arXiv subjects

Martin A. Zwaan

Publications and source records attributed to Martin A. Zwaan.

28 records · Page 2Linked to original sources

arXiv2018

A high-resolution mosaic of the neutral hydrogen in the M81 triplet

We present a 3x3 degrees, 105-pointing, high-resolution neutral hydrogen (HI) mosaic of the M81 galaxy triplet (including the galaxies M81, M82 and NGC 3077, as well as dwarf galaxy NGC 2976) obtained with the Very Large Array (VLA) C and D arrays. This uniformly covers the entire area and velocity range of the triplet with a resolution of ~20'' or ~420 pc. The data reveal many small-scale anomalous velocity features highlighting the complexity of the interacting M81 triplet. We compare our data with Green Bank Telescope (GBT) observations of the same area. This provides evidence for a substantial reservoir of low-column density gas in the northern part of the triplet, probably associated with M82. Such a reservoir is not found in the southern part. We report a number of kpc-sized low-mass HI clouds with HI masses of a few times 10^6 Msun. Their dynamical masses are much larger than their baryonic masses, which could indicate the presence of dark matter if the clouds are rotationally supported. However, due to their spatial and kinematical association with HI tidal features, it is more likely that the velocity widths indicate tidal effects or streaming motions. We do not find any clouds not associated with tidal features down to an HI mass limit of a few times 10^4 Msun. We compare the HI column densities with resolved stellar density maps and find a star formation threshold around 3-6 10^20 cm-2$. We find that extreme velocity dispersions can be explained by a superposition of multiple components along the line of sight near M81 as well as winds or outflows around M82. The velocity dispersions found are high enough that these processes could explain the linewidths of Damped-Lyman-alpha absorbers observed at high redshift.

astro-ph.GA↗

arXiv2018

Molecular Emission from a Galaxy Associated with a z~2.2 Damped Lyman-alpha Absorber

Using the Atacama Large Millimeter/sub-millimeter Array, we have detected CO(3-2) line and far-infrared continuum emission from a galaxy associated with a high-metallicity ([M/H] = -0.27) damped Ly-alpha absorber (DLA) at z =2.19289. The galaxy is located 3.5" away from the quasar sightline, corresponding to a large impact parameter of 30 kpc at the DLA redshift. We use archival Very Large Telescope-SINFONI data to detect Halpha emission from the associated galaxy, and find that the object is dusty, with a dust-corrected star formation rate of 110 +60 -30 Msun/yr. The galaxy's molecular mass is large, Mmol = (1.4 +- 0.2) x 10^11 x (α_CO/4.3) x (0.57/r_31) Msun, supporting the hypothesis that high-metallicity DLAs arise predominantly near massive galaxies. The excellent agreement in redshift between the CO(3-2) line emission and low-ion metal absorption (~40 km/s) disfavors scenarios whereby the gas probed by the DLA shows bulk motion around the galaxy. We use Giant Metrewave Radio Telescope HI 21cm absorption spectroscopy to find that the HI along the DLA sightline must be warm, with a stringent lower limit on the spin temperature of T_s > 1895 x (f/0.93) K. The detection of CI absorption in the DLA, however, also indicates the presence of cold neutral gas. To reconcile these results requires that the cold components in the DLA contribute little to the HI column density, yet contain roughly 50% of the metals of the absorber, underlining the complex multi-phase nature of the gas surrounding high-z galaxies.

astro-ph.GA↗

arXiv2017

New ALMA and Fermi/LAT Observations of the large-scale jet of PKS 0637-752 Strengthen the Case Against the IC/CMB Model

The Chandra X-ray observatory has discovered several dozen anomalously X-ray-bright jets associated with powerful quasars. A popular explanation for the X-ray flux from the knots in these jets is that relativistic synchrotron-emitting electrons inverse-Compton scatter Cosmic Microwave Background (CMB) photons to X-ray energies (the IC/CMB model). This model predicts a high gamma-ray flux which should be detectable by the Fermi Large Area Telescope (LAT) for many sources. GeV-band upper limits from Fermi/LAT for the well-known anomalous X-ray jet in PKS 0637-752 were previously shown in Meyer et al., (2015) to violate the predictions of the IC/CMB model. Previously, measurements of the jet synchrotron spectrum, important for accurately predicting the gamma-ray flux level, were lacking between radio and infrared wavelengths. Here we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of the large-scale jet at 100, 233, and 319 GHz which further constrain the synchrotron spectrum, supporting the previously published empirical model. We also present updated limits from the Fermi/LAT using the new `Pass 8' calibration and approximately 30% more time on source. With these deeper limits we rule out the IC/CMB model at the 8.7 sigma level. Finally, we demonstrate that complete knowledge of the synchrotron SED is critical in evaluating the IC/CMB model.

astro-ph.HE↗

arXiv2017

Spatially resolved star formation relation in two HI-rich galaxies with central post-starburst signature

E+A galaxies are post-starburst systems that are identified from their optical spectra. These galaxies contain a substantial young A-type stellar component, but have only little ongoing star formation (SF). HI 21-cm line emission is found in approximately half of the nearby E+A galaxies, indicating that they contain a reservoir of gas that could fuel active SF. Here, we study two HI-rich galaxies, which show a typical E+A spectrum at the centre and SF at larger radii. We present new high spatial resolution radio interferometric observations of the HI 21-cm emission line using the VLA and of the CO(1-0) emission line using ALMA. We combine these data sets to predict the SFR and show that it does not correlate well with the SFR derived from H alpha on sub-kpc scales. We apply a recently developed statistical model for the small scale behaviour of the SF relation to predict and interpret the observed scatter. We find smoothly distributed, regularly rotating HI gas. The CO(1-0) emission line is not detected for both galaxies. The derived upper limit on the CO mass implies a molecular gas depletion time shorter than 20 Myr. However, due to the low metallicity, the CO-to-H2 conversion factor is highly uncertain. In the relations between the H alpha-based SFR and the HI mass, we observe a substantial scatter we demonstrate results from small-number statistics of independent star-forming regions on sub-kpc scales. This finding adds to the existing literature reporting a scale dependence of the molecular SF relation, showing that the atomic and molecular phases are both susceptible to the evolutionary 'cycling' of individual regions. This suggests that the atomic gas reservoirs host substantial substructure, which should be observable with future high-resolution observations. (abridged)

astro-ph.GA↗

arXiv2015

ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253

We present spatially resolved ($\sim$50 pc) imaging of molecular gas species in the central kiloparsec of the nearby starburst galaxy NGC 253, based on observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA). A total of 50 molecular lines are detected over a 13 GHz bandwidth imaged in the 3 mm band. Unambiguous identifications are assigned for 27 lines. Based on the measured high CO/C$^{17}$O isotopic line ratio ($\gtrsim$350), we show that $^{12}$CO(1-0) has moderate optical depths. A comparison of the HCN and HCO$^{+}$ with their $^{13}$C-substituted isotopologues shows that the HCN(1-0) and HCO$^{+}$(1-0) lines have optical depths at least comparable to CO(1-0). H$^{13}$CN/H$^{13}$CO$^{+}$ (and H$^{13}$CN/HN$^{13}$C) line ratios provide tighter constraints on dense gas properties in this starburst. SiO has elevated abundances across the nucleus. HNCO has the most distinctive morphology of all the bright lines, with its global luminosity dominated by the outer parts of the central region. The dramatic variation seen in the HNCO/SiO line ratio suggests that some of the chemical signatures of shocked gas are being erased in the presence of dominating central radiation fields (traced by C$_{2}$H and CN). High density molecular gas tracers (including HCN, HCO$^+$, and CN) are detected at the base of the molecular outflow. We also detect hydrogen $β$ recombination lines that, like their $α$ counterparts, show compact, centrally peaked morphologies, distinct from the molecular gas tracers. A number of sulfur based species are mapped (CS, SO, NS, C$_{2}$S, H$_{2}$CS and CH$_{3}$SH) and have morphologies similar to SiO.

astro-ph.GA↗

arXiv2013

Neutral atomic hydrogen (HI) gas evolution in field galaxies at z ~ 0.1 and 0.2

We measure the neutral atomic hydrogen (HI) gas content of field galaxies at intermediate redshifts of z ~ 0.1 and z ~ 0.2 using hydrogen 21-cm emission lines observed with the Westerbork Synthesis Radio Telescope (WSRT). In order to make high signal-to-noise ratio detections, an HI signal stacking technique is applied: HI emission spectra from multiple galaxies, optically selected by the CNOC2 redshift survey project, are co-added to measure the average HI mass of galaxies in the two redshift bins. We calculate the cosmic HI gas densities (Ω_{HI}) at the two redshift regimes and compare those with measurements at other redshifts to investigate the global evolution of the HI gas density over cosmic time. From a total of 59 galaxies at z ~ 0.1 we find Ω_{HI} = (0.33 $\pm$ 0.05) ~ $\times$ 10$^{-3}$, and at z ~ 0.2 we find Ω_{HI} = (0.34 $\pm$ 0.09) ~ $\times$ 10$^{-3}$, based on 96 galaxies. These measurements help bridge the gap between high-z damped Lyman-$α$ observations and blind 21-cm surveys at $z=$ 0. We find that our measurements of Ω_{HI} at z ~ 0.1 and 0.2 are consistent with the HI gas density at z ~ 0 and that all measurements of Ω_{HI} from 21-cm emission observations at $z \la$ ~ 0.2 are in agreement with no evolution of the HI gas content in galaxies during the last 2.4 Gyr.

astro-ph.CO↗

arXiv2013

The cold gas content of post-starburst galaxies

Post-starburst galaxies, or E+A galaxies, are characterized by optical spectra showing strong Balmer absorption lines, indicating a young stellar population, and little or no emission lines, implying no active star formation. These galaxies are interpreted as a transitional population between star-forming, disk-dominated galaxies and spheroidal quiescent, non-star forming galaxies. Here, we present single dish HI 21-cm emission line measurements of a sample of eleven of these galaxies at redshifts z<0.05. We detect H I emission in six of the E+A galaxies. In combination with earlier studies, the total number of E+A galaxies with measured cold gas components is now eleven. Roughly half of the E+As studied so far have detectable HI. The gas fractions of these galaxies, measured with respect to their stellar mass, are between 1 and 10 percent and are at the high end of the gas fractions measured in gas-bearing early type galaxies and typically lower than seen in late-type galaxies with comparable stellar masses. This finding is consistent with the idea that E+As are currently evolving from the blue cloud to the red sequence. However, the question of why the star formation has ceased in these galaxies while a significant gas reservoir is still present can only be answered by higher spatial resolution observations of the cold gas.

astro-ph.CO↗

arXiv2009

The Halo Occupation Distribution of HI Galaxies

We perform an analysis of the spatial clustering properties of HI selected galaxies from the HI Parkes All Sky Survey (HIPASS) using the formalism of the halo occupation distribution (HOD). The resulting parameter constraints show that the fraction of satellite galaxies (i.e. galaxies which are not the central member of their host dark matter halo) among HIPASS galaxies is <20%, and that satellite galaxies are therefore less common in HIPASS than in optically selected galaxy redshift surveys. Moreover the lack of fingers-of-god in the redshift space correlation function of HIPASS galaxies may indicate that the HI rich satellites which do exist are found in group mass rather than cluster mass dark matter halos. We find a minimum halo mass for HIPASS galaxies at the peak of the redshift distribution of M~10^11 solar masses, and show that less than 10% of baryons in HIPASS galaxies are in the form of HI. Quantitative constraints on HOD models from HIPASS galaxies are limited by uncertainties introduced through the small survey volume. However our results imply that future deeper surveys will allow the distribution of HI with environment to be studied in detail via clustering of HI galaxies.

astro-ph.CO↗

arXiv2006

The Weak Clustering of Gas-Rich Galaxies

We examine the clustering properties of HI-selected galaxies through an analysis of the HI Parkes All-Sky Survey Catalogue (HICAT) two-point correlation function. Various sub-samples are extracted from this catalogue to study the overall clustering of HI-rich galaxies and its dependence on luminosity, HI gas mass and rotational velocity. These samples cover the entire southern sky Dec < 0 deg, containing up to 4,174 galaxies over the radial velocity range 300-12,700 km/s. A scale length of r_0 = 3.45 +/- 0.25 Mpc/h and slope of gamma = 1.47 +/- 0.08 is obtained for the HI-rich galaxy real-space correlation function, making gas-rich galaxies among the most weakly clustered objects known. HI-selected galaxies also exhibit weaker clustering than optically selected galaxies of comparable luminosities. Good agreement is found between our results and those of synthetic HI-rich galaxy catalogues generated from the Millennium Run CDM simulation. Bisecting HICAT using different parameter cuts, clustering is found to depend most strongly on rotational velocity and luminosity, while the dependency on HI mass is marginal. Splitting the sample around v_rot = 108 km/s, a scale length of r_0 = 2.86 +/- 0.46 Mpc/h is found for galaxies with low rotational velocities compared to r_0 = 3.96 +/- 0.33 Mpc/h for the high rotational velocity sample.

astro-ph↗

arXiv2000

Constraints on the Space Density of Extragalactic HVCs

High Velocity Clouds (HVCs) have recently attracted renewed attention as being long lived, massive dark matter dominated clouds of primordial composition distributed throughout the Local Group. In this picture the HVCs would contain a few times 10^7 M_sun of HI and would be at distances of a few hundred kpc to 1.5 Mpc from the Local Group barycenter. If this extragalactic interpretation of HVCs is true, similar clouds are expected in other galaxy groups and around galaxies. We discuss the limits blind HI surveys and QSO absorption line studies put on this proposed population of clouds.

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