CHANG-ES Krause, Marita; Irwin, Judith; Schmidt, Philip ...
Astronomy and astrophysics (Berlin),
07/2020, Letnik:
639
Journal Article
Recenzirano
Odprti dostop
Context.
The magnetic field in spiral galaxies is known to have a large-scale spiral structure along the galactic disk and is observed as X-shaped in the halo of some galaxies. While the disk field ...can be well explained by dynamo action, the three-dimensional structure of the halo field and its physical nature are still unclear.
Aims.
As first steps towards understanding the halo fields, we want to clarify whether or not the observed X-shaped field is a wide-spread pattern in the halos of spiral galaxies. We also aim to investigate whether these halo fields are simply turbulent fields ordered by compression or shear (anisotropic turbulent fields), or have a large-scale regular structure.
Methods.
Analysis of the Faraday rotation in the halo is used as a tool to distinguish anisotropic turbulent fields from large-scale magnetic fields. However, this has been challenging until recently because of the faint halo emission in linear polarization. Our sensitive VLA broadband observations in
C
-band and
L
-band of 35 spiral galaxies seen edge-on (called CHANG-ES) allowed us to perform rotation measure synthesis (RM synthesis) in their halos and to analyze the results. We further accomplished a stacking of the observed polarization maps of 28 CHANG-ES galaxies in
C
-band.
Results.
Though the stacked edge-on galaxies were of different Hubble type, and had differing star formation activity and interaction activity, the stacked image clearly reveals an X-shaped structure of the apparent magnetic field. We detected a large-scale (coherent) halo field in all 16 galaxies that have extended polarized emission in their halos. We detected large-scale field reversals in all of their halos. In six galaxies, these are along lines that are approximately perpendicular to the galactic midplane (vertical RMTL) with about 2 kpc separation. Only in NGC 3044 and possibly in NGC 3448 did we observe vertical giant magnetic ropes (GMR) similar to those detected recently in NGC 4631.
Conclusions.
The observed X-shaped structure of the halo field seems to be an underlying feature of spiral galaxies. It can be regarded as the two-dimensional projection of the regular magnetic field which we found to have scales of typically 1 kpc or larger observed over several kiloparsecs. The ordered magnetic field extends far out in the halo and beyond. We detected large-scale magnetic field reversals in the halo that may indicate that GMR are more or less tightly wound. With these discoveries, we hope to stimulate model simulations for the halo magnetic field that should also explain the determined asymmetry of the polarized intensity (PI).
ABSTRACT
We consider the largest sample of 561 edge-on galaxies observed with integral field units by the MaNGA survey and find 300 galaxies where the ionized gas shows a negative vertical gradient ...(lag) in its rotational speed. We introduce the stop altitude as the distance to the galactic mid-plane at which the gas rotation should stop in the linear approximation. We find correlations between the lags, stop altitude and galactic mass, stellar velocity dispersion, and overall Sersic index. We do not find any correlation of the lags or stop altitude with the star formation activity in the galaxies. We conclude that low-mass galaxies (log(M*/M⊙) < 10) with low-Sersic index and with low-stellar velocity dispersion possess a wider ‘zone of influence’ in the extragalactic gas surrounding them with respect to higher mass galaxies that have a significant spherical component. We estimated the trend of the vertical rotational gradient with radius and find it flat for most of the galaxies in our sample. A small subsample of galaxies with negative radial gradients of lag has an enhanced fraction of objects with aged low-surface brightness structures around them (e.g. faint shells), which indicates that noticeable accretion events in the past affected the extraplanar gas kinematics and might have contributed to negative radial lag gradients. We conclude that an isotropic accretion of gas from the circumgalactic medium plays a significant role in the formation of rotation velocity lags.
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 in
C
-band and 1.4 ± 0.7 kpc in
L
-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 height
h˜
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.
An oxygen abundance gradient into the outer disc of M81 Patterson, Maria T.; Walterbos, Rene A. M.; Kennicutt, Robert C. ...
Monthly Notices of the Royal Astronomical Society,
20/May , Letnik:
422, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The extended H i disc and tidal tails of M81 present an interesting environment to study the effects of galaxy interaction on star formation and chemical evolution of the outer disc of a large spiral ...galaxy. We present Hα imaging of the outer disc of M81 and luminosities for 40 H ii regions out to ∼3 R
25. We have also obtained MMT spectra for 21 H ii regions out to more than twice R
25. We derive strong-line oxygen abundances for all H ii regions using R
23-based and N ii/O ii-based calibrations and electron temperature abundances for seven regions spanning a galactocentric distance between 5.7 and 32 kpc. We also comment on the abundances of H ii regions near KDG 61 and the 'tidal dwarf' candidate HoIX. Our results constitute the most radially extended metallicity study for M81 to date. With this extended data set, we find an overall oxygen abundance gradient of Δ(log (O/H))/ΔR
G∼−0.013 dex kpc−1 over the entire radial range. This is significantly flatter than what has been found in previous studies, which were limited to the optical disc. From our temperature-based abundances, we find Δ(log (O/H))/ΔR
G∼−0.020 dex kpc−1 and present the possibility of a broken gradient from these data, but note the need to obtain more temperature-based abundances at intermediate galactocentric distances (∼10-20 kpc) to verify whether or not this may be the case. We discuss our main result of a rather flat gradient for M81 in the context of simulations and observations of abundance gradients in other galaxies. We find that the shallow abundance gradient of M81 is likely a result of the interaction history of this galaxy.
We present measurements of the stellar and gaseous velocities in the central 5' of the Local Group spiral M33. The data were obtained with the ARC 3.5 m telescope. Blue and red spectra with ...resolutions from 2 to 4 AA covering the principal gaseous emission and stellar absorption lines were obtained along the major and minor axes and six other position angles. The observed radial velocities of the ionized gas along the photometric major axis of M33 remain flat at 22 km s super(-1) all the way into the center, while the stellar velocities show a gradual rise from 0 to 22 km s super(-1) over that same region. The central star cluster is at or very close to the dynamical center, with a velocity that is in accordance with M33's systemic velocity to within our uncertainties. Velocities on the minor axis are nonzero out to about 1' from the center in both the stars and gas. Together with the major-axis velocities, they point at significant deviations from circular rotation. The most likely explanation for the bulk of the velocity patterns are streaming motions along a weak inner bar with a position angle close to that of the minor axis, as suggested by previously published IR photometric images. The presence of bar Imprints in M33 implies that all major Local Group galaxies are barred. The noncircular motions over the inner 200 pc make it difficult to constrain the shape of M33's inner dark matter halo profile. If the noncircular motions we find in this nearby Sc galaxy are present in other more distant late-type galaxies, they might be difficult to recognize.
ABSTRACT
Counter-rotating components in galaxies are one of the most direct forms of evidence for past gas accretion or merging. We discovered 10 edge-on disc gaseous counter-rotators in a sample of ...523 edge-on galaxies identified in the final MaNGA (Mapping Nearby Galaxies at APO) IFU sample. The counter-rotators tend to located in small groups. The gaseous counter-rotators have intermediate stellar masses and and located in the green valley and red sequence of the colour–magnitude diagram. The average vertical extents of the stellar and ionized gas discs are the same as for the rest of the sample while their radial gas and stellar distributions are more centrally concentrated. This may point at angular momentum loss during the formation process of the counter-rotating discs. The counter-rotators have low gas and dust content, weak emission-line strengths, and low star formation rates. This suggests that the formation of counter-rotators may be an efficient way to quench galaxies. One counter-rotator, SDSS J080016.09+292817.1 (Galaxy F), has a post-starburst region and a possible AGN at the centre. Another counter-rotator, SDSS J131234.03+482159.8 (Galaxy H), is identified as a potential ongoing galaxy interaction with its companion satellite galaxy, a gas-rich spiral galaxy. This may be representative case of a gaseous counter-rotator forming through a merger origin. However, tidal distortions expected in mergers are only found in a few of the galaxies and we cannot rule out direct gas accretion as another formation mechanism.
We report the discovery of Andromeda X, a new dwarf spheroidal satellite of M31, based on stellar photometry from the Sloan Digital Sky Survey. Using follow-up imaging data we have estimated its ...distance and other physical properties. We find that Andromeda X has a dereddened central surface brightness of k sub(v,0) 6 26.7 mag arcsec super(-2) and a total apparent magnitude of V sub(tot) 6 16.1, which at the derived distance modulus, (m - M) sub(0) 6 24.12-24.34, yields an absolute magnitude of M sub(v) 6 -8.1 c 0.5; these values are quite comparable to those of Andromeda IX, a previously discovered low-luminosity M31 satellite. The discoveries of Andromeda X and of numerous other extremely faint satellites around M31 and the Milky Way in the past few years suggest that such objects may be plentiful in the Local Group.
ABSTRACT
Only about 19 Galactic and 25 extragalactic bonafide luminous blue variables (LBVs) are known to date. This incomplete census prevents our understanding of this crucial phase of massive star ...evolution which leads to the formation of heavy binary black holes via the classical channel. With large samples of LBVs one could better determine the duration and maximum stellar luminosity which characterize this phase. We search for candidate LBVs (cLBVs) in a new galaxy, NGC 7793. For this purpose, we combine high spatial resolution images from two Hubble Space Telescope (HST) programs with optical spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE). By combining PSF-fitting photometry measured on F547M, F657N, and F814W images, with restrictions on point-like appearance (at HST resolution) and H α luminosity, we find 100 potential cLBVs, 36 of which fall in the MUSE fields. Five of the latter 36 sources are promising cLBVs which have MV ≤ −7 and a combination of: H α with a P-Cygni profile; no O i$\, \lambda 6300$ emission; weak or no O iii$\, \lambda 5007$ emission; large N ii/H α relative to H ii regions; and S ii$\, \lambda 6716$/S ii$\, \lambda 6731\sim 1$. It is not clear if these five cLBVs are isolated from O-type stars, which would favour the binary formation scenario of LBVs. Our study, which approximately covers one fourth of the optical disc of NGC 7793, demonstrates how by combining the above HST surveys with multi-object spectroscopy from 8-m class telescopes, one can efficiently find large samples of cLBVs in nearby galaxies.
We introduce a new survey to map the radio continuum halos of a sample of 35 edge-on spiral galaxies at 1.5 GHz and 6 GHz in all polarization products. The survey is exploiting the new wide bandwidth ...capabilities of the Karl G. Jansky Very Large Array (i.e., the Expanded Very Large Array) in a variety of array configurations (B, C, and D) in order to compile the most comprehensive data set yet obtained for the study of radio halo properties. This is the first survey of radio halos to include all polarization products. In this first paper, we outline the scientific motivation of the survey, the specific science goals, and the expected improvements in noise levels and spatial coverage from the survey. Our goals include investigating the physical conditions and origin of halos, characterizing cosmic-ray transport and wind speed, measuring Faraday rotation and mapping the magnetic field, probing the in-disk and extraplanar far-infrared-radio continuum relation, and reconciling non-thermal radio emission with high-energy gamma-ray models. The sample size allows us to search for correlations between radio halos and other properties, including environment, star formation rate, and the presence of active galactic nuclei. In a companion paper (Paper II) we outline the data reduction steps and present the first results of the survey for the galaxy, NGC 4631.
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