arXiv subjects

Bernd Vollmer

Publications and source records attributed to Bernd Vollmer.

16 recordsLinked to original sources

arXiv2026

ALMA-JELLY II. Constraining the radial profiles of the quenching timescales in ram-pressure-stripped galaxies

We investigate the environmental quenching histories and spatial progression of ram-pressure stripping in 11 galaxies in the nearby Coma, Norma, and A1367 clusters. As part of the ALMA-JELLY large program, we combine high-sensitivity VLT/MUSE integral-field spectroscopy with ultraviolet-to-far-infrared photometry in 37 spatially resolved disc apertures. The sample spans $\log(M_*/\mathrm{M}\odot)\sim7.5-10.5$ and includes galaxies undergoing active or recent ram-pressure stripping, identified through prominent H$α$ or UV tails. We use Bayesian spectrophotometric modelling with exponentially declining star-formation histories and show with mock data that combining spectra and photometry breaks parameter degeneracies and improves quenching-age constraints. All galaxies show quenched outer discs and relatively old ($\lesssim1$ Gyr) stellar populations. Massive galaxies ($\log(M_*/\mathrm{M}_\odot)>9.5$) exhibit clear outside-in quenching: inner regions quench about 500 Myr later than their outskirts. The small scatter in quenching age with distance from the truncation radius, despite differences in environment and stripping geometry, points to a common evolutionary sequence. Low-mass and dwarf galaxies instead show weak radial gradients, with quenching occurring more rapidly, typically within 300 Myr.

astro-ph.GA↗

arXiv2025

ALMA Sub-parsec Resolution Dense Molecular Line Observations of the NGC 1068 Nucleus

We present the results of our ALMA observations of the dense molecular HCN J=4-3 and HCO$^{+}$ J=4-3 lines at $\lesssim$1 pc ($\lesssim$14 mas) resolution in the nuclear region of the nearby ($\sim$14 Mpc) well-studied AGN NGC 1068. Both emission lines are clearly detected around the AGN along an almost east-west direction, which we ascribe to the dusty molecular torus. The HCN J=4-3 emission is brighter than the HCO$^{+}$ J=4-3 emission in the compact ($\lesssim$3-5 pc) torus region. Apparent counter-rotation between the inner ($\lesssim$2 pc) and outer ($\gtrsim$2 pc) parts of the western torus, previously seen in $\sim$1.5 pc-resolution HCN J=3-2 and HCO$^{+}$ J=3-2 data, is also confirmed in our new $\lesssim$1 pc-resolution HCN J=4-3 and HCO$^{+}$ J=4-3 data. We apply a physically counter-rotating torus model, in which a compact dense gas clump collided with the western side of the existing rotating torus from the opposite direction, and we find that this model largely reproduces the observed properties of the combined new $\lesssim$1 pc-resolution HCN J=4-3 and HCO$^{+}$ J=4-3 data, and the previously obtained $\lesssim$1.5 pc-resolution HCN J=3-2 and HCO$^{+}$ J=3-2 data.

astro-ph.GA↗

arXiv2025

FAST Reveals the Extended HI Halo and Accretion Signatures of NGC2683

We present the results of our recent HI observations conducted on the edge-on galaxy NGC2683 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). In comparison to previous observations made by the VLA, FAST has detected a more extensive distribution of HI. Particularly noteworthy is that the detections made by FAST extend approximately four times farther than those of the VLA in the vertical direction from the galactic plane. The total HI flux measured for NGC2683 amounts to $F_{\rm HI} = 112.1\,\rm{Jy\,km\,s^{-1}}$ (equivalent to a total HI mass of $M_{\rm HI} = 2.32 \times 10^9\,{\rm M_\odot}$), which is slightly higher than that detected by VLA. FAST has also identified three dwarf galaxies in close proximity to NGC2683, namely KK69, NGC2683dw1 (hereafter dw1), and NGC2683dw3$?$ (hereafter dw3$?$). dw3$?$ is situated within the extended HI distribution of NGC2683 in projection and lies near the tail of KK69 extending towards NGC2683. These observations suggest that dw3$?$ is likely a result of the accretion process from NGC2683 to KK69. Furthermore, FAST has detected three high-velocity clouds (HVCs), with complex B potentially undergoing accretion with NGC2683. Based on the model from Vollmer et al. 2016 and incorporating the HI halo component, we found that the model with the added HI halo aligns more closely with our FAST observations in NGC2683. The estimated mass of this HI halo is $3 \times 10^8\,{\rm M_\odot}$, constituting approximately 13% of the total HI mass of the galaxy. We suggest that the origination of this HI halo is more likely attributed to external gas accretion.

astro-ph.GA↗

arXiv2016

CHANG-ES VII: Magnetic outflows from the Virgo cluster galaxy NGC 4388

We investigate the effects of ram pressure on the ordered magnetic field of a galaxy hosting a radio halo and strong nuclear outflows. New radio images in total and polarized intensity of the edge-on Virgo galaxy NGC\,4388 were obtained within the CHANG-ES EVLA project. The unprecedented noise level reached allows us to detect striking new features of the ordered magnetic field. The nuclear outflow extends far into the halo to about 5\,kpc from the center and is spatially correlated with the $\rm{H}α$ and X-ray emission. For the first time, the southern outflow is detected. Above and below both spiral arms we find extended blobs of polarized emission with an ordered field oriented perpendicular to the disk. The synchrotron lifetime of the cosmic ray electrons (CREs) in these regions yields a mean outflow velocity of $(270\pm70)\kms$, in agreement with a galactic wind scenario. The observed symmetry of the polarized halo features in NGC 4388 excludes a compression of the halo gas by the ram pressure of the intra-cluster medium (ICM). The assumption of equilibrium between the halo pressure and the ICM ram pressure yields an estimate of the ICM density that is consistent with both the ICM density derived from X-ray observations and the recent \textit{Planck} Sunyaev-Zel'dovich measurements. The detection of a faint radio halo around cluster galaxies could thus be used for an estimate of ICM ram pressure.

astro-ph.GA↗

arXiv2013

Transformation of a Virgo Cluster Dwarf Irregular Galaxy by Ram Pressure Stripping: IC3418 and its Fireballs

We present optical imaging and spectroscopy and HI imaging of the Virgo Cluster galaxy IC 3418, which is likely a "smoking gun" example of the transformation of a dwarf irregular into a dwarf elliptical galaxy by ram pressure stripping. IC 3418 has a spectacular 17 kpc length UV-bright tail comprised of knots, head-tail, and linear stellar features. The only H-alpha emission arises from a few HII regions in the tail, the brightest of which are at the heads of head-tail UV sources whose tails point toward the galaxy ("fireballs"). Several of the elongated tail sources have H-alpha peaks outwardly offset by 80-150 pc from the UV peaks, suggesting that gas clumps continue to accelerate through ram pressure, leaving behind streams of newly formed stars which have decoupled from the gas. Absorption line strengths, measured from Keck DEIMOS spectra, together with UV colors, show star formation stopped 300+/-100 Myr ago in the main body, and a strong starburst occurred prior to quenching. While neither H-alpha nor HI emission are detected in the main body of the galaxy, we have detected 4x10^7 M_sun of HI from the tail with the VLA. The gas consumption timescale in the tail is relatively long, implying that most of the stripped gas does not form stars but joins the ICM. The velocities of tail HII regions, measured from Keck LRIS spectra, extend only a small fraction of the way to the cluster velocity, suggesting that star formation does not happen in more distant parts of the tail. Stars in the outer tail have velocities exceeding the escape speed, but some in the inner tail should fall back into the galaxy, forming halo streams. One likely fallback stream is identified.

astro-ph.CO↗

arXiv2013

The Next Generation Virgo Cluster Survey. IV. NGC 4216: A Bombarded Spiral in the Virgo Cluster

We present an investigation into the origins of a series of interlaced narrow filamentary stellar structures, loops and plumes in the vicinity of the Virgo Cluster, edge-on spiral galaxy, NGC 4216 that were previously identified by the Blackbird Telescope. Using the deeper, higher-resolution and precisely calibrated optical CFHT/MegaCam images obtained as part of the Next Generation Virgo Cluster Survey (NGVS), we confirm the previously identified features and identify a few additional structures. The NGVS data allowed us to make a physical study of these low-surface brightness features and investigate their origin. The likely progenitors of the structures were identified as either already catalogued VCC dwarfs or newly discovered satellites caught in the act of being destroyed. They have the same g-i color index and likely contain similar stellar populations. The alignment of three dwarfs along an apparently single stream is intriguing, and we cannot totally exclude that these are second-generation dwarf galaxies being born inside the filament from the debris of an original dwarf. The observed complex structures, including in particular a stream apparently emanating from a satellite of a satellite, point to a high rate of ongoing dwarf destruction/accretion in the region of the Virgo Cluster where NGC 4216 is located. We discuss the age of the interactions and whether they occurred in a group that is just falling into the cluster and shows signs of so-called "pre-processing" before it gets affected by the cluster environment, or in a group which already ventured towards the central regions of Virgo Cluster.

astro-ph.CO↗

arXiv2011

Caught in the Act: Strong, Active Ram Pressure Stripping in Virgo Cluster Spiral NGC 4330

We present a multi-wavelength study of NGC 4330, a highly-inclined spiral galaxy in the Virgo Cluster which is a clear example of strong, ongoing ICM-ISM ram pressure stripping. The HI has been removed from well within the undisturbed old stellar disk, to 50% - 65% of R_25. Multi-wavelength data (WIYN BVR and H-alpha, VLA 21-cm HI and radio continuum, and GALEX NUV and FUV) reveal several one-sided extraplanar features likely caused by ram pressure at an intermediate disk-wind angle. At the leading edge of the interaction, the H-alpha and dust extinction curve sharply out of the disk in a remarkable and distinctive "upturn" feature that may be generally useful as a diagnostic indicator of active ram pressure. On the trailing side, the ISM is stretched out in a long tail which contains 10% of the galaxy's total HI emission, 6 - 9% of its NUV-FUV emission, but only 2% of the H-alpha. The centroid of the HI tail is downwind of the UV/H-alpha tail, suggesting that the ICM wind has shifted most of the ISM downwind over the course of the past 10 - 300 Myr. Along the major axis, the disk is highly asymmetric in the UV, but more symmetric in H-alpha and HI, also implying recent changes in the distributions of gas and star formation. The UV-optical colors indicate very different star formation histories for the leading and trailing sides of the galaxy. On the leading side, a strong gradient in the UV-optical colors of the gas-stripped disk suggests that it has taken 200-400 Myr to strip the gas from a radius of >8 to 5 kpc, but on the trailing side there is no age gradient. All our data suggest a scenario in which NGC 4330 is falling into cluster center for first time and has experienced a significant increase in ram pressure over the last 200-400 Myr.

astro-ph.CO↗

arXiv2010

VIVA, VLA Imaging of Virgo spirals in Atomic gas: I. The Atlas & The HI Properties

We present the result of a new VLA HI Imaging survey of Virgo galaxies, VIVA (the VLA Imaging survey of Virgo galaxies in Atomic gas). The survey includes high resolution HI data of 53 carefully selected late type galaxies (48 spirals and 5 irregular systems). The goal is to study environmental effects on HI gas properties of cluster galaxies to understand which physical mechanisms affect galaxy evolution in different density regions, and to establish how far out the impact of the cluster reaches. As a dynamically young cluster, Virgo contains examples of galaxies experiencing a variety of environmental effects. Its nearness allows us to study each galaxy in great detail. We have selected Virgo galaxies with a range of star formation properties in low to high density regions (at the projected distance from M87, d_87=0.3-3.3 Mpc). Contrary to pr evious studies, more than half of the galaxies in the sample (~60%) are fainter than 12 mag in B_T. Overall, the selected galaxies represent the late type Virgo galaxies (S0/a to Sd/Irr) down to m_p<~14.6 fairly well in morphological type, systemic velocity, subcluster membership, HI mass and deficiency. In this paper (VIVA I: the atlas and the HI properties), we present HI maps and properties, and describe the HI morphology and kinematics of individual galaxies in detail (abbreviated).

astro-ph.CO↗

arXiv2007

Virgo Galaxies with Long One-Sided HI Tails

In a new HI imaging survey of Virgo galaxies (VIVA: VLA Imaging of Virgo galaxies in Atomic gas), we find 7 spiral galaxies with long HI tails. The morphology varies but all the tails are extended well beyond the optical radii on one side. These galaxies are found in intermediate-low density regions (0.6-1 Mpc in projection from M87). The tails are all pointing roughly away from M87, suggesting that these tails may have been created by a global cluster mechanism. While the tidal effects of the cluster potential are too small, a rough estimate suggests that simple ram-pressure stripping indeed could have formed the tails in all but two cases. At least three systems show HI truncation to within the stellar disk, providing evidence for a gas-gas interaction. Although most of these galaxies do not appear disturbed optically, some have close neighbors, suggesting that tidal interactions may have moved gas outwards making it more susceptible to the ICM ram-pressure or viscosity. Indeed, a simulation study of one of the tail galaxies, NGC 4654, suggests that the galaxy is most likely affected by the combined effect of a gravitational interaction and ram-pressure stripping. We conclude that these one-sided HI tail galaxies have recently arrived in the cluster, falling in on highly radial orbits. It appears that galaxies begin to lose their gas already at intermediate distances from the cluster center through ram-pressure or turbulent viscous stripping and tidal interactions with neighbours, or a combination of both.

astro-ph↗

arXiv2005

Observations of Stripped Edge-on Virgo Cluster Galaxies

We present observations of highly inclined, HI deficient, Virgo cluster spiral galaxies. Our high-resolution VLA HI observations of edge-on galaxies allow us to distinguish extraplanar gas from disk gas. All of our galaxies have truncated H-alpha disks, with little or no disk gas beyond a truncation radius. While all the gas disks are truncated, the observations show evidence for a continuum of stripping states: symmetric, undisturbed truncated gas disks indicate galaxies that were stripped long ago, while more asymmetric disks suggest ongoing or more recent stripping. We compare these timescale estimates with results obtained from two-dimensional stellar spectroscopy of the outer disks of galaxies in our sample. One of the galaxies in our sample, NGC 4522 is a clear example of active ram-pressure stripping, with 40% of its detected HI being extraplanar. As expected, the outer disk stellar populations of this galaxy show clear signs of recent (and, in fact, ongoing) stripping. Somewhat less expected, however, is the fact that the spectrum of the outer disk of this galaxy, with very strong Balmer absorption and no observable emission, would be classified as ``k+a'' if observed at higher redshift. Our observations of NGC 4522 and other galaxies at a range of cluster radii allow us to better understand the role that clusters play in the structure and evolution of disk galaxies.

astro-ph↗

arXiv2005

A High Resolution HI Study of Selected Virgo Galaxies - Preliminary Results on Gas Morphology & Extra-Planar Gas

We present preliminary results of VLA HI imaging of selected Virgo cluster galaxies. The goal is to study environmental effects on galaxy evolution. Our sample of 41 galaxies is spread throughout the cluster and spans a wide range in star formation properties. Here we present the total HI maps of 13 galaxies. We find a number of galaxies with extended HI tails, almost all pointing away from the cluster center. Truncated HI disks are found close to the center but also in the outer region. Some galaxies near the cluster center show compression of the gas on one side. Multiwavelength data of NGC 4569 and kinematics on NGC 4396 indicate that some of the HI is extra-planar. These preliminary results on the HI morphology already suggest that a variety of environmental effects such as ICM-ISM interactions, harassment, tidal interactions or mergers may be at work to affect the evolution of galaxies.

astro-ph↗

arXiv2005

Dense Cloud Ablation and Ram Pressure Stripping of the Virgo Spiral NGC 4402

We present optical, HI and radio continuum observations of the highly inclined Virgo Cluster Sc galaxy NGC 4402, which show evidence for ram-pressure stripping and dense cloud ablation. VLA HI and radio continuum maps show a truncated gas disk and emission to the northwest of the main disk emission. In particular, the radio continuum emission is asymmetrically extended to the north and skewed to the west. The Halpha image shows numerous HII complexes along the southern edge of the gas disk, possibly indicating star formation triggered by the ICM pressure. BVR images at 0.5" resolution obtained with the WIYN Tip-Tilt Imager show a remarkable dust lane morphology: at half the optical radius, the dust lane of the galaxy curves up and out of the disk, matching the HI morphology. Large dust plumes extend upward for ~1.5 kpc from luminous young star clusters at the SE edge of the truncated gas disk. These star clusters are very blue, indicating very little dust reddening, which suggests dust blown away by an ICM wind at the leading edge of the interaction. To the south of the main ridge of interstellar material, where the galaxy is relatively clean of gas and dust, we have discovered 1 kpc long linear dust filaments with a position angle that matches the extraplanar radio continuum tail; we interpret this angle as the projected ICM wind direction. One of the observed dust filaments has an HII region at its head. We interpret these dust filaments as large, dense clouds which were initially left behind as the low-density ISM is stripped, but are then ablated by the ICM wind. These results provide striking new evidence on the fate of molecular clouds in stripped cluster galaxies.

astro-ph↗

arXiv2004

Torus models for obscuration in type 2 AGN

We discuss a clumpy model of obscuring dusty tori around AGN. Cloud-cloud collisions lead to an effective viscosity and a geometrically thick accretion disk, which has the required properties of a torus. Accretion in the combined gravitational potential of central black hole and stellar cluster generates free energy, which is dissipated in collisions, and maintains the thickness of the torus. A quantitative treatment for the torus in the prototypical Seyfert 2 nucleus of NGC 1068 together with a radiative transfer calculation for NIR re-emission from the torus is presented.

astro-ph↗

arXiv2004

Spiral Galaxy - ICM Interactions in the Virgo Cluster

We discuss HI and optical evidence for ongoing ICM-ISM interactions in 6 HI-deficient Virgo cluster spiral galaxies. One of the clearest cases is the highly inclined Virgo galaxy NGC 4522, which has a normal stellar disk but a truncated gas disk, and lots of extraplanar gas right next to the gas truncation radius in the disk. Unusually strong HI, H-alpha and radio continuum emission are all detected from the extraplanar gas. The radio continuum polarized flux and spectral index peak on the side opposite the extraplanar gas, suggesting ongoing pressure by the ICM. Four other HI-deficient edge-on Virgo spirals show evidence of extraplanar ISM gas or exhibit asymmetries in their disk HI distributions, but contain much less extraplanar HI than NGC 4522. Comparison with recent simulations suggests this difference may be evolutionary, with large surface densities of extraplanar gas observed only in early phases of an ICM-ISM interaction. In NGC 4569, the H-alpha image shows 2 effects of ICM pressure on the galaxy ISM. An anomalous arm of HII regions, possibly extraplanar, emerges from the edge of a truncated H-alpha disk. This resembles the arms seen in simulations which are formed by the combined effects of wind pressure plus rotation. An extended nebulosity near the minor axis, also in the NW, is interpreted as a starburst outflow bubble disturbed by ICM wind pressure.

astro-ph↗

arXiv2004

VLA HI Observations of Gas Stripping in the Virgo Cluster Spiral NGC 4522

We present VLA HI observations at ~20"=1.5 kpc resolution of the highly inclined, HI-deficient Virgo cluster spiral galaxy NGC 4522, which is one of the clearest and nearest cases of ongoing ICM-ISM stripping. HI is abundant and spatially coincident with the stellar disk in the center, but beyond R = 3 kpc the HI distribution in the disk is sharply truncated and the only HI is extraplanar, and all on the northwest side. The kinematics and the morphology of the HI appear more consistent with ongoing stripping, and less consistent with gas fall-back which may occur long after peak pressure. Much of the extraplanar gas exhibits a modest net blueshift with respect to the galaxy's disk rotational velocities, consistent with gas accelerated toward the mean cluster velocity. The SW side of the galaxy has less HI in the disk but more HI in the halo, suggesting more effective gas removal on the side of the galaxy which is rotating into the ICM wind. The galaxy is 3.3 degrees ~800 kpc from M87, somewhat outside the region of strongest cluster X-ray emission. The ram pressure may be significantly stronger than standard values, due to large bulk motions and local density enhancements of the ICM gas, which may occur in a dynamic, shock-filled ICM experiencing subcluster merging. The HI and H-alpha distributions are similar, implying that the star-forming molecular ISM has been effectively stripped from the outer disk of the galaxy along with the HI.

astro-ph↗

arXiv1999

The Minispiral in the Galactic Center revisited

We present the results of a re-examination of a [Ne II] line emission data cube (λ12.8 μm) and discuss the kinematic structure of the inner \sim 3 \times 4 pc of the Galaxy. The quality of [Ne II] as a tracer of ionized gas is examined by comparing it to radio data. A three dimensional representation of the data cube allows us to disentangle features which are projected onto the same location on the sky. A model of gas streams in different planes is fitted to the data. We find that most of the material is located in a main plane which itself is defined by the inner edge of the Circum-Nuclear Disk in the Galactic Center. Finally, we present a possible three dimensional model of the gas streams.

astro-ph↗