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The effect of dust attenuation on a galaxy's light depends on a number of physical properties, such as geometry and dust composition, both of which can vary across the faces of galaxies. To investigate this variation, we continue analysis on star-forming regions in 29 galaxies studied previously. We analyse these regions using Swift /UV Optical Telescope and Wide-field Infrared Survey Explorer images, as well as Sloan Digital Sky Survey/Mapping Nearby Galaxies at Apache Point Observatory emission line maps to constrain the relationship between the infrared excess (IRX) and the ultraviolet spectral index, β, for each star-forming region. This relationship can be used to constrain which dust attenuation law is appropriate for the region. We find that the value of Dn (4000) for a region is correlated with both IRX and β, and that the gas-phase metallicity is strongly correlated with the IRX. This correlation between metallicity and IRX suggests that regardless of aperture, metal-rich regions have steeper attenuation curves. We also find that integrated galactic light follows nearly the same IRX–β relationship as that found for kpc-sized star-forming regions. This similarity may suggest that the attenuation law followed by the galaxy is essentially the same as that followed by the regions, although the relatively large size of our star-forming regions complicates this interpretation because optical opacity and attenuation curves have been observed to vary within individual galaxies. [ABSTRACT FROM AUTHOR]

We develop a novel approach to measure the dust attenuation properties of galaxies, including the dust opacity, the shape of the attenuation curve, and the strength of the 2175 Å absorption feature. From an observed spectrum, the method uses a model-independent approach to derive a relative attenuation curve with an absolute amplitude calibrated using NIR photometry. The dust-corrected spectrum is fitted with stellar population models to derive the dust-free model spectrum, which is compared with the observed SED/spectrum from NUV to NIR to determine the dust attenuation properties. We apply this method to investigate dust attenuation on kiloparsec scales using a sample of 134 galaxies with integral field spectroscopy from MaNGA, NIR imaging from 2MASS, and NUV imaging from Swift/UVOT. We find that the attenuation curve slope and the 2175 Å bump in both the optical and NUV span a wide range at kiloparsec scales. The slope is shallower at higher optical opacity, regardless of the specific star formation rate (sSFR), the minor-to-major axis ratio (b / a) of the galaxies, and the location of spaxels within individual galaxies. The 2175 Å bump presents a strong negative correlation with the sSFR, while the correlations with the optical opacity, b / a, and the locations within individual galaxies are all weak. All of these trends appear to be independent of the stellar mass of the galaxies. Our results support the scenario that the variation of the 2175 Å bump is driven predominantly by processes related to star formation, such as the destruction of small dust grains by UV radiation in star-forming regions. [ABSTRACT FROM AUTHOR]

We present an entirely new sample of 388 low-mass galaxies (M ⋆ ≤ 1010 M ⊙) that have spectroscopic signatures indicating the presence of massive black holes (BHs) in the form of active galactic nuclei (AGNs) or tidal disruption events. Of these, 70 have stellar masses in the dwarf galaxy regime with 108 ≲ M ⋆/ M ⊙ ≲ 109.5. We identify the active galaxies by analyzing optical spectra of a parent sample of ∼23,000 low-mass emission-line galaxies in the Galaxy and Mass Assembly (GAMA) Survey Data Release 4, and employing four different diagnostics based on narrow emission-line ratios and the detection of high-ionization coronal lines. We find that 47 of the 388 low-mass active galaxies exhibit broad H α in their spectra, corresponding to virial BH masses in the range M BH ∼ 105.0â€"7.7 M ⊙ with a median BH mass of 〈 M BH〉 ∼ 106.2 M ⊙. Our sample extends to higher redshifts (z ≤ 0.3; 〈 z 〉 = 0.13) than previous samples of AGNs in low-mass/dwarf galaxies based on Sloan Digital Sky Survey spectroscopy, which can be attributed to the spectroscopic limit of GAMA being ∼2 mag deeper. Moreover, our multi-diagnostic approach has revealed low-mass active galaxies spanning a wide range of properties, from blue star-forming dwarfs to luminous “miniquasars” powered by low-mass BHs. As such, this work has implications for BH seeding and AGN feedback at low masses. [ABSTRACT FROM AUTHOR]

Nearby, low-metallicity dwarf starburst galaxies hosting active galactic nuclei (AGNs) offer the best local analogs to study the early evolution of galaxies and their supermassive black holes (BHs). Here we present a detailed multiwavelength investigation of star formation and BH activity in the low-metallicity dwarf–dwarf galaxy merger Mrk 709. Using Hubble Space Telescope Hα and continuum imaging combined with Keck spectroscopy, we determine that the two dwarf galaxies are likely in the early stages of a merger (i.e., their first pass) and discover a spectacular ∼10 kpc long string of young massive star clusters (t ≲ 10 Myr; M⋆ ≳ 105M⊙) between the galaxies triggered by the interaction. We find that the southern galaxy, Mrk 709 S, is undergoing a clumpy mode of star formation resembling that seen in high-redshift galaxies, with multiple young clusters/clumps having stellar masses between 107 and 108M⊙. Furthermore, we present additional evidence for a low-luminosity AGN in Mrk 709 S (first identified by Reines et al. using radio and X-ray observations), including the detection of the coronal [Fe x ] optical emission line. The work presented here provides a unique glimpse into processes key to hierarchical galaxy formation and BH growth in the early universe. [ABSTRACT FROM AUTHOR]

Massive black holes (BHs) in dwarf galaxies can provide strong constraints on BH seeds; however, reliably detecting them is notoriously difficult. High-resolution radio observations were recently used to identify accreting massive BHs in nearby dwarf galaxies, with a significant fraction found to be non-nuclear. Here we present the first results of our optical follow-up of these radio-selected active galactic nuclei (AGNs) in dwarf galaxies using integral field unit (IFU) data from Gemini-North. We focus on the dwarf galaxy J1220+3020, which shows no clear optical AGN signatures in its nuclear Sloan Digital Sky Survey spectrum covering the radio source. With our new IFU data, we confirm the presence of an active BH via the AGN coronal line [Fe x ] and enhanced [O i ] emission coincident with the radio source. Furthermore, we detect broad Hα emission and estimate a BH mass of MBH = 104.9M⊙. We compare the narrow emission line ratios to standard BPT diagnostics and shock models. Spatially resolved BPT diagrams show some AGN signatures, particularly in [O i ]/Hα, but overall do not unambiguously identify the AGN. A comparison of our data to shock models clearly indicates shocked emission surrounding the AGN. The physical model most consistent with the data is an active BH with a radiatively inefficient accretion flow that both photoionizes and shock-excites the surrounding gas. We conclude that feedback is important in radio-selected BHs in dwarf galaxies and that radio surveys may probe a population of low accretion rate BHs in dwarf galaxies that cannot be detected through optical surveys alone. [ABSTRACT FROM AUTHOR]

We present MCSED , a new spectral energy distribution (SED) fitting code that mates flexible stellar evolution calculations with the Markov Chain Monte Carlo algorithms of the software package emcee. It takes broad-, intermediate-, and narrowband photometry, emission-line fluxes, and/or absorption-line spectral indices and returns probability distributions and covariance plots for all model parameters. It includes a variety of dust attenuation curves with parameters for varying the UV slopes and bump strengths, a prescription for continuum and polycyclic aromatic hydrocarbon emission from dust, models for continuum and line emission from ionized gas, options for fixed and variable stellar metallicity, and a selection of star formation rate (SFR) histories. The code is well suited for exploring parameter interdependencies in sets of galaxies with known redshifts for which there is multiband photometry and/or spectroscopy. We apply MCSED to a sample of ∼2000 galaxies in the five CANDELS fields that were selected via their strong [O iii ] λ5007 emission, and we explore the systematic behavior of their SEDs. We find that the galaxies become redder with stellar mass due to both increasing internal attenuation and a greater population of older stars. The slope of the UV extinction curve also changes with stellar mass, and at least some galaxies exhibit an extinction excess at 2175 Å. Finally, we demonstrate that below M ≲ 109M⊙, the shape of the star-forming galaxy main sequence is highly dependent on the galaxies' assumed SFR history, as calculations that assume a constant SFR produce stellar masses that are ∼1 dex smaller than those found using more realistic SFR histories. [ABSTRACT FROM AUTHOR]

The attenuation of light from star-forming galaxies is correlated with a multitude of physical parameters including star formation rate, metallicity and total dust content. This variation in attenuation is even more evident on kiloparsec scales, which is the relevant size for many current spectroscopic integral field unit surveys. To understand the cause of this variation, we present and analyse Swift /UVOT near-UV (NUV) images and SDSS/MaNGA emission-line maps of 29 nearby (z < 0.084) star-forming galaxies. We resolve kiloparsec-sized star-forming regions within the galaxies and compare their optical nebular attenuation (i.e. the Balmer emission line optical depth, |$\tau ^{l}_{B}\equiv \tau _{\textrm {H}\beta }-\tau _{\textrm {H}\alpha }$|⁠) and NUV stellar continuum attenuation (via the NUV power-law index, β) to the attenuation law described by Battisti et al. We show the data agree with that model, albeit with significant scatter. We explore the dependence of the scatter of the β– |$\tau ^{l}_{B}$| measurements from the star-forming regions on different physical parameters, including distance from the nucleus, star formation rate and total dust content. Finally, we compare the measured |$\tau ^{l}_{B}$| and β values for the individual star-forming regions with those of the integrated galaxy light. We find a strong variation in β between the kiloparsec scale and the larger galaxy scale that is not seen in |$\tau ^{l}_{B}$|⁠. We conclude that the sightline dependence of UV attenuation and the reddening of β due to the light from older stellar populations could contribute to the scatter in the β– |$\tau ^{l}_{B}$| relation. [ABSTRACT FROM AUTHOR]