[en] We present sensitive CO (J = 1 $\to <!--\; ???\; -->$ 0) emission line observations of the three metal-poor dwarf irregular galaxies Leo P (Z ∼ 3% Z_⊙), Sextans A (Z ∼ 7.5% Z_⊙), and Sextans B (Z ∼ 7.5% Z_⊙), all obtained with the Combined Array for Millimeter-wave Astronomy interferometer. While no CO emission was detected, the proximity of the three systems allows us to place very stringent (4σ) upper limits on the CO luminosity (L_CO) in these metal-poor galaxies. We find the CO luminosities to be L_CO < 2900 K km s⁻¹ pc² for Leo P, L_CO < 12,400 K km s⁻¹ pc² for Sextans A, and L_CO < 9700 K km s⁻¹ pc² for Sextans B. Comparison of our results with recent observational estimates of the factor for converting between L_CO and the mass of molecular hydrogen, as well as theoretical models, provides further evidence that either the CO-to-H₂ conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydrogen.

[en] Using the Sloan Digital Sky Survey (SDSS) data release 4 (DR 4), we investigate the spatial distribution of low and high surface brightness galaxies (LSBGs and HSBGs, respectively). In particular, we focus our attention on the influence of interactions between galaxies on the star formation strength in the redshift range 0.01 < z < 0.1. With cylinder counts and projected distance to the first and fifth nearest neighbor as environment tracers, we find that LSBGs tend to have a lack of companions compared to HSBGs at small scales (<2Mpc). Regarding the interactions, we have evidence that the fraction of LSBGs with strong star formation activity increases when the distance between pairs of galaxies (r_p ) is smaller than about four times the Petrosian radius (r₉₀) of one of the components. Our results suggest that, rather than being a condition for their formation, the isolation of LSBGs is more connected with their survival and evolution. The effect of the interaction on the star formation strength, measured by the average value of the birthrate parameter b, seems to be stronger for HSBGs than for LSBGs. The analysis of our population of LSBGs and HSBGs hosting an active galactic nucleus (AGN) show that, regardless of the mass range, the fraction of LSBGs having an AGN is lower than the corresponding fraction of HSBGs with an AGN. Also, we observe that the fraction of HSBGs and LSBGs having an AGN increases with the bulge luminosity. These results, and those concerning the star-forming properties of LSBGs as a function of the environment, fit with the scenario proposed by some authors where, below a given threshold of surface mass density, low surface brightness disks are unable to propagate instabilities, preventing the formation and evolution of massive black holes in the centers of LSBGs.

[en] The blue compact dwarf galaxy I Zw 18 is one of the most metal-poor systems known in the local universe (12+log(O/H) = 7.17). In this work we study I Zw 18 using data from Spitzer, Herschel Space Telescope, and IRAM Plateau de Bure Interferometer. Our data set includes the most sensitive maps of I Zw 18, to date, in both the far-infrared and the CO J = 1 → 0 transition. We use dust emission models to derive a dust mass upper limit of only M_dust ≤ 1.1 × 10⁴ M_☉ (3σ limit). This upper limit is driven by the non-detection at 160 μm, and it is a factor of 4-10 times smaller than previous estimates (depending on the model used). We also estimate an upper limit to the total dust-to-gas mass ratio of M_Dust/M_gas ≤ 5.0 × 10^–5. If a linear correlation between the dust-to-gas mass ratio and metallicity (measured as O/H) were to hold, we would expect a ratio of 3.9 × 10^–4. We also show that the infrared spectral energy distribution is similar to that of starbursting systems.

[en] This study explores the effects of different assumptions and systematics on the determination of the local, spatially resolved star formation law. Using four star formation rate (SFR) tracers (Hα with azimuthally averaged extinction correction, mid-infrared 24 μm, combined Hα and mid-infrared 24 μm, and combined far-ultraviolet and mid-infrared 24 μm), several fitting procedures, and different sampling strategies, we probe the relation between SFR and molecular gas at various spatial resolutions (500 pc and larger) and surface densities (Σ_H2)approx. 10-245 M_sun pc^-2) within the central ~6.5 kpc in the disk of NGC 4254. We explore the effect of diffuse emission using an unsharp masking technique with varying kernel size. The fraction of diffuse emission, f_DE, thus determined is a strong inverse function of the size of the filtering kernel. We find that in the high surface brightness regions of NGC 4254 the form of the molecular gas star formation law is robustly determined and approximately linear (∼0.8-1.1) and independent of the assumed fraction of diffuse emission and the SFR tracer employed. When the low surface brightness regions are included, the slope of the star formation law depends primarily on the assumed fraction of diffuse emission. In such a case, results range from linear when the fraction of diffuse emission in the SFR tracer is f_DE ∼< 30% (or when diffuse emission is removed in both the star formation and the molecular gas tracer) to super-linear (∼1.4) when f_DE ∼> 50%. We find that the tightness of the correlation between gas and star formation varies with the choice of star formation tracer. The 24 μm SFR tracer by itself shows the tightest correlation with the molecular gas surface density, whereas the Hα corrected for extinction using an azimuthally averaged correction shows the highest dispersion. We find that for R < 0.5R₂₅ the local star formation efficiency is constant and similar to that observed in other large spirals, with a molecular gas depletion time τ_dep ∼ 2 Gyr.

[en] We present a detailed study of the molecular gas content and stellar population properties of three massive galaxies at 1 < z < 1.3 that are in different stages of quenching. The galaxies were selected to have quiescent optical/near-infrared spectral energy distribution and relatively bright emission at 24 μm, and show remarkably diverse properties. CO emission from each of the three galaxies is detected in deep NOEMA observations, allowing us to derive molecular gas fractions M _gas/M _* of 13%–23%. We also reconstruct the star formation histories by fitting models to the observed photometry and optical spectroscopy, finding evidence for recent rejuvenation in one object, slow quenching in another, and rapid quenching in the third system. To better constrain the quenching mechanism we explore the depletion times for our sample and other similar samples at z ∼ 0.7 from the literature. We find that the depletion times are highly dependent on the method adopted to measure the star formation rate: using the UV+IR luminosity we obtain depletion times about 6 times shorter than those derived using dust-corrected [O ii] emission. When adopting the star formation rates from spectral fitting, which are arguably more robust, we find that recently quenched galaxies and star-forming galaxies have similar depletion times, while older quiescent systems have longer depletion times. These results offer new, important constraints for physical models of galaxy quenching.

[en] The [C ii] fine-structure transition at 158 μm is frequently the brightest far-infrared line in galaxies. Due to its low ionization potential, C⁺ can trace the ionized, atomic, and molecular phases of the ISM. We present velocity-resolved [C ii] and [N ii] pointed observations from SOFIA/GREAT on ∼500 pc scales in the nearby galaxies M101 and NGC 6946 and investigate the multiphase origin of [C ii] emission over a range of environments. We show that ionized gas makes a negligible contribution to the [C ii] emission in these positions using [N ii] observations. We spectrally decompose the [C ii] emission into components associated with the molecular and atomic phases using existing CO (2–1) and H i data and show that a peak signal-to-noise ratio of 10–15 is necessary for a reliable decomposition. In general, we find that in our pointings ≳50% of the [C ii] emission arises from the atomic phase, with no strong dependence on star formation rate, metallicity, or galactocentric radius. We do find a difference between pointings in these two galaxies, where locations in NGC 6946 tend to have larger fractions of [C ii] emission associated with the molecular phase than in M101. We also find a weak but consistent trend for fainter [C ii] emission to exhibit a larger contribution from the atomic medium. We compute the thermal pressure of the cold neutral medium through the [C ii] cooling function and find $\mathrm{l}\mathrm{o}\mathrm{g}(P_{\mathrm{t}\mathrm{h}}/k)=3.8\text{--}4.6[\mathrm{K}{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->3}]$, a value slightly higher than similar determinations, likely because our observations are biased toward star-forming regions.

[en] We present electron temperature (T _e) maps for the edge-on system Mrk 1486, affording “direct-method” gas-phase metallicity measurements across 5.″8 (4.1 kpc) along the minor axis and 9.″9 (6.9 kpc) along the major axis. These maps, enabled by strong detections of the [O iii] λ4363 auroral emission line across a large spatial extent of Mrk 1486, reveal a clear negative minor-axis T _e gradient in which temperature decreases with increasing distance from the disk plane. We find that the lowest metallicity spaxels lie near the extremes of the major axis, while the highest metallicity spaxels lie at large spatial offsets along the minor axis. This is consistent with a picture in which low-metallicity inflows dilute the metallicity at the edges of the major axis of the disk, while star formation drives metal-enriched outflows along the minor axis. We find that the outflow metallicity in Mrk 1486 is 0.20 dex (1.6 times) higher than the average insterstellar medium (ISM) metallicity, and more than 0.80 dex (6.3 times) higher than metal-poor inflowing gas, which we observe to be below 5% Z _⊙. This is the first example of metallicity measurements made simultaneously for inflowing, outflowing, and inner disk ISM gas using consistent T _e-based methodology. These measurements provide unique insight into how baryon-cycle processes contribute to the assembly of a galaxy like Mrk 1486.

[en] We present spatially resolved ALMA observations of the CO $J=3-<!--\; ???\; -->2$ emission line in two massive galaxies at z = 2.5 on the star-forming main sequence. Both galaxies have compact dusty star-forming cores with effective radii of $R_{\mathrm{e}}=1.3\pm <!--\; ??\; -->0.1\mathrm{k}\mathrm{p}\mathrm{c}$ and $R_{\mathrm{e}}=1.2\pm <!--\; ??\; -->0.1\mathrm{k}\mathrm{p}\mathrm{c}$ in the 870 μm continuum emission. The spatial extent of star-forming molecular gas is also compact with $R_{\mathrm{e}}=1.9\pm <!--\; ??\; -->0.4\mathrm{k}\mathrm{p}\mathrm{c}$ and $R_{\mathrm{e}}=2.3\pm <!--\; ??\; -->0.4\mathrm{k}\mathrm{p}\mathrm{c}$, but more extended than the dust emission. Interpreting the observed position–velocity diagrams with dynamical models, we find the starburst cores to be rotation dominated with the ratio of the maximum rotation velocity to the local velocity dispersion of $v_{max}/{\mathrm{\sigma <!--\; ??\; -->}}_0={7.0}_{-<!--\; ???\; -->2.8}^{+2.5}$ ($v_{max}={386}_{-<!--\; ???\; -->32}^{+36}$ km s⁻¹) and $v_{max}/{\mathrm{\sigma <!--\; ??\; -->}}_0={4.1}_{-<!--\; ???\; -->1.5}^{+1.7}$ ($v_{max}={391}_{-<!--\; ???\; -->41}^{+54}$ km s⁻¹). Given that the descendants of these massive galaxies in the local universe are likely ellipticals with $v/\mathrm{\sigma <!--\; ??\; -->}$ nearly an order of magnitude lower, the rapidly rotating galaxies would lose significant net angular momentum in the intervening time. The comparisons among dynamical, stellar, gas, and dust mass suggest that the starburst CO-to-H₂ conversion factor of ${\mathrm{\alpha <!--\; ??\; -->}}_{\mathrm{C}\mathrm{O}}=0.8M_{\odot <!--\; ???\; -->}$ (K km s⁻¹ pc⁻²)⁻¹ is appropriate in the spatially resolved cores. The dense cores are likely to be formed in extreme environments similar to the central regions of local ultraluminous infrared galaxies. Our work also demonstrates that a combination of medium-resolution CO and high-resolution dust continuum observations is a powerful tool for characterizing the dynamical state of molecular gas in distant galaxies.

[en] We present 0.″2-resolution Atacama Large Millimeter/submillimeter Array observations at 870 μm for 25 Hα-seleced star-forming galaxies around the main sequence at z = 2.2–2.5. We detect significant 870 μm continuum emission in 16 (64%) of these galaxies. The high-resolution maps reveal that the dust emission is mostly radiated from a single region close to the galaxy center. Exploiting the visibility data taken over a wide uv distance range, we measure the half-light radii of the rest-frame far-infrared emission for the best sample of 12 massive galaxies with log(M_*/M_⊙) > 11. We find nine galaxies to be associated with extremely compact dust emission with R_{1/2,870 μm} < 1.5 kpc, which is more than a factor of 2 smaller than their rest-optical sizes, $⟨<!--\; ???\; -->R_{1/2,1.6\mathrm{\mu <!--\; ??\; -->}\mathrm{m}}⟩<!--\; ???\; -->=3.2\mathrm{k}\mathrm{p}\mathrm{c}$, and is comparable with optical sizes of massive quiescent galaxies at similar redshifts. As they have an exponential disk with Sérsic index of $⟨<!--\; ???\; -->n_{1.6\mathrm{\mu <!--\; ??\; -->}\mathrm{m}}⟩<!--\; ???\; -->=1.2$ in the rest-optical, they are likely to be in the transition phase from extended disks to compact spheroids. Given their high star formation rate surface densities within the central 1 kpc of $⟨<!--\; ???\; -->\mathrm{\Sigma <!--\; ??\; -->}{\mathrm{S}\mathrm{F}\mathrm{R}}_{1\mathrm{k}\mathrm{p}\mathrm{c}}⟩<!--\; ???\; -->=40$ M_⊙ yr⁻¹ kpc⁻², the intense circumnuclear starbursts can rapidly build up a central bulge with ΣM_{*,1 kpc} > 10¹⁰ M_⊙ kpc⁻² in several hundred megayears, i.e., by z ∼ 2. Moreover, ionized gas kinematics reveal that they are rotation supported with an angular momentum as large as that of typical star-forming galaxies at z = 1–3. Our results suggest that bulges are commonly formed in extended rotating disks by internal processes, not involving major mergers.

[en] We present 0.″2 resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations at 870 μm in a stellar mass–selected sample of 85 massive ($M_{\star <!--\; ???\; -->}><!--\; >\; -->{10}^{11}{M}_{\odot <!--\; ???\; -->}$) star-forming galaxies (SFGs) at $z=1.9\text{--}2.6$ in the CANDELS/3D-Hubble Space Telescope fields of UDS and GOODS-S. We measure the effective radius of the rest-frame far-infrared (FIR) emission for 62 massive SFGs. They are distributed over wide ranges of FIR size from $R_{\mathrm{e},\mathrm{F}\mathrm{I}\mathrm{R}}=0.4\mathrm{k}\mathrm{p}\mathrm{c}$ to $R_{\mathrm{e},\mathrm{F}\mathrm{I}\mathrm{R}}=6\mathrm{k}\mathrm{p}\mathrm{c}$. The effective radius of the FIR emission is smaller by a factor of ${2.3}_{-<!--\; ???\; -->1.0}^{+1.9}$ than the effective radius of the optical emission and is smaller by a factor of ${1.9}_{-<!--\; ???\; -->1.0}^{+1.9}$ than the half-mass radius. Taking into account potential extended components, the FIR size would change only by ∼10%. By combining the spatial distributions of the FIR and optical emission, we investigate how galaxies change the effective radius of the optical emission and the stellar mass within a radius of 1 kpc, $M_{1\mathrm{k}\mathrm{p}\mathrm{c}}$. The compact starburst puts most of the massive SFGs on the mass–size relation for quiescent galaxies (QGs) at z ∼ 2 within 300 Myr if the current star formation activity and its spatial distribution are maintained. We also find that within 300 Myr, ∼38% of massive SFGs can reach the central mass of $M_{1\mathrm{k}\mathrm{p}\mathrm{c}}={10}^{10.5}{M}_{\odot <!--\; ???\; -->}$, which is around the boundary between massive SFGs and QGs. These results suggest an outside-in transformation scenario in which a dense core is formed at the center of a more extended disk, likely via dissipative in-disk inflows. Synchronized observations at ALMA 870 μm and James Webb Space Telescope 3–4 μm will explicitly verify this scenario.