Mid-infrared Spectral Energy Distributions Research Articles

Active Galactic Nuclei in a Mid-infrared Selected Galaxy Sample at z > 0.13: [Ne v]λ3426 Line Emission as a Benchmark

We present a 24 μm-selected spectroscopic sample z > 0.13 (median 〈z〉 = 0.41) in the Lockman Hole field, consisting of 4035 spectra. Our aim is to identify active galactic nuclei (AGNs) and determine their fraction in this mid-infrared-selected sample. In this work, we use the [Ne v]λ3426 emission line to spectroscopically identify AGNs. Combined with broad-line Type I AGNs selected in our previous study, our sample consists of 887 (∼22%) spectroscopically confirmed AGNs. We perform a stacking analysis on the remaining spectra and find that in various mid-infrared (MIR) wedge-selected AGN candidates, the stacked spectra still show significant [Ne v]λ3426 emission. In contrast, no clear [Ne v]λ3426 signal is detected in non-AGN candidates falling outside the wedges. Assuming a range of AGN MIR spectral energy distribution (SED) slope of −0.3 <α < 0.7, and an average star-forming relation derived from 65 star-forming templates, we develop a robust method to separate the AGN and star-forming contributions to the MIR SEDs using the rest-frame L 12/L 1.6 versus L 4.5/L 1.6 diagram. We separate the objects into bins of L 12 and find that the AGN fraction increases with increasing L 12. We also find that the stacked [Ne v]λ3426 strength scales with L 12. The pure AGN luminosity at 12 μm exhibits a positive correlation with the star formation rates, indicating possible coevolution and common gas supply between the AGNs and their host galaxies. Varying population properties across the redshift range explored contribute to the observed correlation.

The Hawaii Infrared Parallax Program. VI. The Fundamental Properties of 1000+ Ultracool Dwarfs and Planetary-mass Objects Using Optical to Mid-infrared Spectral Energy Distributions and Comparison to BT-Settl and ATMO 2020 Model Atmospheres

We derive the bolometric luminosities (L bol) of 865 field-age and 189 young ultracool dwarfs (spectral types M6–T9, including 40 new discoveries presented here) by directly integrating flux-calibrated optical to mid-infrared (MIR) spectral energy distributions (SEDs). The SEDs consist of low-resolution (R ∼ 150) near-infrared (NIR; 0.8–2.5μm) spectra (including new spectra for 97 objects), optical photometry from the Pan-STARRS1 survey, and MIR photometry from the CatWISE2020 survey and Spitzer/IRAC. Our L bol calculations benefit from recent advances in parallaxes from Gaia, Spitzer, and UKIRT, as well as new parallaxes for 19 objects from CFHT and Pan-STARRS1 presented here. Coupling our L bol measurements with a new uniform age analysis for all objects, we estimate substellar masses, radii, surface gravities, and effective temperatures (T eff) using evolutionary models. We construct empirical relationships for L bol and T eff as functions of spectral type and absolute magnitude, determine bolometric corrections in optical and infrared bandpasses, and study the correlation between evolutionary model-derived surface gravities and NIR gravity classes. Our sample enables a detailed characterization of BT-Settl and ATMO 2020 atmospheric model systematics as a function of spectral type and position in the NIR color–magnitude diagram. We find the greatest discrepancies between atmospheric and evolutionary model-derived T eff (up to 800 K) and radii (up to 2.0 R Jup) at the M/L spectral type transition boundary. With 1054 objects, this work constitutes the largest sample to date of ultracool dwarfs with determinations of their fundamental parameters.

Water in the terrestrial planet-forming zone of the PDS 70 disk.

Terrestrial and sub-Neptune planets are expected to form in the inner (less than 10 AU) regions of protoplanetary disks1. Water plays a key role in their formation2-4, although it is yet unclear whether water molecules are formed in situ or transported from the outer disk5,6. So far Spitzer Space Telescope observations have only provided water luminosity upper limits for dust-depleted inner disks7, similar to PDS 70, the first system with direct confirmation of protoplanet presence8,9. Here we report JWST observations of PDS 70, a benchmark target to search for water in a disk hosting a large (approximately 54 AU) planet-carved gap separating an inner and outer disk10,11. Our findings show water in the inner disk of PDS 70. This implies that potential terrestrial planets forming therein have access to a water reservoir. The column densities of water vapour suggest in-situ formation via a reaction sequence involving O, H2 and/or OH, and survival through water self-shielding5. This is also supported by the presence of CO2 emission, another molecule sensitive to ultraviolet photodissociation. Dust shielding, and replenishment of both gas and small dust from the outer disk, may also play a role in sustaining the water reservoir12. Our observations also reveal a strong variability of the mid-infrared spectral energy distribution, pointing to a change of inner disk geometry.

Metallicity–PAH Relation of MIR-selected Star-forming Galaxies in AKARI North Ecliptic Pole-wide Survey

We investigate the variation in the mid-infrared spectral energy distributions of 373 low-redshift (z < 0.4) star-forming galaxies, which reflects a variety of polycyclic aromatic hydrocarbon (PAH) emission features. The relative strength of PAH emission is parameterized as q PAH, which is defined as the mass fraction of PAH particles in the total dust mass. With the aid of continuous mid-infrared photometric data points covering 7–24 μm and far-infrared flux densities, q PAH values are derived through spectral energy distribution fitting. The correlation between q PAH and other physical properties of galaxies, i.e., gas-phase metallicity (), stellar mass, and specific star-formation rate (sSFR) are explored. As in previous studies, q PAH values of galaxies with high metallicity are found to be higher than those with low metallicity. The strength of PAH emission is also positively correlated with the stellar mass and negatively correlated with the sSFR. The correlation between q PAH and each parameter still exists even after the other two parameters are fixed. In addition to the PAH strength, the application of metallicity-dependent gas-to-dust mass ratio appears to work well to estimate gas mass that matches the observed relationship between molecular gas and physical parameters. The result obtained will be used to calibrate the observed PAH luminosity-total infrared luminosity relation, based on the variation of MIR-FIR SED, which is used in the estimation of hidden star formation.

Multiwavelength Analysis of a Nearby Heavily Obscured AGN in NGC 449

We present the multiwavelength analysis of a heavily obscured active galactic nucleus (AGN) in NGC 449. We first constructed a broadband X-ray spectrum using the latest NuSTAR and XMM-Newton data. Its column density (≃1024 cm−2) and photon index (Γ ≃ 2.4) were reliably obtained by analyzing the broadband X-ray spectrum. However, the scattering fraction and the intrinsic X-ray luminosity could not be well constrained. Combined with the information obtained from the mid-infrared spectrum and spectral energy distribution fitting, we derived its intrinsic X-ray luminosity (≃8.54 × 1042 erg s−1) and scattering fraction (f scat ≃ 0.26%). In addition, we also derived the following results. (1) The mass accretion rate of the central AGN is about 2.54 × 10−2 M ⊙ yr−1, and the Eddington ratio is 8.39 × 10−2. (2) The torus of this AGN has a high gas-to-dust ratio (N H/A V = 8.40 × 1022 cm−2 mag−1). (3) The host galaxy and central AGN are both in the early stage of coevolution.

The stellar populations of quiescent ultra-diffuse galaxies from optical to mid-infrared spectral energy distribution fitting

ABSTRACT We use spectral energy distribution (SED) fitting to place constraints on the stellar population properties of 29 quiescent ultra-diffuse galaxies (UDGs) across different environments. We use the fully Bayesian routine PROSPECTOR coupled with archival data in the optical, near, and mid-infrared from Spitzer and Wide-field Infrared Survey Explorer under the assumption of an exponentially declining star formation history. We recover the stellar mass, age, metallicity, dust content, star formation time scales, and photometric redshifts (photo-zs) of the UDGs studied. Using the mid-infrared data, we probe the existence of dust in UDGs. Although its presence cannot be confirmed, we find that the inclusion of small amounts of dust in the models brings the stellar populations closer to those reported with spectroscopy. Additionally, we fit the redshifts of all galaxies. We find a high accuracy in recovering photo-zs compared to spectroscopy, allowing us to provide new photo-z estimates for three field UDGs with unknown distances. We find evidence of a stellar population dependence on the environment, with quiescent field UDGs being systematically younger than their cluster counterparts. Lastly, we find that all UDGs lie below the mass–metallicity relation for normal dwarf galaxies. Particularly, the globular cluster (GC)-poor UDGs are consistently more metal-rich than GC-rich ones, suggesting that GC-poor UDGs may be puffed-up dwarfs, while most GC-rich UDGs are better explained by a failed galaxy scenario. As a byproduct, we show that two galaxies in our sample, NGC 1052-DF2 and NGC 1052-DF4, share equivalent stellar population properties, with ages consistent with 8 Gyr. This finding supports formation scenarios where the galaxies were formed together.

A multiwavelength study of star formation in nearby galaxies: evidence for inside-out growth of the stellar disc

ABSTRACT Global, radial, and local photometric measurements of several nearby low-mass galaxies are presented. Multiwavelength observations are used to trace the distributions of old and young stars, with the help of far-ultraviolet (FUV) to mid-infrared spectral energy distributions to derive star formation histories (SFHs). In addition to the surface brightness profiles, catalogues of UV-selected sources are created to better understand regions of galaxies with recent star formation. The clustering of these UV sources are studied in the dwarf irregular UGC 08188 (IC 4182) using a two-point correlation function. We find that the reddest of these UV sources are not clustered on any scale, but the bluer sources are clustered on scales approximately less than 1 kpc. We also find a possible negative radial gradient in the SFH parameter that marks the beginning of the younger stellar population’s formation (tburst) when analysing the entire sample of 3165 UV sources found across a total of 34 galaxies, giving quantitative evidence for the inside-out growth of galactic discs – a natural consequence of galaxy evolution in a Λ cold dark matter universe.

Passive spiral galaxies deeply captured by Subaru Hyper Suprime-Cam

Abstract This paper presents a thousand passive spiral galaxy samples at z = 0.01–0.3 based on a combined analysis of the Third Public Data Release of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP PDR3) and the GALEX–SDSS–WISE Legacy Catalog (GSWLC-2). Among 54871 gri galaxy cutouts taken from the HSC-SSP PDR3 over 1072 deg2, we conducted a search with deep-learning morphological classification for candidates of passive spirals below the star-forming main sequence derived by ultraviolet to mid-infrared spectral energy distribution fitting in the GSWLC-2. We then classified the candidates into 1100 passive spirals and 1141 secondary samples based on visual inspections. Most of the latter cases are considered to be passive ringed S0 or pseudo-ringed galaxies. The remaining secondary samples have ambiguous morphologies, including two peculiar objects with diamond-shaped stellar wings. The selected passive spirals have a similar distribution to the general quiescent galaxies on the EWHδ–Dn4000 diagram and concentration indices. Moreover, we detected an enhanced passive fraction of spiral galaxies in X-ray clusters. Passive spirals in galaxy clusters are preferentially located in the midterm or late infall phase on the phase–space diagram, supporting the ram pressure scenario, which has been widely advocated in previous studies. The source catalog and gri-composite images are available on the HSC-SSP PDR3 website 〈https://hsc.mtk.nao.ac.jp/ssp/data-release/〉. Future updates, including integration with a citizen science project dedicated to the HSC data, will achieve more effective and comprehensive classifications.

Dust models for the extinction of Type IIn supernova SN 2010jl

ABSTRACT The unusual extinction curves of SN 2010jl provide an excellent opportunity to investigate the properties of dust formed by core-collapse supernovae. By using a series of dust models with different compositions and grain size distributions, we fit the extinction curves of SN 2010jl and find that a silicate–graphite mixture dust model characterized by exponentially cut-off power-law size distributions can well reproduce its unusual extinction curves. The best-fitting results show that the extinctions derived from the dust models are consistent with the observed values at all epochs. However, the total-to-selective extinction ratio RV is ∼2.8–3.1, which is significantly smaller than the value of RV ≈ 6.4 derived by Gall et al. The best-fitting models indicate that the dust grains around SN 2010jl are possibly composed of small-sized astronomical silicate grains and micron-sized graphite grains. In addition, by fitting the optical to mid-infrared spectral energy distribution, we find that the dust mass around SN 2010jl increases with time, up to $0.005\, \mathrm{ M}_{\odot }$ around 1300 d after peak brightness, which is consistent with previous estimates (Gall et al.; Sarangi et al.).

Radio emission from dust-obscured galaxies

ABSTRACT The coevolution of galaxies and their central supermassive black holes is a subject of intense research. A class of objects, the dust-obscured galaxies (DOGs) are particularly interesting in this respect as they are thought to represent a short evolutionary phase when violent star formation activity in the host galaxy may coexist with matter accretion on to the black hole powering the active nucleus. Here, we investigate different types of DOGs classified by their mid-infrared spectral energy distributions to reveal whether they can be distinguished by their arcsec-scale radio properties. Radio emission is unaffected by dust obscuration and may originate from both star formation and an active nucleus. We analyse a large sample of 661 DOGs complied from the literature and find that only a small fraction of them (∼2 per cent) are detected with flux densities exceeding ∼1 mJy in the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. These radio-detected objects are almost exclusively ‘power-law’ DOGs. Stacking analysis of the FIRST image cutouts centred on the positions of individually radio-undetected sources suggests that weak radio emission is present in ‘power-law’ DOGs. On the other hand, radio emission from ‘bump’ DOGs is only marginally detected in the median-stacked FIRST image.

A multiwavelength study of star formation in 15 local star-forming galaxies

ABSTRACT We have fit the far-ultraviolet to mid-infrared spectral energy distributions (SEDs) for several nearby galaxies (&amp;lt;20 Mpc). Global, radial, and local photometric measurements are explored to better understand how SED-derived star formation histories (SFHs) and classic star formation rate tracers manifest at different scales. Surface brightness profiles and radial SED fitting provide insight into stellar population gradients in stellar discs and haloes. A double exponential SFH model is used in the SED fitting to better understand the distributions of young versus old populations throughout these galaxies. Different regions of a galaxy often have undergone very different SFHs, either in strength, rate, timing, or some combination of all these factors. An analysis of individual stellar complexes within these galaxies shows a relationship between the ages of stellar clusters and how these clusters are distributed throughout the galaxy. These star formation properties are presented alongside previously published H i observations to provide a holistic picture of a small sample of nearby star-forming galaxies. The results presented here show that there is a wide variety of star formation gradients and average stellar age distributions that can manifest in a ΛCDM universe.

A Complete 16 μm Selected Galaxy Sample at z ∼ 1: Mid-infrared Spectral Energy Distributions

Abstract We describe a complete, flux-density-limited sample of galaxies at redshift 0.8 &lt; z &lt; 1.3 selected at 16 μm. At the selection wavelength near 8 μm rest, the observed emission comes from both dust heated by intense star formation and active galactic nuclei (AGNs). Fitting the spectral energy distributions (SEDs) of the sample galaxies to local-galaxy templates reveals that more than half the galaxies have SEDs dominated by star formation. About one-sixth of the galaxy SEDs are dominated by an AGN, and nearly all of the rest of the SEDs are composite. Comparison with X-ray and far-infrared observations shows that combinations of luminosities at rest-frame 4.5 and 8 μm give good measures of both AGN luminosity and star formation rate. The sample galaxies mostly follow the established star-forming main sequence for z = 1 galaxies, but of the galaxies more than 0.5 dex above that main sequence, more than half have AGN-type SEDs. Similarly, the most luminous AGNs tend to have higher star formation rates than the main-sequence value. Galaxies with stellar masses &gt;1011 M ⊙ are unlikely to host an AGN. About 1% of the sample galaxies show an SED with dust emission typical of neither star formation nor an AGN.

Infrared colours and spectral energy distributions of hard X-ray selected obscured and Compton-thick active galactic nuclei

ABSTRACT We investigate the infrared colours and spectral energy distributions (SEDs) of 338 X-ray selected active galactic nuclei (AGNs) from the Swift/Burst Alert Telescope (BAT) 105-month survey catalogue, which have been detected using AKARI, in order to find new selection criteria for Compton-thick AGNs. By combining data from the Galaxy Evolution Explorer (GALEX), the Sloan Digital Sky Survey (SDSS) Data Release 14 (DR14), the Two-Micron All Sky Survey (2MASS), the Wide-field Infrared Survey Explorer (WISE), AKARI and Herschel for the first time, we perform ultraviolet (UV) to far-infrared (FIR) SEDs, fitting 158 Swift/BAT AGNs using cigale and constraining the AGN model parameters of obscured and Compton-thick AGNs. The comparison of average SEDs shows that while the mid-infrared (MIR) SEDs are similar for the three AGN populations, the optical/UV and FIR regions have differences. We measure the dust luminosity, the pure AGN luminosity and the total infrared luminosity. We examine the relationships between the measured infrared luminosities and the hard X-ray luminosity in the 14–195 keV band. We show that the average covering factor of Compton-thick AGNs is higher compared with the obscured and unobscured AGNs. We present new infrared selection criteria for Compton-thick AGNs based on MIR and FIR colours ([9–22 μm] &amp;gt; 3.0 and [22–90 μm] &amp;lt; 2.7) from WISE and AKARI. We find two known Compton-thick AGNs that are not included in the Swift/BAT sample. We conclude that MIR colours covering 9.7-μm silicate absorption and the MIR continuum could be promising new tools to identify Compton-thick AGNs.

Intermediate-mass Early-type Disk Galaxies in the Virgo Cluster. II. Near-Infrared Spectra and Evidence for Differences in Evolution* † ‡ † ‡

Abstract We discuss near-infrared (NIR) slit spectra of six early-type disk galaxies in the Virgo Cluster that were examined previously at visible/red wavelengths. Despite having similar intrinsic luminosities, colors, and morphologies, the NIR spectrophotometric properties of these galaxies indicate that they are not a homogeneous ensemble differing only in terms of luminosity-weighted age and metallicity. While the depth of the CO(2,0) band is consistent with the centers of these galaxies having solar or slightly sub-solar luminosity-weighted metallicities, galaxy-to-galaxy variations in the depth of the Na i 2.21 μm doublet are found, with Na i2.21 μm lines in three galaxies (NGC 4491, NGC 4584, and NGC 4620) that are deeper than those predicted for a solar chemical mixture and a solar-neighborhood mass function. In contrast, the Ca i2.26 μm triplet shows good galaxy-to-galaxy agreement but tends to be deeper than the model prediction. Considering the depth of the NaD lines, the deep Na i2.21 μm lines are tentatively attributed to a bottom-heavy mass function. This is counter to observed trends between mass function slope and velocity dispersion, and so the possibility of a super-solar [Na/Fe] is also discussed. Two of the three galaxies with deep Na i 2.21 μm (NGC 4584 and NGC 4620) have Sérsic exponents that are consistent with a classical bulge. As for NGC 4491, its central NIR spectrum contains prominent emission lines. The relative strengths of Brγ and H2S(1), the presence of [Fe ii] emission, and the mid-infrared spectral-energy distribution are all consistent with intense recent star formation near the center of that galaxy. The NIR spectrum of NGC 4584 is devoid of line emission in the NIR, suggesting that star formation does not power the emission detected at visible wavelengths from that galaxy. Wavelengths that contain the Ballick–Ramsey C2 band at 1.76 μm are matched by moderately metal-poor E-MILES model spectra with an age of 2 Gyr. The radial age trends in these galaxies are in the opposite sense to those in late-type disk galaxies, and it is concluded that they have evolved in a cluster environment for at least several Gyr.

A NuSTAR and XMM-Newton Study of the Two Most Actively Star-forming Green Pea Galaxies (SDSS J0749+3337 and SDSS J0822+2241)

Abstract We explore X-ray evidence for the presence of active galactic nuclei (AGNs) in the two most actively star-forming Green Pea galaxies (GPs), SDSS J0749+3337 and SDSS J0822+2241, which have star formation rates (SFRs) of 123 M ⊙ yr−1 and 78 M ⊙ yr−1, respectively. The GPs have red mid-infrared (MIR) spectral energy distributions and higher 22 μm luminosities than expected from a proxy of the SFR (Hα luminosity), consistent with hosting AGNs with 2–10 keV luminosities of ∼1044 erg s−1. We thus obtain and analyze the first hard (&gt;10 keV) X-ray data observed with NuSTAR and archival XMM-Newton data below 10 keV. From the NuSTAR ≈20 ks data, however, we find no significant hard X-ray emission. By contrast, soft X-ray emission with 0.5–8 keV luminosities of ≈1042 erg s−1 is significantly detected in both targets, which can be explained only by star formation (SF). A possible reason for the lack of clear evidence is that a putative AGN torus absorbs most of the X-ray emission. Applying a smooth-density AGN torus model, we determine minimum hydrogen column densities along the equatorial plane ( ) consistent with the nondetection. The results indicate cm−2 for SDSS J0749+3337 and cm−2 for SDSS J0822+2241. Therefore, the GPs may host such heavily obscured AGNs. Otherwise, no AGN exists and the MIR emission is ascribed to SF. Active SF in low-mass galaxies is indeed suggested to reproduce red MIR colors. This would imply that diagnostics based on MIR photometry data alone may misidentify such galaxies as AGNs.

Does AGN fraction depend on redshift or luminosity? An extinction-free test by 18-band near- to mid-infrared SED fitting in the AKARI NEP wide field

Abstract Revealing what fraction of galaxies harbor active galactic nuclei (AGNs) is central to understanding the black hole accretion history of the universe. However, optical and soft X-ray surveys miss the most highly obscured AGNs. Infrared (IR), instead, is more robust against absorption. Previous IR photometric surveys, however, only had four or five filters in mid-IR. Our AKARI North Ecliptic Pole wide field sample has 18 filters in mid-IR (nine from AKARI, four from WISE, and five from Spitzer), for the first time allowing a sophisticated mid-IR spectral energy distribution (SED) fitting diagnosis for a statistical number of sources (89178 over 5.4 deg2). By using an SED fitting technique, we investigated the evolution of AGN fraction as a function of redshift and IR (8–1000 μm) luminosity in an extinction-free way. We found that the AGN fraction (FAGN) shows no sign of strong redshift evolution. Instead, FAGN increases with increasing IR luminosity in all redshift bins (0 &amp;lt; $z$ &amp;lt; 2).

Extreme submillimetre starburst galaxies

We have used two catalogues, a Herschel catalogue selected at 500 μm (HerMES) and an IRAS catalogue selected at 60 μm (RIFSCz), to contrast the sky at these two wavelengths. Both surveys demonstrate the existence of “extreme” starbursts, with star-formation rates (SFRs) &gt; 5000 M⊙ yr−1. The maximum intrinsic star-formation rate appears to be ~30 000 M⊙ yr−1. The sources with apparent SFR estimates higher than this are in all cases either lensed systems, blazars, or erroneous photometric redshifts. At redshifts between three and five, the time-scale for the Herschel galaxies to make their current mass of stars at their present rate of star formation is ~108 yr, so these galaxies are making a significant fraction of their stars in the current star-formation episode. Using dust mass as a proxy for gas mass, the Herschel galaxies at redshift three to five have gas masses comparable to their mass in stars. Of the 38 extreme starbursts in our Herschel survey for which we have more complete spectral energy distribution (SED) information, 50% show evidence for QSO-like optical emission, or exhibit AGN dust tori in the mid-infrared SEDs. In all cases however the infrared luminosity is dominated by a starburst component. We derive a mean covering factor for AGN dust as a function of redshift and derive black hole masses and black hole accretion rates. There is a universal ratio of black-hole mass to stellar mass in these high redshift systems of ~10−3, driven by the strong period of star-formation and black-hole growth at z = 1−5.

A Study of Two Diffuse Dwarf Galaxies in the Field

Abstract We present optical long-slit spectroscopy and far-ultraviolet to mid-infrared spectral energy distribution fitting of two diffuse dwarf galaxies, LSBG-285 and LSBG-750, which were recently discovered by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We measure redshifts using Hα line emission and find that these galaxies are at comoving distances of ≈25 and ≈41 Mpc, respectively, after correcting for the local velocity field. They have effective radii of r eff = 1.2 and 1.8 kpc and stellar masses of M ⋆ ≈ (2–3) × 107 M ⊙. There are no massive galaxies ( ) within a comoving separation of at least 1.5 Mpc from LSBG-285 and 2 Mpc from LSBG-750. These sources are similar in size and surface brightness to ultradiffuse galaxies, except they are isolated, star-forming objects that were optically selected in an environmentally blind survey. Both galaxies likely have low stellar metallicities [Z ⋆/Z ⊙] &lt; −1.0 and are consistent with the stellar mass–metallicity relation for dwarf galaxies. We set an upper limit on LSBG-750's rotational velocity of ∼50 km s−1, which is comparable to dwarf galaxies of similar stellar mass with estimated halo masses &lt;1011 M ⊙. We find tentative evidence that the gas-phase metallicities in both of these diffuse systems are high for their stellar mass, though a statistically complete, optically selected galaxy sample at very low surface brightness will be necessary to place these results into context with the higher surface brightness galaxy population.

AKARI mid-infrared slitless spectroscopic survey of star-forming galaxies at z ≲ 0.5

Context. Deep mid-infrared (MIR) surveys have revealed numerous strongly star-forming galaxies at redshift z ≲ 2. Their MIR fluxes are produced by a combination of continuum and polycyclic aromatic hydrocarbon (PAH) emission features. The PAH features can dominate the total MIR flux, but are difficult to measure without spectroscopy. Aims. We aim to study star-forming galaxies by using a blind spectroscopic survey at MIR wavelengths to understand evolution of their star formation rate (SFR) and specific SFR (SFR per stellar mass) up to z ≃ 0.5, by paying particular attention to their PAH properties. Methods. We conducted a low-resolution (R ≃ 50) slitless spectroscopic survey at 5–13 μm of 9 μm flux-selected sources (&gt;0.3 mJy) around the north ecliptic pole with the infrared camera (IRC) onboard AKARI. After removing 11 AGN candidates by using the IRC photometry, we identify 48 PAH galaxies with PAH 6.2, 7.7, and 8.6 μm features at z &lt; 0.5. The rest-frame optical–MIR spectral energy distributions (SEDs) based on CFHT and IRC imaging covering 0.37–18 μm were produced, and analysed in conjunction with the PAH spectroscopy. We defined the PAH enhancement by using the luminosity ratio of the 7.7 μm PAH feature over the 3.5 μm stellar component of the SEDs. Results. The rest-frame SEDs of all PAH galaxies have a universal shape with stellar and 7.7 μm bumps, except that the PAH enhancement significantly varies as a function of the PAH luminosities. We identify a PAH-enhanced population at z ≳ 0.35, whose SEDs and luminosities are typical of luminous infrared galaxies. They show particularly larger PAH enhancement at high luminosity, implying that they are vigorous star-forming galaxies with elevated specific SFR. Our composite starburst model that combines a very young and optically very thick starburst with a very old population can successfully reproduce most of their SED characteristics, although we cannot confirm this optically think component from our spectral analysis.

Star-forming Galaxies as AGN Imposters? A Theoretical Investigation of the Mid-infrared Colors of AGNs and Extreme Starbursts

Abstract We conduct for the first time a theoretical investigation of the mid-infrared spectral energy distribution (SED) produced by dust heated by an active galactic nucleus (AGN) and an extreme starburst. These models employ an integrated modeling approach using photoionization and stellar population synthesis models in which both the line and emergent continuum is predicted from gas exposed to the ionizing radiation from a young starburst and an AGN. In this work, we focus on the infrared colors from the Wide-field Infrared Survey Explorer, predicting the dependence of the colors on the input radiation field, the interstellar medium conditions, the obscuring column, and the metallicity. We find that an extreme starburst can mimic an AGN in two band mid-infrared color cuts employed in the literature. However, the three-band color cuts employed in the literature require starbursts with extremely high ionization parameters or gas densities. We show that the extreme mid-infrared colors seen in some blue compact dwarf galaxies are not due to metallicity but rather a combination of high ionization parameters and high column densities. Based on our theoretical calculations, we present a theoretical mid-infrared color cut that will exclude even the most extreme starburst that we have modeled in this work. The theoretical AGN demarcation region presented here can be used to identify elusive AGN candidates for future follow-up studies with the James Webb Space Telescope. The full suite of simulated SEDs are available online.