Emission-line Diagnostic Diagrams Research Articles

MAMBO: An empirical galaxy and AGN mock catalogue for the exploitation of future surveys

Current and future large surveys will produce unprecedented amounts of data. Realistic simulations have become essential for the design and development of these surveys, as well as for the interpretation of the results. We present MAMBO, a flexible and efficient workflow to build empirical galaxy and active galactic nucleus (AGN) mock catalogues that reproduce the physical and observational properties of these sources. We started with simulated dark matter (DM) haloes, to preserve the link with the cosmic web, and we populated them with galaxies and AGN using abundance matching techniques. We followed an empirical methodology, using stellar mass functions, host galaxy AGN mass functions, and AGN accretion rate distribution functions studied at different redshifts to assign, among other properties, stellar masses, the fraction of quenched galaxies, or the AGN activity (demography, obscuration, multiwavelength emission, etc.). As a proof test, we applied the method to a Millennium DM lightcone of 3.14 $ deg^2$ up to a redshift of $z=10$ and down to stellar masses of $ M \, M_ We show that the AGN population from the mock lightcone presented here reproduces with good accuracy various observables, such as state-of-the-art luminosity functions in the X-ray up to $z 7$ and in the ultraviolet up to $z 5$, optical/near-infrared colour-colour diagrams, and narrow emission line diagnostic diagrams. Finally, we demonstrate how this catalogue can be used to make useful predictions for large surveys. Using Euclid as a case example, we compute, among other forecasts, the expected surface densities of galaxies and AGN detectable in the Euclid $H_ E $ band. We find that Euclid might observe (on $H_ E $ only) about $10^ $ and $8 $ type 1 and 2 AGN, respectively, and $2 $ galaxies at the end of its $ deg^2$ Wide survey, in good agreement with other published forecasts.

Constraints on the narrow-line region of the X-ray quasi-periodic eruption source GSN 069

ABSTRACT The origins of quasi-periodic eruptions (QPEs) are poorly understood, although most theoretical explanations invoke an accretion disc around a supermassive black hole. The gas and stellar environments in the galactic nuclei of these sources are also poorly constrained. In this paper, we present an analysis of archival Hubble Space Telescope (HST) images to study the narrow-line [O iii] emission in the QPE source GSN 069. We find strong evidence for a compact nuclear [O iii] emission region of size ≲ 35 pc, overlaid on top of extended [O iii] emission up to 1 kpc away from the nucleus. The age of the accretion system is estimated to be between 10 and 100 yr. The [O iii] luminosity of the compact region was measured to be $(2.1 \pm 0.3) \times 10^{40}\, \rm erg\, s^{-1}$. Based on cloudy simulations, we constrain that the [O iii] emitting gas has a hydrogen number density in the range $2 \times 10^{3} \lt n_{\rm H} \lesssim 10^{8}\, \rm cm^{-3}$ and volume filling factor fV < 4 × 10−3. We suggest that the dense gas in the nuclear region of GSN 069 originates from molecular clouds (with total mass $\gtrsim 3 \times 10^{3}\, {\rm M}_{\odot }$), which are freshly ionized by the soft X-ray photons from the accretion disc. We predict possible evolution of the compact narrow-line region on emission-line diagnostic diagrams, and hence future HST or integral-field unit observations can be used to further pin down the age of this puzzling system.

Star formation in outer rings of S0 galaxies

Aims. Though S0 galaxies are usually thought to be “red and dead”, they often present weak star formation organized in ring structures in their outer disks. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. The nearby luminous S0 galaxy NGC 1211 is studied here. Methods. By applying Fabry-Perot scanning interferometry at the Russian 6m telescope, we measured the gas rotation in the outer ring of the galaxy. We studied the pattern and properties of the brightest star formation regions with the tunable filter Mapper of Narrow Galaxy Lines (MaNGaL) at the 2.5 m telescope of the Caucasian Mountain Observatory of the SAI MSU. Results. The gas excitation is strongly inhomogeneous along the ring: in the eastern part, HII-type excitation dominates, while the western part of the ring is partly excited by shocks, as revealed by the emission-line diagnostic diagram. The gas metallicity of the emission clumps in the outer ring also has a broad distribution. The star formation rate is currently falling and is in general too weak for the galaxy gas content. We conclude that the gas in the ring has been accreted recently, about 300 Myr ago, probably from an inclined direction, and that the accretion event has provoked a brief, ineffective starburst.

MaNGA AGN dwarf galaxies (MAD) – I. A new sample of AGNs in dwarf galaxies with spatially-resolved spectroscopy

ABSTRACT The finding of active galactic nuclei (AGNs) in dwarf galaxies has important implications for galaxy evolution and supermassive black hole formation models. Yet, how AGNs in dwarf galaxies form is still debated, in part due to scant demographics. We use the MaNGA survey, comprising ∼10 000 galaxies at z < 0.15, to identify AGN dwarf galaxies using a spaxel-by-spaxel classification in three spatially-resolved emission-line diagnostic diagrams (the [N ii]-, [S ii]-, and [O i]-BPT) and the WHAN diagram. This yields a sample of 664 AGN dwarf galaxies, the largest to date, and an AGN fraction of $\sim 20~{{\ \rm per\ cent}}$ that is significantly larger than that of single-fibre-spectroscopy studies (i.e. $\sim 1~{{\ \rm per\ cent}}$). This can be explained by the lower bolometric luminosity (<1042 erg s−1) and accretion rate (sub-Eddington) of the MaNGA AGN dwarf galaxies. We additionally identify 1176 SF-AGNs (classified as star-forming in the [N ii]-BPT but as AGNs in the [S ii]- and [O i]-BPT), 122 Composite, and 173 LINER sources. The offset between the optical centre of the galaxy and the median position of the AGN spaxels is more than 3 arcsec for ∼62% of the AGNs, suggesting that some could be off-nuclear. We also identify seven new broad-line AGNs with log MBH = 5.0–5.9 M⊙. Our results show how integral-field spectroscopy is a powerful tool for uncovering faint and low-accretion AGNs and better constraining the demographics of AGNs in dwarf galaxies.

Evidence for a large off-centered galactic outflow and its connection to the extraplanar diffuse ionized gas in IC 1553

Aims. We analyze a MUSE optical integral field spectrum of the star-forming edge-on galaxy IC 1553 in order to study its extraplanar diffuse ionized gas (eDIG) and the processes shaping its disk-halo interface. Methods. We extracted the optical emission line properties from the integral field spectrum and generated the commonly used emission line diagnostic diagrams in order to analyze the ionization conditions and the distribution of the eDIG. Furthermore, we performed gravitational potential fitting to investigate the kinematics of a suspected galactic outflow. Results. We find that the eDIG scale height has a maximum value of approximately 1.0 kpc and decreases roughly linearly with the radial distance from the galactic center in projection. The ionization state of the eDIG is not consistent with a pure photoionization scenario and instead requires a significant contribution from shock ionization. This, in addition to the gas kinematics, strongly suggests the presence of a galactic scale outflow, the origin of which lies at least 1.4 kpc away from the galactic center. The inferred shock velocity in the eDIG of approximately 225 km s−1 is comparable to the escape velocity estimated from our potential modelling. The asymmetric distribution of currently star-forming clusters produces a range of different ionization conditions in the eDIG. As a result, the vertical emission line profiles vary quantitatively and qualitatively along the major axis of the galaxy. This analysis illustrates that it is crucial in studies of the eDIG to use observations that take the spatial and kinematical distributions into account, such as those done with integral field units, to form an accurate picture of the relevant physical properties.

The Star Formation Across Cosmic Time (SFACT) Survey. III. Spectroscopy of the Initial Catalog of Emission-line Objects

The Star Formation Across Cosmic Time (SFACT) survey is a new narrowband survey designed to detect emission-line galaxies (ELGs) and quasi-stellar objects (QSOs) over a wide range of redshifts in discrete redshift windows. The survey utilizes the WIYN 3.5 m telescope and the Hydra multifiber positioner to perform efficient follow-up spectroscopy on galaxies identified in the imaging part of the survey. Because the objects in the SFACT survey are selected by their strong emission lines, it is possible to obtain useful spectra for even the faintest of our sources (r ∼ 25). Here, we present the 453 objects that have spectroscopic data from the three SFACT pilot-study fields, 415 of which are confirmed ELGs. The methodology for processing and measuring these data is outlined in this paper, and example spectra are displayed for each of the three primary emission lines used to detect objects in the survey (Hα, [O III]λ5007, and [O II]λ3727). Spectra of additional QSOs and nonprimary-emission-line detections are also shown as examples. The redshift distribution of the pilot-study sample is examined, and the ELGs are placed in different emission-line diagnostic diagrams in order to distinguish the star-forming galaxies from the active galactic nuclei.

Mapping the stellar population and gas excitation of MaNGA galaxies with megacubes. Results for AGN versus control sample

ABSTRACT We present spaxel-by-spaxel stellar population fits for the ∼10 000 MaNGA data cubes. We provide multiple extension fits files, nominated as megacubes, with maps of several properties as well as emission-line profiles that are provided for each spaxel. All the megacubes are available through a web interface (https://manga.linea.org.br/ or http://www.if.ufrgs.br/~riffel/software/megacubes/). We also defined a final Active Galactic Nuclei (AGN) sample, as well as a control sample matching the AGN host galaxy properties. We have analysed the stellar populations and spatially resolved emission-line diagnostic diagrams of these AGNs and compared them with the control galaxies sample. We find that the relative fractions of young (t ≤56 Myr) and intermediate-age (100 Myr ≤t ≤ 2 Gyr) show predominantly a positive gradient for both AGNs and controls. The relative fraction of intermediate-age stellar population is higher in AGN hosts when compared to the control sample, and this difference becomes larger for higher [O iii] luminosity AGNs. We attribute this to the fact that extra gas is available in these more luminous sources and that it most likely originates from mass-loss from the intermediate-age stars. The spatially resolved diagnostic diagrams reveal that the AGN emission is concentrated in the inner 0.5 Re (effective radius) region of the galaxies, showing that the AGN classification is aperture dependent and that emission-line ratios have to be taken together with the H α equivalent width for proper activity classification. We present a composite ‘BPT+WHAN’ diagram that produces a more comprehensive mapping of the gas excitation.

The environment of AGN dwarf galaxies at z ∼ 0.7 from the VIPERS survey

ABSTRACT Dwarf galaxies are ideal laboratories to study the relationship between the environment and active galactic nucleus (AGN) activity. However, the type of environments in which dwarf galaxies hosting AGN reside is still unclear and limited to low-redshift studies (${z\lt 0.5}$). We use the VIMOS Public Extragalactic Redshift Survey (VIPERS) to investigate, for the first time, their environments at $0.5\lt {z}\lt 0.9$. We select a sample of 12 942 low-mass ($\rm {log}(\mathit{ M}_\mathrm{*}/M_{\odot })\le 10$) galaxies and use the emission-line diagnostic diagram to identify AGN. We characterize their local environments as the galaxy density contrast, δ, derived from the fifth nearest neighbour method. Our work demonstrates that AGN and non-AGN dwarf galaxies reside in similar environments at intermediate redshift suggesting that the environment is not an important factor in triggering AGN activity already since ${z=0.9}$. Dwarf galaxies show a strong preference for low-density environments, independently of whether they host an AGN or not. Their properties do not change when moving to denser environments, suggesting that dwarf galaxies are not gas-enriched due to environmental effects. Moreover, AGN presence does not alter host properties supporting the scenario that AGN feedback does not impact the star formation of the host. Lastly, AGNs are found to host overmassive black holes. This is the first study of dwarf galaxies hosting AGNs at ${z\gt 0.5}$. The next generation of deep surveys will reveal whether or not such lack of environmental trends is common also for faint higher redshift dwarf galaxy populations.

Optical and JWST Mid-IR Emission Line Diagnostics for Simultaneous IMBH and Stellar Excitation in z ∼ 0 Dwarf Galaxies*

Abstract Current observational facilities have yet to conclusively detect 103–104 M ⊙ intermediate-mass black holes (IMBHs) that fill in the evolutionary gap between seed black holes in the early universe and z ∼ 0 supermassive black holes. Dwarf galaxies present an opportunity to reveal active IMBHs amidst persistent star formation. We introduce photoionization simulations tailored to address key physical uncertainties: coincident versus noncoincident mixing of IMBH and starlight excitation, open versus closed geometries of surrounding gas clouds, and different shapes of the spectral energy distribution of active galactic nuclei (AGN). We examine possible AGN emission line diagnostics in the optical and mid-IR, and find that the diagnostics are often degenerate with respect to the investigated physical uncertainties. In spite of these setbacks, and in contrast to recent work, we are able to show that [O iii]/Hβ typically remains bright for dwarf AGN powered by IMBHs down to 103 M ⊙. Dwarf AGN are predicted to have inconsistent star-forming and Seyfert/LINER classifications using the most common optical diagnostics. In the mid-IR, [O iv] 25.9 μm and [Ar ii] 6.98 μm are less sensitive to physical uncertainties than are optical diagnostics. Based on these emission lines, we provide several diagrams of mid-IR emission line diagnostic diagrams with demarcations for separating starbursts and AGN with varying levels of activity. The diagrams are valid over a wide range of ionization parameters and metallicities out to z ∼ 0.1, so will prove useful for future JWST observations of local dwarf AGN in the search for IMBHs. We make our photoionization simulation suite freely available.

Deepest far ultraviolet view of a central field in the Coma cluster by AstroSat UVIT

Abstract We present analysis of the far ultraviolet (FUV) emission of sources in the central region of the Coma cluster ( $z=0.023$ ) using the data taken by the UVIT aboard the multi-wavelength satellite mission AstroSat. We find a good correlation between the UVIT FUV flux and the fluxes in both wavebands of the Galex mission, for the common sources. We detect stars and galaxies, amongst which the brightest ( $r \lesssim 17$ mag) galaxies in the field of view are mostly members of the Coma cluster. We also detect three quasars ( $z = 0.38, 0.51, 2.31$ ), one of which is likely the farthest object observed by the UVIT so far. In almost all the optical and UV colour-colour and colour-magnitude planes explored in this work, the Coma galaxies, other galaxies and bright stars could be separately identified, but the fainter stars and quasars often coincide with the faint galaxies. We have also investigated galaxies with unusual FUV morphology which are likely to be galaxies experiencing ram-pressure stripping in the cluster. Amongst others, two confirmed cluster members which were not investigated in the literature earlier, have been found to show unusual FUV emission. All the distorted sources are likely to have fallen into the cluster recently, and hence have not virialised yet. A subset of our data have optical spectroscopic information available from the archives. For these sources ( ${\sim} 10\%$ of the sample), we find that 17 galaxies identify as star-forming, 18 as composite and 13 as host galaxies for active galactic nuclei, respectively on the emission-line diagnostic diagram.

Compact Elliptical Galaxies Hosting Active Galactic Nuclei in Isolated Environments

Abstract We present the discovery of rare active galactic nuclei (AGNs) in nearby (z < 0.05) compact elliptical galaxies (cEs) located in isolated environments. Using spectroscopic data from the Sloan Digital Sky Survey (SDSS) Data Release 12, four AGNs were identified based on the optical emission-line diagnostic diagram. SDSS optical spectra of AGNs show the presence of distinct narrow-line emissions. Utilizing the black hole (BH) mass–stellar velocity dispersion scaling relation and the correlation between the narrow L([O iii])/L(Hβ) line ratio and the width of the broad Hα emission line, we estimated the BH masses of the cEs to be in the range of 7 × 105–8 × 107 M ⊙. The observed surface brightness profiles of the cEs were fitted with a double Sérsic function using the Dark Energy Camera Legacy Survey r-band imaging data. Assuming the inner component as the bulge, the K-band bulge luminosity was also estimated from the corresponding Two Micron All Sky Survey images. We found that our cEs follow the observed BH mass–stellar velocity dispersion and BH mass–bulge luminosity scaling relations, albeit there was a large uncertainty in the derived BH mass of one cE. In view of the observational properties of BHs and those of the stellar populations of cEs, we discuss the proposition that cEs in isolated environments are bona fide low-mass early-type galaxies (i.e., a nature origin).

A search for active galactic nuclei in low-mass compact galaxies

ABSTRACT Low-mass compact galaxies (ultracompact dwarfs, UCDs, and compact ellipticals, cEs) populate the stellar size–mass plane between globular clusters and early-type galaxies. Known to be formed either in situ with an intrinsically low mass or resulting from the stripping of a more massive galaxy, the presence of a supermassive or an intermediate-mass black hole (BH) could help discriminate between these possible scenarios. With this aim, we have performed a multiwavelength search of active BH activity, i.e. active galactic nuclei (AGN), in a sample of 937 low-mass compact galaxies (580 UCDs and 357 cEs). This constitutes the largest study of AGN activity in these types of galaxies. Based on their X-ray luminosity, radio luminosity, and morphology, and/or optical emission line diagnostic diagrams, we find a total of 11 cEs that host an AGN. We also study for the first time the location of both low-mass compact galaxies (UCDs and cEs) and dwarf galaxies hosting AGN on the BH–galaxy scaling relations, finding that low-mass compact galaxies tend to be overmassive in the BH mass–stellar mass plane but not as much in the BH mass–stellar velocity dispersion correlation. This, together with available BH mass measurements for some of the low-mass compact galaxies, supports a stripping origin for the majority of these objects that would contribute to the scatter seen at the low-mass end of the BH–galaxy scaling relations. However, the differences are too large to be explained solely by this scatter, and thus our results suggest that a flattening at such low masses is also plausible, happening at a velocity dispersion of ∼20–40 km s−1.

Optical emission lines in the most massive galaxies: Morphology, kinematics, and ionisation properties

In order to better characterise the upper end of the galaxy stellar mass range, the MUSE Most Massive Galaxies (M3G) Survey targeted the most massive galaxies (M > 1012 M⊙) found in the densest known clusters of galaxies at z ∼ 0.046. The sample is composed of 25 early-type galaxies: 14 brightest cluster galaxies (BCGs), of which 3 are in the densest region of the Shapley super cluster (SSC), and 11 massive satellites in the SSC. In the present work we use MUSE data to derive the spatial distribution and kinematics of the gas and discuss its ionisation mechanism and origin in the optical wavelength range. We fit the continuum of the spectra using an extensive library of single stellar population models and model the emission lines employing up to three Gaussian functions. In the M3G sample, ionised gas was detected in five BCGs, of which one is in the densest region of the SSC, and six massive satellites in the SSC. Among these objects, [O I] and [N I] were detected in three BCGs and one satellite. The gas is centrally concentrated in almost all objects, except for two BCGs that show filaments and two massive satellites with extended emission. Moreover, the emission line profiles of three BCGs present redshifted and/or blueshifted components. The presence of dust was revealed by analysing Balmer line ratios, obtaining a mean E(B − V) of 0.2–0.3. The emission-line diagnostic diagrams show predominately low-ionisation nuclear emission-line regions line ratios with little contamination from star formation. In the M3G sample, the gas was detected in 80% of fast rotators and 35% of slow rotators. The orientations of stellar and gaseous rotations are aligned with respect to each other for 60% of satellites and 25% of BCGs. The presence of misalignments points to an external origin of the gas for three BCGs and two satellites. On the other hand, some of these systems are characterised by triaxial and prolate-like stellar rotation, which could support an internal origin of the gas even in the case of misalignments.

Hidden AGNs in Dwarf Galaxies Revealed by MaNGA: Light Echoes, Off-nuclear Wanderers, and a New Broad-line AGN

Abstract Active galactic nuclei (AGNs) in dwarf galaxies could host the relics of those early universe seed black holes that did not grow into supermassive black holes. Using MaNGA integral field unit (IFU) spectroscopy we have found a sample of 37 dwarf galaxies that show AGN ionization signatures in spatially resolved emission line diagnostic diagrams. The AGN signatures are largely missed by integrated emission line diagnostics for 23 of them. The bolometric luminosity of these 23 new AGN candidates is ≲1040 erg s−1, fainter than that of single-fiber SDSS AGNs, X-ray AGNs, and radio AGNs in dwarf galaxies, which supports IFU spectroscopy as a powerful tool for identifying hidden and faint AGNs in dwarf galaxies. The AGN emission is in most cases offset from the optical center of the dwarf galaxy and shows a symmetric morphology, which indicates that either the AGNs are off-nuclear, that the central emission of the galaxy is dominated by star formation, or that the AGNs are turned off and we are observing a past ionization burst. One of the new AGNs shows a broad Hα emission line component, from which we derive a black hole mass in the realm of intermediate-mass black holes. This constitutes the first hidden type 1 AGN discovered in a dwarf galaxy based on IFU spectroscopy. The finding of this sample of hidden and faint AGNs has important implications for population studies of AGNs in dwarf galaxies and for seed black hole formation models.

Exploring the differences of integrated and spatially resolved analysis using integral field unit data: the case of Abell 14

ABSTRACT We present a new approach to study planetary nebulae using integral field spectroscopy. VLT@VIMOS datacube of the planetary nebula Abell 14 is analysed in three different ways by extracting: (i) the integrated spectrum, (ii) one-dimensional simulated long-slit spectra for different position angles, and (iii) spaxel-by-spaxel spectra. These data are used to build emission-line diagnostic diagrams and explore the ionization structure and excitation mechanisms combining data from one- and three-dimensional photoionization models. The integrated and 1D simulated spectra are suitable for developing diagnostic diagrams, while the spaxel spectra can lead to misinterpretation of the observations. We find that the emission-line ratios of Abell 14 are consistent with UV photoionised emission; however, there are some pieces of evidence of an additional thermal mechanism. The chemical abundances confirm its previous classification as a Type I planetary nebula, without spatial variation. We find, though, variation in the ionization correction factors as a function of the slit’s position angle. The star at the geometric centre of Abell 14 has an A5 spectral type with an effective temperature of Teff = 7909 ± 135 K and surface gravity log(g) = 1.4 ± 0.1 cm s−2. Hence, this star cannot be responsible for the ionization state of the nebula. Gaia parallaxes of this star yield distances between 3.6 and 4.5 kpc in good agreement with the distance derived from a three-dimensional photoionization modelling of Abell 14, indicating the presence of a binary system at the centre of the planetary nebula.

UV bright red-sequence galaxies: how do UV upturn systems evolve in redshift and stellar mass?

ABSTRACT The so-called ultraviolet (UV) upturn of elliptical galaxies is a phenomenon characterized by the up-rise of their fluxes in bluer wavelengths, typically in the 1200–2500 Å range. This work aims at estimating the rate of occurrence of the UV upturn over the entire red-sequence population of galaxies that show significant UV emission. This assessment is made considering it as function of three parameters: redshift, stellar mass, and – what may seem counter-intuitive at first – emission-line classification. We built a multiwavelength spectrophotometric catalogue from the Galaxy Mass Assembly survey, together with aperture-matched data from Galaxy Evolution Explorer Medium-Depth Imaging Survey (MIS) and Sloan Digital Sky Survey, covering the redshift range between 0.06 and 0.40. From this sample, we analyse the UV emission among UV bright galaxies, by selecting those that occupy the red-sequence locus in the (NUV− r) × (FUV−NUV) chart; then, we stratify the sample by their emission-line classes. To that end, we make use of emission-line diagnostic diagrams, focusing the analysis in retired/passive lineless galaxies. Then, a Bayesian logistic model was built to simultaneously deal with the effects of all galaxy properties (including emission-line classification or lack thereof). The main results show that retired/passive systems host an up-rise in the fraction of UV upturn for redshifts between 0.06 and 0.25, followed by an in-fall up to 0.35. Additionally, we show that the fraction of UV upturn hosts rises with increasing stellar mass.

Radio spectral index distribution of SDSS-FIRST sources across optical diagnostic diagrams

Context. The empirical relations between supermassive black holes and their host spheroids point towards the crucial role of galactic nuclei in affecting the properties of their hosts. A detailed understanding of how the activity of a galactic nucleus regulates the growth of its host is still missing. Aims. To understand the activity and the types of accretion of supermassive black holes in different hosts, it is essential to study the radio-optical properties of a large sample of extragalactic sources. In particular, we aim to study the radio spectral index trends across the optical emission line diagnostic diagrams to search for potential (anti)correlations. Methods. To this goal, we combined flux densities from the radio FIRST survey at 1.4 GHz (with the flux density range 10 mJy ≤ F1.4 ≤ 100 mJy) for 396 SDSS sources at intermediate redshift (0.04 ≤ z ≤ 0.4) with the Effelsberg radiotelescope measurements at 4.85 GHz and 10.45 GHz. The information about the optical emission-line ratios is obtained from the SDSS-DR7 catalogue. Results. Using the Effelsberg data, we were able to infer the two-point radio spectral index distributions for star-forming galaxies, composite galaxies (with a combined contribution to the line emission from the star formation and AGN activity), Seyferts, and low ionization narrow emission region (LINER) galaxies. Conclusions. While studying the distribution of steep, flat, and inverted sources across optical diagnostic diagrams, we found three distinct classes of radio emitters for our sample: (i) sources with a steep radio index, high ionization ratio, and high radio loudness, (ii) sources with a flat radio index, lower ionization ratio, and intermediate radio loudness, (iii) sources with an inverted radio index, low ionization ratio, and low radio loudness. The classes (i), (ii), and (iii) cluster mainly along the transition from Seyfert to LINER sources in the optical diagnostic (Baldwin, Phillips & Telervich; BPT) diagram. We interpret these groups as a result of the recurrent nuclear-jet activity.

Unsupervised Classification of Galaxies. I. Independent Component Analysis Feature Selection

Subjective classification of galaxies can mislead us in the quest for the formation and evolution of galaxies since this is necessarily limited to a few features. The human mind is unable to comprehend the complex correlations in a manifold parameter space, and multivariate analyses are the best tools for understanding the differences among various kinds of objects. In this series of papers, an objective classification of 362,923 galaxies from the Value Added Galaxy Catalog is carried out with the help of two methods of multivariate analysis. First, Independent Component Analysis is used to determine a set of derived independent components that are linear combinations of 47 observed features (namely, ionized lines, Lick indices, photometric and morphological properties, star formation rates, etc.) of the galaxies. Subsequently, a K-means cluster analysis is applied to the nine independent components to obtain 10 distinct, homogeneous groups. In this first paper, we describe the methods and the main results. It appears that the nine Independent Components represent a complete physical description of galaxies (velocity dispersion, ionization, metallicity, surface brightness, and structure). We find that our 10 groups can be essentially placed into traditional and empirical classes (from color–magnitude and emission-line diagnostic diagrams, early versus late types) despite the classical corresponding features (color, line ratios, and morphology) not being significantly correlated with the nine Independent Components. A more detailed physical interpretation of the groups will be performed in subsequent papers.

The MOSDEF Survey: Sulfur Emission-line Ratios Provide New Insights into Evolving Interstellar Medium Conditions at High Redshift∗

Abstract We present results on the emission-line properties of 1.3 ≤ z ≤ 2.7 galaxies drawn from the complete the MOSFIRE Deep Evolution Field (MOSDEF) survey. Specifically, we use observations of the emission-line diagnostic diagram of [O iii]λ 5007/Hβ versus [S ii]λλ6717,6731/Hα, i.e., the “[S ii] BPT diagram,” to gain insight into the physical properties of high-redshift star-forming regions. High-redshift MOSDEF galaxies are offset toward lower [S ii]λλ6717,6731/Hα at fixed [O iii]λ5007/Hβ, relative to local galaxies from the Sloan Digital Sky Survey (SDSS). Furthermore, at fixed [O iii]λ5007/Hβ, local SDSS galaxies follow a trend of decreasing [S ii]λλ6717,6731/Hα as the surface density of star formation (ΣSFR) increases. We explain this trend in terms of the decreasing fractional contribution from diffuse ionized gas (f DIG) as ΣSFR increases in galaxies, which causes galaxy-integrated line ratios to shift toward the locus of pure H ii-region emission. The z ∼ 0 relationship between f DIG and ΣSFR implies that high-redshift galaxies have lower f DIG values than typical local systems, given their significantly higher typical ΣSFR. When an appropriate low-redshift benchmark with zero or minimal f DIG is used, high-redshift MOSDEF galaxies appear offset toward higher [S ii]λλ6717,6731/Hα and/or [O iii]λ 5007/Hβ. The joint shifts of high-redshift galaxies in the [S ii] and [N ii] BPT diagrams are best explained in terms of the harder spectra ionizing their star-forming regions at fixed nebular oxygen abundance (expected for chemically young galaxies), as opposed to large variations in N/O ratios or higher ionization parameters. The evolving mixture of H ii regions and diffuse ionized gas is an essential ingredient of our description of the interstellar medium over cosmic time.

Evidence for Shock-heated Gas in the Taffy Galaxies and Bridge from Optical Emission-line IFU Spectroscopy

Abstract We present optical integral field unit observations of the Taffy system (UGC 12914/15), named for the radio emission that stretches between the two galaxies. Given that these gas-rich galaxies are believed to have recently collided head-on, the pair exhibits a surprisingly normal total (sub-LIRG) IR luminosity (L FIR ∼ 4.5 × 1010 L ⊙). Previous observations have demonstrated that a large quantity of molecular and neutral gas has been drawn out of the galaxies into a massive multiphase bridge. We present, for the first time, spatially resolved spectroscopy of the ionized gas in the system. The results show that the ionized gas is highly disturbed kinematically, with gas spread in two main filaments between the two galaxies. The line profiles exhibit widespread double components in both the bridge and parts of the disks of the galaxies. We investigate the spatial distribution of the excitation properties of the ionized gas using emission-line diagnostic diagrams and conclude that a large quantity (up to 40%) of the emission from the entire system is consistent with gas heated in ∼200 km s−1 shocks. While the shocked gas is mainly associated with the bridge, there is a significant amount of shocked gas associated with both galaxies. Confirming other multiwavelength indicators, the results suggest that the effects of shocks and turbulence can continue to be felt in a high-speed galaxy collision long after the collision has occurred. The persistence of shocks in the Taffy system may explain the relatively low current star formation rates in the system as a whole.