arXiv subjects

Alec S. Hirschauer

Publications and source records attributed to Alec S. Hirschauer.

At least 37 records · Page 2Linked to original sources

arXiv2024

JWST MIRI and NIRCam Unveil Previously Unseen Infrared Stellar Populations in NGC 6822

NGC 6822 is a nearby (~490 kpc) non-interacting low-metallicity (0.2 Zsolar) dwarf galaxy which hosts several prominent H ii regions, including sites of highly embedded active star formation. In this work, we present an imaging survey of NGC 6822 conducted with the NIRCam and MIRI instruments onboard JWST. We describe the data reduction, source extraction, and stellar population identifications from combined near- and mid-infrared (IR) photometry. Our NIRCam observations reach seven magnitudes deeper than previous JHKs surveys of this galaxy, which were sensitive to just below the tip of the red giant branch (TRGB). These JWST observations thus reveal for the first time in the near-IR the red clump stellar population and extend nearly three magnitudes deeper. In the mid-IR, we observe roughly two magnitudes below the TRGB with the MIRI F770W and F1000W filters. With these improvements in sensitivity, we produce a catalogue of ~900,000 point sources over an area of ~ 6.0 x 4.3 arcmin2. We present several NIRCam and MIRI colour-magnitude diagrams and discuss which colour combinations provide useful separations of various stellar populations to aid in future JWST observation planning. Finally, we find populations of carbon- and oxygen-rich asymptotic giant branch stars which will assist in improving our understanding of dust production in low-metallicity, early Universe analogue galaxies

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arXiv2024

Young Stellar Objects in NGC 346: A JWST NIRCam/MIRI Imaging Survey

We present a JWST imaging survey with NIRCam and MIRI of NGC 346, the brightest star-forming region in the Small Magellanic Cloud (SMC). By combining aperture and point spread function (PSF) photometry of eleven wavelength bands across these two instruments, we have detected more than 200,000 unique sources. Using near-infrared (IR) color analysis, we observe various evolved and young populations, including 196 young stellar objects (YSOs) and pre-main sequence stars suitable for forthcoming spectroscopic studies. We expand upon this work, creating mid-IR color-magnitude diagrams and determining color cuts to identify 833 reddened sources which are YSO candidates. We observe that these candidate sources are spatially associated with regions of dusty, filamentary nebulosity. Furthermore, we fit model YSO spectral energy distributions (SEDs) to a selection of sources with detections across all of our MIRI bands. We classify with a high degree of confidence 23 YSOs in this sample and estimate their radii, bolometric temperatures, luminosities, and masses. We detect YSOs approaching 1 solar mass, the lowest-mass extragalactic YSOs confirmed to date.

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arXiv2024

The Size-Linewidth Relation and Signatures of Feedback from Quiescent to Active Star Forming Regions in the LMC

To investigate the effects of stellar feedback on the gravitational state of giant molecular clouds (GMCs), we study $^{12}$CO and $^{13}$CO ALMA maps of nine GMCs distributed throughout the Large Magellanic Cloud (LMC), the nearest star-forming galaxy to our own. We perform noise and resolution matching on the sample, working at a common resolution of 3.5 arcseconds (0.85 pc at the LMC distance of 50 kpc), and use the \textit{SCIMES} clustering algorithm to identify discrete substructure, or "clumps." We supplement these data with three tracers of recent star formation: $8μ$m surface brightness, continuum-subtracted H$α$ flux, and interstellar radiation field energy density inferred from dust emission. The $^{12}$CO clumps identified cover a range of 3.6 dex in luminosity-based mass and 2.4 dex in average $8μ$m surface brightness, representative of the wide range of conditions of the interstellar medium in the LMC. Our observations suggest evidence for increased turbulence in these clouds. While the turbulent linewidths are correlated with clump surface density, in agreement with previous observations, we find even better correlation with the three star formation activity tracers considered, suggesting stellar energy injection plays a significant role in the dynamical state of the clumps. The excess linewidths we measure do not appear to result from opacity broadening. $^{12}$CO clumps are found to be typically less gravitationally bound than $^{13}$CO clumps, with some evidence of the kinetic-to-gravitational potential energy ratio increasing with star-formation tracers. Further multi-line analysis may better constrain the assumptions made in these calculations.

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arXiv2023

The Star Formation Across Cosmic Time (SFACT) Survey. II. The First Catalog from a New Narrow-Band Survey for Emission-Line Objects

Star Formation Across Cosmic Time (SFACT) is a new narrowband survey designed to detect faint emission-line galaxies and QSOs over a broad range of redshifts. Here we present the first list of SFACT candidates from our pilot-study fields. Using the WIYN 3.5m telescope, we are able to achieve good image quality with excellent depth and routinely detect ELGs to r = 25.0. The limiting line flux of the survey is ~1.0 x 10^16 erg/s/cm^2. SFACT targets three primary emission lines: H-alpha, [O III]5007, and [O II]3727. The corresponding redshift windows allow for the detection of objects at z ~ 0-1. With a coverage of 1.50 square degrees in our three pilot-study fields, a total of 533 SFACT candidates have been detected (355 candidates per square degree). We detail the process by which these candidates are selected in an efficient and primarily automated manner, then tabulate accurate coordinates, broadband photometry, and narrowband fluxes for each source.

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arXiv2023

The Star Formation Across Cosmic Time (SFACT) Survey. I. Survey Description and Early Results from a New Narrow-Band Emission-Line Galaxy Survey

We introduce the Star Formation Across Cosmic Time (SFACT) survey. SFACT is a new narrow-band survey for emission-line galaxies (ELGs) and QSOs being carried out using the wide-field imager on the WIYN 3.5 m telescope. Because of the superior depth and excellent image quality afforded by WIYN, we routinely detect ELGs to r = 25.0. Our survey observations are made using three custom narrow-band filters centered on 6590 A, 6950 A, and 7460 A. Due to the sensitivity of the survey, we are able to simultaneously detect sources via a number of different emission lines over a wide range of redshifts. The principal lines detected in SFACT are H-alpha (redshifts up to 0.144), [O III]5007 (redshifts up to 0.500) and [O II]3727 (redshifts up to 1.015). In this paper we detail the properties of the survey as well as present initial results obtained by analyzing our three pilot-study fields. These fields have yielded a total of 533 ELG candidates in an area of 1.50 square degrees (surface density of 355 ELGs per square degree). Follow-up spectra for a subset of the ELG candidates are also presented. One of the key attributes of the SFACT survey is that the ELGs are detected in discrete redshift windows that will allow us to robustly quantify the properties of the star-forming and AGN populations as a function of redshift to z = 1 and beyond. The planned acquisition of additional narrow-band filters will allow us to expand our survey to substantially higher redshifts.

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arXiv2023

Dissecting the Mid-Infrared Heart of M83 with JWST

We present a first look at the MRS observations of the nucleus of the nearby galaxy M83, taken with MIRI onboard JWST. The observations show a rich set of emission features from the ionized gas, warm molecular gas, and dust. To begin dissecting the complex processes in this part of the galaxy, we divide the observations into four different regions. We find that the strength of the emission features varies strongly from region to region, with the south-east region displaying the weakest features tracing the dust continuum and ISM properties. Comparison between the cold molecular gas traced by the $^{12}$CO (1-0) transition with ALMA and the H$_2$ S(1) transition shows a similar spatial distribution. This is in contrast to the distribution of the much warmer H$_2$ emission from the S(7) transition found to be concentrated around the optical nucleus. We use the rotational emission lines and model the H$_2$ excitation to estimate a total molecular gas mass accounting for the warm H$_2$ component of M($>$50 K)$_{\rm H_{2}}$ = 67.90 ($\pm 5.43$)$\times$10$^{6}$ M$_{\odot}$. We compare this value to the total gas mass inferred by probing the cold H$_2$ gas through the $^{12}$CO (1-0) emission, M(CO)$_{\rm H_{2}}$ = 17.15$\times$10$^{6}$ M$_{\odot}$. We estimate that $\sim$75\% of the total molecular gas mass is contained in the warm H$_2$ component. We also identify [\ion{O}{4}] 25.89 $μ$m and [\ion{Fe}{2}] 25.99 $μ$m emission. We propose that the diffuse [\ion{Fe}{2}] 25.99 $μ$m emission might be tracing shocks created during the interactions between the hot wind produced by the starburst and the much cooler ISM above the galactic plane. More detailed studies are needed to confirm such a scenario.

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arXiv2023

Discovery of dusty sub-solar mass young stellar objects in NGC 346 with JWST/NIRCam

JWST observations of NGC 346, a star-forming region in the metal-poor Small Magellanic Cloud, reveal a substantial population of sub-solar mass young stellar objects (YSOs) with IR excess. We detected $\sim$500 YSOs and pre main sequence (PMS) stars from more than 45,000 unique sources utilizing all four NIRCam wide filters with deep, high-resolution imaging, where ongoing low-mass star formation is concentrated along dust filaments. From these observations, we construct detailed near-IR colour-magnitude diagrams with which preliminary categorizations of YSO classes are made. For the youngest, most deeply-embedded objects, JWST/NIRCam reaches over 10 magnitudes below Spitzer observations at comparable wavelengths, and two magnitudes fainter than HST for more-evolved PMS sources, corresponding to $\sim$0.1 M$_\odot$. For the first time in an extragalactic environment, we detect embedded low-mass star-formation. Furthermore, evidence of IR excess and accretion suggests that dust required for rocky planet formation is present at metallicities as low as 0.2 $Z_\odot$.

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arXiv2022

Searches for Extremely Metal Poor Galaxies using ALFALFA-selected Dwarf Galaxies

We present a study of nearby dwarf galaxies selected from the ALFALFA blind HI survey. A primary goal of the project was to utilize a non-standard selection method with the hope of detecting previously unrecognized extremely metal-poor (XMP) galaxies. The study was motivated by the recent discovery of two XMP galaxies $-$ Leo P and Leoncino $-$ which were both originally found via the ALFALFA survey. We have obtained narrowband H$α$ images for 42 dwarf systems, many of which are located in the local void in front of the Pisces-Perseus Supercluster. Spectra for eleven of the best candidates resulted in the determination of metal abundances for ten of the systems. None were found to be extremely metal poor, although one system (AGC 123350) was found to have an oxygen abundance of log(O/H)+12 = 7.46, or $\sim$6\% solar. One of the galaxies in our sample exhibits a high oxygen abundance for its luminosity, suggesting the possibility that it may have a tidal origin.

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arXiv2022

The 30 Doradus Molecular Cloud at 0.4 pc Resolution with the Atacama Large Millimeter/submillimeter Array: Physical Properties and the Boundedness of CO-emitting Structures

We present results of a wide-field (approximately 60 x 90 pc) ALMA mosaic of CO(2-1) and $^{13}$CO(2-1) emission from the molecular cloud associated with the 30 Doradus star-forming region. Three main emission complexes, including two forming a bowtie-shaped structure extending northeast and southwest from the central R136 cluster, are resolved into complex filamentary networks. Consistent with previous studies, we find that the central region of the cloud has higher line widths at fixed size relative to the rest of the molecular cloud and to other LMC clouds, indicating an enhanced level of turbulent motions. However, there is no clear trend in gravitational boundedness (as measured by the virial parameter) with distance from R136. Structures observed in $^{13}$CO are spatially coincident with filaments and are close to a state of virial equilibrium. In contrast, CO structures vary greatly in virialization, with low CO surface brightness structures outside of the main filamentary network being predominantly unbound. The low surface brightness structures constitute ~10% of the measured CO luminosity; they may be shredded remnants of previously star-forming gas clumps, or alternatively the CO-emitting parts of more massive, CO-dark structures.

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arXiv2021

Hα Dots: Direct-Method Metal Abundances of Low-Luminosity Star-Forming Systems

Utilizing low-luminosity star-forming systems discovered in the H$α$ Dots survey, we present spectroscopic observations undertaken using the KPNO 4m telescope for twenty-six sources. With determinations of robust, "direct"-method metal abundances, we examine the properties of these dwarf systems, exploring their utility in characterizing starburst galaxies at low luminosities and stellar masses. We find that the H$α$ Dots survey provides an effective new avenue for identifying star-forming galaxies in these regimes. In addition, we examine abundance characteristics and metallicity scaling relations with these sources, highlighting a flattening of both the luminosity-metallicity ($L$-$Z$) and stellar mass-metallicity ($M_{*}$-$Z$) relation slopes in these regimes as compared with those utilizing samples covering wider respective dynamic ranges. These local, accessible analogues to the kinds of star-forming dwarfs common at high redshift will help shed light on the building blocks which assembled into the massive galaxies common today.

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arXiv2021

A comprehensive chemical abundance analysis of the extremely metal poor Leoncino Dwarf galaxy (AGC 198691)

We re-examine the extremely metal-poor (XMP) dwarf galaxy AGC 198691 using a high quality spectrum obtained by the LBT's MODS instrument. Previous spectral observations obtained from KOSMOS on the Mayall 4-m and the Blue Channel spectrograph on the MMT 6.5-m telescope did not allow for the determination of sulfur, argon, or helium abundances. We report an updated and full chemical abundance analysis for AGC 198691, including confirmation of the extremely low "direct" oxygen abundance with a value of 12 + log(O/H) = 7.06 $\pm$ 0.03. AGC 198691's low metallicity potentially makes it a high value target for helping determine the primordial helium abundance ($Y_p$). Though complicated by a Na I night sky line partially overlaying the He I $λ$5876 emission line, the LBT/MODS spectrum proved adequate for determining AGC 198691's helium abundance. We employ the recently expanded and improved model of Aver et al. (2021), incorporating higher Balmer and Paschen lines, augmented by the observation of the infrared helium emission line He I $λ$10830 obtained by Hsyu et al. (2020). Applying our full model produced a reliable helium abundance determination, consistent with the expectation for its metallicity. Although this is the lowest metallicity object with a detailed helium abundance, unfortunately, due to its faintness (EW(H$β$) $<$ 100 AA) and the compromised He I $λ$5876, the resultant uncertainty on the helium abundance is too large to allow a significant improvement on the measurement of $Y_p$.

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arXiv2020

The H$α$ Dots Survey. II. A Second List of Faint Emission-Line Objects

We present the second catalog of serendipitously discovered compact extragalactic emission-line sources -- H$α$ Dots. These objects have been discovered in searches of moderately deep narrow-band images acquired for the ALFALFA H$α$ project (Van Sistine et al. 2016). In addition to cataloging 119 new H$α$ Dots, we also present follow-up spectral data for the full sample. These spectra allow us to confirm the nature of these objects as true extragalactic emission-line objects, to classify them in terms of activity type (star forming or AGN), and to identify the emission line via which they were discovered. We tabulate photometric and spectroscopic data for the all objects, and present an overview of the properties of the full H$α$ Dot sample. The H$α$ Dots represent a broad range of star-forming and active galaxies detected via several different emission lines over a wide range of redshifts. The sample includes H$α$-detected blue compact dwarf galaxies at low redshift, [\ion{O}{3}]-detected Seyfert 2 and Green Pea-like galaxies at intermediate redshifts, and QSOs detected via one of several UV emission lines, including Ly$α$. Despite the heterogeneous appearance of the resulting catalog of objects, we show that our selection method leads to well-defined samples of specific classes of emission-line objects with properties that allow for statistical studies of each class.

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arXiv2020

A Demonstration of Extremely Low Latency $γ$-ray, X-Ray & UV Follow-Up of a Millisecond Radio Transient

We report results of a novel high-energy follow-up observation of a potential Fast Radio Burst. The radio burst was detected by VLA/realfast and followed-up by the Neil Gehrels Swift Observatory in very low latency utilizing new operational capabilities of Swift (arXiv:2005.01751), with pointed soft X-ray and UV observations beginning at T0+32 minutes, and hard X-ray/gamma-ray event data saved around T0. These observations are $>10$x faster than previous X-ray/UV follow-up of any radio transient to date. No emission is seen coincident with the FRB candidate at T0, with a 0.2s fluence $5σ$ upper limit of $1.35\times10^{-8}$ erg cm$^{-2}$ (14-195 keV) for a SGR 1935+2154-like flare, nor at T0+32 minutes down to $3σ$ upper limits of 22.18 AB mag in UVOT u band, and $3.33\times10^{-13}$ erg cm$^{-2}$ s$^{-1}$ from 0.3-10 keV for the 2 ks observation. The candidate FRB alone is not significant enough to be considered astrophysical, so this note serves as a technical demonstration. These new Swift operational capabilities will allow future FRB detections to be followed up with Swift at even lower latencies than demonstrated here: 15-20 minutes should be regularly achievable, and 5-10 minutes occasionally achievable. We encourage FRB detecting facilities to release alerts in low latency to enable this science.

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arXiv2020

Dusty Stellar Birth and Death in the Metal-Poor Galaxy NGC 6822

The nearby ($\sim$500 kpc) metal-poor ([Fe/H] $\approx$ -1.2; $Z$ $\approx$ 30% $Z_{\odot}$) star-forming galaxy NGC 6822 has a metallicity similar to systems at the epoch of peak star formation. Through identification and study of dusty and dust-producing stars, it is therefore a useful laboratory to shed light on the dust life cycle in the early Universe. We present a catalog of sources combining near- and mid-IR photometry from the United Kingdom Infrared Telescope (UKIRT; $J$, $H$, and $K$) and the $Spitzer$ $Space$ $Telescope$ (IRAC 3.6, 4.5, 5.8, and 8.0 $μ$m and MIPS 24 $μ$m). This catalog is employed to identify dusty and evolved stars in NGC 6822 utilizing three color-magnitude diagrams (CMDs). With diagnostic CMDs covering a wavelength range spanning the near- and mid-IR, we develop color cuts using kernel density estimate (KDE) techniques to identify dust-producing evolved stars, including red supergiant (RSG) and thermally-pulsing asymptotic giant branch (TP-AGB) star candidates. In total, we report 1,292 RSG candidates, 1,050 oxygen-rich AGB star candidates, and 560 carbon-rich AGB star candidates with high confidence in NGC 6822. Our analysis of the AGB stars suggests a robust population inhabiting the central stellar bar of the galaxy, with a measured global stellar metallicity of [Fe/H] = -1.286 $\pm$ 0.095, consistent with previous studies. In addition, we identify 277 young stellar object (YSO) candidates. The detection of a large number of YSO candidates within a centrally-located, compact cluster reveals the existence of an embedded, high-mass star-formation region that has eluded previous detailed study. Spitzer I appears to be younger and more active than the other prominent star-forming regions in the galaxy.

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arXiv2020

The Leoncino Dwarf Galaxy: Exploring the Low-Metallicity End of the Luminosity-Metallicity and Mass-Metallcity Relations

Extremely metal-poor (XMP) galaxies are low-mass, star-forming galaxies with gas-phase oxygen abundances below 12+log(O/H) = 7.35 (~1/20 Zsun). Galaxy evolution scenarios suggest three pathways to form an XMP: (1) secular evolution at low galaxy masses, (2) slow evolution in voids, or (3) dilution of measured abundances from infall of pristine gas. The recently discovered XMP galaxy Leoncino, with an oxygen abundance below 3% Zsun, provides an opportunity to explore these different scenarios. Using Hubble Space Telescope imaging of the resolved stellar populations of Leoncino, we measure the distance to the galaxy to be D=12.1 (+1.7/-3.4) Mpc and find that Leoncino is located in an under-dense environment. Leoncino has a compact morphology, hosts a population of young, massive stars, has a high gas-to-star mass ratio, and shows signs of interaction with a galaxy nearby on the sky, UGC 5186. Similar to nearly all XMP galaxies known in the nearby universe, Leoncino is offset from the Luminosity-Metallicity (LZ) relation. Yet, Leoncino is consistent with the stellar Mass-Metallicity (MZ) relation defined by Local Volume galaxies. Thus, our results suggest that the offset from the LZ relation is due to higher recent star formation, likely triggered by a minor interaction, while the low oxygen abundance is consistent with the expectation that low-mass galaxies will undergo secular evolution marked by inefficient star formation and metal-loss via galactic winds. This is in contrast to XMP galaxies that are outliers in both the LZ and MZ relations; in such cases, the low oxygen abundances are best explained by dilution due to the infall of pristine gas. We also discuss why quiescent XMP galaxies are underrepresented in current surveys.

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arXiv2019

Asymptotic Giant Branch Stars in the Nearby Dwarf Galaxy Leo P

We have conducted a highly sensitive census of the evolved-star population in the metal-poor dwarf galaxy Leo P and detected four asymptotic giant branch (AGB) star candidates. Leo P is one of the best examples of a nearby analog of high-redshift galaxies because of its primitive metal content (2% of the solar value), proximity, and isolated nature, ensuring a less complicated history. Using medium-band optical photometry from the Hubble Space Telescope (HST), we have classified the AGB candidates by their chemical type. We have identified one oxygen-rich source which appears to be dusty in both the HST and Spitzer observations. Its brightness, however, suggests it may be a planetary nebula or post-AGB object. We have also identified three carbon-rich candidates, one of which may be dusty. Follow-up observations are needed to confirm the nature of these sources and to study the composition of any dust that they produce. If dust is confirmed, these stars would likely be among the most metal-poor examples of dust-producing stars known and will provide valuable insight into our understanding of dust formation at high redshift.

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arXiv2019

The Young Stellar Population of the metal-poor galaxy NGC 6822

We present a comprehensive study of massive young stellar objects (YSOs) in the metal-poor galaxy NGC 6822 using IRAC and MIPS data obtained from the {\em Spitzer Space Telescope}. We find over 500 new YSO candidates in seven massive star-formation regions; these sources were selected using six colour-magnitude cuts. Via spectral energy distribution fitting to the data with YSO radiative transfer models we refine this list, identifying 105 high-confidence and 88 medium-confidence YSO candidates. For these sources we constrain their evolutionary state and estimate their physical properties. The majority of our YSO candidates are massive protostars with an accreting envelope in the initial stages of formation. We fit the mass distribution of the Stage I YSOs with a Kroupa initial mass function and determine a global star-formation rate of 0.039 $M_{\odot} yr^{-1}$. This is higher than star-formation rate estimates based on integrated UV fluxes. The new YSO candidates are preferentially located in clusters which correspond to seven active high-mass star-formation regions which are strongly correlated with the 8 and 24 $μ$m emission from PAHs and warm dust. This analysis reveals an embedded high-mass star-formation region, Spitzer I, which hosts the highest number of massive YSO candidates in NGC 6822. The properties of Spitzer I suggest it is younger and more active than the other prominent H\,{\sc ii} and star-formation regions in the galaxy.

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arXiv2018

Metal Abundances of KISS Galaxies. VI. New Metallicity Relations for the KISS Sample of Star-Forming Galaxies

We present updated metallicity relations for the spectral database of star-forming galaxies (SFGs) found in the KPNO International Spectroscopic Survey (KISS). New spectral observations of emission-line galaxies (ELGs) obtained from a variety of telescope facilities provide oxygen abundance information. A nearly four-fold increase in the number of KISS objects with robust metallicities relative to our previous analysis provides for an empirical abundance calibration to compute self-consistent metallicity estimates for all SFGs in the sample with adequate spectral data. In addition, a sophisticated spectral energy distribution (SED) fitting routine has provided robust calculations of stellar mass. With these new and/or improved galaxy characteristics, we have developed luminosity-metallicity ($L$-$Z$) relations, mass-metallicity ($M_{*}$-$Z$) relations, and the so-called Fundamental Metallicity Relation (FMR) for over 1,450 galaxies from the KISS sample. This KISS $M_{*}$-$Z$ relation is presented for the first time and demonstrates markedly lower scatter than the KISS $L$-$Z$ relation. We find that our relations agree reasonably well with previous publications, modulo modest offsets due to differences in the SEL metallicity calibrations used. We illustrate an important bias present in previous $L$-$Z$ and $M_{*}$-$Z$ studies involving direct-method ($T_{e}$) abundances that may result in systematically lower slopes in these relations. Our KISS FMR shows consistency with those found in the literature, albeit with a larger scatter. This is likely a consequence of the KISS sample being biased toward galaxies with high levels of activity.

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