our preferred solution. In this model, a shorter gas accretion time-scale at larger radii is required. This suggests that ‘outside–in quenching’ governs the star formation processes of low-mass satellite galaxies in dense environments.

New constraints on reionization from a redshift-independent efficiency model

AbstractWe present an empirical model built on a high-resolution N-body dark matter simulation. We assume a redshift-independent star-formation efficiency for each halo to convert the accretion rate into a star-formation rate. Our model is calibrated using the z = 4 UV luminosity function (UVLF) and successfully predicts the observed UVLF at z = 5 – 10. We present predictions at z = 5 – 10 for UV luminosity and stellar mass functions, JWST number counts, the stellar-to-halo mass relation and star-formation histories. We combine this model with bleeding-edge reionization constraints (from z > 7 quasars, z ∼ 7 Ly α line-profiles, the updated Planck τ) to find new perspectives on the Epoch of Reionization (EoR). We find MUV < − 13.5 galaxies need an average fesc = 0.22 ± 0.05 to drive reionization and a highly compressed timeline: the IGM neutral fraction is [0.9, 0.5, 0.1] at z = [8.4 ± 0.2, 7.0 ± 0.2, 6.3 ± 0.2]. Inspired by the newly assembled sample of Lyman Continuum leakers that unanimously displays higher-than-average star-formation surface density (sigma), we fit a model tying fesc to sigma. Since sigma grows by > 2.5 dex over z = 0 – 8, our model explains the humble values of fesc at low-z. We find, strikingly, that < 5% of galaxies with MUV < − 18 account for > 80% of the reionization budget. We predict leakers like COLA1 (z = 6.6, MUV = − 21.5) become common towards the EoR and that the protagonists of reionization are not hiding across the faint-end of the luminosity function but are already known to us.