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Gas in Void Galaxies
Void galaxies, residing within the deepest underdensities of the Cosmic Web, present an ideal population for the study of galaxy formation and evolution in an environment undisturbed by the complex processes modifying galaxies in clusters and groups, and provide an observational test for theories of cosmological structure formation. We investigate the neutral hydrogen properties (i.e. content, morphology, kinematics) of void galaxies, both individually and systematically, using a combination of observations and simulations, to form a more complete understanding of the nature of these systems. We investigate in detail the H I morphology and kinematics of two void galaxies. One is an isolated polar disk galaxy in a diffuse cosmological wall situated between two voids. The considerable gas mass and apparent lack of stars in the polar disk, coupled with the general underdensity of the environment, supports recent theories of cold flow accretion as an alternate formation mechanism for polar disk galaxies.
We also examine KK 246, the only confirmed galaxy located within the nearby Tully Void. It is a dwarf galaxy with an extremely extended H I disk and signs of an H I cloud with anomalous velocity. It also exhibits clear misalignment between the kinematical major and minor axes, and a general misalignment between the H I and optical major axes. The relative isolation and extreme underdense environment make these both very interesting cases for examining the role of gas accretion in galaxy evolution. To study void galaxies as a population, we have carefully selected a sample of 60 galaxies that reside in the deepest underdensities of geometrically identified voids within the SDSS. We have imaged this new Void Galaxy Survey in H I at the Westerbork Synthesis Radio Telescope with a typical resolution of 8 kpc, probing a volume of 1.2 Mpc and 12,000 km sâ»Âč surrounding each galaxy. We reach H I mass limits of 2 x 10âžM_sun and column density sensitivities of 5 x 10Âčâč cmâ»ÂČ.
We find that the void galaxies are generally gas rich, low luminosity, blue disk galaxies, but identify three as early type galaxies. The void galaxy optical and H I properties are not unusual for their luminosity and morphology. The small scale clustering in the void is similar to that in higher density regions, and we identify 18 H I rich neighboring galaxies in the voids. Two of these are systems of three galaxies linearly aligned and joined by a H I bridge, suggestive of filamentary formation within the void. We find no population of H I rich low luminosity galaxies within the observed voids that are not close companions of the targeted sample. Finally, to put these observations in a theoretical context, we analyze a (120 hâ»Âč Mpc)Âł adaptive mesh refinement hydrodynamic simulation that contains a high resolution subvolume centered on a ~30 Mpc diameter void. We construct mock observations with ~1 kpc resolution of the stellar and gas properties of these systems which reproduce the range of colors and luminosities observed in the SDSS for nearby galaxies, however we find no strong trends with density. We also make predictions for a significant population of low luminosity (M_r = -14) dwarf galaxies that is preferentially located in low density regions and specifically in the void center
Attenuation modified by DIG and dust as seen in M31
The spatial distribution of dust in galaxies affects the global attenuation,
and hence inferred properties, of galaxies. We trace the spatial distribution
of dust in five fields (at 0.6-0.9 kpc scale) of M31 by comparing optical
attenuation with the total dust mass distribution. We measure the attenuation
from the Balmer decrement using Integral Field Spectroscopy and the dust mass
from Herschel far-IR observations. Our results show that M31's dust attenuation
closely follows a foreground screen model, contrary to what was previously
found in other nearby galaxies. By smoothing the M31 data we find that spatial
resolution is not the cause for this difference. Based on the emission line
ratios and two simple models, we conclude that previous models of dust/gas
geometry need to include a weakly or non-attenuated diffuse ionized gas (DIG)
component. Due to the variation of dust and DIG scale heights with galactic
radius, we conclude that different locations in galaxies will have different
vertical distributions of gas and dust and therefore different measured
attenuation. The difference between our result in M31 with that found in other
nearby galaxies can be explained by our fields in M31 lying at larger galactic
radii than the previous studies that focused on the centres of galaxies.Comment: 20 pages, 13 figures, ApJ accepted and in pres
Simulated void galaxies in the standard cold dark matter model
We analyze a (120 h^{-1} Mpc)^3 adaptive mesh refinement hydrodynamic
simulation that contains a higher-resolution 31 x 31 x 35 h^{-3} Mpc subvolume
centered on a ~30 Mpc diameter void. Our detailed ~1 kpc resolution allows us
to identify 1300 galaxies within this void to a limiting halo mass of ~10^{10}
M_sun. Nearly 1000 galaxies are found to be in underdense regions, with 300
galaxies residing in regions less than half the mean density of the simulation
volume. We construct mock observations of the stellar and gas properties of
these systems, and reproduce the range of colors and luminosities observed in
the SDSS for nearby (z < 0.03) galaxies. We find no trends with density for the
most luminous (M_r
-16), though they are less reliably resolved, typically appear bluer, with
higher rates of star formation and specific star formation and lower mean
stellar ages than galaxies in average density environments. We find a
significant population of low luminosity (M_r ~ -14) dwarf galaxies that is
preferentially located in low density regions and specifically in the void
center. This population may help to reduce, but not remove, the discrepancy
between the predicted and observed number of void galaxies.Comment: 23 pages, 14 figures, submitted to Ap
The Origins of [CII] Emission in Local Star-forming Galaxies
The [CII] 158um fine-structure line is the brightest emission line observed
in local star-forming galaxies. As a major coolant of the gas-phase
interstellar medium, [CII] balances the heating, including that due to
far-ultraviolet photons, which heat the gas via the photoelectric effect.
However, the origin of [CII] emission remains unclear, because C+ can be found
in multiple phases of the interstellar medium. Here we measure the fractions of
[CII] emission originating in the ionized and neutral gas phases of a sample of
nearby galaxies. We use the [NII] 205um fine-structure line to trace the
ionized medium, thereby eliminating the strong density dependence that exists
in the ratio of [CII]/[NII] 122um. Using the FIR [CII] and [NII] emission
detected by the KINGFISH and Beyond the Peak Herschel programs, we show that
60-80% of [CII] emission originates from neutral gas. We find that the fraction
of [CII] originating in the neutral medium has a weak dependence on dust
temperature and the surface density of star formation, and a stronger
dependence on the gas-phase metallicity. In metal-rich environments, the
relatively cooler ionized gas makes substantially larger contributions to total
[CII] emission than at low abundance, contrary to prior expectations.
Approximate calibrations of this metallicity trend are provided.Comment: 8 pages, accepted for publication in Ap
Investigating the Drivers of Electron Temperature Variations in H ii Regions with Keck-KCWI and VLT-MUSE
H ii region electron temperatures are a critical ingredient in metallicity determinations, and recent observations have revealed systematic variations in the temperatures measured using different ions. We present electron temperatures (T e ) measured using the optical auroral lines ([N ii]λ5756, [O ii]λ λ7320, 7330, [S ii]λ λ4069, 4076, [O iii]λ4363, and [S iii]λ6312) for a sample of H ii regions in seven nearby galaxies. We use observations from the Physics at High Angular resolution in Nearby Galaxies survey (PHANGS) obtained with integral field spectrographs on Keck (Keck Cosmic Web Imager) and the Very Large Telescope (Multi-Unit Spectroscopic Explorer). We compare the different T e measurements with H ii region and ISM environmental properties such as electron density, ionization parameter, molecular gas velocity dispersion, and stellar association/cluster mass and age obtained from PHANGS. We find that the temperatures from [O ii] and [S ii] are likely overestimated due to the presence of electron density inhomogeneities in H ii regions. We measure high [O iii] temperatures in a subset of regions with high molecular gas velocity dispersion and low ionization parameter, which may be explained by the presence of low-velocity shocks. In agreement with previous studies, the T eâT e between [N ii] and [S iii] temperatures have the lowest observed scatter and follow predictions from photoionization modeling, which suggests that these tracers reflect H ii region temperatures across the various ionization zones better than [O ii], [S ii], and [O iii]
PHANGS-MUSE: Detection and Bayesian classification of ~40000 ionised nebulae in nearby spiral galaxies
In this work, we present a new catalogue of >40000 ionised nebulae
distributed across the 19 galaxies observed by the PHANGS-MUSE survey. The
nebulae have been classified using a new model-comparison-based algorithm that
exploits the odds ratio principle to assign a probabilistic classification to
each nebula in the sample. The resulting catalogue is the largest catalogue
containing complete spectral and spatial information for a variety of ionised
nebulae available so far in the literature. We developed this new algorithm to
address some of the limitations of the traditional classification criteria,
such as their binarity, the sharpness of the involved limits, and the limited
amount of data they rely on for the classification. The analysis of the
catalogue shows that the algorithm performs well when selecting H II regions.
We can recover their luminosity function, and its properties are in line with
what is available in the literature. We also identify a rather significant
population of shock-ionised regions (mostly composed of supernova remnants), an
order of magnitude larger than any other homogeneous catalogue of supernova
remnants currently available in the literature. The number of supernova
remnants we identify per galaxy is in line with results in our Galaxy and other
very nearby sources. However, limitations in the source detection algorithm
result in an incomplete sample of planetary nebulae, even though their
classification seems robust. Finally, we demonstrate how applying a correction
for the contribution of the diffuse ionised gas to the nebulae's spectra is
essential to obtain a robust classification of the objects and how a correct
measurement of the extinction using DIG-corrected line fluxes prompts the use
of a higher theoretical Ha/Hb ratio (3.03) than what is commonly used when
recovering the E(B-V) via the Balmer decrement technique in massive
star-forming galaxies.Comment: 58 pages, 46 figures. Paper accepted for pubblications in A&A. The
catalogue will be available via the CDS or at the following link:
http://dx.doi.org/10.11570/23.000
Measuring the mixing scale of the ISM within nearby spiral galaxies
The spatial distribution of metals reflects, and can be used to constrain, the processes of chemical enrichment and mixing. Using PHANGS-MUSE optical integral field spectroscopy, we measure the gas-phase oxygen abundances (metallicities) across 7138 H II regions in a sample of eight nearby disc galaxies. In Paper I, we measure and report linear radial gradients in the metallicities of each galaxy, and qualitatively searched for azimuthal abundance variations. Here, we examine the 2D variation in abundances once the radial gradient is subtracted, (O/H), in order to quantify the homogeneity of the metal distribution and to measure the mixing scale over which H II region metallicities are correlated. We observe low (0.03â0.05 dex) scatter in (O/H) globally in all galaxies, with significantly lower (0.02â0.03 dex) scatter on small (<600 pc) spatial scales. This is consistent with the measurement uncertainties, and implies the 2D metallicity distribution is highly correlated on scales of 600 pc. We compute the two-point correlation function for metals in the disc in order to quantify the scale lengths associated with the observed homogeneity. This mixing scale is observed to correlate better with the local gas velocity dispersion (of both cold and ionized gas) than with the star formation rate. Selecting only H II regions with enhanced abundances relative to a linear radial gradient, we do not observe increased homogeneity on small scales. This suggests that the observed homogeneity is driven by the mixing introducing material from large scales rather than by pollution from recent and on-going star formation.k. KK and FS gratefully acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) in the form of an Emmy Noether Research Group (grant number KR4598/2-1, PI: Kreckel). SCOG and RSK acknowledge support from the DFG via SFB 881 âThe Milky Way
Systemâ (project-ID 138713538; subprojects B1, B2, and B8) and
from the Heidelberg cluster of excellence EXC 2181-390900948
âSTRUCTURES: A unifying approach to emergent phenomena in the physical world, mathematics, and complex dataâ, funded by the German Excellence Strategy. RSK furthermore thanks for funding from the European Research Council via the ERC Synergy Grant ECOGAL (grant 855130). ER acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), funding reference number RGPIN-2017-03987. FB acknowledges funding from the European Research Council (ERC) under the European Unionâs Horizon 2020 research and innovation programme (grant agreement no. 726384/Empire). JMDK and MC gratefully acknowledge funding from the DFG through an Emmy Noether Research Group (grant number KR4801/1-1). JMDK, MC, and JJK gratefully acknowledge funding from the DFG through the
DFG Sachbeihilfe (grant number KR4801/2-1). JMDK gratefully acknowledges funding from the ERC under the European Unionâs Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907). EW acknowledges support from the DFG via SFB 881 âThe Milky Way Systemâ (project-ID 138713538; subproject P2). TGW acknowledges funding from the ERC under the European Unionâs Horizon 2020 research
and innovation programme (grant agreement no. 694343
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