arXiv subjects

K. Grasha

Publications and source records attributed to K. Grasha.

At least 37 records · Page 2Linked to original sources

arXiv2022

Star Cluster Formation and Evolution in M101: An Investigation with the Legacy Extragalactic UV Survey

We present Hubble Space Telescope WFC3/UVIS (F275W, F336W) and ACS/WFC optical (F435W, F555W, and F814W) observations of the nearby grand-design spiral galaxy M101 as part of the Legacy Extragalactic UV Survey (LEGUS). Compact sources detected in at least four bands were classified by both human experts and the convolutional neural network StarcNet. Human experts classified the 2,351 brightest sources, retrieving $N_{c} = 965$ star clusters. StarcNet, trained on LEGUS data not including M101, classified all 4,725 sources detected in four bands, retrieving $N_{c} = 2,270$ star clusters. The combined catalog represents the most complete census to date of compact star clusters in M101. We find that for the 2,351 sources with both a visual- and ML-classification StarcNet is able to reproduce the human classifications at high levels of accuracy ($\sim 80-90\%$), which is equivalent to the level of agreement between human classifiers in LEGUS. The derived cluster age distribution implies a disruption rate of $dN/dτ\propto τ^{-0.45 \pm 0.14}$ over $10^{7} < τ< 10^{8.5}$yr for cluster masses $\geq 10^{3.55} M_{\odot}$ for the central region of M101 and $dN/dτ\propto τ^{-0.02 \pm 0.15}$ for cluster masses $\geq 10^{3.38} M_{\odot}$ in the northwest region of the galaxy. The trends we recover are weaker than those of other nearby spirals (e.g. M51) and starbursts, consistent with the M101 environment having a lower-density interstellar medium, and providing evidence in favor of environmentally-dependent cluster disruption in the central, southeast, and northwest regions of M101.

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arXiv2022

Variations in the $Σ_{\rm SFR} {-} Σ_{\rm mol} {-} Σ_{\rm \star}$ plane across galactic environments in PHANGS galaxies

There exists some consensus that stellar mass surface density ($Σ_{*}$) and molecular gas mass surface density ($Σ_{\rm mol}$) are the main quantities responsible for locally setting the star formation rate. This regulation is inferred from locally resolved scaling relations between these two quantities and the star formation rate surface density ($Σ_{\rm SFR}$). However, the universality of these relations is debated. Here, we probe the interplay between these three quantities across different galactic environments at a spatial resolution of 150 pc. We perform a hierarchical Bayesian linear regression to find the best set of parameters $C_{*}$, $C_{\rm mol}$, and $C_{\rm norm}$ that describe the star-forming plane conformed by these quantities, such that $\log Σ_{\rm SFR} = C_{*} \log Σ_{*} + C_{\rm mol} \log Σ_{\rm mol} + C_{\rm norm}$, and explore variations in the determined parameters across galactic environments, focusing our analysis on the $C_{*}$ and $C_{\rm mol}$ slopes. We find signs of variations in the posterior distributions of $C_{*}$ and $C_{\rm mol}$ across different galactic environments. Bars show the most negative value of $C_{*}$, a sign of longer depletion times, while spiral arms show the highest $C_{*}$ among all environments. We conclude that systematic variations in the interplay of $Σ_{*}$, $Σ_{\rm mol}$ and $Σ_{\rm SFR}$ across galactic environments exist at a spatial resolution of 150 pc, and we interpret these variations as produced by an additional mechanism regulating the formation of stars that is not captured by either $Σ_{*}$ or $Σ_{\rm mol}$. We find that these variations correlate with changes in the star formation efficiency across environments, which could be linked to the dynamical state of the gas that prevents it from collapsing and forming stars, or to changes in the molecular gas fraction.

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arXiv2022

Linking stellar populations to HII regions across nearby galaxies: I. Constraining pre-supernova feedback from young clusters in NGC1672

One of the fundamental factors regulating the evolution of galaxies is stellar feedback. However, we still do not have strong observational constraints on the relative importance of the different feedback mechanisms (e.g. radiation, ionised gas pressure, stellar winds) in driving HII region evolution and molecular cloud disruption. In this letter, we constrain the relative importance of the various feedback mechanisms from young massive star populations by resolving HII regions across the disk of the nearby star-forming galaxy NGC 1672. We combine measurements of ionised gas nebular lines obtained by PHANGS-MUSE, with high-resolution imaging from the HST in both the narrow-band Hα and broad-band filters. We identify a sample of 40 isolated, compact HII regions in the HST Hα image, for which we measure the sizes that were previously unresolved in seeing-limited ground-based observations. Additionally, we identify the ionisation source(s) for each HII region from catalogues produced as part of the PHANGS-HST survey. We find that the HII regions investigated are mildly dominated by thermal or wind pressure, yet their elevation above the radiation pressure is within the expected uncertainty range. We see that radiation pressure provides a substantially higher contribution to the total pressure than previously found in the literature over similar size scales. In general, we find higher pressures within more compact HII regions, which is driven by the inherent size scaling relations of each pressure term, albeit with significant scatter introduced by the variation in the stellar population properties (e.g. luminosity, mass, age, metallicity). For nearby galaxies, here we provide a promising approach that could yield the statistics required to map out how the importance of different stellar feedback mechanisms evolve over the lifetime of an HII region.

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arXiv2022

Metallicity, ionization parameter, and pressure variations of HII regions in the TYPHOON spiral galaxies

We present a spatially-resolved HII region study of the gas-phase metallicity, ionization parameter, and ISM pressure maps of 6 local star-forming and face-on spiral galaxies from the TYPHOON program. Self-consistent metallicity, ionization parameter, and pressure maps are calculated simultaneously through an iterative process to provide useful measures of the local chemical abundance and its relation to localized ISM properties. We constrain the presence of azimuthal variations in metallicity by measuring the residual metallicity offset $Δ$(O/H) after subtracting the linear fits to the radial metallicity profiles. We however find weak evidence of azimuthal variations in most of the galaxies, with small (mean 0.03 dex) scatter. The galaxies instead reveal that HII regions with enhanced and reduced abundances are found distributed throughout the disk. While the spiral pattern plays a role in organizing the ISM, it alone does not establish the relatively uniform azimuthal variations we observe. Differences in the metal abundances are more likely driven by the strong correlations with the local physical conditions. We find a strong and positive correlation between the ionization parameter and the local abundances as measured by the relative metallicity offset $Δ$(O/H), indicating a tight relationship between local physical conditions and their localized enrichment of the ISM. Additionally, we demonstrate the impact of unresolved observations on the measured ISM properties by rebinning the datacubes to simulate low-resolution (1 kpc) observations, typical of large IFU surveys. We find that the ionization parameter and ISM pressure diagnostics are impacted by the loss of resolution such that their measured values are larger relative to the measured values on sub-HII region scales.

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arXiv2022

The First Large Absorption Survey in HI (FLASH): I. Science Goals and Survey Design

We describe the scientific goals and survey design of the First Large Absorption Survey in HI (FLASH), a wide field survey for 21-cm line absorption in neutral atomic hydrogen (HI) at intermediate cosmological redshifts. FLASH will be carried out with the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope and is planned to cover the sky south of $δ\approx +40$deg at frequencies between 711.5 and 999.5MHz. At redshifts between $z = 0.4$ and $1.0$ (look back times of 4 - 8Gyr), the HI content of the Universe has been poorly explored due to the difficulty of carrying out radio surveys for faint 21-cm line emission and, at ultra-violet wavelengths, space-borne searches for Damped Lyman-$α$ absorption in quasar spectra. The ASKAP wide field of view and large spectral bandwidth, in combination with a radio-quiet site, will enable a search for absorption lines in the radio spectra of bright continuum sources over 80% of the sky. This survey is expected to detect at least several hundred intervening 21-cm absorbers, and will produce an HI-absorption-selected catalogue of galaxies rich in cool, star-forming gas, some of which may be concealed from optical surveys. Likewise, at least several hundred associated 21-cm absorbers are expected to be detected within the host galaxies of radio sources at $0.4 < z < 1.0$, providing valuable kinematical information for models of gas accretion and jet-driven feedback in radio-loud active galactic nuclei. FLASH will also detect OH 18-cm absorbers in diffuse molecular gas, megamaser OH emission, radio recombination lines, and stacked HI emission.

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arXiv2021

Comparing the pre-SNe feedback and environmental pressures for 6000 HII regions across 19 nearby spiral galaxies

The feedback from young stars (i.e. pre-supernova) is thought to play a crucial role in molecular cloud destruction. In this paper, we assess the feedback mechanisms acting within a sample of 5810 HII regions identified from the PHANGS-MUSE survey of 19 nearby ($<$ 20 Mpc) star-forming, main sequence spiral galaxies (log($M_\star$/M$_\odot$)= 9.4 $-$ 11). These optical spectroscopic maps are essential to constrain the physical properties of the HII regions, which we use to investigate their internal pressure terms. We estimate the photoionised gas ($P_\mathrm{therm}$), direct radiation ($P_\mathrm{rad}$), and mechanical wind pressure ($P_\mathrm{wind}$), which we compare to the confining pressure of their host environment ($P_\mathrm{de}$). The HII regions remain unresolved within our ${\sim}50{-}100$ pc resolution observations, so we place upper ($P_\mathrm{max}$) and lower ($P_\mathrm{min}$) limits on each of the pressures by using a minimum (i.e. clumpy structure) and maximum (i.e. smooth structure) size, respectively. We find that the $P_\mathrm{max}$ measurements are broadly similar, and for $P_\mathrm{min}$ the $P_\mathrm{therm}$ is mildly dominant. We find that the majority of HII regions are over-pressured, $P_\mathrm{tot}/P_\mathrm{de} = (P_\mathrm{therm}+P_\mathrm{wind}+P_\mathrm{rad})/P_\mathrm{de} > 1$, and expanding, yet there is a small sample of compact HII regions with $P_\mathrm{tot,max}/P_\mathrm{de} < 1$ ($\sim$1% of the sample). These mostly reside in galaxy centres ($R_\mathrm{gal}<1$kpc), or, specifically, environments of high gas surface density; log($Σ_\mathrm{gas}/\mathrm{M_\odot} \mathrm{pc}^{-2}$)$\sim$2.5 (measured on kpc-scales). Lastly, we compare to a sample of literature measurements for $P_\mathrm{therm}$ and $P_\mathrm{rad}$ to investigate how dominant pressure term transitions over around 5dex in spatial dynamic range and 10 dex in pressure.

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arXiv2021

Star formation scaling relations at ~100 pc from PHANGS: Impact of completeness and spatial scale

Aims: The complexity of star formation at the physical scale of molecular clouds is not yet fully understood. We investigate the mechanisms regulating the formation of stars in different environments within nearby star-forming galaxies from the PHANGS sample. Methods: Integral field spectroscopic data and radio-interferometric observations of 18 galaxies were combined to explore the existence of the resolved star formation main sequence (rSFMS), resolved Kennicutt-Schmidt relation (rKS), and resolved molecular gas main sequence (rMGMS), and we derived their slope and scatter at spatial resolutions from 100 pc to 1 kpc (under various assumptions). Results: All three relations were recovered at the highest spatial resolution (100 pc). Furthermore, significant variations in these scaling relations were observed across different galactic environments. The exclusion of non-detections has a systematic impact on the inferred slope as a function of the spatial scale. Finally, the scatter of the $Σ_\mathrm{mol. gas + stellar}$ versus $Σ_\mathrm{SFR}$ correlation is smaller than that of the rSFMS, but higher than that found for the rKS. Conclusions: The rMGMS has the tightest relation at a spatial scale of 100 pc (scatter of 0.34 dex), followed by the rKS (0.41 dex) and then the rSFMS (0.51 dex). This is consistent with expectations from the timescales involved in the evolutionary cycle of molecular clouds. Surprisingly, the rKS shows the least variation across galaxies and environments, suggesting a tight link between molecular gas and subsequent star formation. The scatter of the three relations decreases at lower spatial resolutions, with the rKS being the tightest (0.27 dex) at a spatial scale of 1 kpc. Variation in the slope of the rSFMS among galaxies is partially due to different detection fractions of $Σ_\mathrm{SFR}$ with respect to $Σ_\mathrm{stellar}$.

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arXiv2021

Dense molecular gas properties on 100 pc scales across the disc of NGC 3627

It is still poorly constrained how the densest phase of the interstellar medium varies across galactic environment. A large observing time is required to recover significant emission from dense molecular gas at high spatial resolution, and to cover a large dynamic range of extragalactic disc environments. We present new NOrthern Extended Millimeter Array (NOEMA) observations of a range of high critical density molecular tracers (HCN, HNC, HCO+) and CO isotopologues (13CO, C18O) towards the nearby (11.3 Mpc), strongly barred galaxy NGC 3627. These observations represent the current highest angular resolution (1.85"; 100 pc) map of dense gas tracers across a disc of a nearby spiral galaxy, which we use here to assess the properties of the dense molecular gas, and their variation as a function of galactocentric radius, molecular gas, and star formation. We find that the HCN(1-0)/CO(2-1) integrated intensity ratio does not correlate with the amount of recent star formation. Instead, the HCN(1-0)/CO(2-1) ratio depends on the galactic environment, with differences between the galaxy centre, bar, and bar end regions. The dense gas in the central 600 pc appears to produce stars less efficiently despite containing a higher fraction of dense molecular gas than the bar ends where the star formation is enhanced. In assessing the dynamics of the dense gas, we find the HCN(1-0) and HCO+(1-0) emission lines showing multiple components towards regions in the bar ends that correspond to previously identified features in CO emission. These features are co-spatial with peaks of Halpha emission, which highlights that the complex dynamics of this bar end region could be linked to local enhancements in the star formation.

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arXiv2021

New Constraints on the $^{12}$CO(2-1)/(1-0) Line Ratio Across Nearby Disc Galaxies

Both the CO(2-1) and CO(1-0) lines are used to trace the mass of molecular gas in galaxies. Translating the molecular gas mass estimates between studies using different lines requires a good understanding of the behaviour of the CO(2-1)-to-CO(1-0) ratio, $R_{21}$. We compare new, high quality CO(1-0) data from the IRAM 30-m EMPIRE survey to the latest available CO(2-1) maps from HERACLES, PHANGS-ALMA, and a new IRAM 30-m M51 Large Program. This allows us to measure $R_{21}$ across the full star-forming disc of nine nearby, massive, star-forming spiral galaxies at 27" (${\sim} 1{-}2$ kpc) resolution. We find an average $R_{21} = 0.64\pm0.09$ when we take the luminosity-weighted mean of all individual galaxies. This result is consistent with the mean ratio for disc galaxies that we derive from single-pointing measurements in the literature, $R_{\rm 21, lit}~=~0.59^{+0.18}_{-0.09}$. The ratio shows weak radial variations compared to the point-to-point scatter in the data. In six out of nine targets the central enhancement in $R_{21}$ with respect to the galaxy-wide mean is of order $\sim 10{-}20\%$. We estimate an azimuthal scatter of $\sim$20% in $R_{21}$ at fixed galactocentric radius but this measurement is limited by our comparatively coarse resolution of 1.5 kpc. We find mild correlations between $R_{21}$ and CO brightness temperature, IR intensity, 70-to-160$ μ$m ratio, and IR-to-CO ratio. All correlations indicate that $R_{21}$ increases with gas surface density, star formation rate surface density, and the interstellar radiation field.

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arXiv2021

Stromlo Stellar Tracks: non-solar scaled abundances for massive stars

We present the Stromlo Stellar Tracks, a set of stellar evolutionary tracks, computed by modifying the Modules for Experiments in Stellar Astrophysics (MESA) 1D stellar evolution package, to fit the Galactic Concordance abundances for hot ($T > 8000$ K) massive ($\geq 10M_\odot$) Main-Sequence (MS) stars. Until now, all stellar evolution tracks are computed at solar, scaled-solar, or alpha-element enhanced abundances, and none of these models correctly represent the Galactic Concordance abundances at different metallicities. This paper is the first implementation of Galactic Concordance abundances to the stellar evolution models. The Stromlo tracks cover massive stars ($10\leq M/M_\odot \leq 300$) with varying rotations ($v/v_{\rm crit} = 0.0, 0.2, 0.4$) and a finely sampled grid of metallicities ($-2.0 \leq {\rm [Z/H]} \leq +0.5$; $Δ{\rm [Z/H]} = 0.1$) evolved from the pre-main sequence to the end of $^{12}$Carbon burning. We find that the implementation of Galactic Concordance abundances is critical for the evolution of main-sequence, massive hot stars in order to estimate accurate stellar outputs (L, T$_{\rm eff}$, $g$), which, in turn, have a significant impact on determining the ionizing photon luminosity budgets. We additionally support prior findings of the importance that rotation plays on the evolution of massive stars and their ionizing budget. The evolutionary tracks for our Galactic Concordance abundance scaling provide a more empirically motivated approach than simple uniform abundance scaling with metallicity for the analysis of HII regions and have considerable implications in determining nebular emission lines and metallicity. Therefore, it is important to refine the existing stellar evolutionary models for comprehensive high-redshift extragalactic studies. The Stromlo tracks are publicly available to the astronomical community online.

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arXiv2020

Looking for obscured young star clusters in NCG 1313

Using recently acquired HST NIR observations (J, Pa$β$ and H bands) of the nearby galaxy NGC1313, we investigate the timescales required by a young star cluster to emerge from its natal cloud. We search for extincted star clusters, potentially embedded in their natal cloud as either: 1. compact sources in regions with high H$α$/Pa$β$ extinctions; 2. compact HII regions that appear as point-like sources in the Pa$β$ emission map. The NUV--optical--NIR photometry of the candidate clusters is used to derive their ages, masses and extinctions via a least$-χ^2$ SED broad and narrow--band fitting process. The 100 clusters in the final samples have masses in the range $\rm \log_{10}(M/M_\odot)=2.5-3.5$ and moderate extinctions, $\rm E(B-V)\lesssim1.0$ mag. Focusing on the young clusters ($0-6$ Myr) we derive a weak correlation between extinction and age of the clusters. Almost half of the clusters have low extinctions, $\rm E(B-V)<0.25$ mag, already at very young ages ($\le3$ Myr), suggesting that dust is quickly removed from clusters. A stronger correlation is found between the morphology of the nebular emission (compact, partial or absent, both in H$α$ and Pa$β$) and cluster age. Relative fractions of clusters associated with a specific nebular morphology is used to estimate the typical timescales for clearing the natal gas cloud, resulting between 3 and 5 Myr, $\sim1$ Myr older than what estimated from NUV--optical--based cluster studies. This difference hints to a bias for optically--only based studies, which JWST will address in the coming years.

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arXiv2020

The Age-Dependence of Mid-Infrared Emission Around Young Star Clusters

Using the star cluster catalogs from the Hubble Space Telescope program Legacy ExtraGalactic UV survey (LEGUS) and 8 $μ$m images from the IRAC camera on the Spitzer Space Telescope for 5 galaxies within 5 Mpc, we investigate how the 8 $μ$m dust luminosity correlates with the stellar age on the 30--50 pc scale of star forming regions. We construct a sample of 97 regions centered at local peaks of 8 $μ$m emission, each containing one or more young star cluster candidates from the LEGUS catalogs. We find a tight anti-correlation with a Pearson correlation coefficient of $r=-0.84\pm0.05$ between the mass-normalized dust-only 8 $μ$m luminosity and the age of stellar clusters younger than 1 Gyr; the 8 $μ$m luminosity decreases with increasing age of the stellar population. Simple assumptions on a combination of stellar and dust emission models reproduce the observed trend. We also explore how the scatter of the observed trend depends on assumptions of stellar metallicity, PAH abundance, fraction of stellar light absorbed by dust, and instantaneous versus continuous star formation models. We find that variations in stellar metallicity have little effect on the scatter, while PAH abundance and the fraction of dust-absorbed light bracket the full range of the data. We also find that the trend is better explained by continuous star formation, rather than instantaneous burst models. We ascribe this result to the presence of multiple star clusters with different ages in many of the regions. Upper limits of the dust-only 8 $μ$m emission as a function of age are provided.

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arXiv2019

Spatial segregation of massive clusters in dwarf galaxies

The relative average minimum projected separations of star clusters in the Legacy ExtraGalactic UV Survey (LEGUS) and in tidal dwarfs around the interacting galaxy NGC 5291 are determined as a function of cluster mass to look for cluster-cluster mass segregation. Class 2 and 3 LEGUS clusters, which have a more irregular internal structure than the compact and symmetric class 1 clusters, are found to be mass segregated in low mass galaxies, which means that the more massive clusters are systematically bunched together compared to the lower mass clusters. This mass segregation is not present in high-mass galaxies nor for class 1 clusters. We consider possible causes for this segregation including differences in cluster formation and scattering in the shallow gravitational potentials of low mass galaxies.

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arXiv2019

Mapping metallicity variations across nearby galaxy disks

The distribution of metals within a galaxy traces the baryon cycle and the buildup of galactic disks, but the detailed gas phase metallicity distribution remains poorly sampled. We have determined the gas phase oxygen abundances for 7,138 HII regions across the disks of eight nearby galaxies using VLT/MUSE optical integral field spectroscopy as part of the PHANGS-MUSE survey. After removing the first order radial gradients present in each galaxy, we look at the statistics of the metallicity offset (Delta O/H) and explore azimuthal variations. Across each galaxy, we find low (sigma=0.03-0.05 dex) scatter at any given radius, indicative of efficient mixing. We compare physical parameters for those HII regions that are 1 sigma outliers towards both enhanced and reduced abundances. Regions with enhanced abundances have high ionization parameter, higher Halpha luminosity, lower Halpha velocity dispersion, younger star clusters and associated molecular gas clouds show higher molecular gas densities. This indicates recent star formation has locally enriched the material. Regions with reduced abundances show increased Halpha velocity dispersions, suggestive of mixing introducing more pristine material. We observe subtle azimuthal variations in half of the sample, but can not always cleanly associate this with the spiral pattern. Regions with enhanced and reduced abundances are found distributed throughout the disk, and in half of our galaxies we can identify subsections of spiral arms with clearly associated metallicity gradients. This suggests spiral arms play a role in organizing and mixing the ISM.

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arXiv2019

H$α$ Morphologies of Star Clusters: A LEGUS study of HII region evolution timescales and stochasticity in low mass clusters

The morphology of HII regions around young star clusters provides insight into the timescales and physical processes that clear a cluster's natal gas. We study ~700 young clusters (<10Myr) in three nearby spiral galaxies (NGC 7793, NGC 4395, and NGC 1313) using Hubble Space Telescope (HST) imaging from LEGUS (Legacy ExtraGalactic Ultraviolet Survey). Clusters are classified by their H$α$ morphology (concentrated, partially exposed, no-emission) and whether they have neighboring clusters (which could affect the clearing timescales). Through visual inspection of the HST images, and analysis of ages, reddenings, and stellar masses from spectral energy distributions fitting, together with the (U-B), (V-I) colors, we find: 1) the median ages indicate a progression from concentrated (~3 Myr), to partially exposed (~4 Myr), to no H$α$ emission (>5Myr), consistent with the expected temporal evolution of HII regions and previous results. However, 2) similarities in the age distributions for clusters with concentrated and partially exposed H$α$ morphologies imply a short timescale for gas clearing (<1Myr). 3) our cluster sample's median mass is ~1000 M, and a significant fraction (~20%) contain one or more bright red sources (presumably supergiants), which can mimic reddening effects. Finally, 4) the median E(B-V) values for clusters with concentrated H$α$ and those without H$α$ emission appear to be more similar than expected (~0.18 vs. ~0.14, respectively), but when accounting for stochastic effects, clusters without H$α$ emission are less reddened. To mitigate stochastic effects, we experiment with synthesizing more massive clusters by stacking fluxes of clusters within each H$α$ morphological class. Composite isolated clusters also reveal a color and age progression for H$α$ morphological classes, consistent with analysis of the individual clusters.

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arXiv2019

An ALMA-HST Study of Millimeter Dust Emission and Star Clusters

We present results from a joint ALMA-HST study of the nearby spiral galaxy NGC 628. We combine the HST LEGUS database of over 1000 stellar clusters in NGC 628 with ALMA Cycle 4 millimeter/submillimeter observations of the cold dust continuum that span ~15 square kpc including the nuclear region and western portions of the galaxy's disk. The resolution -- 1.1" or approximately 50 pc at the distance of NGC 628 -- allows us to constrain the spatial variations in the slope of the millimeter dust continuum as a function of the ages and masses of the nearby stellar clusters. Our results indicate an excess of dust emission in the millimeter assuming a typical cold dust model for a normal star-forming galaxy, but little correlation of the dust continuum slope with stellar cluster age or mass. For the depth and spatial coverage of these observations, we cannot substantiate the millimeter/submillimeter excess arising from the processing of dust grains by the local interstellar radiation field. We detect a bright unknown source in NGC 628 in ALMA bands 4 and 7 with no counterparts at other wavelengths from ancillary data. We speculate this is possibly a dust obscured supernova.

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arXiv2019

SIGNALS: I. Survey Description

SIGNALS, the Star formation, Ionized Gas, and Nebular Abundances Legacy Survey, is a large observing program designed to investigate massive star formation and HII regions in a sample of local extended galaxies. The program will use the imaging Fourier transform spectrograph SITELLE at the Canada-France-Hawaii Telescope. Over 355 hours (54.7 nights) have been allocated beginning in fall 2018 for eight consecutive semesters. Once completed, SIGNALS will provide a statistically reliable laboratory to investigate massive star formation, including over 50 000 resolved HII regions : the largest, most complete, and homogeneous database of spectroscopically and spatially resolved extragalactic HII regions ever assembled. For each field observed, three datacubes covering the spectral bands of the filters SN1 (363 -386 nm), SN2 (482 - 513 nm), and SN3 (647 - 685 nm) are gathered. The spectral resolution selected for each spectral band is 1000, 1000, and 5000, respectively. As defined, the project sample will facilitate the study of small-scale nebular physics and many other phenomena linked to star formation at a mean spatial resolution of 20 pc. This survey also has considerable legacy value for additional topics including planetary nebulae, diffuse ionized gas, andsupernova remnants. The purpose of this paper is to present a general outlook of the survey, notably the observing strategy, galaxy sample, and science requirements.

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arXiv2019

MAGPHYS+photo-z: Constraining the Physical Properties of Galaxies with Unknown Redshifts

We present an enhanced version of the multiwavelength spectral modeling code MAGPHYS that allows the estimation of galaxy photometric redshift and physical properties (e.g., stellar mass, star formation rate, dust attenuation) simultaneously, together with robust characterization of their uncertainties. The self-consistent modeling over ultraviolet to radio wavelengths in MAGPHYS+photo-z is unique compared to standard photometric redshift codes. The broader wavelength consideration is particularly useful for breaking certain degeneracies in color vs. redshift for dusty galaxies with limited observer-frame ultraviolet and optical data (or upper limits). We demonstrate the success of the code in estimating redshifts and physical properties for over 4,000 infrared-detected galaxies at 0.4<z<6.0 in the COSMOS field with robust spectroscopic redshifts. We achieve high photo-z precision ($σ_{Δz/(1+z_{spec})}\lesssim0.04$), high accuracy (i.e., minimal offset biases; median$(Δz/(1+z_{spec}))\lesssim0.02$), and low catastrophic failure rates ($η\simeq4\%$) over all redshifts. Interestingly, we find that a weak 2175A absorption feature in the attenuation curve models is required to remove a subtle systematic photo-z offset ($z_{phot}-z_{spec}\simeq-0.03$) that occurs when this feature is not included. As expected, the accuracy of derived physical properties in MAGPHYS+photo-z decreases strongly as redshift uncertainty increases. The all-in-one treatment of uncertainties afforded with this code is beneficial for accurately interpreting physical properties of galaxies in large photometric datasets. Finally, we emphasize that MAGPHYS+photo-z is not intended to replace existing photo-z codes, but rather offer flexibility to robustly interpret physical properties when spectroscopic redshifts are unavailable. The MAGPHYS+photo-z code is publicly available online.

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