The traditional picture of post-starburst galaxies as dust- and gas-poor merger remnants, rapidly transitioning to quiescence, has been recently challenged. Unexpected detections of a significant ...interstellar medium (ISM) in many post-starburst galaxies raise important questions. Are they truly quiescent, and if so, what mechanisms inhibit further star formation? What processes dominate their ISM energetics? We present an infrared spectroscopic and photometric survey of 33 E+A post-starbursts selected by the Sloan Digital Sky Survey, aimed at resolving these questions. We find compact, warm dust reservoirs with high PAH abundances and total gas and dust masses significantly higher than expected from stellar recycling alone. Both polycyclic aromatic hydrocarbon (PAH)/total infrared (TIR) and dust-to-burst stellar mass ratios are seen to decrease with post-burst age, indicative of the accumulating effects of dust destruction and an incipient transition to hot, early-type ISM properties. Their infrared spectral properties are unique, with dominant PAH emission, very weak nebular lines, unusually strong H2 rotational emission, and deep C ii deficits. There is substantial scatter among star formation rate (SFR) indicators, and both PAH and TIR luminosities provide overestimates. Even as potential upper limits, all tracers show that the SFR has typically experienced a decline of more than two orders of magnitude since the starburst and that the SFR is considerably lower than expected given both their stellar masses and molecular gas densities. These results paint a coherent picture of systems in which star formation was, indeed, rapidly truncated, but in which the ISM was not completely expelled, and is instead supported against collapse by latent or continued injection of turbulent or mechanical heating. The resulting aging burst populations provide a "high-soft" radiation field that seemingly dominates the E+A galaxies' unusual ISM energetics.
Abstract
We present the discovery of a candidate ultra-faint Milky-Way satellite, Eridanus IV (DELVE J0505−0931), detected in photometric data from the DECam Local Volume Exploration survey (DELVE). ...Eridanus IV is a faint (
M
V
= − 4.7 ± 0.2), extended (
r
1
/
2
=
75
−
13
+
16
pc
), and elliptical (
ϵ
= 0.54 ± 0.1) system at a heliocentric distance of
76.7
−
6.1
+
4.0
kpc
, with a stellar population that is well described by an old, metal-poor isochrone (age of
τ
∼ 13.0 Gyr and metallicity of Fe/H ≲ − 2.1 dex). These properties are consistent with the known population of ultra-faint Milky-Way satellite galaxies. Eridanus IV is also prominently detected using proper-motion measurements from Gaia Early Data Release 3, with a systemic proper motion of
(
μ
α
cos
δ
,
μ
δ
)
=
(
+
0.25
±
0.06
,
−
0.10
±
0.05
)
mas yr
−1
measured from its horizontal branch and red-giant-branch member stars. We find that the spatial distribution of likely member stars hints at the possibility that the system is undergoing tidal disruption.
We report results from a systematic wide-area search for faint dwarf galaxies at heliocentric distances from 0.3 to 2 Mpc using the full six years of data from the Dark Energy Survey (DES). Unlike ...previous searches over the DES data, this search specifically targeted a field population of faint galaxies located beyond the Milky Way virial radius. We derive our detection efficiency for faint, resolved dwarf galaxies in the Local Volume with a set of synthetic galaxies and expect our search to be complete to \(M_V\) ~ \((-7, -10)\) mag for galaxies at \(D = (0.3, 2.0)\) Mpc respectively. We find no new field dwarfs in the DES footprint, but we report the discovery of one high-significance candidate dwarf galaxy at a distance of \(2.2\substack{+0.05\\-0.12}\) Mpc, a potential satellite of the Local Volume galaxy NGC 55, separated by \(47\) arcmin (physical separation as small as 30 kpc). We estimate this dwarf galaxy to have an absolute V-band magnitude of \(-8.0\substack{+0.5\\-0.3}\) mag and an azimuthally averaged physical half-light radius of \(2.2\substack{+0.5\\-0.4}\) kpc, making this one of the lowest surface brightness galaxies ever found with \(\mu = 32.3\) mag \({\rm arcsec}^{-2}\). This is the largest, most diffuse galaxy known at this luminosity, suggesting possible tidal interactions with its host.
We present the discovery of a candidate ultra-faint Milky Way satellite, Eridanus IV (DELVE J0505\(-\)0931), detected in photometric data from the DECam Local Volume Exploration survey (DELVE). ...Eridanus IV is a faint (\(M_V = -4.7 \pm 0.2\)), extended (\(r_{1/2} = 75^{+16}_{-13}\) pc), and elliptical (\(\epsilon = 0.54 \pm 0.1\)) system at a heliocentric distance of \(76.7^{+4.0}_{-6.1}\) kpc, with a stellar population that is well-described by an old, metal-poor isochrone (age of \(\tau \sim 13.0\) Gyr and metallicity of \({\rm Fe/H \lesssim -2.1}\) dex). These properties are consistent with the known population of ultra-faint Milky Way satellite galaxies. Eridanus IV is also prominently detected using proper motion measurements from Gaia Early Data Release 3, with a systemic proper motion of \((\mu_{\alpha} \cos \delta, \mu_{\delta}) = (+0.25 \pm 0.06, -0.10 \pm 0.05)\) mas yr\(^{-1}\) measured from its horizontal branch and red giant branch member stars. We find that the spatial distribution of likely member stars hints at the possibility that the system is undergoing tidal disruption.
The DECam Local Volume Exploration survey (DELVE) is a 126-night survey program on the 4-m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. DELVE seeks to understand the ...characteristics of faint satellite galaxies and other resolved stellar substructures over a range of environments in the Local Volume. DELVE will combine new DECam observations with archival DECam data to cover ~15000 deg\(^2\) of high-Galactic-latitude (|b| > 10 deg) southern sky to a 5\(\sigma\) depth of g,r,i,z ~ 23.5 mag. In addition, DELVE will cover a region of ~2200 deg\(^2\) around the Magellanic Clouds to a depth of g,r,i ~ 24.5 mag and an area of ~135 deg\(^2\) around four Magellanic analogs to a depth of g,i ~ 25.5 mag. Here, we present an overview of the DELVE program and progress to date. We also summarize the first DELVE public data release (DELVE DR1), which provides point-source and automatic aperture photometry for ~520 million astronomical sources covering ~5000 deg\(^2\) of the southern sky to a 5\(\sigma\) point-source depth of g=24.3, r=23.9, i=23.3, and z=22.8 mag. DELVE DR1 is publicly available via the NOIRLab Astro Data Lab science platform.
The traditional picture of post-starburst galaxies as dust- and gas-poor merger remnants, rapidly transitioning to quiescence, has been recently challenged. Unexpected detections of a significant ISM ...in many post-starbursts raise important questions. Are they truly quiescent and, if so, what mechanisms inhibit further star formation? What processes dominate their ISM energetics? We present an infrared spectroscopic and photometric survey of 33 SDSS-selected E+A post-starbursts, aimed at resolving these questions. We find compact, warm dust reservoirs with high PAH abundances, and total gas and dust masses significantly higher than expected from stellar recycling alone. Both PAH/TIR and dust-to-burst stellar mass ratios are seen to decrease with post-burst age, indicative of the accumulating effects of dust destruction and an incipient transition to hot, early-type ISM properties. Their infrared spectral properties are unique, with dominant PAH emission, very weak nebular lines, unusually strong H\(_{2}\) rotational emission, and deep \({\rm C\, II}\) deficits. There is substantial scatter among SFR indicators, and both PAH and TIR luminosities provide overestimates. Even as potential upper limits, all tracers show that the SFR has typically experienced a more than two order-of-magnitude decline since the starburst, and that the SFR is considerably lower than expected given both their stellar masses and molecular gas densities. These results paint a coherent picture of systems in which star formation was, indeed, rapidly truncated, but in which the ISM was \(\textit{not}\) completely expelled, and is instead supported against collapse by latent or continued injection of turbulent or mechanical heating. The resulting aging burst populations provide a "high-soft" radiation field which seemingly dominates the E+As' unusual ISM energetics.
Free-living nitrogen fixation (FLNF) in the rhizosphere, or N fixation by heterotrophic bacteria living on/near root surfaces, is ubiquitous and a significant source of N in some terrestrial systems. ...FLNF is also of interest in crop production as an alternative to chemical fertilizer, potentially reducing production costs and ameliorating negative environmental impacts of fertilizer N additions. Despite this interest, a mechanistic understanding of controls (e.g., carbon, oxygen, nitrogen, and nutrient availability) on FLNF in the rhizosphere is lacking but necessary. FLNF is distinct from and occurs under more diverse and dynamic conditions than symbiotic N fixation; therefore, predicting FLNF rates and understanding controls on FLNF has proven difficult. This has led to large gaps in our understanding of FLNF, and studies aimed at identifying controls on FLNF are needed. Here, we provide a mechanistic overview of FLNF, including how various controls may influence FLNF in the rhizosphere in comparison with symbiotic N fixation occurring in plant nodules where environmental conditions are moderated by the plant. We apply this knowledge to a real-world example, the bioenergy crop switchgrass (
), to provide context of how FLNF may function in a managed system. We also highlight future challenges to assessing FLNF and understanding how FLNF functions in the environment and significantly contributes to plant N availability and productivity.
Soil microbes release a variety of aminopeptidase enzymes to degrade proteins into their constituent amino acid monomers, thereby increasing amino acid availability in the near-cell environment. We ...investigated how inorganic N availability controls the substrate induction response of aminopeptidase enzymes by conducting protein amendment experiments via gelatin addition to fertilized and unfertilized soil from a long-term N addition study. Protein addition triggered a measurable increase in enzyme activity, which we interpreted as an induction response, in all five of the aminopeptidase enzymes we investigated. The magnitude of these induction responses did not differ by fertilization regime even when unamended aminopeptidase activity differed between long-term fertilization treatments. Though soil microbes are thought to release aminopeptidase enzymes as a means of cellular N acquisition at steady-state concentrations, our results suggest that they use the same enzymes to access protein-derived C as an energetic resource at non-steady-state conditions, such as those triggered by the substrate induction response. The substrate induction response of aminopeptidase enzymes is therefore best viewed as a means of microbial competition for the substantial energetic resources available in protein subsidies.
•We investigated controls on five different soil aminopeptidase enzymes.•Long term fertilization repressed activity for some aminopeptidase enzymes.•Gelatin addition caused induction responses in all aminopeptidase enzymes.•Induction responses were of equal magnitude in fertilized and unfertilized soil.•Microbes use protein subsidies as energy sources regardless of N availability.
ABSTRACT
Soil microorganisms play a key role in driving major biogeochemical cycles and in global responses to climate change. However, understanding and predicting the behavior and function of these ...microorganisms remains a grand challenge for soil ecology due in part to the microscale complexity of soils. It is becoming increasingly clear that understanding the microbial perspective is vital to accurately predicting global processes. Here, we discuss the microbial perspective including the microbial habitat as it relates to measurement and modeling of ecosystem processes. We argue that clearly defining and quantifying the size, distribution and sphere of influence of microhabitats is crucial to managing microbial activity at the ecosystem scale. This can be achieved using controlled and hierarchical sampling designs. Model microbial systems can provide key data needed to integrate microhabitats into ecosystem models, while adapting soil sampling schemes and statistical methods can allow us to collect microbially-focused data. Quantifying soil processes, like biogeochemical cycles, from a microbial perspective will allow us to more accurately predict soil functions and address long-standing unknowns in soil ecology.
The microbial perspective, including microscale microbial interactions, is a crucial component of the soil environment that must be considered when measuring soil processes and developing predictive ecosystem models.
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