ALBERT

Description: We study the spatially resolved excitation properties of the ionized gas in a sample of 646 galaxies using integral field spectroscopy data from the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) programme. Making use of Baldwin–Philips–Terlevich diagnostic diagrams we demonstrate the ubiquitous presence of extended (kpc scale) low-ionization emission-line regions (LIERs) in both star-forming and quiescent galaxies. In star-forming galaxies LIER emission can be associated with diffuse ionized gas, most evident as extraplanar emission in edge-on systems. In addition, we identify two main classes of galaxies displaying LIER emission: ‘central LIER’ (cLIER) galaxies, where central LIER emission is spatially extended, but accompanied by star formation at larger galactocentric distances, and ‘extended LIER’ (eLIER) galaxies, where LIER emission is extended throughout the whole galaxy. In eLIER and cLIER galaxies, LIER emission is associated with radially flat, low H α equivalent width of line emission (〈3 Å) and stellar population indices demonstrating the lack of young stellar populations, implying that line emission follows tightly the continuum due to the underlying old stellar population. The H α surface brightness radial profiles are always shallower than 1/r 2 and the line ratio [O iii ] 5007/[O ii ] 3727,29 (a tracer of the ionization parameter of the gas) shows a flat gradient. This combined evidence strongly supports the scenario in which LIER emission is not due to a central point source but to diffuse stellar sources, the most likely candidates being hot, evolved (post-asymptotic giant branch) stars. Shocks are observed to play a significant role in the ionization of the gas only in rare merging and interacting systems.

Description: 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 H ii 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 H ii regions harbour 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 –1 with respect to the local circular rotation of the gas. H i observations of HCG 91 from the Very Large Array and broad-band optical images from Pan-STARRS (Panoramic Survey Telescope And Rapid Response System) 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 points towards infalling and collapsing extraplanar gas clouds at the disc–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 disc locally, and on sub-kpc scales.

Description: 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 250 000 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.

Description: We examine the H i -based star formation efficiency ( ${\rm SFE}_{{\rm H\,\small {I}}}$ ), the ratio of star formation rate to the atomic hydrogen (H i ) mass, in the context of a constant stability star-forming disc model. Our observations of H i -selected galaxies show ${\rm SFE}_{{\rm H\,\small {I}}}$ to be fairly constant (log ${\rm SFE}_{{\rm H\,\small {I}}}=-9.65$  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 disc and that stars form within the molecular gas in this disc. 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 disc. For high-mass galaxies such as the Milky Way, we find that either prescription predicts ${\rm SFE}_{{\rm H\,\small {I}}}$ similar to the observations. However, the hydrostatic pressure prescription is a more accurate ${\rm SFE}_{{\rm H\,\small {I}}}$ predictor for low-mass galaxies. Our model is the first model that links the uniform ${\rm SFE}_{{\rm H\,\small {I}}}$ observed in galaxies at low redshifts to star-forming discs with constant marginal stability. While the rotational amplitude V max is the primary driver of disc structure in our model, we find that the specific angular momentum of the galaxy may play a role in explaining a weak correlation between ${\rm SFE}_{{\rm H\,\small {I}}}$ and effective surface brightness of the disc.

Description: We present a systematic analysis of the rotation curves of 187 galaxies with stellar masses greater than 10 10 M , with atomic gas masses from the GALEX Arecibo Sloan Survey (GASS) and with follow-up long-slit spectroscopy from the MMT. Our analysis focuses on stellar rotation curves derived by fitting stellar template spectra to the galaxy spectra binned along the slit. In this way, we are able to obtain accurate rotation velocity measurements for a factor of 2 more galaxies than possible with the Hα line. Galaxies with high atomic gas mass fractions are the most dark-matter-dominated galaxies in our sample and have dark matter halo density profiles that are to first order well described by Navarro–Frenk–White profiles with an average concentration parameter of 10. The inner slopes of the rotation curves correlate more strongly with stellar population age than with galaxy mass or structural parameters. At fixed stellar mass, the rotation curves of more actively star-forming galaxies have steeper inner slopes than less actively star-forming galaxies. The ratio between the galaxy specific angular momentum and the total specific angular momentum of its dark matter halo, R j , correlates strongly with galaxy mass, structure and gas content. Low-mass, disc-dominated galaxies with atomic gas mass fractions greater than 20 per cent have median values of R j of around 1, but massive, bulge-dominated galaxies have R j  = 0.2–0.3. We argue that these trends can be understood in a picture where gas inflows triggered by disc instabilities lead to the formation of passive, bulge-dominated galaxies with low specific angular momentum.

Description: We study the stellar mass Tully–Fisher relation (TFR; stellar mass versus rotation velocity) for a morphologically blind selection of emission line galaxies in the field at redshifts 0.1 〈  z  〈 0.375. Kinematics ( g , V rot ) are measured from emission lines in Keck/DEIMOS spectra and quantitative morphology is measured from V - and I -band Hubble images. We find a transition stellar mass in the TFR, log M * /M  = 9.5. Above this mass, nearly all galaxies are rotation dominated, on average more morphologically disc-like according to quantitative morphology, and lie on a relatively tight TFR. Below this mass, the TFR has significant scatter to low rotation velocity and galaxies can either be rotation-dominated discs on the TFR or asymmetric or compact galaxies which scatter off. We refer to this transition mass as the ‘mass of disc formation’, M df because above it all star-forming galaxies form discs (except for a small number of major mergers and highly star-forming systems), whereas below it a galaxy may or may not form a disc.

Description: Radio emission from radio-quiet quasars may be due to star formation in the quasar host galaxy, to a jet launched by the supermassive black hole, or to relativistic particles accelerated in a wide-angle radiatively driven outflow. In this paper, we examine whether radio emission from radio-quiet quasars is a byproduct of star formation in their hosts. To this end, we use infrared spectroscopy and photometry from Spitzer and Herschel to estimate or place upper limits on star formation rates in hosts of ~300 obscured and unobscured quasars at z 〈 1. We find that low-ionization forbidden emission lines such as [Ne ii ] and [Ne iii ] are likely dominated by quasar ionization and do not provide reliable star formation diagnostics in quasar hosts, while polycyclic aromatic hydrocarbon (PAH) emission features may be suppressed due to the destruction of PAH molecules by the quasar radiation field. While the bolometric luminosities of our sources are dominated by the quasars, the 160 μm fluxes are likely dominated by star formation, but they too should be used with caution. We estimate median star formation rates to be 6–29 M  yr –1 , with obscured quasars at the high end of this range. This star formation rate is insufficient to explain the observed radio emission from quasars by an order of magnitude, with log ( L radio, obs / L radio, SF ) = 0.6–1.3 depending on quasar type and star formation estimator. Although radio-quiet quasars in our sample lie close to the 8–1000 μm infrared/radio correlation characteristic of the star-forming galaxies, both their infrared emission and their radio emission are dominated by the quasar activity, not by the host galaxy.

Description: We analyse a sample of 30 000 nearby obscured active galactic nuclei (AGNs) with optical spectra from Sloan Digital Sky Survey (SDSS) and mid-IR photometry from Wide-field Infrared Survey Explorer . Our aim is to investigate the AGN host galaxy properties with mid-IR luminosities as AGN activity indicator, and to compare with previous studies based on [O iii ] emission lines. First we find that the [3.4]–[4.6] colour has weak dependence on host stellar age, but strong dependence on AGN activity. We then use a ‘pair-matching’ technique to subtract the host 4.6 μm contribution. By combining Seyferts with a sample of SDSS quasars at z  〈 0.7, we show that the [O iii ] and the intrinsic AGN 4.6 μm luminosities correlate roughly linearly over 4 orders of magnitude, but with substantial scatter. We also compare the partition functions of the total integrated 4.6 μm and [O iii ] line luminosities from Seyferts and a sub-population of low ionization nuclear emission-line regions (LINERs) with significant nuclear 4.6 μm emission, as a function of a variety of host galaxy properties, finding that they are identical. We conclude, therefore, that [O iii ] as an AGN indicator shows no particular biases as compared to the 4.6 μm luminosity. Our results also demonstrate that some LINERs do fit in with the expectations of the simple Unified Model.

Description: We develop a model for the distribution of the interstellar medium (ISM) and star formation in galaxies based on recent studies that indicate that galactic discs stabilize to a constant stability parameter, which we combine with prescriptions of how the phases of the ISM are determined and for the star formation law (SFL). The model predicts the gas surface mass density and star formation intensity of a galaxy given its rotation curve, stellar surface mass density and the gas velocity dispersion. This model is tested on radial profiles of neutral and molecular ISM surface mass density and star formation intensity of 12 galaxies selected from the H i Nearby Galaxy Survey sample. Our tests focus on intermediate radii (0.3 to 1 times the optical radius) because there are insufficient data to test the outer discs and the fits are less accurate in detail in the centre. Nevertheless, the model produces reasonable agreement with the ISM mass and star formation rate integrated over the central region in all but one case. To optimize the model, we evaluate four recipes for the stability parameter, three recipes for apportioning the ISM into molecular and neutral components, and eight versions of the SFL. We find no clear-cut best prescription for the two-fluid (gas and stars) stability parameter Q 2f and therefore for simplicity, we use the Wang and Silk approximation ( Q WS ). We found that an empirical scaling between the molecular-to-neutral ISM ratio ( R mol ) and the stellar surface mass density proposed by Leroy et al. works marginally better than the other two prescriptions for this ratio in predicting the ISM profiles, and noticeably better in predicting the star formation intensity from the ISM profiles produced by our model with the SFLs we tested. Thus, in the context of our modelled ISM profiles, the linear molecular SFL and the two-component SFL work better than the other prescriptions we tested. We incorporate these relations into our ‘constant Q disc’ model.