We present an atlas of ultraviolet and infrared images of ∼15,750 local (d 50 Mpc) galaxies, as observed by NASA's Wide-field Infrared Survey Explorer (WISE) and Galaxy Evolution Explorer (GALEX) ...missions. These maps have matched resolution (FWHM 7 5 and 15″), matched astrometry, and a common procedure for background removal. We demonstrate that they agree well with resolved intensity measurements and integrated photometry from previous surveys. This atlas represents the first part of a program (the z = 0 Multiwavelength Galaxy Synthesis) to create a large, uniform database of resolved measurements of gas and dust in nearby galaxies. The images and associated catalogs will be publicly available at the NASA/IPAC Infrared Science Archive. This atlas allows us estimate local and integrated star formation rates (SFRs) and stellar masses (M ) across the local galaxy population in a uniform way. In the appendix, we use the population synthesis fits of Salim et al. to calibrate integrated M and SFR estimators based on GALEX and WISE. Because they leverage a Sloan Digital Sky Survey (SDSS)-based training set of >100,000 galaxies, these calibrations have high precision and allow us to rigorously compare local galaxies to SDSS results. We provide these SFR and M estimates for all galaxies in our sample and show that our results yield a "main sequence" of star-forming galaxies comparable to previous work. We also show the distribution of intensities from resolved galaxies in NUV-to-WISE1 versus WISE1-to-WISE3 space, which captures much of the key physics accessed by these bands.
The Spatially Resolved Dust-to-metals Ratio in M101 Chiang, I-Da; Sandstrom, Karin M.; Chastenet, Jérémy ...
Astrophysical journal/The Astrophysical journal,
10/2018, Letnik:
865, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The dust-to-metals ratio describes the fraction of heavy elements contained in dust grains, and its variation provides key insights into the life cycle of dust. We measure the dust-to-metals ratio in ...M101, a nearby galaxy with a radial metallicity (Z) gradient spanning ∼1 dex. We fit the spectral energy distribution of dust from 100 to 500 m with five variants of the modified blackbody dust emission model in which we vary the temperature distribution and how emissivity depends on wavelength. Among them, the model with a single-temperature blackbody modified by a broken power-law emissivity gives the statistically best fit and physically most plausible results. Using these results, we show that the dust-to-gas ratio is proportional to . This implies that the dust-to-metals ratio is not constant in M101, but decreases as a function of radius, which is equivalent to a lower fraction of metals trapped in dust at low metallicity (large radius). The dust-to-metals ratio in M101 remains at or above what would be predicted by the minimum depletion level of metals observed in the Milky Way. Our current knowledge of the metallicity-dependent CO-to-H2 conversion factor suggests that variations in the conversion factor cannot be responsible for the trends in dust-to-metals ratio we observe. This change of dust-to-metals ratio is significantly correlated with the mass fraction of molecular hydrogen, which suggests that the accretion of gas-phase metals onto existing dust grains could contribute to a variable dust-to-metals ratio.
We utilize archival far-infrared maps from the Herschel Space Observatory in four Local Group galaxies (Small and Large Magellanic Clouds (SMC and LMC), M31, and M33). We model their spectral energy ...distribution from 100 to 500 m using a single-temperature modified blackbody emission with a fixed emissivity index of β = 1.8. From the best-fit model, we derive the dust temperature, , and the dust mass surface density, , at 13 pc resolution for SMC and LMC, and at 167 pc resolution for all targets. This measurement allows us to build the distribution of dust mass and luminosity as functions of dust temperature and mass surface density. We compare those distribution functions among galaxies and between regions in a galaxy. We find that LMC has the highest mass-weighted average , while M31 and M33 have the lowest mass-weighted average . Within a galaxy, star-forming regions have higher and relative to the overall distribution function, because of more intense heating by young stars and higher gas mass surface density. When we degrade the resolutions to mimic distant galaxies, the mass-weighted mean temperature gets warmer as the resolution gets coarser, meaning that the temperatures derived from unresolved observations are systematically higher than those in highly resolved observations. As an implication, the total dust mass is lower (underestimated) in coarser resolutions. This resolution-dependent effect is more prominent in clumpy star-forming galaxies (SMC, LMC, and M33) and less prominent in a more quiescent massive spiral (M31).
We present maps of the dust properties in the Small and Large Magellanic Clouds (SMC, LMC) from fitting Spitzer and Herschel observations with the Draine & Li dust model. We derive the abundance of ...the small carbonaceous grain (or polycyclic aromatic hydrocarbon; PAH) component. The global PAH fraction ( , the fraction of the dust mass in the form of PAHs) is smaller in the SMC ( %) than in the LMC ( %). We measure the PAH fraction in different gas phases (H ii regions, ionized gas outside of H ii regions, molecular gas, and diffuse neutral gas). H ii regions appear as distinctive holes in the spatial distribution of the PAH fraction. In both galaxies, the PAH fraction in the diffuse neutral medium is higher than in the ionized gas, but similar to the molecular gas. Even at equal radiation field intensity, the PAH fraction is lower in the ionized gas than in the diffuse neutral gas. We investigate the PAH life-cycle as a function of metallicity between the two galaxies. The PAH fraction in the diffuse neutral medium of the LMC is similar to that of the Milky Way (∼4.6%), while it is significantly lower in the SMC. Plausible explanations for the higher PAH fraction in the diffuse neutral medium of the LMC compared to the SMC include: more effective PAH production by fragmentation of large grains at higher metallicity, and/or the growth of PAHs in molecular gas.
We measure the CO-to-H2 conversion factor (αCO) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from far-infrared emission as a tracer of the gas surface density and ...assuming a constant dust-to-metal ratio. In total, we have ∼790 and ∼610 independent measurements of αCO for CO (2–1) and (1–0), respectively. The mean values for αCO (2–1) and αCO (1–0) are 9.3−5.4+4.6 and 4.2−2.0+1.9M⊙pc−2(Kkms−1)−1, respectively. The CO-intensity-weighted mean is 5.69 for αCO (2–1) and 3.33 for αCO (1–0). We examine how αCO scales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength (U¯). Among them, U¯, ΣSFR, and the integrated CO intensity (WCO) have the strongest anticorrelation with spatially resolved αCO. We provide linear regression results to αCO for all quantities tested. At galaxy-integrated scales, we observe significant correlations between αCO and WCO, metallicity, U¯, and ΣSFR. We also find that αCO in each galaxy decreases with the stellar mass surface density (Σ⋆) in high-surface-density regions (Σ⋆ ≥ 100 M⊙ pc−2), following the power-law relations αCO(2–1)∝Σ⋆−0.5 and αCO(1–0)∝Σ⋆−0.2. The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease in αCO with increasing Σ⋆ as a result of higher velocity dispersion compared to isolated, self-gravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction in αCO. The decrease in αCO at high Σ⋆ is important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge “Milky Way–like” to “starburst-like” conversion factors.
Abstract
Ratios of polycyclic aromatic hydrocarbon (PAH) vibrational bands are a promising tool for measuring the properties of the PAH population and their effect on star formation. The photometric ...bands of the MIRI and NIRCam instruments on JWST provide the opportunity to measure PAH emission features across entire galaxy disks at unprecedented resolution and sensitivity. Here we present the first results of this analysis in a sample of three nearby galaxies: NGC 628, NGC 1365, and NGC 7496. Based on the variations observed in the 3.3, 7.7, and 11.3
μ
m features, we infer changes to the average PAH size and ionization state across the different galaxy environments. High values of F335M
PAH
/F1130W and low values of F1130W/F770W are measured in H
ii
regions in all three galaxies. This suggests that these regions are populated by hotter PAHs, and/or that the PAH ionization fraction is larger. We see additional evidence of heating and/or changes in PAH size in regions with higher molecular gas content as well as increased ionization in regions with higher H
α
intensity.
Context. Dust modeling is crucial to infer dust properties and budget for galaxy studies. However, there are systematic disparities between dust grain models that result in corresponding systematic ...differences in the inferred dust properties of galaxies. Quantifying these systematics requires a consistent fitting analysis. Aims. We compare the output dust parameters and assess the differences between two dust grain models, the DustEM model and THEMIS. In this study, we use a single fitting method applied to all the models to extract a coherent and unique statistical analysis. Methods. We fit the models to the dust emission seen by Spitzer and Herschel in the Small and Large Magellanic Clouds (SMC and LMC). The observations cover the infrared (IR) spectrum from a few microns to the sub-millimeter range. For each fitted pixel, we calculate the full n-D likelihood based on a previously described method. The free parameters are both environmental (U, the interstellar radiation field strength; αISRF, power-law coefficient for a multi-U environment; Ω∗, the starlight strength) and intrinsic to the model (Yi: abundances of the grain species i; αsCM20, coefficient in the small carbon grain size distribution). Results. Fractional residuals of five different sets of parameters show that fitting THEMIS brings a more accurate reproduction of the observations than the DustEM model. However, independent variations of the dust species show strong model-dependencies. We find that the abundance of silicates can only be constrained to an upper-limit and that the silicate/carbon ratio is different than that seen in our Galaxy. In the LMC, our fits result in dust masses slightly lower than those found in the literature, by a factor lower than 2. In the SMC, we find dust masses in agreement with previous studies.
Determining how the galactic environment, especially the high gas densities and complex dynamics in bar-fed galaxy centers, alters the star formation efficiency (SFE) of molecular gas is critical to ...understanding galaxy evolution. However, these same physical or dynamical effects also alter the emissivity properties of CO, leading to variations in the CO-to-H2 conversion factor (αCO) that impact the assessment of the gas column densities and thus of the SFE. To address such issues, we investigate the dependence of αCO on the local CO velocity dispersion at 150 pc scales using a new set of dust-based αCO measurements and propose a new αCO prescription that accounts for CO emissivity variations across galaxies. Based on this prescription, we estimate the SFE in a sample of 65 galaxies from the PHANGS–Atacama Large Millimeter/submillimeter Array survey. We find increasing SFE toward high-surface-density regions like galaxy centers, while using a constant or metallicity-based αCO results in a more homogeneous SFE throughout the centers and disks. Our prescription further reveals a mean molecular gas depletion time of 700 Myr in the centers of barred galaxies, which is overall three to four times shorter than in nonbarred galaxy centers or the disks. Across the galaxy disks, the depletion time is consistently around 2–3 Gyr, regardless of the choice of αCO prescription. All together, our results suggest that the high level of star formation activity in barred centers is not simply due to an increased amount of molecular gas, but also to an enhanced SFE compared to nonbarred centers or disk regions.
Abstract The PHANGS survey uses Atacama Large Millimeter/submillimeter Array, Hubble Space Telescope, Very Large Telescope, and JWST to obtain an unprecedented high-resolution view of nearby ...galaxies, covering millions of spatially independent regions. The high dimensionality of such a diverse multiwavelength data set makes it challenging to identify new trends, particularly when they connect observables from different wavelengths. Here, we use unsupervised machine-learning algorithms to mine this information-rich data set to identify novel patterns. We focus on three of the PHANGS-JWST galaxies, for which we extract properties pertaining to their stellar populations; warm ionized and cold molecular gas; and polycyclic aromatic hydrocarbons (PAHs), as measured over 150 pc scale regions. We show that we can divide the regions into groups with distinct multiphase gas and PAH properties. In the process, we identify previously unknown galaxy-wide correlations between PAH band and optical line ratios and use our identified groups to interpret them. The correlations we measure can be naturally explained in a scenario where the PAHs and the ionized gas are exposed to different parts of the same radiation field that varies spatially across the galaxies. This scenario has several implications for nearby galaxies: (i) The uniform PAH ionized fraction on 150 pc scales suggests significant self-regulation in the interstellar medium, (ii) the PAH 11.3/7.7 μ m band ratio may be used to constrain the shape of the non-ionizing far-ultraviolet to optical part of the radiation field, and (iii) the varying radiation field affects line ratios that are commonly used as PAH size diagnostics. Neglecting this effect leads to incorrect or biased PAH sizes.
Abstract The Small Magellanic Cloud (SMC) shows a large variation in ultraviolet (UV) dust extinction curves, ranging from Milky Way (MW) like to significantly steeper curves with no detectable 2175 ...Å bump. This result is based on a sample of only nine sight lines. From Hubble Space Telescope Space Telescope Imaging Spectrograph and IUE spectra of OB stars, we have measured UV extinction curves along 32 SMC sight lines where eight of these curves were published previously. We find 16 sight lines with steep extinction with no detectable 2175 Å bump, four sight lines with MW-like extinction with a detectable 2175 Å bump, two sight lines with fairly flat UV extinction and weak/absent 2175 Å bumps, and 10 sight lines with unreliable curves due to low SMC dust columns. Our expanded sample shows that the sight lines with and without the 2175 Å bump are located throughout the SMC and not limited to specific regions. The average extinction curve of the 16 bump-less sight lines is very similar to the previous average based on four sight lines. We find no correlation between dust column and the strength of the 2175 Å bump. We test the hypothesis that the 2175 Å bump is due to the same dust grains that are responsible for the mid-infrared carbonaceous (polycyclic aromatic hydrocarbon) emission features and find they are correlated, confirming recent work in the MW. Overall, the slope of the UV extinction increases as the amplitudes of the 2175 Å bump and far-ultraviolet curvature decrease. Finally, the UV slope is correlated with N (H i )/ A ( V ) and the 2175 Å bump and nonlinear far-ultraviolet rise amplitudes are anticorrelated with N (H i )/ A ( V ).
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