arXiv subjects

Helga Dénes

Publications and source records attributed to Helga Dénes.

32 records · Page 2Linked to original sources

arXiv2024

WALLABY Pilot Survey: the Tully-Fisher relation in the NGC 4808, Vela and NGC 5044 fields

The Tully-Fisher Relation (TFR) is a well-known empirical relationship between the luminosity of a spiral galaxy and its circular velocity, allowing us to estimate redshift independent distances. Here we use high signal-to-noise HI 21-cm integrated spectra from the second pilot data release (PDR2, 180 deg2) of the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY). In order to prepare for the full WALLABY survey, we have investigated the TFR in phase 2 of the pilot survey with a further three fields. The data were obtained with wide-field Phased Array Feeds on the Australian Square Kilometre Array Pathfinder (ASKAP) and have an angular resolution of 30 arcsec and a velocity resolution of ~4 km/s. Galaxy luminosities have been measured from the Wide-field Infrared Survey Explorer (WISE), and optical galaxy inclinations from the Dark Energy Camera Legacy Survey. We present TFRs for wavelengths from 0.8-3.4μm. We examine sources of galaxy inclination data and investigate magnitudes from the DECam Local Volume Exploration Survey (DELVE) and DENIS catalogues and the 4HS target catalogue based on the VISTA Hemisphere Survey (VHS). We consider the baryonic TFR. These are all of interest for TFR using the full WALLABY survey of 200,000 galaxies. We demonstrate that WALLABY TFR distances can take their place among state of the art studies of the local velocity field.

astro-ph.GA↗

arXiv2023

A new method for spatially resolving the turbulence driving mixture in the ISM with application to the Small Magellanic Cloud

Turbulence plays a crucial role in shaping the structure of the interstellar medium. The ratio of the three-dimensional density contrast ($σ_{ρ/ρ_0}$) to the turbulent sonic Mach number ($\mathcal{M}$) of an isothermal, compressible gas describes the ratio of solenoidal to compressive modes in the turbulent acceleration field of the gas, and is parameterised by the turbulence driving parameter: $b=σ_{ρ/ρ_0}/\mathcal{M}$. The turbulence driving parameter ranges from $b=1/3$ (purely solenoidal) to $b=1$ (purely compressive), with $b=0.38$ characterising the natural mixture (1/3~compressive, 2/3~solenoidal) of the two driving modes. Here we present a new method for recovering $σ_{ρ/ρ_0}$, $\mathcal{M}$, and $b$, from observations on galactic scales, using a roving kernel to produce maps of these quantities from column density and centroid velocity maps. We apply our method to high-resolution HI emission observations of the Small Magellanic Cloud (SMC) from the GASKAP-HI survey. We find that the turbulence driving parameter varies between $b\sim 0.3$ and $b\sim 1.0$ within the main body of the SMC, but the median value converges to $b\sim0.51$, suggesting that the turbulence is overall driven more compressively ($b>0.38$). We observe no correlation between the $b$ parameter and HI or H$α$ intensity, indicating that compressive driving of HI turbulence cannot be determined solely by observing HI or H$α$ emission density, and that velocity information must also be considered. Further investigation is required to link our findings to potential driving mechanisms such as star-formation feedback, gravitational collapse, or cloud-cloud collisions.

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arXiv2023

WALLABY Pilot Survey: Hydra Cluster Galaxies UV and HI morphometrics

Galaxy morphology in atomic hydrogen (HI) and in the ultra-violet (UV) are closely linked. This has motivated their combined use to quantify morphology over the full H i disk for both H i and UV imaging. We apply galaxy morphometrics: Concentration, Asymmetry, Gini, M20 and Multimode-Intensity-Deviation statistics to the first moment-0 maps of the WALLABY survey of galaxies in the Hydra cluster center. Taking advantage of this new HI survey, we apply the same morphometrics over the full HI extent on archival GALEX FUV and NUV data to explore how well HI truncated, extended ultraviolet disk (XUV) and other morphological phenomena can be captured using pipeline WALLABY data products. Extended HI and UV disks can be identified relatively straightforward from their respective concentration. Combined with WALLABY HI, even the shallowest GALEX data is sufficient to identify XUV disks. Our second goal is to isolate galaxies undergoing ram-pressure stripping in the H i morphometric space. We employ four different machine learning techniques, a decision tree, a k-nearest neighbour, a support-vector machine, and a random forest. Up to 80% precision and recall are possible with the Random Forest giving the most robust results.

astro-ph.GA↗

arXiv2023

The Apertif Radio Transient System (ARTS): Design, Commissioning, Data Release, and Detection of the first 5 Fast Radio Bursts

Fast Radio Bursts must be powered by uniquely energetic emission mechanisms. This requirement has eliminated a number of possible source types, but several remain. Identifying the physical nature of Fast Radio Burst (FRB) emitters arguably requires good localisation of more detections, and broadband studies enabled by real-time alerting. We here present the Apertif Radio Transient System (ARTS), a supercomputing radio-telescope instrument that performs real-time FRB detection and localisation on the Westerbork Synthesis Radio Telescope (WSRT) interferometer. It reaches coherent-addition sensitivity over the entire field of the view of the primary dish beam. After commissioning results verified the system performed as planned, we initiated the Apertif FRB survey (ALERT). Over the first 5 weeks we observed at design sensitivity in 2019, we detected 5 new FRBs, and interferometrically localised each of these to 0.4--10 sq. arcmin. All detections are broad band and very narrow, of order 1 ms duration, and unscattered. Dispersion measures are generally high. Only through the very high time and frequency resolution of ARTS are these hard-to-find FRBs detected, producing an unbiased view of the intrinsic population properties. Most localisation regions are small enough to rule out the presence of associated persistent radio sources. Three FRBs cut through the halos of M31 and M33. We demonstrate that Apertif can localise one-off FRBs with an accuracy that maps magneto-ionic material along well-defined lines of sight. The rate of 1 every ~7 days next ensures a considerable number of new sources are detected for such study. The combination of detection rate and localisation accuracy exemplified by the 5 first ARTS FRBs thus marks a new phase in which a growing number of bursts can be used to probe our Universe.

astro-ph.HE↗

arXiv2022

WALLABY Pilot Survey: HI gas kinematics of galaxy pairs in cluster environment

We examine the HI gas kinematics of galaxy pairs in two clusters and a group using Australian Square Kilometre Array Pathfinder (ASKAP) WALLABY pilot survey observations. We compare the HI properties of galaxy pair candidates in the Hydra I and Norma clusters, and the NGC 4636 group, with those of non-paired control galaxies selected in the same fields. We perform HI profile decomposition of the sample galaxies using a tool, {\sc baygaud} which allows us to de-blend a line-of-sight velocity profile with an optimal number of Gaussian components. We construct HI super-profiles of the sample galaxies via stacking of their line profiles after aligning the central velocities. We fit a double Gaussian model to the super-profiles and classify them as kinematically narrow and broad components with respect to their velocity dispersions. Additionally, we investigate the gravitational instability of HI gas disks of the sample galaxies using Toomre Q parameters and HI morphological disturbances. We investigate the effect of the cluster environment on the HI properties of galaxy pairs by dividing the cluster environment into three subcluster regions (i.e., outskirts, infalling and central regions). We find that the denser cluster environment (i.e., infalling and central regions) is likely to impact the HI gas properties of galaxies in a way of decreasing the amplitude of the kinematically narrow HI gas ($M_{\rm{narrow}}^{\rm{HI}}$/$M_{\rm{total}}^{\rm{HI}}$), and increasing the Toomre Q values of the infalling and central galaxies. This tendency is likely to be more enhanced for galaxy pairs in the cluster environment.

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arXiv2022

A cloud-cloud collision in Sgr B2? 3D simulations meet SiO observations

We compare the properties of shocked gas in Sgr B2 with maps obtained from 3D simulations of a collision between two fractal clouds. In agreement with $^{13}$CO(1-0) observations, our simulations show that a cloud-cloud collision produces a region with a highly turbulent density substructure with an average $N_{\rm H2}\gtrsim 5\times10^{22}\,\rm cm^{-2}$. Similarly, our numerical multi-channel shock study shows that colliding clouds are efficient at producing internal shocks with velocities of $5-50\,\rm km\,s^{-1}$ and Mach numbers of $\sim4-40$, which are needed to explain the $\sim 10^{-9}$ SiO abundances inferred from our SiO(2-1) IRAM observations of Sgr B2. Overall, we find that both the density structure and the shocked gas morphology in Sgr B2 are consistent with a $\lesssim 0.5\,\rm Myr$-old cloud-cloud collision. High-velocity shocks are produced during the early stages of the collision and can ignite star formation, while moderate- and low-velocity shocks are important over longer time-scales and can explain the extended SiO emission in Sgr B2.

astro-ph.GA↗

arXiv2022

GASKAP-HI Pilot Survey Science III: An unbiased view of cold gas in the Small Magellanic Cloud

We present the first unbiased survey of neutral hydrogen (HI) absorption in the Small Magellanic Cloud (SMC). The survey utilises pilot HI observations with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope as part of the Galactic ASKAP HI (GASKAP-HI) project whose dataset has been processed with the GASKAP-HI absorption pipeline, also described here. This dataset provides absorption spectra towards 229 continuum sources, a 275% increase in the number of continuum sources previously published in the SMC region, as well as an improvement in the quality of absorption spectra over previous surveys of the SMC. Our unbiased view, combined with the closely matched beam size between emission and absorption, reveals a lower cold gas faction (11%) than the 2019 ATCA survey of the SMC and is more representative of the SMC as a whole. We also find that the optical depth varies greatly between the SMC's bar and wing regions. In the bar we find that the optical depth is generally low (correction factor to the optically thin column density assumption of $\mathcal{R}_{\rm HI} \sim 1.04$) but increases linearly with column density. In the wing however, there is a wide scatter in optical depth despite a tighter range of column densities.

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arXiv2020

Chromatic periodic activity down to 120 MHz in a Fast Radio Burst

Fast radio bursts (FRBs) are extragalactic astrophysical transients whose brightness requires emitters that are highly energetic, yet compact enough to produce the short, millisecond-duration bursts. FRBs have thus far been detected between 300 MHz and 8 GHz, but lower-frequency emission has remained elusive. A subset of FRBs is known to repeat, and one of those sources, FRB 20180916B, does so with a 16.3 day activity period. Using simultaneous Apertif and LOFAR data, we show that FRB 20180916B emits down to 120 MHz, and that its activity window is both narrower and earlier at higher frequencies. Binary wind interaction models predict a narrower periodic activity window at lower frequencies, which is the opposite of our observations. Our detections establish that low-frequency FRB emission can escape the local medium. For bursts of the same fluence, FRB 20180916B is more active below 200 MHz than at 1.4 GHz. Combining our results with previous upper-limits on the all-sky FRB rate at 150 MHz, we find that there are 3-450 FRBs/sky/day above 50 Jy ms at 90% confidence. We are able to rule out the scenario in which companion winds cause FRB periodicity. We also demonstrate that some FRBs live in clean environments that do not absorb or scatter low-frequency radiation.

astro-ph.HE↗

arXiv2020

Structure and kinematics of shocked gas in Sgr B2: further evidence of a cloud-cloud collision from SiO emission maps

We present SiO J=2-1 maps of the Sgr B2 molecular cloud, which show shocked gas with a turbulent substructure comprising at least three cavities at velocities of [10,40] km s$^{-1}$ and an arc at velocities of [-20,10] km s$^{-1}$. The spatial anti-correlation of shocked gas at low and high velocities, and the presence of bridging features in position-velocity diagrams suggest that these structures formed in a cloud-cloud collision. Some of the known compact HII regions spatially overlap with sites of strong SiO emission at velocities of [40,85] km s$^{-1}$, and are between or along the edges of SiO gas features at [100,120] km s$^{-1}$, suggesting that the stars responsible for ionizing the compact HII regions formed in compressed gas due to this collision. We find gas densities and kinetic temperatures of the order of $n_{\rm H_2}\sim 10^5\rm cm^{-3}$ and $\sim$30 K, respectively, towards three positions of Sgr B2. The average values of the SiO relative abundances, integrated line intensities, and line widths are $\sim$10$^{-9}$, $\sim$11 K km s$^{-1}$, and $\sim$31 km s$^{-1}$, respectively. These values agree with those obtained with chemical models that mimic grain sputtering by C-type shocks. A comparison of our observations with hydrodynamical simulations shows that a cloud-cloud collision that took place $\lesssim$ 0.5 Myr ago can explain the density distribution with a mean column density of $\bar{N}_{\rm H_2}\gtrsim 5\times10^{22}$ cm$^{-2}$, and the morphology and kinematics of shocked gas in different velocity channels. Colliding clouds are efficient at producing internal shocks with velocities $\sim$5-50 km $s^{-1}$. High-velocity shocks are produced during the early stages of the collision and can readily ignite star formation, while moderate- and low-velocity shocks are important over longer timescales and can explain the widespread SiO emission in Sgr B2.

astro-ph.GA↗

arXiv2019

Mapping Spatial Variations of HI Turbulent Properties in the Small and Large Magellanic Cloud

We developed methods for mapping spatial variations of the spatial power spectrum (SPS) and structure function (SF) slopes, with a goal of connecting neutral hydrogen (HI) statistical properties with the turbulent drivers. The new methods were applied on the HI observations of the Small and Large Magellanic Clouds (SMC and LMC). In the case of the SMC, we find highly uniform turbulent properties of HI, with no evidence for local enhancements of turbulence due to stellar feedback. Such properties could be caused by a significant turbulent driving on large-scales. Alternatively, a significant line-of-sight depth of the SMC could be masking out localized regions with a steeper SPS slope caused by stellar feedback. In contrast to the SMC, the LMC HI shows a large diversity in terms of its turbulent properties. Across most of the LMC, the small-scale SPS slope is steeper than the large-scale slope due to the presence of the HI disk. On small spatial scales, we find several areas of localized steepening of the SPS slope around major HII regions, with the 30 Doradus region being the most prominent. This is in agreement with predictions from numerical simulations which suggest steepening of the SPS slope due to stellar feedback eroding and destroying interstellar clouds. We also find localized steepening of the large-scale SPS slope in the outskirts of the LMC. This is likely caused by the flaring of the HI disk, or alternatively ram-pressure stripping of the LMC disk due to the interactions with the surrounding halo gas.

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arXiv2018

Angular momentum regulates HI gas content and HI central hole size in the discs of spirals

The neutral atomic hydrogen (HI) content of spiral galaxies has been observed to vary with environment, with more HI-deficient spirals residing in high density environments. This can be attributed to environmental effects such as ram pressure stripping and tidal interactions, which remove HI from the discs of galaxies. However, some spirals in low-density environments have also been observed to have relatively low HI mass fractions. The low densities of the Intergalactic Medium and lack of nearby galaxies in such environments make ram pressure stripping and tidal interactions unlikely candidates of gas removal. What then could be making these spirals HI deficient? Obreschkow et al. introduced a parameter-free model for the neutral atomic gas fraction ($f_{atm}$), in a symmetric equilibrium disc as a function of the global atomic stability parameter ($q$), which depends on specific angular momentum. In order to examine if this model accounts for HI-deficient galaxies in low-density environments, we have used the $M_{HI} ~-$ M$_{R}$ scaling relation to select six HI-deficient spiral galaxies and observed them with the ATCA. By measuring their $f_{atm}$ and $q$ values we find that the galaxies owe their observed HI deficiencies to low specific angular momenta. Additionally, we also find that the central HI hole sizes of our sample galaxies are related to their $q$ values, following the prediction of Obreschkow et al. This result brings to light the importance of angular momentum in understanding the physics of the interstellar medium in the discs of galaxies and consequently their evolution.

astro-ph.GA↗

arXiv2014

New HI scaling relations to probe the HI content of galaxies via global HI-deficiency maps

We present new multi-wavelength scaling relations between the neutral hydrogen content (HI) and the stellar properties of nearby galaxies selected from the HI Parkes All-Sky Survey (HIPASS). We use these new scaling relations to investigate the environmental dependency of the HI content of galaxies. We find that galaxies in high density environments tend to have on average less HI than galaxies with the same stellar mass in the low density environment. Our new HI scaling relations allow us to identify individual galaxies, as well as group/cluster environments, that have an `anomalous' HI content. We map the global distribution of HI-deficient and HI-excess galaxies on the sky and compare it to the large scale structure of galaxies. We find galaxy clusters to be HI-deficient, and we identify that the regions surrounding clusters tend to be HI-excess. Finally, we demonstrate the potential of using HI scaling relations to predict future HI surveys based on an optical redshift survey. We apply our scaling relations to 16709 galaxies in the 6dF Galaxy Survey (6dFGS) that lie in the HIPASS volume and compare our predictions to the measurements. We find that scaling relations are good method to estimate the outcome of HI surveys.

astro-ph.GA↗

arXiv2013

Finding galaxies with unusual HI content

Observations show that galaxies in galaxy clusters are strongly influenced by their environment. There is growing evidence that some galaxies in groups show similar properties to galaxies in clusters, such as redder colours and gas deficiency, highlighting that environmental processes are also effective on galaxy group scales. The question is though, which mechanisms are important in low density environments? To answer this, we identify gas deficient galaxies to investigate recent or ongoing environmental processes, such as gas stripping. We are using scaling relations between the neutral hydrogen (HI) content and optical properties of galaxies to identify galaxies with significantly less HI than an average galaxy of the same type. We derive new, multi wavelength HI-optical scaling relations using the HI Parkes All Sky Survey (HIPASS) with optical and near infrared datasets. We also show our preliminary results from observations of a sample of 6 HI-deficient galaxies, which we identified in low density environments.

astro-ph.GA↗

arXiv2013

Choirs, HI Galaxy Groups: Catalogue and Detection of Star-forming Dwarf Group Members

Hα observations centred on galaxies selected from the HI Parkes All Sky Survey (HIPASS, Barnes et al. 2001) typically show one and sometimes two star-forming galaxies within the approximately 15-arcminute beam of the Parkes 64-m HI detections. In our Survey of Ionization in Neutral Gas Galaxies (SINGG, Meurer et al. 2006) we found fifteen cases of HIPASS sources containing four or more emission line galaxies (ELGs). We name these fields Choir groups. In the most extreme case we found a field with at least nine ELGs. In this paper we present a catalogue of Choir group members in the context of the wider SINGG sample. The dwarf galaxies in the Choir groups would not be individually detectable in HIPASS at the observed distances if they were isolated, but are detected in SINGG narrow-band imaging due to their membership of groups with sufficiently large total HI mass. The ELGs in these groups are similar to the wider SINGG sample in terms of size, Hα equivalent width, and surface brightness. Eight of these groups have two large spiral galaxies with several dwarf galaxies and may be thought of as morphological analogues of the Local Group. However, on average our groups are not significantly HI-deficient, suggesting that they are at an early stage of assembly, and more like the M81 group. The Choir groups are very compact at typically only 190 kpc in projected distance between the two brightest members. They are very similar to SINGG fields in terms of star formation efficiency (the ratio of star formation rate to HI mass; SFE), showing an increasing trend in SFE with stellar mass.

astro-ph.CO↗