SFR Relation Research Articles

Trading oxygen for iron. I. The [O/Fe]-specific star formation rate relation of galaxies

Our current knowledge of the star-forming metallicity of galaxies relies primarily on gas-phase oxygen abundance measurements. However, these do not always allow an accurate description of differences in stellar evolution and feedback, which are driven by variations in iron abundance. alpha -elements (such as oxygen) and iron are produced by sources that operate on different timescales and the link between them is not straightforward. We explore the origin of the O/Fe --specific SFR (sSFR) relation, linking chemical abundances to galaxy formation timescales. This relation is adhered to by star-forming galaxies across redshifts according to cosmological simulations and basic theoretical expectations. Its apparent universality makes it suitable for trading the readily available oxygen for iron abundance. We show that the relation is determined by the relative iron production efficiency of core-collapse and type Ia supernovae and the delay-time distribution of the latter ---uncertain factors that could be constrained empirically with the O/Fe ---sSFR relation. We compile and homogenise a literature sample of star-forming galaxies with observational iron abundance determinations to place first constraints on the O/Fe --sSFR relation over a wide range of sSFR. The relation shows a clear evolution towards lower O/Fe with decreasing sSFR and a flattening above log$_ $(sSFR/yr)>-9. These results are broadly consistent with expectations, but better constraints are needed to inform the models. We independently derive the relation from old Milky Way stars and find remarkable agreement between the two, as long as the recombination-line absolute oxygen abundance scale is used in conjunction with stellar metallicity measurements.

The power spectrum of extended [C ii] haloes around high redshift galaxies

ABSTRACT Atacama Large Millimeter/sub-millimeter Array (ALMA) observations have detected extended (≃10 kpc) [C$\scriptstyle \rm II$] haloes around high-redshift (z ≳ 5) star-forming galaxies. If such extended structures are common, they may have an impact on the line intensity mapping (LIM) signal. We compute the LIM power spectrum including both the central galaxy and the [C $\scriptstyle \rm II$] halo, and study the detectability of such signal in an ALMA LIM survey. We model the central galaxy and the [C $\scriptstyle \rm II$] halo brightness with a $\rm S\acute{e}rsic$ + exponential profile. The model has two free parameters: the effective radius ratio $f_{R_{\rm e}}$, and the central surface brightness ratio, fΣ, between the two components. [C $\scriptstyle \rm II$] haloes can significantly boost the LIM power spectrum signal. For example, for relatively compact [C $\scriptstyle \rm II$] haloes ($f_\Sigma =0.4$, $f_{R_{\rm e}}=2.0$), the signal is boosted by ≃20 times; for more extended and diffuse haloes ($f_\Sigma =0.1, f_{R_{\rm e}}=6.0$), the signal is boosted by ≃100 times. For the ALMA ASPECS survey (resolution θbeam = 1.13 arcsec, survey area $\Omega _{\rm survey}=2.9\, \rm arcmin^{2}$) the [C $\scriptstyle \rm II$] power spectrum is detectable only if the deL14d [C $\scriptstyle \rm II$]–SFR relation holds. However, with an optimized survey (θbeam = 0.232 arcsec, $\Omega _{\rm survey}=2.0\, \rm deg^{2}$), the power spectrum is detectable for all the [C $\scriptstyle \rm II$]–SFR relations considered in this paper. Such a survey can constrain $f_\Sigma$ ($f_{R_{\rm e}}$) with a relative uncertainty of $\sim 15~{{\ \rm per\ cent}}$ ($\sim 10~{{\ \rm per\ cent}}$). A successful LIM experiment will provide unique constraints on the nature, origin, and frequency of extended [C $\scriptstyle \rm II$] haloes, and the [C $\scriptstyle \rm II$]–SFR relation at early times.

ALMA as a Redshift Machine: Using [C ii] to Efficiently Confirm Galaxies in the Epoch of Reionization

The [C ii]158μm line has long been proposed as a promising line to spectroscopically confirm galaxies in the epoch of reionization. In this paper, we present the results of new ALMA observations spectral scanning for [C ii] in six particularly luminous Lyman-break galaxies at z ∼ 7. The six sources were drawn from a sample of bright z ∼ 7 galaxies identified using the wide-area optical, near-IR, and Spitzer/IRAC data over the COSMOS/UltraVISTA field and were targeted on the basis of tight constraints on their redshifts from their IRAC [3.6]–[4.5] colors. We detect significant (>9σ) [C ii] lines in three of our six targets (50%) cospatial with the rest-UV emission from the ground/space-based near-IR imaging. The luminosities of the [C ii] lines lie in the range 5.6–8.8 × 108 L ⊙, consistent with the local [C ii]–SFR relation. Meanwhile, their [C ii]/L IR ∼ 1–3 × 10−3 ratios are slightly elevated compared to local (U)LIRGS. This could be due to lower dust-to-gas or dust-to-metal ratios. We also find that our sources display a large kinematic diversity, with one source showing signs of rotation, one source a likely major merger, and one dispersion-dominated source that might contain a bright star-forming clump. Our results highlight the effectiveness of spectral scans with ALMA in spectroscopically confirming luminous galaxies in the epoch of reionization, something that is being be applied on a significantly larger sample in the ongoing REBELS large program.

Constraints on galactic outflows from the metallicity–stellar mass–SFR relation of EAGLE simulation andSDSSgalaxies

ABSTRACTStellar feedback-driven outflows regulate the stellar formation and chemical enrichment of galaxies, yet the underlying dependence of mass outflow rate on galaxy properties remains largely unknown. We develop a simple yet comprehensive non-equilibrium chemical evolution model (NE-CEM) to constrain the mass-loading factor η of outflows using the metallicity-stellar mass–SFR relation observed by Sloan Digital Sky Survey (SDSS) at z = 0. Our NE-CEM predicts the chemical enrichment by explicitly tracking both the histories of star formation and mass-loading. After exploring the eagle simulation, we discover a compact yet flexible model that accurately describes the average star formation histories of galaxies. Applying a novel method of chemically measuring η to eagle, we find η can be parametrized by its dependence on stellar mass and specific SFR as $\log \eta \propto M_*^{\alpha }s{\mathrm{SFR}}^{\beta }$, with α = − 0.12 and β = 0.32 in eagle. Our chemically inferred η agrees remarkably well with the kinematic measurements by Mitchell et al. After extensive tests with eagle, we apply an NE-CEM Bayesian analysis to the SDSS data, yielding a tight constraint of $\log (\eta /0.631) = 0.731{\pm }0.002\times (M_*/10^{9.5}\, \mathrm{M}_{\odot })^{-0.222\pm 0.004} (s{\mathrm{SFR}}/10^{-9.5}\, \mathrm{yr}^{-1})^{0.078\pm 0.003}$, in good agreement with the down-the-barrel measurements. Our best-fitting NE-CEM not only accurately describes the metallicity-stellar mass–SFR relation at z = 0, but also successfully reproduce the so-called ‘fundamental metallicity relation’ at higher redshifts. Our results reveal that different galaxies form stars and enrich their gas in a non-equilibrium but strikingly coherent fashion across cosmic time.

Deep ALMA redshift search of a z ∼ 12 GLASS-JWST galaxy candidate

ABSTRACT The JWST has discovered a surprising abundance of bright galaxy candidates in the very early universe (≤500 Myr after the Big Bang), calling into question current galaxy formation models. Spectroscopy is needed to confirm the primeval nature of these candidates, as well as to understand how the first galaxies form stars and grow. Here we present deep spectroscopic and continuum ALMA observations towards GHZ2/GLASS-z12, one of the brightest and most robust candidates at z > 10, identified in the GLASS-JWST Early Release Science Program. We detect a 5.8σ line, offset 0${_{.}^{\prime\prime}}$5 from the JWST position of GHZ2/GLASS-z12, that associating it with the [O iii] 88 $\mu {\rm m}$ transition, implies a spectroscopic redshift of z = 12.117 ± 0.001. We verify the detection using extensive statistical tests. The oxygen line luminosity places GHZ2/GLASS-z12 above the [O iii]-SFR relation for metal-poor galaxies, implying an enhancement of [O iii] emission in this system while the JWST-observed emission is likely a lower-metallicity region. The lack of dust emission seen by these observations is consistent with the blue UV slope observed by JWST, which suggest little dust attenuation in galaxies at this early epoch. Further observations will unambiguously confirm the redshift and shed light on the origins of the wide and offset line and physical properties of this early galaxy. This work illustrates the synergy between JWST and ALMA, and paves the way for future spectroscopic surveys of z > 10 galaxy candidates.

MASCOT: molecular gas depletion times and metallicity gradients – evidence for feedback in quenching active galaxies

ABSTRACT We present results from the first public data release of the MaNGA-ARO Survey of CO Targets (MASCOT), focusing our study on galaxies whose star formation rates and stellar masses place them below the ridge of the star-forming main sequence. In optically selected type 2 AGN/low-ionization nuclear emission regions (LINERs)/Composites, we find an empirical relation between gas-phase metallicity gradients ∇Z and global molecular gas depletion times $t_{\rm dep} = M_{\rm H_2} /{\rm SFR}$ with ‘more quenched’ systems showing flatter/positive gradients. Our results are based on the O3N2 metallicity diagnostic (applied to star-forming regions within a given galaxy), which was recently suggested to also be robust against emission by diffuse ionized gas (DIG) and LINERs. We conduct a systematic investigation into possible drivers of the observed ∇Z − tdep relation (ouflows, gas accretion, in situ star formation, mergers, and morphology). We find a strong relation between ∇Z or tdep and centralized outflow strength traced by the [O iii] velocity broadening. We also find signatures of suppressed star formation in the outskirts in AGN-like galaxies with long depletion times and an enhancement of metals in the outer regions. We find no evidence of inflows impacting the metallicity gradients, and none of our results are found to be significantly affected by merger activity or morphology. We thus conclude that the observed ∇Z–tdep relation may stem from a combination of metal redistribution via weak feedback, and a connection to in situ star formation via a resolved mass-metallicity–SFR relation.

Bayesian hierarchical modelling of the M*–SFR relation from 1 ≲ z ≲ 6 in ASTRODEEP

ABSTRACT The Hubble Frontier Fields represent the opportunity to probe the high-redshift evolution of the main sequence of star-forming galaxies to lower masses than possible in blank fields thanks to foreground lensing of massive galaxy clusters. We use the beagle SED-fitting code to derive stellar masses, $\rm{{M_{\star }}}=\log ({\it M}/{\rm{M_{\odot }}})$, SFRs, $\rm{{\Psi }}=\log (\rm{{\psi }}/{\rm{M_{\odot }}}\, {\rm{yr}}^{-1})$, and redshifts from galaxies within the astrodeep catalogue. We fit a fully Bayesian hierarchical model of the main sequence over 1.25 < z < 6 of the form $\rm{{\Psi }}= \rm{\alpha _\mathrm{9.7}}(z) + \rm{\beta }({\rm{M_{\star }}}-9.7) + \mathcal {N}(0,\rm{\sigma }^2)$ while explicitly modelling the outlier distribution. The redshift-dependent intercept at $\rm{{M_{\star }}}=9.7$ is parametrized as $\rm{\alpha _\mathrm{9.7}}(z) = \log [{\it N}(1+{\it z})^{\rm{\gamma }}] + 0.7$. Our results agree with an increase in normalization of the main sequence to high redshifts that follows the redshift-dependent rate of accretion of gas on to dark matter haloes with $\rm{\gamma }=2.40^{+0.18}_{-0.18}$. We measure a slope and intrinsic scatter of $\rm{\beta }=0.79^{+0.03}_{-0.04}$ and $\rm{\sigma }=0.26^{+0.02}_{-0.02}$. We find that the sampling of the SED provided by the combination of filters (Hubble + ground-based Ks-band + Spitzer 3.6 and 4.5 μm) is insufficient to constrain M⋆ and Ψ over the full dynamic range of the observed main sequence, even at the lowest redshifts studied. While this filter set represents the best current sampling of high-redshift galaxy SEDs out to z > 3, measurements of the main sequence to low masses and high redshifts still strongly depend on priors employed in SED fitting (as well as other fitting assumptions). Future data sets with JWST should improve this.

The PAU survey: measurements of the 4000 Å spectral break with narrow-band photometry

ABSTRACT The D4000 spectral break index is one of the most important features in the visible spectrum, as it is a proxy for stellar ages and is also used in galaxy classification. However, its direct measurement has always been reserved to spectroscopy. Here, we present a general method to directly measure the D4000 with narrow-band (NB) photometry; it has been validated using realistic simulations, and then evaluated with PAUS NBs, cross-matched with VIPERS spectra (iAB < 22.5, 0.562 < z < 0.967). We also reconstruct the D4000 with the SED-fitting code cigale; the use of PAUS NBs instead of broad-bands significantly improves the SED fitting results. For D4000n, the direct measurement has $\rm \langle SNR \rangle \sim 4$, but we find that for iAB < 21 all direct D4000 measurements have $\rm SNR\gt 3$. The cigale D4000n has $\rm \langle SNR \rangle \sim 20$, but underestimates the error by >50 per cent. Furthermore, the direct method recreates well the D4000–SFR relation, as well as the D4000–mass relation for blue galaxies (for red galaxies, selection effects impact the results). On the other hand, cigale accurately classifies galaxies into red and blue populations. We conclude that the direct measurement of D4000 with narrow-band photometry is a promising tool to determine average properties of galaxy samples, with results compatible with spectroscopy.

The ALMA REBELS Survey. Epoch of Reionization giants: Properties of dusty galaxies at z ≈ 7

ABSTRACT We analyse FIR dust continuum measurements for 14 galaxies (redshift z ≈ 7) in the ALMA Reionization Era Bright Emission Line Survey (REBELS) Large Program to derive their physical properties. Our model uses three input data, i.e. (a) the UV spectral slope, β, (b) the observed UV continuum flux at 1500 Å, F1500, (c) the observed continuum flux at $\approx 158\, \mu$m, F158, and considers Milky Way (MW) and SMC extinction curves, along with different dust geometries. We find that REBELS galaxies have 28−90.5 per cent of their star formation obscured; the total (UV+IR) star formation rates are in the range $31.5 \lt {\rm SFR}/({\rm M}_\odot \, {\rm yr}^{-1}) \lt 129.5$. The sample-averaged dust mass and temperature are $(1.3\pm 1.1)\times 10^7 \, \mathrm{M}_\odot$ and 52 ± 11 K, respectively. However, in some galaxies dust is particularly abundant (REBELS-14, $M^{\prime }_{\rm d} \approx 3.4 \times 10^7 \, \mathrm{M}_\odot$), or hot (REBELS-18, $T^{\prime }_{\rm d} \approx 67$ K). The dust distribution is compact (<0.3 kpc for 70 per cent of the galaxies). The inferred dust yield per supernova is $0.1 \le y_{\rm d}/\, \mathrm{M}_\odot \le 3.3$, with 70 per cent of the galaxies requiring $y_{\rm d} \lt 0.25 \, \mathrm{M}_\odot$. Three galaxies (REBELS-12, 14, 39) require $y_{\rm d} \gt 1 \, \mathrm{M}_\odot$, which is likely inconsistent with pure SN production, and might require dust growth via accretion of heavy elements from the interstellar medium. With the SFR predicted by the model and a MW extinction curve, REBELS galaxies detected in [C ii] nicely follow the local LCII−SFR relation, and are approximately located on the Kennicutt–Schmidt relation. The sample-averaged gas depletion time is $0.11\, y_{\rm P}^{-2}$ Gyr, where yP is the ratio of the gas-to-stellar distribution radius. For some systems, a solution simultaneously matching the observed (β, F1500, F158) values cannot be found. This occurs when the index Im = (F158/F1500)/(β − βint), where βint is the intrinsic UV slope, exceeds $I_m^{*}\approx 1120$ for an MW curve. For these objects, we argue that the FIR and UV emitting regions are not co-spatial, questioning the use of the IRX–β relation.

The effect of active galactic nuclei on the cold interstellar medium in distant star-forming galaxies

In the framework of a systematic study with the ALMA interferometer of IR-selected main-sequence and starburst galaxies at z ∼ 1 − 1.7 at typical ∼1″ resolution, we report on the effects of mid-IR- and X-ray-detected active galactic nuclei (AGN) on the reservoirs and excitation of molecular gas in a sample of 55 objects. We find widespread detectable nuclear activity in ∼30% of the sample. The presence of dusty tori influences the IR spectral energy distribution of galaxies, as highlighted by the strong correlation among the AGN contribution to the total IR luminosity budget (fAGN = LIR, AGN/LIR), its hard X-ray emission, and the Rayleigh-Jeans to mid-IR (S1.2 mm/S24 μm) observed color, with evident consequences on the ensuing empirical star formation rate estimates. Nevertheless, we find only marginal effects of the presence and strength of AGN on the carbon monoxide CO (J = 2, 4, 5, 7) or neutral carbon ([C I](3P1 − 3P0), [C I](3P2 − 3P1)) line luminosities and on the derived molecular gas excitation as gauged by line ratios and the full spectral line energy distributions. The [C I] and CO emission up to J = 5, 7 thus primarily traces the properties of the host in typical IR luminous galaxies. However, our analysis highlights the existence of a large variety of line luminosities and ratios despite the homogeneous selection. In particular, we find a sparse group of AGN-dominated sources with the highest LIR, AGN/LIR, SFR ratios, ≳3, that are more luminous in CO (5−4) than what is predicted by the L′CO(5-4)−LIR, SFR relation, which might be the result of the nuclear activity. For the general population, our findings translate into AGN having minimal effects on quantities such as gas and dust fractions and star formation efficiencies. If anything, we find hints of a marginal tendency of AGN hosts to be compact at far-IR wavelengths and to display 1.8 times larger dust optical depths. In general, this is consistent with a marginal impact of the nuclear activity on the gas reservoirs and star formation in average star-forming AGN hosts with LIR > 5 × 1011 L⊙, typically underrepresented in surveys of quasars and submillimeter galaxies.

Modelling the M*–SFR relation at high redshift: untangling factors driving biases in the intrinsic scatter measurement

ABSTRACT We present a method to self-consistently propagate stellar-mass [$\hbox{$\hbox{${\rm M}$}_{\star }$}=\log (\hbox{${\rm M}$}/\hbox{${\rm M}_{\odot }$})$] and star-formation-rate [$\hbox{${\Psi }$}=\log (\hbox{${\psi }$}/\hbox{${\rm M}_{\odot }$}\, {\rm yr}^{-1}$)] uncertainties on to intercept (α), slope (β), and intrinsic-scatter (σ) estimates for a simple model of the main sequence of star-forming galaxies, where $\hbox{${\Psi }$}= \alpha + \beta \hbox{$\hbox{${\rm M}$}_{\star }$}+ \mathcal {N}(0,\sigma)$. To test this method and compare it with other published methods, we construct mock photometric samples of galaxies at z ∼ 5 based on idealized models combined with broad- and medium-band filters at wavelengths 0.8–5 μm. Adopting simple Ψ estimates based on dust-corrected ultraviolet luminosity can underestimate σ. We find that broad-band fluxes alone cannot constrain the contribution from emission lines, implying that strong priors on the emission-line contribution are required if no medium-band constraints are available. Therefore, at high redshifts, where emission lines contribute a higher fraction of the broad-band flux, photometric fitting is sensitive to Ψ variations on short (∼10 Myr) time-scales. Priors on age imposed with a constant (or rising) star formation history (SFH) do not allow one to investigate a possible dependence of σ on $\hbox{${\rm M}$}_{\star }$ at high redshifts. Delayed exponential SFHs have less constrained priors, but do not account for Ψ variations on short time-scales, a problem if σ increases due to stochasticity of star formation. A simple SFH with current star formation decoupled from the previous history is appropriate. We show that, for simple exposure-time calculations assuming point sources, with low levels of dust, we should be able to obtain unbiased estimates of the main sequence down to $\mathrm{ log}(\hbox{${\rm M}$}/\hbox{${\rm M}_{\odot }$})\sim 8$ at z ∼ 5 with the James Webb Space Telescope while allowing for stochasticity of star formation.

Predictions of the L [C ii]–SFR and [Cii] Luminosity Function at the Epoch of Reionization

Abstract We present the first predictions for the L [C ii]–SFR relation and [Cii] luminosity function (LF) in the epoch of reionization (EOR) based on cosmological hydrodynamics simulations using the simba suite plus radiative transfer calculations via sígame. The sample consists of 11,137 galaxies covering halo mass log M halo ∈ [9, 12.4] M ⊙, star formation rate SFR ∈ [0.01, 330] M ⊙ yr−1, and metallicity 〈Z gas〉SFR ∈ [0.1, 1.9] Z ⊙. The simulated L [C ii]–SFR relation is consistent with the range observed, but with a spread of ≃0.3 dex at the high end of SFR (>100 M ⊙ yr−1) and ≃0.6 dex at the lower end, and there is tension between our predictions and the values of L [C ii] above 108.5 L ⊙ observed in some galaxies reported in the literature. The scatter in the L [C ii]–SFR relation is mostly driven by galaxy properties, such that at a given SFR galaxies with higher molecular mass and metallicity have higher L [C ii]. The [Cii] LF predicted by simba is consistent with the upper limits placed by the only existing untargeted flux-limited [Cii] survey at the EOR and those predicted by semianalytic models. We compare our results with existing models and discuss the differences responsible for the discrepant slopes in the L [C ii]–SFR relation.

Deep into the structure of the first galaxies: SERRA views

We study the formation and evolution of a sample of Lyman Break Galaxies in the Epoch of Reionization by using high-resolution ($\sim 10 \,{\rm pc}$), cosmological zoom-in simulations part of the SERRA suite. In SERRA, we follow the interstellar medium (ISM) thermo-chemical non-equilibrium evolution, and perform on-the-fly radiative transfer of the interstellar radiation field (ISRF). The simulation outputs are post-processed to compute the emission of far infrared lines ([CII], [NII], and [OIII]). At $z=8$, the most massive galaxy, `Freesia', has an age $t_\star \simeq 409\,{\rm Myr}$, stellar mass $M_{\star} \simeq 4.2\times 10^9 {\rm M}_{\odot}$, and a star formation rate ${\rm SFR} \simeq 11.5\,{\rm M}_{\odot}{\rm yr}^{-1}$, due to a recent burst. Freesia has two stellar components (A and B) separated by $\simeq 2.5\, {\rm kpc}$; other 11 galaxies are found within $56.9 \pm 21.6 \, {\rm kpc}$. The mean ISRF in the Habing band is $G = 7.9\, G_0$ and is spatially uniform; in contrast, the ionisation parameter is $U = 2^{+20}_{-2} \times 10^{-3}$, and has a patchy distribution peaked at the location of star-forming sites. The resulting ionising escape fraction from Freesia is $f_{\rm esc}\simeq 2\%$. While [CII] emission is extended (radius 1.54 kpc), [OIII] is concentrated in Freesia-A (0.85 kpc), where the ratio $\Sigma_{\rm [OIII]}/\Sigma_{\rm [CII]} \simeq 10$. As many high-$z$ galaxies, Freesia lies below the local [CII]-SFR relation. We show that this is the general consequence of a starburst phase (pushing the galaxy above the Kennicutt-Schmidt relation) which disrupts/photodissociates the emitting molecular clouds around star-forming sites. Metallicity has a sub-dominant impact on the amplitude of [CII]-SFR deviations.

Evident black hole-bulge coevolution in the distant universe

ABSTRACT Observations in the local universe show a tight correlation between the masses of supermassive black holes (SMBHs; MBH) and host-galaxy bulges (Mbulge), suggesting a strong connection between SMBH and bulge growth. However, direct evidence for such a connection in the distant universe remains elusive. We have studied sample-averaged SMBH accretion rate ($\overline{\rm BHAR}$) for bulge-dominated galaxies at z = 0.5–3. While previous observations found $\overline{\rm BHAR}$ is strongly related to host-galaxy stellar mass (M⋆) for the overall galaxy population, our analyses show that, for the bulge-dominated population, $\overline{\rm BHAR}$ is mainly related to SFR rather than M⋆. This ${\overline{\rm BHAR}}$–SFR relation is highly significant, e.g. 9.0σ (Pearson statistic) at z = 0.5–1.5. Such a $\overline{\rm BHAR}$–SFR connection does not exist among our comparison sample of galaxies that are not bulge dominated, for which M⋆ appears to be the main determinant of SMBH accretion. This difference between the bulge-dominated and comparison samples indicates that SMBHs only coevolve with bulges rather than the entire galaxies, explaining the tightness of the local MBH−Mbulge correlation. Our best-fitting ${\overline{\rm BHAR}}$–SFR relation for the bulge-dominated sample is ${\log \overline{\rm BHAR}= \log \mathrm{SFR} - (2.48\pm 0.05)}$ (solar units). The best-fitting $\overline{\rm BHAR}/\mathrm{SFR}$ ratio (10−2.48) for bulge-dominated galaxies is similar to the observed MBH/Mbulge values in the local universe. Our results reveal that SMBH and bulge growth are in lockstep, and thus non-causal scenarios of merger averaging are unlikely the origin of the MBH−Mbulge correlation. This lockstep growth also predicts that the MBH−Mbulge relation should not have strong redshift dependence.

Kiloparsec-scale gaseous clumps and star formation at z = 5–7

We investigate the morphology of the [CII] emission in a sample of "normal" star-forming galaxies at $5<z<7.2$ in relation to their UV (rest-frame) counterpart. We use new ALMA observations of galaxies at $z\sim6-7$ as well as a careful re-analysis of archival ALMA data. In total 29 galaxies were analysed, 21 of which are detected in [CII]. For several of the latter the [CII] emission breaks into multiple components. Only a fraction of these [CII] components, if any, is associated with the primary UV systems, while the bulk of the [CII] emission is associated either with fainter UV components, or not associated with any UV counterpart at the current limits. By taking into account the presence of all these components, we find that the L$_{\rm [CII]}$-SFR relation at early epochs is fully consistent with the local relation, but it has a dispersion of 0.48$\pm$0.07 dex, which is about two times larger than observed locally. We also find that the deviation from the local L$_{\rm [CII]}$-SFR relation has a weak anti-correlation with the EW(Ly$\alpha$). The morphological analysis also reveals that [CII] emission is generally much more extended than the UV emission. As a consequence, these primordial galaxies are characterised by a [CII] surface brightness generally much lower than expected from the local ${\rm \Sigma_{[CII}-\Sigma_{SFR}}$ relation. These properties are likely a consequence of a combination of different effects, namely: gas metallicity, [CII] emission from obscured star-forming regions, strong variations of the ionisation parameter, and circumgalactic gas in accretion or ejected by these primeval galaxies.

ALMA Detection of Extended [C ii] Emission in Himiko at z = 6.6

Abstract Himiko is one of the most luminous Lyα emitters at z = 6.595. It has three star-forming clumps detected in the rest-frame UV, with a total SFR = 20 M ⊙ yr−1. We report the Atacama Large Millimeter/submillimeter Array (ALMA) detection of the [C ii]158 μm line emission in this Galaxy with a significance of 8σ. The total [C ii] luminosity (L [C ii] = 1.2 × 108 L ⊙) is fully consistent with the local L [C ii]–SFR relation. The ALMA high-angular resolution reveals that the [C ii] emission is made of two distinct components. The brightest [C ii] clump is extended over 4 kpc and is located on the peak of the Lyα nebula, which is spatially offset by 1 kpc relative to the brightest UV clump. The second [C ii] component is spatially unresolved (size &lt;2 kpc) and coincident with one of the three UV clumps. While the latter component is consistent with the local L [C ii]–SFR relation, the other components are scattered above and below the local relation. We shortly discuss the possible origin of the [C ii] components and their relation with the star-forming clumps traced by the UV emission.

Interpreting ALMA observations of the ISM during the epoch of reionization

We present cosmological, radiation-hydrodynamics simulations of galaxy formation during the epoch of reionization in an effort towards modelling the interstellar medium (ISM) and interpreting ALMA observations. Simulations with and without stellar radiation are compared at large (Mpc), intermediate (tens of kpc), and small (sub kpc) scales. At large scales, the dense regions around galaxies reionize first before UV photons penetrate the voids; however, considerable amounts of neutral gas remain present within the haloes. The spatial distribution of neutral gas is highly dynamic and is anti-correlated with the presence of stars older than a few Myrs. For our specific feedback implementation, most of the metals remain inside the virial radii of haloes and they are proportionally distributed over the ionized and neutral medium by mass. For our most massive galaxy with ${\rm M_h}\sim10^{11}$M$_{\odot}$, the majority of the CII and OI mass are associated with cold neutral clumps. NII is more diffuse and arises in warmer gas while OIII arises in hotter gas with a higher ionization parameter, produced by photo-heating and supernovae. If smaller pockets of high metallicity gas exist in the ISM, the emission from these ions may be observable by ALMA while the low metallicity of the galaxy may cause these systems to fall below the local [CII]-SFR relation. The presence of dust can cause spatial offsets between UV/Ly$\alpha$ and [CII] emission as suggested by the recent observations of Maiolino et al. [OIII] may be spatially offset from both of these components since it arises from a different part of density-temperature phase-space.

Zooming on the internal structure of z≃6 galaxies

We present zoom-in, AMR, high-resolution ($\simeq 30$ pc) simulations of high-redshift ($z \simeq 6$) galaxies with the aim of characterizing their internal properties and interstellar medium. Among other features, we adopt a star formation model based on a physically-sound molecular hydrogen prescription, and introduce a novel scheme for supernova feedback, stellar winds and dust-mediated radiation pressure. In the zoom-in simulation the target halo hosts "Dahlia", a galaxy with a stellar mass $M_*=1.6\times 10^{10}$M$_\odot$, representative of a typical $z\sim 6$ Lyman Break Galaxy. Dahlia has a total H2 mass of $10^{8.5}$M$_\odot$, that is mainly concentrated in a disk-like structure of effective radius $\simeq 0.6$ kpc and scale height $\simeq 200$ pc. Frequent mergers drive fresh gas towards the center of the disk, sustaining a star formation rate per unit area of $\simeq 15 $M$_\odot$ yr$^{-1}$ kpc$^{-2}$. The disk is composed by dense ($n \gtrsim 25$ cm$^{-3}$), metal-rich ($Z \simeq 0.5 $ Z$_\odot$) gas, that is pressure-supported by radiation. We compute the $158\mu$m [CII] emission arising from {Dahlia}, and find that $\simeq 95\%$ of the total [CII] luminosity ($L_{[CII]}\simeq10^{7.5}$ L$_\odot$) arises from the H2 disk. Although $30\%$ of the CII mass is transported out of the disk by outflows, such gas negligibly contributes to [CII] emission, due to its low density ($n \lesssim 10$ cm$^{-3}$) and metallicity ($Z\lesssim 10^{-1}$Z$_\odot$). Dahlia is under-luminous with respect to the local [CII]-SFR relation; however, its luminosity is consistent with upper limits derived for most $z\sim6$ galaxies.

Enhanced X-ray emission from Lyman break analogues and a possibleLX–SFR–metallicity plane

The source of energetic photons that heated and reionized the early Universe remains uncertain. Early galaxies had low metallicity and recent population synthesis calculations suggest that the number and luminosity of high-mass X-ray binaries are enhanced in star-forming galaxies with low metallicity, offering a potentially important and previously overlooked source of heating and reionization. Lyman break analogue (LBA) galaxies are local galaxies that strongly resemble the high-redshift, star-forming Lyman Break Galaxies and have been suggested as local analogues to these metal-deficient galaxies found in the early Universe. We studied a sample of ten LBAs in order to measure the relation between star formation rate and X-ray luminosity. We found that for LBAs with metallicities in the range $12+\log_{10}({\rm O/H}) = 8.15-8.80$, the $L_X-$SFR relation was $\log_{10} (L_X/{\rm SFR}\, {[\rm erg\ s^{-1}\ M_{\odot}^{-1}\ yr]}) = 39.85(\pm 0.10)$ in the $0.5-8$~keV band with a dispersion of $\sigma = 0.25$~dex. This is an enhancement of nearly a factor of $2$ in the $L_{0.5-8\text{keV}}$--SFR relation relative to results for nearby, near-solar metallicity galaxies. The enhancement is significant at the 98.2\% level ($2.4\sigma$). Our enhanced $L_X/{\rm SFR}$ relation is consistent with the metallicity-dependent predicted value from population synthesis models. We discuss the possibility of a $L_X$--SFR--Metallicity plane for star-forming galaxies. These results are important to our understanding of reionization and the formation of early galaxies.

ON THE [C ii]–SFR RELATION IN HIGH REDSHIFT GALAXIES

After two ALMA observing cycles, only a handful of [CII] $158\,\mu m$ emission line searches in z>6 galaxies have reported a positive detection, questioning the applicability of the local [CII]-SFR relation to high-z systems. To investigate this issue we use the Vallini et al. 2013 (V13) model, based on high-resolution, radiative transfer cosmological simulations to predict the [CII] emission from the interstellar medium of a z~7 (halo mass $M_h=1.17\times10^{11}M_{\odot}$) galaxy. We improve the V13 model by including (a) a physically-motivated metallicity (Z) distribution of the gas, (b) the contribution of Photo-Dissociation Regions (PDRs), (c) the effects of Cosmic Microwave Background on the [CII] line luminosity. We study the relative contribution of diffuse neutral gas to the total [CII] emission ($F _{diff}/F_{tot}$) for different SFR and Z values. We find that the [CII] emission arises predominantly from PDRs: regardless of the galaxy properties, $F _{diff}/F_{tot}\leq 10$% since, at these early epochs, the CMB temperature approaches the spin temperature of the [CII] transition in the cold neutral medium ($T_{CMB}\sim T_s^{CNM}\sim 20$ K). Our model predicts a high-z [CII]-SFR relation consistent with observations of local dwarf galaxies ($0.02<Z/Z_{\odot}<0.5$). The [CII] deficit suggested by actual data ($L_{CII}<2.0\times 10^7 L_{\odot}$ in BDF3299 at z~7.1) if confirmed by deeper ALMA observations, can be ascribed to negative stellar feedback disrupting molecular clouds around star formation sites. The deviation from the local [CII]-SFR would then imply a modified Kennicutt-Schmidt relation in z>6 galaxies. Alternatively/in addition, the deficit might be explained by low gas metallicities ($Z<0.1 Z_{\odot}$).