Star-forming Systems Research Articles

XXL-HSC: Link between AGN activity and star formation in the early Universe (z ⩾ 3.5)

In this work, our aim is to investigate the star formation rate (SFR) of the host galaxies of active galactic nuclei (AGNs) in the early Universe. To this end, we constructed a sample of 149 luminous (L2 − 10 keV > 1044 erg s−1) X-ray AGNs at z ≥ 3.5 selected in three fields with different depths and observed areas from the Chandra COSMOS Legacy survey, XMM-XXL North, and eROSITA Final Equatorial-Depth Survey. We built their spectral energy distributions (SED) using the available multi-wavelength photometry from X-rays up to the far-IR. Then, we estimated the stellar mass, M*, and the SFR of the AGNs using the X-CIGALE SED fitting algorithm. After applying several quality criteria, we ended up with 89 high-z sources. More than half (55%) of the X-ray sample have spectroscopic redshifts. Based on our analysis, our high-z X-ray AGNs live in galaxies with a median M* = 5.6 × 1010 M⊙ and SFR* ≈ 240 M⊙yr−1. The majority of the high-z sources (∼89%) were found inside or above the main sequence (MS) of star-forming galaxies. Estimations of the normalised SFR, SFRNORM, defined as the ratio of the SFR of AGNs, to the SFR of MS galaxies, show that the SFR of AGNs is enhanced by a factor of ∼1.8 compared to non-AGN star-forming systems. Combining our results with previous studies at lower redshifts, we confirmed that SFRNORM does not evolve with redshift. Using the specific black hole accretion rate (i.e. LX divided by M*), λBHAR, which can be used as a tracer of the Eddington ratio, we found that the majority of AGNs that lie inside or above the MS have higher specific accretion rates compared to sources below the MS. Finally, we found indications that the SFR of the most massive AGN host galaxies (log (M*/M⊙) > 1011.5 − 12) remains roughly constant as a function of M*, in agreement with the SFR of MS star-forming galaxies.

Evidence That Shocks Destroy Small PAH Molecules in Low-luminosity Active Galactic Nuclei

We combined mapping-mode mid-infrared Spitzer spectra with complementary infrared imaging to perform a spatially resolved study of polycyclic aromatic hydrocarbon (PAH) emission from the central regions of 66 nearby galaxies, roughly evenly divided into star-forming systems and low-luminosity active galactic nuclei (AGNs). In conjunction with similar measurements available for quasars, we aim to understand the physical properties of PAHs across a broad range of black hole accretion power, with the goal of identifying observational diagnostics that can be used to probe the effect of AGNs on the host galaxy. Whereas the PAH emission correlates tightly with far-ultraviolet luminosity in star-forming regions, the spatially resolved regions of AGNs tend to be PAH deficient. Moreover, AGN regions exhibit on average smaller PAH 6.2 μm/7.7 μm and larger PAH 11.3 μm/7.7 μm band ratios. Although the current data are highly restrictive, they suggest that these anomalous PAH band ratios cannot be explained by the effects of the AGN radiation field alone. Instead, they hint that small grains may be destroyed by the combined effects of radiative processes and shocks, which are plausibly linked to jets and outflows preferentially associated with highly sub-Eddington, radiatively inefficient AGNs. While quasars also present a PAH deficit and unusual PAH band ratios, their characteristics differ in detail compared to those observed in more weakly accreting AGNs, a possible indicator of fundamental differences in their modes of energy feedback.

Early Results from GLASS-JWST. I: Confirmation of Lensed z ≥ 7 Lyman-break Galaxies behind the Abell 2744 Cluster with NIRISS

We present the first search for z ≥ 7, continuum-confirmed Lyman break sources with NIRISS/WFS spectroscopy over the Abell 2744 Frontier Fields cluster, as part of the GLASS-JWST-ERS survey. With ∼15 hr of preimaging and multiangle grism exposures in the F115W, F150W, and F200W filters, we describe the general data handling (i.e., reduction, cleaning, modeling, and extraction processes) and analysis for the GLASS-JWST survey. We showcase the power of JWST to peer deep into reionization, when most intergalactic hydrogen is neutral, by confirming two galaxies at z = 8.04 ± 0.15 and z = 7.90 ± 0.13 by means of their Lyman breaks. Fainter continuum spectra are observed in both the F150W and F200W bands, indicative of blue (−1.69 and −1.33) UV slopes and moderately bright absolute magnitudes (−20.37 and −19.68 mag). We do not detect strong Lyα in either galaxy, but do observe tentative (∼2.7–3.8σ) He ii λ1640 Å, O iii]λλ1661,1666 Å, and N iii]λλ1747,1749 Å line emission in one, suggestive of low-metallicity, star-forming systems with possible nonthermal contributions. These novel observations provide a first look at the extraordinary potential of JWST/NIRISS for confirming representative samples of bright z ≥ 7 sources in the absence of strong emission lines, and gain unprecedented insight into their contributions toward cosmic reionization.

Observations and Simulations of Radio Emission and Magnetic Fields in Minkowski's Object

We combine new data from the Karl G. Jansky Very Large Array with previous radio observations to create a more complete picture of the ongoing interactions between the radio jet from galaxy NGC 541 and the star-forming system known as Minkowski’s Object (MO). We then compare those observations with synthetic radio data generated from a new set of magnetohydrodynamic simulations of jet–cloud interactions specifically tailored to the parameters of MO. The combination of radio intensity, polarization, and spectral index measurements all convincingly support the interaction scenario and provide additional constraints on the local dynamical state of the intracluster medium and the time since the jet–cloud interaction first began. In particular, we show that only a simulation with a bent radio jet can reproduce the observations.

Comparison of the star formation in X-ray-selected AGN in eFEDS with that of star-forming galaxies

We use approximately 1800 X-ray active galactic nuclei (AGN) in the eROSITA Final Equatorial-Depth Survey (eFEDS) that span over two orders of magnitude in X-ray luminosity, LX, 2−10 keV ≈ 1043−45 erg s−1, and compare their star-formation rate (SFR) to that of non-AGN star-forming systems, at 0.5 < z < 1.5. To this end, we compiled a reference galaxy catalogue of about 17 000 sources. Both samples have the same photometric coverage, from optical to far-infrared. We construct the spectral energy distributions (SEDs) of all sources and fit them using the CIGALE code, using the same templates and parametric grid for both samples, and accounting for the mass incompleteness and exclude quiescent systems from both datasets. This allows us to compare the SFR of the two populations in a uniform manner, minimising systematic effects. Based on our analysis, AGN at low and moderate LX (LX, 2−10 keV < 1044 erg s−1) have SFR that is lower than, or at most equal to, that of star-forming galaxies, in agreement with previous studies. The large number of luminous X-ray AGN available in the eFEDS dataset enable us to expand our investigations at higher LX to test previous, tentative results. At LX, 2−10 keV > 1044.2 erg s−1, the SFR of AGN appears enhanced by ∼30% compared to that of star-forming sources, for systems with stellar mass in the range of 10.5 < log [M*(M⊙)] < 11.5, confirming indications found in previous studies. The most massive sources, log [M*(M⊙)] > 11.5, present a flat SFRnorm–LX relation up to LX, 2−10 keV ∼ 1044.5 erg s−1, with SFR similar to that of star-forming galaxies. However, at higher LX (LX, 2−10 keV ∼ 1045 erg s−1), we find indications that the SFR of these massive AGN hosts may be enhanced compared to that of non-AGN systems.

The interstellar medium of high-redshift galaxies: Gathering clues from C III] and [C II] lines

Context.A tight relation between [C II] 158 μm line luminosity and the star formation rate (SFR) has been observed for local galaxies. At high redshift (z > 5), galaxies instead deviate downwards from the local Σ[C II]− ΣSFRrelation. This deviation might be caused by different interstellar medium (ISM) properties in galaxies at early epochs.Aims.To test this hypothesis, we combined the [C II] and SFR data with C III] 1909 Å line observations and our physical models. We additionally investigated how ISM properties, such as burstiness,κs, total gas density,n, and metallicity,Z, affect the deviation from the Σ[C II]− ΣSFRrelation in these sources.Methods.We present the VLT/X-shooter observations targeting the C III]λ1909 line emission in three galaxies at 5.5 < z < 7.0. We include archival X-shooter data of two other sources at 5.5 < z < 7.0 and the VLT/MUSE archival data of six galaxies atz ∼ 2. We extend our sample of galaxies with eleven star-forming systems at 6 < z < 7.5, with either C III] or [C II] detection reported in the literature.Results.We detected C III]λλ1907, 1909 line emission in HZ10 and we derived the intrinsic, integrated flux of the C III]λ1909 line. We constrained the ISM properties for our sample of galaxies,κs,n, andZ, by applying our physically motivated model based on the MCMC algorithm. For the most part, high-zstar-forming galaxies show subsolar metallicities. The majority of the sources have log(κs) ≳ 1, that is, they overshoot the Kennicutt–Schmidt (KS) relation by about one order of magnitude.Conclusions.Our findings suggest that the whole KS relation might be shifted upwards at early times. Furthermore, all the high-zgalaxies of our sample lie below the Σ[C II]− ΣSFRlocal relation. The total gas density,n, shows the strongest correlation with the deviation from the local Σ[C II]− ΣSFRrelation, namely, low-density high-zsystems have lower [C II] surface brightness, in agreement with theoretical models.

Definitive upper bound on the negligible contribution of quasars to cosmic reionization

Cosmic (hydrogen) reionization marks one of the major phase transitions of the universe at redshift z >= 6. During this epoch, hydrogen atoms in the intergalactic medium (IGM) were ionized by Lyman continuum (LyC) photons. However, it remains challenging to identify the major sources of the LyC photons responsible for reionization. In particular, individual contributions of quasars (or active galactic nuclei, AGNs) and galaxies are still under debate. Here we construct the far-ultraviolet (far-UV) luminosity function for type 1 quasars at z >= 6 that spans 10 magnitudes (-19 < M_UV < -29), conclusively showing that quasars made a negligible contribution to reionization. We mainly search for quasars in the low-luminosity range of M_UV > -23 mag that is critical to determine quasars' total LyC photon production but has been barely explored previously. We find that the quasar population can only provide less than 7% (95% confidence level) of the total photons needed to keep the universe ionized at z = 6.0 - 6.6. Our result suggests that galaxies, presumably low-luminosity star-forming systems, are the major sources of hydrogen reionization.

Semi-analytic spectral fitting: simultaneously modelling the mass accumulation and chemical evolution in MaNGA spiral galaxies

Abstract We develop a novel semi-analytic spectral fitting approach to quantify the star-formation histories (SFHs) and chemical enrichment histories (ChEHs) of individual galaxies. We construct simple yet general chemical evolution models that account for gas inflow and outflow processes as well as star formation, to investigate the evolution of merger-free star-forming systems. These models are fitted directly to galaxies’ absorption-line spectra, while their emission lines are used to constrain current gas phase metallicity and star formation rate. We apply this method to spiral galaxies selected from the SDSS-IV MaNGA survey. By fitting the co-added absorption-line spectra for each galaxy, and using the emission-line constraints on present-day metallicity and star formation, we reconstruct both the SFHs and the ChEHs for all objects in the sample. We can use these reconstructions to obtain archaeological measures of derived correlations such as the mass–metallicity relation at any redshift, which compare favourably with direct observations. We find that both the SFHs and ChEHs have strong mass dependence: massive galaxies accumulate their stellar masses and become enriched earlier. This mass dependence causes the observed flattening of the mass–metallicity relation at lower redshifts. The model also reproduces the observed gas-to-stellar mass ratio and its mass dependence. Moreover, we are able to determine that more massive galaxies have earlier gas infall times and shorter infall time-scales, and that the early chemical enrichment of low-mass galaxies is suppressed by strong outflows, while outflows are not very significant in massive galaxies.

The physical connection between central stellar surface density and stellar spin in SAMI and MaNGA nearby galaxies

ABSTRACT The stellar surface density within the inner 1 kpc (Σ1) has become a popular tool for understanding the growth of galaxies and its connection with the quenching of star formation. The emerging picture suggests that building a central dense core is a necessary condition for quenching. However, it is not clear whether changes in Σ1 trace changes in stellar kinematics and the growth of dispersion-dominated bulges. In this paper, we combine imaging from the Sloan Digital Sky Survey with stellar kinematics from the Sydney-AAO Multi-object Integral-field unit and Mapping Nearby Galaxies at Apache Point Observatory surveys to quantify the correlation between Σ1 and the proxy for stellar spin parameter within one effective radius (λre) for 1599 nearby galaxies. We show that, on the star-forming main sequence and at fixed stellar mass, changes in Σ1 are mirrored by changes in λre. While forming stars, main-sequence galaxies remain rotationally-dominated systems, with their Σ1 increasing but their stellar spin staying either constant or slightly increasing. The picture changes below the main sequence, where Σ1 and λre are no longer correlated. Passive systems show a narrower range of Σ1, but a wider range of λre compared to star-forming galaxies. Our results indicate that, from a structural point of view, passive galaxies are a more heterogeneous population than star-forming systems, and may have followed a variety of evolutionary paths. This also suggests that, if dispersion-dominated bulges still grow significantly at z ∼ 0, this generally takes place during, or after, the quenching phase.

EMPRESS. VI. Outflows Investigated in Low-mass Galaxies with M ∗ = 104–107 M ⊙: Weak Feedback in Low-mass Galaxies?

We study emission line profiles of 21 nearby low-mass (M * = 104–107 M ⊙) galaxies in deep medium-high resolution spectra taken with Magellan/MagE. These low-mass galaxies are actively star-forming systems with high specific star formation rates of ∼100–1000 Gyr−1 that are well above the star formation main sequence and its extrapolation. We identify broad-line components of Hα and [O iii]λ5007 emission in 14 out of the 21 galaxies that cannot be explained by the MagE instrumental profile or the natural broadening of line emission. We conduct double-Gaussian profile fitting to the emission of the 14 galaxies, and find that the broad-line components have line widths significantly larger than those of the narrow-line components, indicative of galactic outflows. The broad-line components have moderately large line widths of ∼100 km s−1. We estimate the maximum outflow velocities v max and obtain values of ≃60–200 km s−1, which are found to be comparable to or slightly larger than the escape velocities. Positive correlations of v max with star formation rates, stellar masses, and circular velocities extend down into this low-mass regime. Broad- to narrow-line flux ratios (BNRs) are generally found to be smaller than those of massive galaxies. The small v max and BNRs suggest that the mass-loading factors η can be as small as 0.1–1 or below, in contrast to the large η of energy-driven outflows predicted by numerical simulations.

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

As part of the Virgo Cluster Survey Tracing Ionised Gas Emission, a blind narrow-band Hα+[NII] imaging survey of the Virgo cluster carried out with MegaCam at the CFHT, we discovered eight massive (1010 ≲ Mstar ≲ 1011 M⊙) lenticular galaxies with prominent ionised gas emission features in their inner (a few kiloparsec) regions. These features are either ionised gas filaments similar to those observed in cooling flows (two galaxies), or they are thin discs with sizes 0.7 ≲ R(Hα)≲2.0 kpc (six galaxies), thus significantly smaller than those of the stellar disc (R(Hα)≃7 − 22%Riso(r)). The morphological properties of these discs are similar to those of the dust seen in absorption in high-resolution HST images. Using a unique set of multifrequency data, including new or archival ASTROSAT/UVIT, GALEX, HST, CFHT,Spitzer, andHerschelimaging data, combined with IFU (MUSE, ALMA) and long-slit (SOAR) spectroscopy, we show that while the gas that is located within these inner discs is photoionised by young stars, which signals ongoing star formation, the gas in the filamentary structures is shock ionised. The star formation surface brightness of these discs is similar to that observed in late-type galaxies. Because of their reduced size, however, these lenticular galaxies are located below the main sequence of unperturbed or cluster star-forming systems. By comparing the dust masses measured from absorption maps in optical images, from the Balmer decrement, or estimated by fitting the UV-to-far-IR spectral energy distribution of the target galaxies, we confirm that the dust masses derived from optical attenuation maps are heavily underestimated because of geometrical effects due to the relative distribution of the absorbing dust and the emitting stars. We also show that these galaxies have gas-to-dust ratios ofG/D≃ 8030320, and that the star formation within these discs follows the Schmidt relation, but with an efficiency that is reduced by a factor of ∼2.5. Using our unique set of multifrequency data, we discuss the possible origin of the ionised gas in these objects, which suggests multiple and complex formation scenarios for massive lenticular galaxies in clusters.

Successive deuteration in low-mass star-forming regions: The case of D2-methanol (CHD2OH) in IRAS 16293-2422

Context. Di-deuterated molecules are observed in the earliest stages of star formation at abundances of a few percent relative to their nondeuterated isotopologs, which is unexpected considering the scarcity of deuterium in the interstellar medium. With sensitive observations leading to the detection of a steadily increasing number of di-deuterated species, it is becoming possible to explore successive deuteration chains. Aims. The accurate quantification of the column density of di-deuterated methanol is a key piece of the puzzle that is missing in the otherwise thoroughly constrained family of D-bearing methanol in the deeply embedded low-mass protostellar system and astrochemical template source IRAS 16293-2422. A spectroscopic dataset for astrophysical purposes was built for CHD2OH and made publicly available to facilitate the accurate characterization of this species in astrochemical surveys. Methods. The newly computed line list and partition function were used to search for CHD2OH toward IRAS 16293-2422 A and B in data from the Atacama Large Millimeter/submillimeter Array (ALMA) Protostellar Interferometric Line Survey (PILS). Only nonblended, optically thin lines of CHD2OH were used for the synthetic spectral fitting. Results. The constructed spectroscopic database contains line frequencies and strengths for 7417 transitions in the 0–500 GHz frequency range. ALMA-PILS observations in the 329–363 GHz range were used to identify 105 unique, nonblended, optically thin line frequencies of CHD2OH for synthetic spectral fitting. The derived excitation temperatures and column densities yield high D/H ratios of CHD2OH in IRAS 16293-2422 A and B of 7.5 ± 1.1% and 7.7 ± 1.2%, respectively. Conclusions. Deuteration in IRAS 16293-2422 is not higher than in other low-mass star-forming regions (L483, SVS13-A, NGC 1333-IRAS2A, -IRAS4A, and -IRAS4B). Di-deuterated molecules consistently have higher D/H ratios than their mono-deuterated counterparts in all low-mass protostars, which may be a natural consequence of H–D substitution reactions as seen in laboratory experiments. The Solar System’s natal cloud, as traced by comet 67P/Churyumov–Gerasimenko, may have had a lower initial abundance of D, been warmer than the cloud of IRAS 16293-2422, or been partially reprocessed. In combination with accurate spectroscopy, a careful spectral analysis, and the consideration of the underlying assumptions, successive deuteration is a robust window on the physicochemical provenance of star-forming systems.

Hα Dots: Direct-method Metal Abundances of Low-luminosity Star-forming Systems

Abstract Utilizing low-luminosity star-forming systems discovered in the Hα Dots survey, we present spectroscopic observations undertaken using the Kitt Peak National Observatory 4 m telescope for 26 sources. With determinations of robust, “direct”-method metal abundances, we examine the properties of these dwarf systems, exploring their utility in characterizing starburst galaxies at low luminosities and stellar masses. We find that the Hα Dots survey provides an effective new avenue for identifying star-forming galaxies in these regimes. In addition, we examine abundance characteristics and metallicity scaling relations with these sources, highlighting a flattening of both the luminosity–metallicity (L–Z) and stellar mass–metallicity (M *–Z) relation slopes in these regimes as compared with those utilizing samples covering wider respective dynamic ranges. These local, accessible analogs to the kinds of star-forming dwarfs common at high redshift will help shed light on the building blocks that assembled into the massive galaxies common today.

Non-Gaussianity of optical emission lines in SDSS star-forming galaxies and its implications on galactic outflows

Abstract The shape of emission lines in the optical spectra of star-forming galaxies reveals the kinematics of the diffuse gaseous component. We analyse the shape of prominent emission lines in a sample of $\sim$ 53000 star-forming galaxies from the Sloan Digital Sky Survey, focusing on departures from gaussianity. Departures from a single gaussian profile allow us to probe the motion of gas and to assess the role of outflows. The sample is divided into groups according to their stellar velocity dispersion and star formation rate (SFR). The spectra within each group are stacked to improve the signal-to-noise ratio of the emission lines, to remove individual signatures, and to enhance the effect of SFR on the shapes of the emission lines. The moments of the emission lines, including kurtosis and skewness, are determined. We find that most of the emission lines in strong star-forming systems unequivocally feature negative kurtosis. This signature is present in $\mathrm{H}\unicode{x03B2}$ , $\mathrm{H}\unicode{x03B1}$ , [N ii], and [S ii] in massive galaxies with high SFRs. We attribute it as evidence of radial outflows of ionised gas driven by the star formation of the galaxies. Also, most of the emission lines in low-mass systems with high SFRs feature negative skewness, and we interpret it as evidence of dust obscuration in the galactic disk. These signatures are however absent in the [O iii] line, which is believed to trace a different gas component. The observed trend is significantly stronger in face-on galaxies, indicating that star formation drives the outflows along the galactic rotation axis, presumably the path of least resistance. The data suggest that outflows driven by star formation exert accumulated impacts on the interstellar medium, and the outflow signature is more evident in older galaxies as they have experienced a longer total duration of star formation.

Spectroscopy of an extreme [O iii] emitting active galactic nucleus at z = 3.212: implications for the reionization era

ABSTRACT Reionization-era galaxies often display intense nebular emission lines, both in rest-frame optical ([O iii] + H β) and ultraviolet (UV; C iii], C iv). How such strong nebular emission is powered remains unclear, with both active galactic nuclei (AGNs) and hot stars considered equally viable. The UV continuum slopes of these early systems tend to be very blue (β &amp;lt; −2), reflecting minimal dust obscuration, young ages, and low metallicities. This contrasts with narrow-lined AGNs at $z$ ∼ 2−3, whose UV slopes are significantly redder (β &amp;gt; −1) than typical star-forming systems in the reionization era. To investigate the properties of AGNs in the reionization era, we have conducted a search for potential examples of rare analogues with blue continua at intermediate redshift ($z$ ∼ 2−3). Our goals are to determine whether AGNs with intense line emission and blue continua exist and thereby to establish the range of rest-frame UV and optical line ratios in this population. In this paper, we report the detection of an X-ray luminous AGN at $z$ = 3.21 (UDS-24561) with extreme [O iii] + H β line emission (equivalent width = 1300 Å) and a blue UV continuum slope (β = −2.34). MMT/Binospec and Keck/MOSFIRE spectra indicate rest-frame UV line ratios consistent with AGN photoionization models and rest-frame optical lines with both a narrow component [full width at half-maximum (FWHM) $=154\, {\rm km\, s}^{-1}$] and extended broad wings (FWHM $=977\, {\rm km\, s}^{-1}$), consistent with outflowing gas. We describe how such objects can be identified in future James Webb Space Telescope emission line surveys in the reionization era, thereby providing a valuable census of AGN activity at $z$ &amp;gt; 6 and understanding their contribution to cosmic reionization.

Molecular cloud catalogue from 13CO (1–0) data of the Forgotten Quadrant Survey

Context. New-generation spectroscopic surveys of the Milky Way plane have been revealing the structure of the interstellar medium, allowing the simultaneous study of dense structures from single star-forming objects or systems to entire spiral arms. Aims. The good sensitivity of the new surveys and the development of dedicated algorithms now enable building extensive catalogues of molecular clouds and deriving good estimates of their physical properties. This allows studying the behaviour of these properties across the Galaxy. Methods. We present the catalogue of molecular clouds extracted from the 13CO (1–0) data cubes of the Forgotten Quadrant Survey, which mapped the Galactic plane in the range 220° &lt; l &lt; 240° and −2.​​° 5 &lt; b &lt; 0° in 12CO (1–0) and 13CO (1–0). We compared the properties of the clouds of our catalogue with those of other catalogues. Results. The catalogue contains 87 molecular clouds for which the main physical parameters such as area, mass, distance, velocity dispersion, and virial parameter were derived. These structures are overall less extended and less massive than the molecular clouds identified in the 12CO (1–0) data-set because they trace the brightest and densest part of the 12CO (1–0) clouds. Conversely, the distribution of aspect ratio, equivalent spherical radius, velocity dispersion, and virial parameter in the two catalogues are similar. The mean value of the mass surface density of molecular clouds is 87 ± 55 M⊙ pc−2 and is almost constant across the galactocentric radius, indicating that this parameter, which is a proxy of star formation, is mostly affected by local conditions. Conclusions. In data of the Forgotten Quadrant Survey, we find a good agreement between the total mass and velocity dispersion of the clouds derived from 12CO (1–0) and 13CO (1–0). This is likely because in the surveyed portion of the Galactic plane, the H2 column density is not particularly high, leading to a CO emission with a not very high optical depth. This mitigates the effects of the different line opacities between the two tracers on the derived physical parameters. This is a common feature in the outer Galaxy, but our result cannot be readily generalised to the entire Milky Way because regions with higher particle density could show a different behaviour.

The Observed Cosmic Star Formation Rate Density Has an Evolution that Resembles a Γ(a, bt) Distribution and Can Be Described Successfully by Only Two Parameters

Abstract A debate is emerging regarding the recent inconsistent results of different studies for the cosmic star formation rate density (CSFRD) at high-z. We employ UV and IR data sets to investigate the SFR function (SFRF) at z ∼ 0–9. We find that the SFRFs derived from the dust-corrected UV (UVcorr) data contradict those from IR on some key issues because they are described by different distributions (Schechter versus double power law), imply different physics for galaxy formation (UVcorr data suggest an SFR limit/strong mechanism that diminish the number density of highly star-forming systems with respect to IR) and compare differently with the stellar mass density evolution obtained from spectral energy distribution fitting (UVcorr is in agreement, while IR differs up to 0.5 dex). However, both tracers agree on a constant CSFRD evolution at z ∼ 1–4 and point to a plateau instead of a peak. In addition, using both indicators, we demonstrate that the evolution of the observed CSFRD can be described by only two parameters and a function that has the form of a Gamma distribution (Γ(a, bt)). In contrast to previous parameterizations used in the literature, our framework connects the parameters to physical properties such as the SFR depletion time and cosmic baryonic gas density. The build-up of stellar mass occurs in Γ(a, bt) distributed steps and is the result of gas consumption up to the limit at which no eligible gas for SF at t = ∞ remains, resulting in a final cosmic stellar mass density of ∼ 0.5 × 10 9 M ⊙ Mpc 3 .

EMPRESS. III. Morphology, Stellar Population, and Dynamics of Extremely Metal-poor Galaxies (EMPGs): Are EMPGs Local Analogs of High-z Young Galaxies?*

Abstract We present the morphology and stellar population of 27 extremely metal-poor galaxies (EMPGs) at z ∼ 0 with metallicities of 0.01–0.1 Z ⊙. We conduct multicomponent surface brightness (SB) profile fitting for the deep Subaru/Hyper Suprime-Cam i-band images of the EMPGs with the Galfit software, carefully removing the SB contributions of tails. We find that the EMPGs with a median stellar mass of log ( M * / M ⊙ ) = 6.0 have a median Sérsic index of n = 1.1 and a median effective radius of r e = 200 pc, suggesting that typical EMPGs have a very compact disk. We compare the EMPGs with z ∼ 6 galaxies and local galaxies on the size–mass (r e –M *) diagram, and identify that the majority of the EMPGs have an r e –M * relation similar to z ∼ 0 star-forming galaxies rather than z ∼ 6 galaxies. Not every EMPG is a local analog of high-z young galaxies in the r e –M * relation. A spectrum of one pair of EMPG and tail, so far available, indicates that the tail is dynamically related to the EMPG with a median velocity difference of ΔV = 101 ± 32 km s−1. This moderately large ΔV cannot be explained by the dynamics of the tail, but likely by the infall on the tail. For the first time, we may identify the metal-poor star-forming system just now infalling into the tail.

Giant star-forming complexes in high-z main-sequence galaxy analogues: the internal structure of clumps in DYNAMO galaxies

ABSTRACT To indirectly study the internal structure of giant clumps in main sequence galaxies at z ∼ 1–3, we target very turbulent and gas-rich local analogues from the DYNAMO sample with the Hubble Space Telescope, over a wavelength range of ∼200–480 nm. We present a catalogue of 58 clumps identified in six DYNAMO galaxies, including the WFC3/UVIS F225W, F336W, and F467M photometry where the (225–336) and (336–467) colours are sensitive to extinction and stellar population age, respectively. We measure the internal colour gradients of clumps themselves to study their age and extinction properties. We find a marked colour trend within individual clumps, where the resolved colour distributions show that clumps generally have bluer (336–467) colours (denoting very young ages) in their centres than at their edges, with little variation in the (225–336) colour associated with extinction. Furthermore, we find that clumps whose colours suggest they are older, are preferentially located closer towards the centres of their galaxies, and we find no young clumps at small galactocentric distances. Both results are consistent with simulations of high-redshift star-forming systems that show clumps form via violent disc instability, and through dynamic processes migrate to the centres of their galaxies to contribute to bulge growth on time-scales of a few 100 Myr, while continually forming stars in their centres. When we compare the DYNAMO clumps to those in these simulations, we find the best agreement with the long-lived clumps.

Star formation quenching stages of active and non-active galaxies

The mechanisms that bring galaxies to strongly reduce their star formation activity (star-formation quenching) remain poorly understood. To better study galaxy evolution, we propose a classification based on maps of ionised hydrogen distribution traced by the kiloparsec-resolved, equivalent width of Hα maps, and the nuclear activity of the galaxies using information from the Baldwin-Philips-Terlevich diagnostic diagrams. Using these tools, we group a sample of 238 galaxies from the Calar Alto Legacy Integral Field spectroscopy Area survey in six quenching stages (QSs): (i) objects dominated by recent star formation; (ii) systems that present a quiescent-nuclear-ring structure in their centre; (iii) galaxies that are centrally quiescent; (iv) galaxies with no clear pattern in their ionisation gas distribution (mixed); (v) systems that posses only a few star-forming regions (nearly retired), or (vi) galaxies that are completely quiescent (fully retired). Regarding their nuclear activity, we further divide the galaxies into two groups: active systems that host a weak or strong active galactic nucleus (AGN) at their centre, and non-active objects. Galaxies grouped into quenching-stage classes occupy specific locations on the star-formation-rate versus stellar mass diagram. The ‘blue cloud’ is populated by the star-forming and the quiescent-nuclear-ring galaxies, the ‘green valley’ is populated by centrally quiescent and mixed systems, and the ‘red sequence’ by the nearly- and fully retired objects. Generally, galaxies that host a weak or strong AGN show properties comparable to those of their non-active counterparts at the same QSs, except for the AGN-hosting star-forming systems. The degree of star-formation quenching increases along the present emission-line pattern sequence from star-forming to fully retired. The proposed emission-line classes reinforce the ‘inside-out’ quenching scenario, which foresees that the suppression of star formation begins from the central regions of the galaxies.