arXiv subjects

Erwin de Blok

Publications and source records attributed to Erwin de Blok.

18 recordsLinked to original sources

arXiv2026

Tracing neutral hydrogen in UGCA 320: A MHONGOOSE perspective on an edge-on dwarf galaxy in a group environment

We present a detailed analysis of the neutral atomic gas (HI) in the dwarf galaxy UGCA 320, observed with the MeerKAT telescope as part of the MHONGOOSE (MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters) programme. In a small group consisting of three dwarf galaxies, all of which contain HI, it is the most massive. Detailed kinematic modelling shows that UGCA 320 contains a substantial amount of (kinematically) anomalous gas (>=20%), at least ~30% of which is likely the result of a tidal interaction with its neighbour UGCA 319. It also reveals that UGCA 320 likely harbours a star-formation driven outflow, and that ~10% of its HI is extra-planar and has a filamentary structure. Although UGCA 320 aligns with established scaling relations from the literature, its neutral hydrogen content is notably complex - shaped by its immediate environment. This underscores the importance of deep, resolved observations and detailed kinematic analyses to capture the nuances of galaxy evolution.

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arXiv2025

Investigating the HI distribution and kinematics of ESO444-G084 and [KKS2000]23: New insights from the MHONGOOSE survey

We present the HI distribution, kinematics, mass modeling, and disk stability of the dwarf irregular galaxies ESO444-G084 and [KKS2000]23 using high-resolution, high-sensitivity MHONGOOSE survey data from MeerKAT. ESO444-G084 shows centrally concentrated HI emission, while [KKS2000]23 exhibits irregular high-density clumps. Total HI fluxes measured down to 10^19 and 10^18 cm^-2 are nearly identical, indicating that the increased HI diameter at lower column densities results mainly from the larger beam, with no significant extra emission detected. We derive total HI masses of (1.1 +/- 0.1) x 10^8 and (6.1 +/- 0.3) x 10^8 solar masses for ESO444-G084 and [KKS2000]23, respectively. Using PyFAT and TiRiFiC, we extract 3D rotation curves that reveal disk-like kinematics in both galaxies. ESO444-G084 shows a warp beyond ~1.8 kpc and a fast-rising curve consistent with a centrally concentrated dark matter distribution, while [KKS2000]23's more gradual rise suggests a more extended halo. Mass modeling with an isothermal halo and stellar mass-to-light ratios of 0.20 for ESO444-G084 and 0.18 for [KKS2000]23 yields consistent results. We analyze disk stability using spatially resolved Toomre Q and gas-to-critical surface density ratios, linking these with H-alpha and FUV-based star formation. ESO444-G084 supports localized star formation despite global stability, while [KKS2000]23 appears gravitationally unstable yet lacks H-alpha, suggesting that turbulence, gas depletion, or past feedback suppresses star formation. No inflows or outflows are detected, indicating internal processes regulate star formation. This study highlights the interplay between HI morphology, kinematics, dark matter distribution, and disk stability, showing how internal processes shape dwarf galaxy evolution.

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arXiv2022

LADUMA: Discovery of a luminous OH megamaser at $z > 0.5$

In the local Universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (HI), radio surveys to probe the cosmic evolution of HI in galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep HI survey, we report the first untargeted detection of an OHM at $z > 0.5$, LADUMA J033046.20$-$275518.1 (nicknamed "Nkalakatha"). The host system, WISEA J033046.26$-$275518.3, is an infrared-luminous radio galaxy whose optical redshift $z \approx 0.52$ confirms the MeerKAT emission line detection as OH at a redshift $z_{\rm OH} = 0.5225 \pm 0.0001$ rather than HI at lower redshift. The detected spectral line has 18.4$σ$ peak significance, a width of $459 \pm 59\,{\rm km\,s^{-1}}$, and an integrated luminosity of $(6.31 \pm 0.18\,{\rm [statistical]}\,\pm 0.31\,{\rm [systematic]}) \times 10^3\,L_\odot$, placing it among the most luminous OHMs known. The galaxy's far-infrared luminosity $L_{\rm FIR} = (1.576 \pm 0.013) \times 10^{12}\,L_\odot$ marks it as an ultra-luminous infrared galaxy; its ratio of OH and infrared luminosities is similar to those for lower-redshift OHMs. A comparison between optical and OH redshifts offers a slight indication of an OH outflow. This detection represents the first step towards a systematic exploitation of OHMs as a tracer of galaxy growth at high redshifts.

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arXiv2022

Kiloparsec-scale imaging of the CO(1-0)-traced cold molecular gas reservoir in a z~3.4 submillimeter galaxy

We present a high-resolution study of the cold molecular gas as traced by CO(1-0) in the unlensed z$\sim$3.4 submillimeter galaxy SMM J13120+4242, using multi-configuration observations with the Karl G. Jansky Very Large Array (JVLA). The gas reservoir, imaged on 0.39" ($\sim$3 kpc) scales, is resolved into two components separated by $\sim$11 kpc with a total extent of 16 $\sim$3 kpc. Despite the large spatial extent of the reservoir, the observations show a CO(1-0) FWHM linewidth of only 267 $\pm$ 64 km s$^{-1}$. We derive a revised line luminosity of L'$_\mathrm{CO(1-0)}$ = (10 $\pm$ 3) $\times$ 10$^{10}$ K km s$^{-1}$ pc$^2$ and a molecular gas mass of M$_\mathrm{gas}$ = (13 $\pm$ 3) $\times$ 10$^{10}$ ($α_\mathrm{CO}$/1) M$_{\odot}$. Despite the presence of a velocity gradient (consistent with previous resolved CO(6-5) imaging), the CO(1-0) imaging shows evidence for significant turbulent motions which are preventing the gas from fully settling into a disk. The system likely represents a merger in an advanced stage. Although the dynamical mass is highly uncertain, we use it to place an upper limit on the CO-to-H$_2$ mass conversion factor $α_\mathrm{CO}$ of 1.4. We revisit the SED fitting, finding that this galaxy lies on the very massive end of the main sequence at z = 3.4. Based on the low gas fraction, short gas depletion time and evidence for a central AGN, we propose that SMM J13120 is in a rapid transitional phase between a merger-driven starburst and an unobscured quasar. The case of SMM J13120 highlights the how mergers may drive important physical changes in galaxies without pushing them off the main sequence.

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arXiv2019

The Local Volume HI Survey (LVHIS)

The `Local Volume HI Survey' (LVHIS) comprises deep HI spectral line and 20-cm radio continuum observations of 82 nearby, gas-rich galaxies, supplemented by multi-wavelength images. Our sample consists of all galaxies with Local Group velocities v_LG < 550 km/s or distances D < 10 Mpc that are detected in the HI Parkes All Sky Survey (HIPASS). Using full synthesis observations in at least three configurations of the Australia Telescope Compact Array (ATCA), we obtain detailed HI maps for a complete sample of gas-rich galaxies with Dec < -30 deg. Here we present a comprehensive LVHIS Galaxy Atlas, including the overall gas distribution, mean velocity field, velocity dispersion, and position-velocity diagrams, together with a homogeneous set of measured and derived galaxy properties. Our primary goal is to investigate the HI morphologies, kinematics, and environment at high resolution and sensitivity. LVHIS galaxies represent a wide range of morphologies and sizes; our measured HI masses range from ~10^7 to 10^10 Msun, based on independent distance estimates. The LVHIS Galaxy Atlas (including FITS files) is available on-line.

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arXiv2017

HI Kinematics, Mass Distribution, and Star Formation Threshold in NGC 6822, using the SKA pathfinder KAT-7

We present high sensitivity HI observations of NGC 6822, obtained with the Karoo Array Telescope (KAT-7). We study the kinematics, the mass distribution, and the star formation thresholds. The KAT-7 short baselines and low system temperature make it sensitive to large-scale, low surface brightness emission. The observations detected $\sim$ 23$\%$ more flux than previous ATCA observations. We fit a tilted ring model to the HI velocity field to derive the rotation curve (RC). The KAT-7 observations allow the measurement of the rotation curve of NGC 6822 out to 5.8 kpc, $\sim$ 1 kpc further than existing measurements. NGC 6822 is seen to be dark matter dominated at all radii. The observationally motivated pseudo-isothermal dark matter (DM) halo model reproduces well the observed RC while the Navarro Frank-White DM model gives a poor fit to the data. We find the best fit mass to light ratio (M/L) of 0.12 $\pm$ 0.01 which is consistent with the literature. The Modified Newtonian Dynamics (MOND) gives a poor fit to our data. We derive the star formation threshold in NGC 6822 using the HI and H$α$ data. The critical gas densities were calculated for gravitational instabilities using the Toomre-Q criterion and the cloud-growth criterion. We found that in regions of star formation, the cloud-growth criterion explains star formation better than the Toomre-Q criterion. This shows that the local shear rate could be a key player in cloud formation for irregular galaxies such as NGC 6822.

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arXiv2015

Comment on "Evidence for dark matter in the inner Milky Way"

This is a brief rebuttal to arXiv:1502.03821, which claims to provide the first observational proof of dark matter interior to the solar circle. We point out that this result is not new, and can be traced back at least a quarter century.

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arXiv2007

Star Formation in NGC 5194 (M51a). II. The Spatially-Resolved Star Formation Law

We have studied the relationship between the star formation rate (SFR) surface density and gas surface density in the spiral galaxy M51a (NGC 5194), using multi-wavelength data obtained as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS). We introduce a new SFR index based on a linear combination of H-alpha emission-line and 24 micron continuum luminosities, that provides reliable extinction-corrected ionizing fluxes and SFR densities over a wide range of dust attenuations. The combination of these extinction-corrected SFR densities with aperture synthesis HI and CO maps has allowed us to probe the form of the spatially-resolved star formation law on scales of 0.5 to 2 kpc. We find that the resolved SFR vs gas surface density relation is well represented by a Schmidt power law, which is similar in form and dispersion to the disk-averaged Schmidt law. We observe a comparably strong correlation of the SFR surface density with the molecular gas surface density, but no significant correlation with the surface density of atomic gas. The best-fitting slope of the Schmidt law varies from N = 1.37 to 1.56, with zeropoint and slope that change systematically with the spatial sampling scale. We tentatively attribute these variations to the effects of areal sampling and averaging of a nonlinear intrinsic star formation law. Our data can also be fitted by an alternative parametrization of the SFR surface density in terms of the ratio of gas surface density to local dynamical time, but with a considerable dispersion.

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arXiv2002

A High-Resolution Rotation Curve of NGC 6822: A Test-case for Cold Dark Matter

We present high resolution rotation curves of the local group dwarf irregular galaxy NGC 6822 obtained with the Australia Telescope Compact Array. Our best curves have an angular resolution of 8'' or 20 pc and contain some 250 independent points. The stellar and gas components of NGC 6822 cannot explain the shape of the curve, except for the very inner regions, and NGC 6822 is consequently very dark matter dominated. There is no evidence for the presence of a steep density cusp down to scales of ~20 pc, contrary to the predictions of Cold Dark Matter.

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arXiv2001

High-resolution rotation curves of LSB galaxies: Mass Models

We present mass models for a sample of 30 high-resolution rotation curves of low surface brightness (LSB) galaxies. We fit both pseudo-isothermal (core-dominated) and Cold Dark Matter (CDM) (cusp-dominated) halos for a wide variety of assumptions about the stellar mass-to-light ratio. We find that the pseudo-isothermal model provides superior fits. CDM fits show systematic deviations from the data, and often have a small statistical likelihood of being the appropriate model. The distribution of concentration parameters is too broad, and has too low a mean, to be explained by $Λ$CDM. This failing becomes more severe as increasing allowance is made for stellar mass: NFW fits require uncomfortably low mass-to-light ratios. In contrast, the maximum disk procedure does often succeed in predicting the inner shape of the rotation curves, but requires uncomfortably large stellar mass-to- light ratios. The data do admit reasonable stellar population mass-to-light ratios if halos have cores rather than cusps.

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arXiv2001

High-resolution rotation curves of low surface brightness galaxies: Data

We present long slit Halpha observations of 50 low surface brightness galaxies. Of these, 36 are of sufficient quality to form rotation curves. These data provide a large increase in the number of low surface brightness galaxies for which accurate rotation curves are available. They also represent an order of magnitude improvement in spatial resolution over previous 21 cm studies (1" to 2" instead of 13" to 45"). The improved resolution and accuracy of the data extend and strengthen the scientific conclusions previously inferred from 21 cm data.

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arXiv2000

The Baryonic Tully-Fisher Relation

We explore the Tully-Fisher relation over five decades in stellar mass in galaxies with circular velocities ranging over 30 < Vc < 300 km/s. We find a clear break in the optical Tully-Fisher relation: field galaxies with Vc < 90 km/s fall below the relation defined by brighter galaxies. These faint galaxies are however very gas rich; adding in the gas mass and plotting baryonic disk mass Md = M* + Mg in place of luminosity restores a single linear relation. The Tully-Fisher relation thus appears fundamentally to be a relation between rotation velocity and total baryonic mass of the form Md = A Vc^4.

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arXiv1998

Testing the Dark Matter Hypothesis with Low Surface Brightness Galaxies and Other Evidence

The severity of the mass discrepancy in spiral galaxies is strongly correlated with the central surface brightness of the disk. Progressively lower surface brightness galaxies have ever larger mass discrepancies. No other parameter (luminosity, size, velocity, morphology) is so well correlated with the magnitude of the mass deficit. The rotation curves of low surface brightness disks thus provide a unique data set with which to probe the dark matter distribution in galaxies. The mass discrepancy is apparent from $R = 0$ giving a nearly direct map of the halo mass distribution. The luminous mass is insignificant. Interpreting the data in terms of dark matter leads to troublesome fine-tuning problems. Different observations require contradictory amounts of dark matter. Structure formation theories are as yet far from able to explain the observations.

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arXiv1998

Testing the Hypothesis of Modified Dynamics with Low Surface Brightness Galaxies and Other Evidence

The rotation curves of low surface brightness galaxies provide a unique data set with which to test alternative theories of gravitation over a large dynamic range in size, mass, surface density, and acceleration. Many clearly fail, including any in which the mass discrepancy appears at a particular length-scale. One hypothesis, MOND [Milgrom 1983, ApJ, 270, 371], is consistent with the data. Indeed, it accurately predicts the observed behavior. We find no evidence on any scale which clearly contradicts MOND, and a good deal which supports it.

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arXiv1997

The Baryon Fraction Distribution and the Tully-Fisher Relation

A number of observations strongly suggest that the baryon fraction is not a universal constant. One obvious interpretation is that there is some distribution of Fb, and the different observations sample different portions of the distribution. However, the small intrinsic scatter in the Tully-Fisher relation requires that the baryon fraction be very nearly universal. It is not easy to resolve this paradox in the framework of the standard picture.

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arXiv1996

Dynamical Stability and Environmental Influences in Low Surface Brightness Disk Galaxies

Using analytic stability criteria, we demonstrate that, due to their low surface mass density and large dark matter content, LSB disks are quite stable against the growth of global nonaxisymmetric modes such as bars. However, depending on their (poorly constrained) stellar velocity dispersions, they may be only marginally stable against local instabilities. We simulate a collision between an LSB and HSB galaxy and find that, while the HSB galaxy forms a strong bar, the response of the LSB disk is milder, manifesting weaker rings and spiral features. The lack of sufficient disk self-gravity to amplify dynamical instabilities naturally explains the rarity of bars in LSB disks. The stability of LSB disks may also inhibit interaction-driven gas inflow and starburst activity in these galaxies.

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arXiv1996

Gas Content and Star Formation Thresholds in the Evolution of Spiral Galaxies

The gas mass fraction (fg) of spiral galaxies is strongly correlated with the central surface brightness of their disks. There exist many dim galaxies with long gas consumption time scales and fg > 0.5. This resolves the gas consumption paradox. The surface density of gas follows the optical surface brightness, but does not vary by as large a factor. This is the signature of a critical density threshold for star formation. Such a mechanism seems to be responsible for the slow evolution of dim galaxies.

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arXiv1996

Dynamical Stability and Galaxy Evolution in LSB Disk Galaxies

We demonstrate that, due to their low surface mass density and large dark matter content, LSB disks are quite stable against the growth of global bar modes. However, they may be only marginally stable against local disk instabilities. We simulate a collision between an LSB and HSB galaxy and find that, while the HSB galaxy forms a strong bar, the response of the LSB disk is milder, in the form of spiral features and an oval distortion. Unlike its HSB counterpart, the LSB disk does not suffer strong inflow of gas into the central regions. The lack of sufficient disk self-gravity to amplify dynamical instabilities makes it difficult to explain strong interaction-driven starbursts in LSB galaxies without invoking mergers.

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