Edge-on Galaxies Research Articles

Probing star formation and ISM properties using galaxy disk inclination

Disk galaxies at intermediate redshift (z ~ 0.7) have been found in previous work to display more optically thick behaviour than their local counterparts in the rest-frame B-band surface brightness, suggesting an evolution in dust properties over the past ~6 Gyr. We compare the measured luminosities of face-on and edge-on star-forming galaxies at different wavelengths (Ultraviolet (UV), mid-infrared (MIR), far-infrared (FIR), and radio) for two well-matched samples of disk-dominated galaxies: a local Sloan Digital Sky Survey (SDSS)-selected sample at z ~ 0.07 and a sample of disks at z ~ 0.7 drawn from Cosmic Evolution Survey (COSMOS). We have derived correction factors to account for the inclination dependence of the parameters used for sample selection. We find that typical galaxies are transparent at MIR wavelengths at both redshifts, and that the FIR and radio emission is also transparent as expected. However, reduced sensitivity at these wavelengths limits our analysis; we cannot rule out opacity in the FIR or radio. Ultra-violet attenuation has increased between z ~ 0 and z ~ 0.7, with the z ~ 0.7 sample being a factor of ~3.4 more attenuated. The larger UV attenuation at z ~ 0.7 can be explained by more clumpy dust around nascent star-forming regions. There is good agreement between the fitted evolution of the normalisation of the SFRUV versus 1 − cos(i) trend (interpreted as the clumpiness fraction) and the molecular gas fraction/dust fraction evolution of galaxies found out to z < 1.

SN 2015ba: a Type IIP supernova with a long plateau

We present optical photometry and spectroscopy from about a week after explosion to $\sim$272 d of an atypical Type IIP supernova, SN 2015ba, which exploded in the edge-on galaxy IC 1029. SN 2015ba is a luminous event with an absolute V-band magnitude of -17.1$\pm$0.2 mag at 50 d since explosion and has a long plateau lasting for $\sim$123 d. The distance to the SN is estimated to be 34.8$\pm$0.7 Mpc using the expanding photosphere and standard candle methods. High-velocity H-Balmer components constant with time are observed in the late-plateau phase spectra of SN 2015ba, which suggests a possible role of circumstellar interaction at these phases. Both hydrodynamical and analytical modelling suggest a massive progenitor of SN 2015ba with a pre-explosion mass of 24-26 M$_\odot$. However, the nebular spectra of SN 2015ba exhibit insignificant levels of oxygen, which is otherwise expected from a massive progenitor. This might be suggestive of the non-monotonical link between O-core masses and the zero-age main-sequence mass of pre-supernova stars and/or uncertainties in the mixing scenario in the ejecta of supernovae.

The Tully–Fisher relation for flat galaxies

We construct a multiparametric Tully-Fisher (TF) relation for a large sample of edge-on galaxies from the Revised Flat Galaxy Catalog using HI data from the EDD database and parameters from the EGIS catalog. We incorporate a variety of additional parameters including structural parameters of edge-on galaxies in different bandpasses. Besides the rotation curve maximum, only the HI-to-optical luminosity ratio and optical colours play a statistically significant role in the multiparametric TF relation. We are able to decrease the standard deviation of the multiparametric TF relation down to 0.32 mag, which is at the level of best modern samples of galaxies used for studies of the matter motion in the Universe via the TF-relation.

A New Quadruple-image Gravitational Lens in an Edge-on Disk Galaxy at z = 0.0956

We report the serendipitous discovery of a quadruply-lensed source behind the z=0.095 edge-on disk galaxy 2MASXJ13170000-1405187, based on public imaging survey data from Pan-STARRS PS1 and the VISTA Hemisphere Survey. Follow-up imaging from Magellan/LDSS3 shows that the background source is spatially extended (i.e. not a QSO), and that two of the lensed images are observed through a prominent dust ring in the disk of the lens galaxy. We summarise results of preliminary modelling, which indicates an Einstein radius of 1.44 arcsec, and a K-band mass-to-light ratio of 0.5, relative to the solar value.

The first VLBI detection of a spiral DRAGN core

Abstract We present the first observation of 0313−192, the archetypal spiral DRAGN, at very long baseline interferometry (VLBI) resolutions. Spiral DRAGNs are Double-lobed Radio sources Associated with Galactic Nuclei (DRAGNs) that are hosted by spiral galaxies. 0313−192 is an edge-on spiral galaxy that appears to host a 360 kpc double-lobed radio source. The core of this galaxy is clearly detected at L, S, and X bands using the Very Long Baseline Array, signifying an ongoing active nucleus in the galaxy. This rules out the possibility that the spiral DRAGN is merely a chance alignment. The radio core has L1.4 GHz ∼ 3.0 × 1023 W Hz−1. Radio components are detected to the south-west of the core, but there are no detections of a counterjet. Assuming a symmetric, relativistic jet, we estimate an upper limit to the inclination angle of θ ≲ 72 deg. The VLBI-detected radio jet components are extremely well aligned with the larger scale radio source suggesting little to no jet disruption or interaction with the interstellar medium of the host galaxy.

Extraplanar H ii Regions in Spiral Galaxies. II. In Situ Star Formation in the Interstellar Thick Disk of NGC 4013

Abstract We present observations of an Hα-emitting knot in the thick disk of NGC 4013, demonstrating it is an H ii region surrounding a cluster of young hot stars z = 860 pc above the plane of this edge-on spiral galaxy. With LBT/MODS spectroscopy we show that this H ii region has an Hα luminosity ∼4–7 times that of the Orion nebula, with an implied ionizing photon production rate log Q 0 ≈ 49.4 (photons s−1). HST/WFPC2 imaging reveals an associated blue continuum source with M V = −8.21 ± 0.24. Together, these properties demonstrate that the H ii region is powered by a young cluster of stars formed in situ in the thick disk, with an ionizing photon flux equivalent to ∼6 O7 V stars. If we assume ≈6 other extraplanar Hα-emitting knots are H ii regions, the total thick disk star formation rate of NGC 4013 is ∼5 × 10−4 M ⊙ yr−1. The star formation likely occurs in the dense clouds of the interstellar thick disk seen in optical images of dust extinction and CO emission.

Gravitational lensing by eigenvalue distributions of random matrix models

We propose to use eigenvalue densities of unitary random matrix ensembles as mass distributions in gravitational lensing. The corresponding lens equations reduce to algebraic equations in the complex plane which can be treated analytically. We prove that these models can be applied to describe lensing by systems of edge-on galaxies. We illustrate our analysis with the Gaussian and the quartic unitary matrix ensembles.

CHANG-ES

Aim.The vertical halo scale height is a crucial parameter to understand the transport of cosmic-ray electrons (CRE) and their energy loss mechanisms in spiral galaxies. Until now, the radio scale height could only be determined for a few edge-on galaxies because of missing sensitivity at high resolution.Methods.We developed a sophisticated method for the scale height determination of edge-on galaxies. With this we determined the scale heights and radial scale lengths for a sample of 13 galaxies from the CHANG-ES radio continuum survey in two frequency bands.Results.The sample average values for the radio scale heights of the halo are 1.1 ± 0.3 kpc inC-band and 1.4 ± 0.7 kpc inL-band. From the frequency dependence analysis of the halo scale heights we found that the wind velocities (estimated using the adiabatic loss time) are above the escape velocity. We found that the halo scale heights increase linearly with the radio diameters. In order to exclude the diameter dependence, we defined a normalized scale heighth˜which is quite similar for all sample galaxies at both frequency bands and does not depend on the star formation rate or the magnetic field strength. However,h˜shows a tight anticorrelation with the mass surface density.Conclusions.The sample galaxies with smaller scale lengths are more spherical in the radio emission, while those with larger scale lengths are flatter. The radio scale height depends mainly on the radio diameter of the galaxy. The sample galaxies are consistent with an escape-dominated radio halo with convective cosmic ray propagation, indicating that galactic winds are a widespread phenomenon in spiral galaxies. While a higher star formation rate or star formation surface density does not lead to a higher wind velocity, we found for the first time observational evidence of a gravitational deceleration of CRE outflow, e.g. a lowering of the wind velocity from the galactic disk.

Improving galaxy morphologies for SDSS with Deep Learning

Abstract We present a morphological catalogue for ∼670 000 galaxies in the Sloan Digital Sky Survey in two flavours: T-type, related to the Hubble sequence, and Galaxy Zoo 2 (GZ2 hereafter) classification scheme. By combining accurate existing visual classification catalogues with machine learning, we provide the largest and most accurate morphological catalogue up to date. The classifications are obtained with Deep Learning algorithms using Convolutional Neural Networks (CNNs). We use two visual classification catalogues, GZ2 and Nair & Abraham (2010), for training CNNs with colour images in order to obtain T-types and a series of GZ2 type questions (disc/features, edge-on galaxies, bar signature, bulge prominence, roundness, and mergers). We also provide an additional probability enabling a separation between pure elliptical (E) from S0, where the T-type model is not so efficient. For the T-type, our results show smaller offset and scatter than previous models trained with support vector machines. For the GZ2 type questions, our models have large accuracy (>97 per cent), precision and recall values (>90 per cent), when applied to a test sample with the same characteristics as the one used for training. The catalogue is publicly released with the paper.

Resolving the disc–halo degeneracy – I: a look at NGC 628

The decomposition of the rotation curve of galaxies into contribution from the disc and dark halo remains uncertain and depends on the adopted mass to light ratio (M/L) of the disc. Given the vertical velocity dispersion of stars and disc scale height, the disc surface mass density and hence the M/L can be estimated. We address a conceptual problem with previous measurements of the scale height and dispersion. When using this method, the dispersion and scale height must refer to the same population of stars. The scale height is obtained from near-IR studies of edge-on galaxies and is weighted towards older kinematically hotter stars, whereas the dispersion obtained from integrated light in the optical bands includes stars of all ages. We aim to extract the dispersion for the hotter stars, so that it can then be used with the correct scale height to obtain the disc surface mass density. We use a sample of planetary nebulae (PNe) as dynamical tracers in the face-on galaxy NGC 628. We extract two different dispersions from its velocity histogram -- representing the older and younger PNe. We also present complementary stellar absorption spectra in the inner regions of this galaxy and use a direct pixel fitting technique to extract the two components. Our analysis concludes that previous studies, which do not take account of the young disc, underestimate the disc surface mass density by a factor of ~ 2. This is sufficient to make a maximal disc for NGC 628 appear like a submaximal disc.

The reports of thick discs’ deaths are greatly exaggerated

Recent studies have made the community aware of the importance of accounting for scattered light when examining low-surface-brightness galaxy features such as thick discs. In our past studies of the thick discs of edge-on galaxies in the Spitzer Survey of Stellar Structure in Galaxies – the S4G – we modelled the point spread function as a Gaussian. In this paper we re-examine our results using a revised point spread function model that accounts for extended wings out to more than 2 .́ 5. We study the 3.6 μm images of 141 edge-on galaxies from the S4G and its early-type galaxy extension. Thus, we more than double the samples examined in our past studies. We decompose the surface-brightness profiles of the galaxies perpendicular to their mid-planes assuming that discs are made of two stellar discs in hydrostatic equilibrium. We decompose the axial surface-brightness profiles of galaxies to model the central mass concentration – described by a Sérsic function – and the disc – described by a broken exponential disc seen edge-on. Our improved treatment fully confirms the ubiquitous occurrence of thick discs. The main difference between our current fits and those presented in our previous papers is that now the scattered light from the thin disc dominates the surface brightness at levels below μ ~ 26 mag arcsec-2. We stress that those extended thin disc tails are not physical, but pure scattered light. This change, however, does not drastically affect any of our previously presented results: 1) Thick discs are nearly ubiquitous. They are not an artefact caused by scattered light as has been suggested elsewhere. 2) Thick discs have masses comparable to those of thin discs in low-mass galaxies – with circular velocities vc< 120 km s-1 – whereas they are typically less massive than the thin discs in high-mass galaxies. 3) Thick discs and central mass concentrations seem to have formed at the same epoch from a common material reservoir. 4) Approximately 50% of the up-bending breaks in face-on galaxies are caused by the superposition of a thin and a thick disc where the scale-length of the latter is the largest.

CHANG-ES X: Spatially Resolved Separation of Thermal Contribution from Radio Continuum Emission in Edge-on Galaxies

Abstract We analyze the application of star formation rate calibrations using Hα and 22 μm infrared (IR) imaging data in predicting the thermal radio component for a test sample of three edge-on galaxies (NGC 891, NGC 3044, and NGC 4631) in the Continuum Halos in Nearby Galaxies—an EVLA Survey (CHANG-ES). We use a mixture of Hα and 24 μm calibration from Calzetti et al. and a linear 22 μm only calibration from Jarrett et al. on the test sample. We apply these relations on a pixel-to-pixel basis to create thermal prediction maps in the two CHANG-ES bands: L and C band (1.5 GHz and 6.0 GHz, respectively). We analyze the resulting nonthermal spectral index maps, and find a characteristic steepening of the nonthermal spectral index with vertical distance from the disk after application of all methods. We find possible evidence of extinction in the 22 μm data as compared to 70 μm Spitzer Multiband Imaging Photometer imaging in NGC 891. We analyze a larger sample of edge-on and face-on galaxy 25–100 μm flux ratios, and find that the ratios for edge-ons are systematically lower by a factor of 1.36, a result we attribute to excess extinction in the mid-IR in edge-ons. We introduce a new calibration for correcting the Hα luminosity for dust when galaxies are edge-on or very dusty.

An ALMA view of star formation efficiency suppression in early-type galaxies after gas-rich minor mergers

Gas-rich minor mergers contribute significantly to the gas reservoir of early-type galaxies (ETGs) at low redshift, yet the star formation efficiency (SFE; the star formation rate divided by the molecular gas mass) appears to be strongly suppressed following some of these events, in contrast to the more well-known merger-driven starbursts. We present observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of six ETGs, which have each recently undergone a gas-rich minor merger, as evidenced by their disturbed stellar morphologies. These galaxies were selected because they exhibit extremely low SFEs. We use the resolving power of ALMA to study the morphology and kinematics of the molecular gas. The majority of our galaxies exhibit spatial and kinematical irregularities, such as detached gas clouds, warps, and other asymmetries. These asymmetries support the interpretation that the suppression of the SFE is caused by dynamical effects stabilizing the gas against gravitational collapse. Through kinematic modelling we derive high velocity dispersions and Toomre Q stability parameters for the gas, but caution that such measurements in edge-on galaxies suffer from degeneracies. We estimate merger ages to be about 100~Myr based on the observed disturbances in the gas distribution. Furthermore, we determine that these galaxies lie, on average, two orders of magnitude below the Kennicutt-Schmidt relation for star-forming galaxies as well as below the relation for relaxed ETGs. We discuss potential dynamical processes responsible for this strong suppression of star formation surface density at fixed molecular gas surface density.

Messier 81’s Planck view versus its halo mapping

This paper is a follow-up of a previous paper about the M 82 galaxy and its halo based on Planck observations. As in the case of M 82, a substantial north-south and east-west temperature asymmetry is also found for the M 81 galaxy, extending up to galactocentric distances of about 1.5°. The temperature asymmetry is almost frequency independent and can be interpreted as a Doppler-induced effect related to the M 81 halo rotation and/or triggered by the gravitational interaction of the galaxies within the M 81 Group. Along with the analogous study of several nearby edge-on spiral galaxies, the cosmic microwave background temperature asymmetry method thus is shown to act as a direct tool to map the galactic haloes and/or the intergalactic bridges, invisible in other bands or by other methods.

Ionised gas structure of 100 kpc in an over-dense region of the galaxy group COSMOS-Gr30 at z ~ 0.7

We report the discovery of a 104 kpc2 gaseous structure detected in [O ii]λλ3727, 3729 in an over-dense region of the COSMOS-Gr30 galaxy group at z ~ 0.725 with deep MUSE Guaranteed Time Observations. We estimate the total amount of diffuse ionised gas to be of the order of (~5 ± 3) × 1010 M⊙ and explore its physical properties to understand its origin and the source(s) of the ionisation. The MUSE data allow the identification of a dozen group members that are embedded in this structure through emission and absorption lines. We extracted spectra from small apertures defined for both the diffuse ionised gas and the galaxies. We investigated the kinematics and ionisation properties of the various galaxies and extended gas regions through line diagnostics (R23, O32, and [O iii]/Hβ) that are available within the MUSE wavelength range. We compared these diagnostics to photo-ionisation models and shock models. The structure is divided into two kinematically distinct sub-structures. The most extended sub-structure of ionised gas is likely rotating around a massive galaxy and displays filamentary patterns that link some galaxies. The second sub-structure links another massive galaxy that hosts an active galactic nucleus (AGN) to a low-mass galaxy, but it also extends orthogonally to the AGN host disc over ~ 35 kpc. This extent is likely ionised by the AGN itself. The location of small diffuse regions in the R23 vs. O32 diagram is compatible with photo-ionisation. However, the location of three of these regions in this diagram (low O32, high R23) can also be explained by shocks, which is supported by their high velocity dispersions. One edge-on galaxy shares the same properties and may be a source of shocks. Regardless of the hypothesis, the extended gas seems to be non-primordial. We favour a scenario where the gas has been extracted from galaxies by tidal forces and AGN triggered by interactions between at least the two sub-structures.

The masses and metallicities of stellar haloes reflect galactic merger histories

There is increasing observational and theoretical evidence for a correlation between the metallicity and the mass of the stellar halo for galaxies with Milky Way-like stellar masses. Using the Illustris cosmological hydrodynamical simulations, we find that this relationship arises because a single massive massive progenitor contributes the bulk of the mass to the accreted stellar component as well as sets its metallicity. Moreover, in the Illustris simulations, this relationship extends over 3 orders of magnitude in accreted stellar mass for central galaxies. We show that for Milky Way-like mass galaxies, the scatter in accreted metallicity at a fixed accreted stellar mass encodes information about the stellar mass of the dominant accreted progenitor, while the radial density and metallicity gradients of the accreted stellar component encodes information about the time of accretion of the dominant progenitor. We demonstrate that for Milky Way-like mass galaxies, the Illustris simulations predict that the metallicity and the stellar mass of the total accreted stellar component can be reconstructed from aperture measurements of the stellar halo along the minor axis of edge-on disk galaxies. These correlations highlight the potential for observational studies of stellar halos to quantify our understanding of the most dominant events in the growth history of galaxies. We explore the implications of our model for our understanding of the accretion histories of the Milky Way, M31 and NGC 5128. In particular, a relatively late and massive accretion is favoured for M31; additionally, we provide a first estimate of the accreted stellar mass for NGC 5128.

Thick Disks in the Hubble Space Telescope Frontier Fields

Abstract Thick disk evolution is studied using edge-on galaxies in two Hubble Space Telescope Frontier Field Parallels. The galaxies were separated into 72 clumpy types and 35 spiral types with bulges. Perpendicular light profiles in F435W, F606W, and F814W (B, V, and I) passbands were measured at 1 pixel intervals along the major axes and fitted to sech2 functions convolved with the instrument line spread function (LSF). The LSF was determined from the average point spread function of ∼20 stars in each passband and field, convolved with a line of uniform brightness to simulate disk blurring. A spread function for a clumpy disk was also used for comparison. The resulting scale heights were found to be proportional to galactic mass, with the average height for a 1010±0.5 M ⊙ galaxy at z = 2 ± 0.5 equal to 0.63 ± 0.24 kpc. This value is probably the result of a blend between thin and thick disk components that cannot be resolved. Evidence for such two-component structure is present in an inverse correlation between height and midplane surface brightness. Models suggest that the thick disk is observed best between the clumps, and there the average scale height is 1.06 ± 0.43 kpc for the same mass and redshift. A 0.63 ± 0.68 mag V − I color differential with height is also evidence for a mixture of thin and thick components.

GRB 161219B/SN 2016jca: A low-redshift gamma-ray burst supernova powered by radioactive heating

Since the first discovery of a broad-lined type Ic supernova (SN) with a long-duration gamma-ray burst (GRB) in 1998, fewer than fifty gamma-ray burst supernovae (GRB-SNe) have been discovered. The intermediate-luminosity Swift GRB 161219B and its associated supernova SN 2016jca, which occurred at a redshift of z=0.1475, represents only the seventh GRB-SN to have been discovered within 1 Gpc, and hence provides an excellent opportunity to investigate the observational and physical properties of these very elusive and rare type of SN. As such, we present optical to near-infrared photometry and optical spectroscopy of GRB 161219B and SN 2016jca, spanning the first three months since its discovery. GRB 161219B exploded in the disk of an edge-on spiral galaxy at a projected distance of 3.4 kpc from the galactic centre. GRB 161219B itself is an outlier of the Amati relation, while SN 2016jca had a rest-frame, peak absolute V-band magnitude of M_V = -19.0, which it reached after 12.5 rest-frame days. We find that the bolometric properties of SN 2016jca are inconsistent with being powered solely by a magnetar central engine, as proposed by other authors, and demonstrate that it was likely powered exclusively by energy deposited by the radioactive decay of nickel and cobalt into their daughter products, which were nucleosynthesized when its progenitor underwent core collapse. We find that 0.22 solar masses of nickel is required to reproduce the peak luminosity of SN 2016jca, and we constrain an ejecta mass of 5.8 solar masses and a kinetic energy of ~5 x 10^52 erg. Finally, we report on a chromatic, pre-maximum bump in the g-band light curve, and discuss its possible origin. [Abridged]

Extraplanar HII regions in the edge-on spiral galaxies NGC 3628 and NGC 4522

Gas infall and outflow are critical for determining the star formation rate and chemical evolution of galaxies but direct measurements of gas flows are diffcult to make. Young massive stars and HII regions in the halos of galaxies are potential tracers for accretion and/or outflows of gas. Gas phase abundances of three HII regions in the lower halos of the edge-on galaxies NGC 3628 and NGC 4522 are determined by analysing optical long-slit spectra. The observed regions have projected distances to the midplane of their host from 1.4 to 3 kpc. With the measured flux densities of the optical nebular emission lines, we derive the oxygen abundance 12 + log(O/H) for the three extraplanar HII regions. The analysis is based on one theoretical and two empirical strong-line calibration methods. The resulting oxygen abundances of the extraplanar HII regions are comparable to the disk HII regions in one case and a little lower in the other case. Since our results depend on the accuracy of the metallicity determinations, we critically discuss the difference of the calibration methods we applied and confirm previously noted offsets. From our measurements, we argue that these three extraplanar HII regions were formed in the disk or at least from disk material. We discuss the processes that could transport disk material into the lower halo of these systems and conclude that gravitational interaction with a companion galaxy is most likely for NGC 3628 while ram pressure is favoured in the case of NGC 4522.

Detection of Extraplanar Diffuse Ionized Gas in M83*

Abstract We present the first kinematic study of extraplanar diffuse ionized gas (eDIG) in the nearby, face-on disk galaxy M83 using optical emission-line spectroscopy from the Robert Stobie Spectrograph on the Southern African Large Telescope. We use a Markov Chain Monte Carlo method to decompose the [N ii] 6548, 6583, Hα, and [S ii] 6717, 6731 emission lines into H ii region and diffuse ionized gas emission. Extraplanar, diffuse gas is distinguished by its emission-line ratios ([N ii]λ6583/Hα ) and its rotational velocity lag with respect to the disk ( km s−1 in projection). With interesting implications for isotropy, the velocity dispersion of the diffuse gas, km s−1, is a factor of a few higher in M83 than in the Milky Way and nearby, edge-on disk galaxies. The turbulent pressure gradient is sufficient to support the eDIG layer in dynamical equilibrium at an electron scale height of kpc. However, this dynamical equilibrium model must be finely tuned to reproduce the rotational velocity lag. There is evidence of local bulk flows near star-forming regions in the disk, suggesting that the dynamical state of the gas may be intermediate between a dynamical equilibrium and a galactic fountain flow. As one of the first efforts to study eDIG kinematics in a face-on galaxy, this study demonstrates the feasibility of characterizing the radial distribution, bulk velocities, and vertical velocity dispersions in low-inclination systems.