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We present observations of CO(1--0) and CO(2--1) lines from the Institut de radioastronomie millim\'etrique (IRAM) 30m telescope toward 20 nearby, optically luminous type 2 quasars (QSO2s) and observations of [C II] 158$\mu$m line from the Stratospheric Observatory For Infrared Astronomy (SOFIA) for 5 QSO2s in the CO sample and 5 type 1 quasars (QSO1s). In the traditional evolutionary scenario explaining different types of QSOs, obscured QSO2s emerge from gas-rich mergers observed as luminous infrared galaxies (LIRGs) and then turn into unobscured QSO1s as the black holes clear out the obscuring material in a blow-out phase. We test the validity of this theoretical prediction by comparing the gas fractions and star formation efficiencies among LIRGs and QSOs. We find that CO luminosity, CO-derived gas masses and gas fractions in QSO1s are consistent with those estimated for QSO2s, while LIRGs exhibit a closer resemblance to QSO2s in terms of CO-derived gas masses and gas fractions, and [C II] luminosity. However, comparisons between [C II] luminosity and star formation tracers such as the CO and infrared luminosity imply additional sources of [C II] emission in QSO1s likely tracing neutral atomic or ionized gas. All three types of galaxies have statistically indistinguishable distributions of star formation efficiency. Our results are consistent with part of the evolutionary scenario where nearby QSO2s could emerge from LIRGs, but they are unlikely to be the precursors of nearby QSO1s., Comment: 32 pages, 10 figures, 7 tables; the complete set of Figure 1 is appended to the end of document. Under review (revisions submitted) by ApJ

Feedback driven massive outflows play a crucial role in galaxy evolution by regulating star formation and influencing the dynamics of surrounding media. Extracting outflow properties from spectral lines is a notoriously difficult process for a number of reasons, including the possibility that a substantial fraction of the outflow is carried by dense gas in a very narrow range in velocity. This gas can hide in spectra with insufficient resolution. Empirically motivated analysis based on the Apparent Optical Depth method, commonly used in the literature, neglects the contribution of this gas, and may therefore underestimate the true gas column density. More complex semi-analytical line transfer (e.g., SALT) models, on the other hand, allow for the presence of this gas by modeling the radial density and velocity of the outflows as power laws. Here we compare the two approaches to quantify the uncertainties in the inferences of outflow properties based on 1-D "down-the-barrel" using the UV spectra of the CLASSY galaxy sample. We find that empirical modeling may significantly underestimate the column densities relative to SALT analysis, particularly in the optically thick regime. We use simulations to show that the main reason for this discrepancy is the presence of large amount of dense material at low velocities, which can be hidden by the finite spectral resolution of the data. The SALT models in turn could over-estimate the column densities if the assumed power laws of the density profiles strong are not a property of actual outflows., Comment: Accepted to the Astrophysical Journal

The [OIII] 5007 A emission line is the most common tracer of warm, ionized outflows in active galactic nuclei across cosmic time. JWST newly allows us to use mid-infrared spectral features at both high spatial and spectral resolution to probe these same winds. Here we present a comparison of ground-based, seeing-limited [OIII] and space-based, diffraction-limited [SIV] 10.51 micron maps of the powerful, kiloparsec-scale outflow in the Type 1 red quasar SDSS J110648.32+480712.3. The JWST data are from the Mid-InfraRed Instrument (MIRI). There is a close match in resolution between the datasets (0."6), in ionization potential of the O$^{+2}$ and S$^{+3}$ ions (35 eV), and in line sensitivity (1e-17 to 2e-17 erg/s/cm$^2$/arcsec$^2$). The [OIII] and [SIV] line shapes match in velocity and linewidth over much of the 20 kpc outflowing nebula, and [SIV] is the brightest line in the rest-frame 3.5-19.5 micron range, demonstrating its usefulness as a mid-IR probe of quasar outflows. [OIII] is nevertheless intriniscally brighter and provides better contrast with the point-source continuum, which is strong in the mid-IR. There is a strong anticorrelation of [OIII]/[SIV] with average velocity, which is consistent with a scenario of differential obscuration between the approaching (blueshifted) and receding (redshifted) sides of the flow. The dust in the wind may also obscure the central quasar, consistent with models that attribute red quasar extinction to dusty winds., Comment: resubmitted to match published version; minor changes

Most of the hydrogen in the intergalactic medium (IGM) was rapidly ionized at high-redshifts. While observations have established that reionization occurred, observational constraints on the emissivity of ionizing photons at high-redshift remains elusive. Here, we present a new analysis of the Low-redshift Lyman Continuum Survey (LzLCS) and archival observations, a combined sample of 89 star-forming galaxies at z~0.3 with Hubble Space Telescope observations of their ionizing continua (or Lyman Continuum, LyC). We find a strong (6$\sigma$ significant) inverse correlation between the continuum slope at 1550\r{A} (defined as F$_\lambda\propto\lambda^{\beta}$) and both the LyC escape fraction (f$_{esc}$) and f$_{esc}$ times the ionizing photon production efficiency ($\xi_{ ion}$). On average, galaxies with redder continuum slopes have smaller f$_{esc}$ than galaxies with bluer slopes due to higher dust attenuation. More than 5% (20%) of the LyC emission escapes galaxies with $\beta$<-2.1 (-2.6). We find strong correlations between $\beta$ and the gas-phase ionization ([OIII]/[OII] flux ratio; at 7.5$\sigma$ significance), galaxy stellar mass (at 5.9$\sigma$), the gas-phase metallicity (at 4.6$\sigma$), and the observed FUV absolute magnitude (M$_{UV}$ at 3.4$\sigma$). Using previous observations of $\beta$ at high-redshift, we estimate the evolution of f$_{esc}$ with both $z$ and M$_{UV}$. The LzLCS suggest that fainter and lower mass galaxies dominate the ionizing photon budget at higher redshift, possibly due to their rapidly evolving metal and dust content. Finally, we use our correlation between $\beta$ and f$_{ esc}\times\xi_{ion}$ to predict the ionizing emissivity of galaxies during the epoch of reionization. Our estimated emissivities match IGM observations, and suggest that star-forming galaxies emit sufficient LyC photons into the IGM to exceed recombinations near redshifts of 7-8., Comment: 14 pages plus appendix. Submitted to MNRAS. Comments encouraged

The properties that govern the production and escape of hydrogen ionizing photons (Lyman continuum, LyC; with energies >13.6 eV) in star-forming galaxies are still poorly understood, but they are key to identifying and characterizing the sources that reionized the Universe. Here we empirically explore the relationship between the hardness of ionizing radiation and the LyC leakage in a large sample of low-$z$ star-forming galaxies from the recent Hubble Space Telescope Low-$z$ Lyman Continuum Survey. Using Sloan Digital Sky Survey stacks and deep XShooter observations, we investigate the hardness of the ionizing spectra ($Q_{\rm He^+}/Q_{\rm H}$) between 54.4 eV (He$^{+}$) and 13.6 eV (H) from the optical recombination lines HeII 4686A and H$\beta$ 4861A for galaxies with LyC escape fractions spanning a wide range, $f_{\rm esc} \rm (LyC) \simeq 0 - 90\%$. We find that the observed intensity of HeII/H$\beta$ is primarily driven by variations in the metallicity, but is not correlated with LyC leakage. Both very strong ($ \simeq 0.5$) and nonleakers ($ < f_{\rm esc} \rm (LyC) > \simeq 0$) present similar observed intensities of HeII/H$\beta$ at comparable metallicity, between $\simeq 0.01$ and $\simeq 0.02$ for $12 + \log({\rm O/H}) > 8.0$ and $<8.0$, respectively. Our results demonstrate that $Q_{\rm He^+}/Q_{\rm H}$ does not correlate with $f_{\rm esc} \rm (LyC)$, which implies that strong LyC emitters do not show harder ionizing spectra than nonleakers at similar metallicity., Comment: 6 pages, 2 figures, 1 tables. Accepted for publication in A&A Letters. v2: updated to match published version, including new title

We present the relation between the star formation rate surface density, $\Sigma_{\rm SFR}$, and the hydrostatic mid-plane pressure, P$_{\rm h}$, for 4260 star-forming regions of kpc size located in 96 galaxies included in the EDGE-CALIFA survey covering a wide range of stellar masses and morphologies. We find that these two parameters are tightly correlated, exhibiting smaller scatter and strong correlation in comparison to other star-forming scaling relations. A power-law, with a slightly sub-linear index, is a good representation of this relation. Locally, the residuals of this correlation show a significant anti-correlation with both the stellar age and metallicity whereas the total stellar mass may also play a secondary role in shaping the $\Sigma_{\rm SFR}$ - P$_{\rm h}$ relation. For our sample of active star-forming regions (i.e., regions with large values of H$\alpha$ equivalent width), we find that the effective feedback momentum per unit stellar mass ($p_\ast/m_\ast$),measured from the P$_{\rm h}$ / $\Sigma_{\rm SFR}$ ratio increases with P$_{\rm h}$. The median value of this ratio for all the sampled regions is larger than the expected momentum just from supernovae explosions. Morphology of the galaxies, including bars, does not seem to have a significant impact in the $\Sigma_{\rm SFR}$ - P$_{\rm h}$ relation. Our analysis suggests that self regulation of the $\Sigma_{\rm SFR}$ at kpc scales comes mainly from momentum injection to the interstellar medium from supernovae explosions. However, other mechanism in disk galaxies may also play a significant role in shaping the $\Sigma_{\rm SFR}$ at local scales. Our results also suggest that P$_{\rm h}$ can be considered as the main parameter that modulates star formation at kpc scales, rather than individual components of the baryonic mass., Comment: 19 pages, 12 figures, submitted to MNRAS November 16 2020

Star formation rate density, $\Sigma_{\rm SFR}$, has shown a remarkable correlation with both components of the baryonic mass at kpc scales (i.e., the stellar mass density, and the molecular gas mass density; $\Sigma_{\ast}$, and $\Sigma_{\rm mol}$, respectively) for galaxies in the nearby Universe. In this study we propose an empirical relation between $\Sigma_{\rm SFR}$ and the baryonic mass surface density ($\Sigma_{\rm b}$ =$\Sigma_{\rm mol,Av}$ + $\Sigma_{\ast}$; where $\Sigma_{\rm mol,Av}$ is the molecular gas density derived from the optical extinction, Av) at kpc scales using the spatially-resolved properties of the MaNGA survey - the largest sample of galaxies observed via Integral Field Spectroscopy (IFS, $\sim$ 8400 objects). We find that $\Sigma_{\rm SFR}$ tightly correlates with $\Sigma_{\rm b}$. Furthermore, we derive an empirical relation between the $\Sigma_{\rm SFR}$ and a second degree polynomial of $\Sigma_{\rm b}$ yielding a one-to-one relation between these two observables. Both, $\Sigma_{\rm b}$ and its polynomial form show a stronger correlation and smaller scatter with respect to $\Sigma_{\rm SFR}$ than the relations derived using the individual components of $\Sigma_{\rm b}$. Our results suggest that indeed these three parameters are physically correlated, suggesting a scenario in which the two components of the baryonic mass regulate the star-formation activity at kpc scales., Comment: 10 pages, 6 Figures, accepted for publication in ApJ

We used VLT/XShooter to target a sample of nearby analogs of Lyman Break Galaxies (LBGs). These Lyman Break Analogs (LBAs) are similar to the LBGs in many of their physical properties. We determine electron temperatures using the weak [O III]4363 emission line, and determine the oxygen abundance (O/H) using the direct and strong line methods. We show that the direct and strong line abundances are consistent with established relations within ~0.2 dex. The analogs have nitrogen-to-oxygen ratios (N/O) and ionization parameters (q) that are, on average, offset with respect to typical local galaxies but similar to galaxies at z ~ 2 and other analogs. The N/O and q excesses correlate with the offsets observed in the strong line ratios, again similar to z ~ 2. The star formation rate surface densities are consistent with the high electron density and ionization, indicating that the interstellar medium (ISM) pressure is set by feedback from the starbursts. For a given O/H, the apparent N/O excess arises due to the offset in O/H with respect to the local mass-metallicity relation. This can be explained by recent inflow of relatively metal-poor gas which lowers O/H while leaving N/O unchanged. The difficulties in determining even basic ISM parameters in these nearby analogs illustrates some of the challenges we face at much higher redshifts, where similar rest-frame optical diagnostics for large samples of galaxies can be accessed with JWST., Comment: ApJ, accepted (25 pages, 21 figures)

A key task of observational extragalactic astronomy is to determine where -- within galaxies of diverse masses and morphologies -- stellar mass growth occurs, how it depends on galaxy properties and what processes regulate star formation. Using spectroscopic indices derived from the stellar continuum at $\sim 4000$\AA, we determine the spatially resolved star-formation histories of 980000 spaxels in 2404 galaxies in the SDSS-IV MaNGA IFU survey. We examine the spatial distribution of star-forming, quiescent, green valley, starburst and post-starburst spaxels as a function of stellar mass and morphology to see where and in what types of galaxy star formation is occurring. The spatial distribution of star-formation is dependent primarily on stellar mass, with a noticeable change in the distribution at \mstar$>10^{10}$\msun. Galaxies above this mass have an increasing fraction of regions that are forming stars with increasing radius, whereas lower mass galaxies have a constant fraction of star forming regions with radius. Our findings support a picture of inside-out growth and quenching at high masses. We find that morphology (measured via concentration) correlates with the fraction of star-forming spaxels, but not with their radial distribution. We find (post-)starburst regions are more common outside of the galaxy centre, are preferentially found in asymmetric galaxies, and have lower gas-phase metallicity than other regions, consistent with interactions triggering starbursts and driving low metallicity gas into regions at $<1.5R_e$., Comment: Accepted for publication in MNRAS. 16 pages, 14 figures

For the first time both H$\alpha$ and far-ultraviolet (FUV) observations from an HI-selected sample are used to determine the dust-corrected star formation rate density (SFRD: $\dot{\rho}$) in the local Universe. Applying the two star formation rate indicators on 294 local galaxies we determine log($\dot{\rho}$$ _{H\alpha}) = -1.68~^{+0.13}_{-0.05}$ [M$_{\odot} $ yr$^{-1} $ Mpc$^{-3}]$ and log($\dot{\rho}_{FUV}$) $ = -1.71~^{+0.12}_{-0.13}$ [M$_\odot $ yr$^{-1} $ Mpc$^{-3}]$. These values are derived from scaling H$\alpha$ and FUV observations to the HI mass function. Galaxies were selected to uniformly sample the full HI mass (M$_{HI}$) range of the HI Parkes All-Sky Survey (M$_{HI} \sim10^{7}$ to $\sim10^{10.7}$ M$_{\odot}$). The approach leads to relatively larger sampling of dwarf galaxies compared to optically-selected surveys. The low HI mass, low luminosity and low surface brightness galaxy populations have, on average, lower H$\alpha$/FUV flux ratios than the remaining galaxy populations, consistent with the earlier results of Meurer. The near-identical H$\alpha$- and FUV-derived SFRD values arise with the low H$\alpha$/FUV flux ratios of some galaxies being offset by enhanced H$\alpha$ from the brightest and high mass galaxy populations. Our findings confirm the necessity to fully sample the HI mass range for a complete census of local star formation to include lower stellar mass galaxies which dominate the local Universe., Comment: 17 pages, 7 figures

We determine the local metallicity of the ionized gas for more than $9.2\times 10^5$ star forming regions (spaxels) located in 1023 nearby galaxies included in the SDSS-IV MaNGA IFU survey. We use the dust extinction derived from the Balmer decrement and stellar template fitting in each spaxel to estimate the local gas and stellar mass densities, respectively. We also use the measured rotation curves to determine the local escape velocity ($\mathrm{V_{esc}}$). We have then analyze the relationships between the local metallicity and both the local gas fraction ($\mu$) and $\mathrm{V_{esc}}$. We find that metallicity decreases with both increasing $\mu$ and decreasing $\mathrm{V_{esc}}$. By examining the residuals in these relations we show that the gas fraction plays a more primary role in the local chemical enrichment than $\mathrm{V_{esc}}$. We show that the gas-regulator model of chemical evolution provides a reasonable explanation of the metallicity on local scales. The best-fit parameters for this model are consistent with metal loss caused by momentum-driven galactic outflows. We also argue that both the gas fraction and local escape velocity are connected to the local stellar surface density, which in turn is a tracer of the epoch at which the dominant local stellar population formed, Comment: 19 Pages, 13 Figures, ApJ accepted

We present the integrated stellar mass-metallicity relation (MZR) for more than 1700 galaxies included in the integral field area SDSS-IV MaNGA survey. The spatially resolved data allow us to determine the metallicity at the same physical scale (effective radius in arcsecs, $\mathrm{R_{eff}}$ ) using a heterogeneous set of ten abundance calibrators. Besides scale factors, the shape of the MZR is similar for all calibrators, consistent with those reported previously using single-fiber and integral field spectroscopy. We compare the residuals of this relation against the star formation rate (SFR) and specific SFR (sSFR). We do not find a strong secondary relation of the MZR with either SFR or the sSFR for any of the calibrators, in contrast with previous single-fiber spectroscopic studies. Our results agree with an scenario in which metal enrichment happens at local scales, with global outflows playing a secondary role in shaping the chemistry of galaxies and cold-gas inflows regulating the stellar formation., Comment: 10 pages, 9 Figures. Accepted for publication in ApJ

We present Herschel PACS observations of the [CII] 158 micron emission line in a sample of 24 intermediate mass (9

Pan-STARRS1 has carried out a set of distinct synoptic imaging sky surveys including the $3\pi$ Steradian Survey and the Medium Deep Survey in 5 bands ($grizy_{P1}$). The mean 5$\sigma$ point source limiting sensitivities in the stacked 3$\pi$ Steradian Survey in $grizy_{P1}$ are (23.3, 23.2, 23.1, 22.3, 21.4) respectively. The upper bound on the systematic uncertainty in the photometric calibration across the sky is 7-12 millimag depending on the bandpass. The systematic uncertainty of the astrometric calibration using the Gaia frame comes from a comparison of the results with Gaia: the standard deviation of the mean and median residuals ($ \Delta ra, \Delta dec $) are (2.3, 1.7) milliarcsec, and (3.1, 4.8) milliarcsec respectively. The Pan-STARRS system and the design of the PS1 surveys are described and an overview of the resulting image and catalog data products and their basic characteristics are described together with a summary of important results. The images, reduced data products, and derived data products from the Pan-STARRS1 surveys are available to the community from the Mikulski Archive for Space Telescopes (MAST) at STScI., Comment: 38 pages, 29 figures, 12 tables

We examine the HI-based star formation efficiency (SFE_HI), the ratio of star formation rate to the atomic Hydrogen (HI) mass, in the context of a constant stability star-forming disk model. Our observations of HI-selected galaxies show SFE to be fairly constant (log SFE_HI = -9.65 yr-1 with a dispersion of 0.3 dex) across ~5 orders of magnitude in stellar masses. We present a model to account for this result, whose main principle is that the gas within galaxies forms a uniform stability disk and that stars form within the molecular gas in this disk. We test two versions of the model differing in the prescription that determines the molecular gas fraction, based on either the hydrostatic pressure, or the stellar surface density of the disk. For high-mass galaxies such as the Milky Way, we find that either prescription predicts SFE_HI similar to the observations. However, the hydrostatic pressure prescription is a more accurate SFE_HI predictor for low-mass galaxies. Our model is the first model that links the uniform SFE_HI observed in galaxies at low redshifts to star-forming disks with constant marginal stability. While the rotational amplitude Vmax is the primary driver of disk structure in our model, we find the specific angular momentum of the galaxy may play a role in explaining a weak correlation between SFE_HI and effective surface brightness of the disk., Comment: 14 pages, 12 figures, accepted for publication in MNRAS

We have mapped the superwind/halo region of the nearby starburst galaxy M82 in the mid-infrared with $Spitzer-IRS$. The spectral regions covered include the H$_2 S(1)-S(3)$, [NeII], [NeIII] emission lines and PAH features. We estimate the total warm H$_2$ mass and the kinetic energy of the outflowing warm molecular gas to be between $M_{warm}\sim5-17\times10^6$ M$_{\odot}$ and $E_{K}\sim6-20\times10^{53}$ erg. Using the ratios of the 6.2, 7.7 and 11.3 micron PAH features in the IRS spectra, we are able to estimate the average size and ionization state of the small grains in the superwind. There are large variations in the PAH flux ratios throughout the outflow. The 11.3/7.7 and the 6.2/7.7 PAH ratios both vary by more than a factor of five across the wind region. The Northern part of the wind has a significant population of PAH's with smaller 6.2/7.7 ratios than either the starburst disk or the Southern wind, indicating that on average, PAH emitters are larger and more ionized. The warm molecular gas to PAH flux ratios (H$_2/PAH$) are enhanced in the outflow by factors of 10-100 as compared to the starburst disk. This enhancement in the H$_2/PAH$ ratio does not seem to follow the ionization of the atomic gas (as measured with the [NeIII]/[NeII] line flux ratio) in the outflow. This suggests that much of the warm H$_2$ in the outflow is excited by shocks. The observed H$_2$ line intensities can be reproduced with low velocity shocks ($v < 40$ km s$^{-1}$) driven into moderately dense molecular gas ($10^2

Galaxies in Hickson Compact Group 91 (HCG 91) were observed with the WiFeS integral field spectrograph as part of our ongoing campaign targeting the ionized gas physics and kinematics inside star forming members of compact groups. Here, we report the discovery of HII regions with abundance and kinematic offsets in the otherwise unremarkable star forming spiral HCG 91c. The optical emission line analysis of this galaxy reveals that at least three HII regions harbor an oxygen abundance ~0.15 dex lower than expected from their immediate surroundings and from the abundance gradient present in the inner regions of HCG 91c. The same star forming regions are also associated with a small kinematic offset in the form of a lag of 5-10 km/s with respect to the local circular rotation of the gas. HI observations of HCG 91 from the Very Large Array and broadband optical images from Pan-STARRS suggest that HCG 91c is caught early in its interaction with the other members of HCG 91. We discuss different scenarios to explain the origin of the peculiar star forming regions detected with WiFeS, and show that evidence point towards infalling and collapsing extra-planar gas clouds at the disk-halo interface, possibly as a consequence of long-range gravitational perturbations of HCG 91c from the other group members. As such, HCG 91c provides evidence that some of the perturbations possibly associated with the early phase of galaxy evolution in compact groups impact the star forming disk locally, and on sub-kpc scales., Comment: 25 pages, 21 figures, MNRAS accepted. Until publication of the article, the interactive component of Figure 4 is available at this URL: http://www.mso.anu.edu.au/~fvogt/website/misc.html

We present the results of an analysis of Pan-STARRS1 Medium Deep Survey multi-band (g,r,i,z,y) images of a sample of 698 low-redshift disk galaxies that span broad ranges in stellar mass, star-formation rate, and bulge/disk ratio. We use population synthesis SED fitting techniques to explore the radial distribution of the light, color, surface mass density, mass/light ratio, and age of the stellar populations. We characterize the structure and stellar content of the galaxy disks out to radii of about twice Petrosian r90, beyond which the halo light becomes significant. We measure normalized radial profiles for sub-samples of galaxies in three bins each of stellar mass and concentration. We also fit radial profiles to each galaxy. The majority of galaxies have down-bending radial surface brightness profiles in the bluer bands with a break radius at roughly r90. However, they typically show single unbroken exponentials in the reddest bands and in the stellar surface mass density. We find that the mass/light ratio and stellar age radial profiles have a characteristic "U-shape". There is a good correlation between the amplitude of the down-bend in the surface brightness profile and the rate of the increase in the M/L ratio in the outer disk. As we move from late- to early-type galaxies, the amplitude of the down-bend and the radial gradient in M/L both decrease. Our results imply a combination of stellar radial migration and suppression of recent star formation can account for the stellar populations of the outer disk., Comment: Accepted for publication in ApJ

We present a study of the prevalence and luminosity of Active Galactic Nuclei (AGN; traced by optical spectra) as a function of both environment and galaxy interactions. For this study we used a sample of more than 250000 galaxies drawn from the Sloan Digital Sky Survey and, crucially, we controlled for the effect of both stellar mass and central star formation activity. Once these two factors are taken into account, the effect of the local density of galaxies and of one-on-one interactions is minimal in both the prevalence of AGN activity and AGN luminosity. This suggests that the level of nuclear activity depends primarily on the availability of cold gas in the nuclear regions of galaxies and that secular processes can drive the AGN activity in the majority of cases. Large scale environment and galaxy interactions only affect AGN activity in an indirect manner, by influencing the central gas supply., Comment: 7 pages, 5 figures, 1 table, accepted for publication in MNRAS

We have carried out extensive and detailed photoionization modeling to successfully constrain the locations of different emission-line galaxies in optical and mid-infrared diagnostic diagrams. Our model grids cover a wide range in parameter space for the active galaxy continuum and starburst galaxies with different stellar population laws and metallicities. We compare the predicted AGN and star-formation mid-infrared line ratios [Ne III]15.56mm/[Ne II]12.81mm and [O IV]25.89mm/[Ne III]15.56mm to the observed values, and find that the best fit for the AGN is via a two-zone approximation. This two-zone approximation is a combination of a matter-bounded component, where [Ne III] and [O IV] are emitted efficiently, and a radiation-bounded component that maximizes [Ne II] emission. We overlay the predictions from this two-zone approximation onto the optical [O III]l5007/Hbeta and [N II]l6583/Halpha diagnostic diagram derived from the Sloan Digital Sky Survey, to find that the high-density and low-ionization radiation-bounded component in our two-zone AGN approximation model provides a good lower limit for [N II] emission. This establishes a new theoretical demarcation line for the minimum AGN contribution in this diagram. This new classification results by a factor of ~1.4 in a higher AGN population than predictions derived from previous divisions of star-forming galaxies. Similarly, we define a maximum AGN contribution in the [O III]/Hbeta and [N II]/Halpha diagram by using a two-zone approximation within a parameter range typical of the narrow-line region., Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal. The paper contains 13 pages, 16 figures

We have conducted a high-resolution spectroscopic study using Spitzer of 18 bulgeless (Sd/Sdm) galaxies that show no definitive signatures of nuclear activity in their optical spectra. This is the first systematic mid-IR search for weak or hidden AGNs in a statistically significant sample of bulgeless disk galaxies. Based on the detection of the high-ionization [NeV] line, we report the discovery of an AGN in one out of the 18 galaxies in the sample. This galaxy, NGC 4178, is a nearby edge-on Sd galaxy, which likely hosts a prominent nuclear star cluster (NSC). The bolometric luminosity of the AGN inferred from the [NeV] luminosity is ~ 8e41 ergs/s. This is almost two orders of magnitude greater than the luminosity of the AGN in NGC 4395, the best studied AGN in a bulgeless disk galaxy. Assuming that the AGN in NGC 4178 is radiating below the Eddington limit, the lower mass limit for the black hole is ~ 6e3M_sun. The fact that none of the other galaxies in the sample shows any evidence for an AGN demonstrates that while the AGN detection rate based on mid-IR diagnostics is high (30-40%) in optically quiescent galaxies with pseudobulges, it drops drastically in Sd/Sdm galaxies. Our observations therefore confirm that AGNs in completely bulgeless disk galaxies are not hidden in the optical but truly are rare. Of the three Sd galaxies with AGNs known so far, all have prominent NSCs, suggesting that in the absence of a well-defined bulge, the galaxy must possess a NSC in order to host an AGN. While the presence of a NSC appears to be a requirement for hosting an AGN in bulgeless galaxies, neither the properties of the NSC nor those of the host galaxy appear exceptional in late-type AGN hosts. The recipe for forming and growing a central black hole in a bulgeless galaxy therefore remains unknown., Comment: 17 pages, 8 figures, accepted for publication in ApJ