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: We present a method to estimate the total gas column density, dust-to-gas and dust-to-metal ratios of distant galaxies from rest-frame optical spectra. The technique exploits the sensitivity of certain optical lines to changes in depletion of metals on to dust grains and uses photoionization models to constrain these physical ratios along with the metallicity and dust column density. We compare our gas column density estimates with $\mathrm{H\,\small {I}}$ and CO gas mass estimates in nearby galaxies to show that we recover their total gas mass surface density to within a factor of 2 up to a total surface gas mass density of ~75 M pc –2 . Our technique is independent of the conversion factor of CO to H 2 and we show that a metallicity-dependent X CO is required to achieve good agreement between our measurements and that provided by CO and $\mathrm{H\,\small {I}}$ . However, we also show that our method cannot be reliably aperture corrected to total integrated gas mass. We calculate dust-to-gas ratios for all star-forming galaxies in the Sloan Digital Sky Survey Data Release 7 and show that the resulting dependence on metallicity agrees well with the trend inferred from modelling of the dust emission of nearby galaxies using far-IR data. We also present estimates of the variation of the dust-to-metal ratio with metallicity and show that this is poorly constrained at metallicities below 50 per cent solar. We conclude with a study of the inventory of gas in the central regions, defined both in terms of a fixed physical radius and as a fixed fraction of the half-light radius, of ~70 000 star-forming galaxies from the Sloan Digital Sky Survey. We show that their central gas content and gas depletion time are not accurately predicted by a single parameter, but in agreement with recent studies we find that a combination of the stellar mass and some measure of central concentration provides a good predictor of gas content in galaxies. We also identify a population of galaxies with low surface densities of stars and very long gas depletion times.

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 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), [Ne ii ], [Ne iii ] emission lines and polycyclic aromatic hydrocarbon (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  ~ 5 and 17  x 10 6 M and E K  ~ 6 and 20  x 10 53  erg. Using the ratios of the 6.2, 7.7 and 11.3 μm 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 5 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 disc 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 disc. This enhancement in the H 2 /PAH ratio does not seem to follow the ionization of the atomic gas (as measured with the [Ne iii ]/[Ne ii ] 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  〈  n H  〈 10 4 cm –3 ) entrained in the outflow.

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.