arXiv subjects

Elias Brinks

Publications and source records attributed to Elias Brinks.

At least 73 records · Page 4Linked to original sources

arXiv2011

Deep Radio Continuum Imaging of the Dwarf Irregular Galaxy IC10: Tracing Star Formation and Magnetic Fields

We exploit the vastly increased sensitivity of the Expanded Very Large Array (EVLA) to study the radio continuum and polarization properties of the post-starburst, dwarf irregular galaxy IC10 at 6 cm, at a linear resolution of ~50 pc. We find close agreement between radio continuum and Halpha emission, from the brightest HII regions to the weaker emission in the disk. A quantitative analysis shows a strictly linear correlation, where the thermal component contributes 50% to the total radio emission, the remainder being due to a non-thermal component with a surprisingly steep radio spectral index of between -0.7 and -1.0 suggesting substantial radiation losses of the cosmic-ray electrons. We confirm and clearly resolve polarized emission at the 10-20% level associated with a non-thermal superbubble, where the ordered magnetic field is possibly enhanced due to the compression of the expanding bubble. A fraction of the cosmic-ray electrons has likely escaped because the measured radio emission is a factor of 3 lower than what is suggested by the Halpha inferred SFR.

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arXiv2011

VII Zw 403: HI Structure in a Blue Compact Dwarf Galaxy

We present optical (UBVJ), ultraviolet (FUV, NUV), and high resolution atomic hydrogen (HI) observations of the nearby blue compact dwarf (BCD), VII Zw 403. We find that VII Zw 403 has a relatively high HI mass-to-light ratio for a BCD. The rotation velocity is nominally 10-15 km/s, but rises to ~20 km/s after correction for the ~8-10 km/s random motions present in the gas. The velocity field is complex; including a variation in the position angle of the major axis going from the NE to the SW parts of the galaxy. Our high resolution HI maps reveal structure in the central gas, including a large, low-density HI depression or hole between the southern and northern halves of the galaxy, coincident with an unresolved x-ray source. Although interactions have been proposed as the triggering mechanism for the vigorous star formation occurring in BCDs, VII Zw 403 does not seem to have been tidally triggered by an external interaction, as we have found no nearby possible perturbers. It also doesn't appear to fall in the set of galaxies that exhibit a strong central mass density concentration, as its optical scale length is large in comparison to similar systems. However, there are some features that are compatible with an accretion event: optical/HI axis misalignment, a change in position angle of the kinematic axis, and a complex velocity field.

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arXiv2011

Studying the spatially resolved Schmidt-Kennicutt law in interacting galaxies: the case of Arp 158

Recent studies have shown that star formation in mergers does not seem to follow the same Schmidt-Kennicutt (KS) relation as in spiral disks, presenting a higher star formation rate (SFR) for a given gas column density. In this paper we study why and how different models of star formation arise. To do so we examine the process of star formation in the interacting system Arp 158 and its tidal debris. We perform an analysis of the properties of specific regions of interest in Arp 158 using observations tracing the atomic and the molecular gas, star formation, the stellar populations as well as optical spectroscopy to determine their exact nature. We also fit their spectral energy distribution with an evolutionary synthesis code. Finally, we compare star formation in these objects to star formation in the disks of spiral galaxies and mergers. Abundant molecular gas is found throughout the system and the tidal tails appear to have many young stars compared to their old stellar content. One of the nuclei is dominated by a starburst whereas the other is an active nucleus. We estimate the SFR throughout the system and find that most regions follow closely the KS relation seen in spiral galaxies with the exception of the nuclear starburst and the tip of one of the tails. We examine whether this diversity is due to uncertainties in the manner the SFR is determined or whether the conditions in the nuclear starburst region are such that it does not follow the same KS law as other regions. Observations of the interacting system Arp 158 provide the first evidence in a resolved fashion that different star-forming regions in a merger may be following different KS laws. This suggests that the physics of the interstellar medium at a scale no larger than 1 kpc, the size of the largest gravitational instabilities and the injection scale of turbulence, determines the origin of these laws.

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arXiv2011

HI asymmetries in the isolated galaxy CIG 292

We present Expanded Very Large Array (EVLA) D-array observations in the 21-cm line of neutral hydrogen (HI) of CIG 292, an isolated SA(s)b galaxy at a distance of ~ 24.3Mpc. From previous HI single dish observations the galaxy was known to have a mildly asymmetric HI profile (A_flux =1.23 {\pm}0.3). Our EVLA observations show there is ~ 12% more HI projected South of the optical centre (approaching velocities) than in the North (receding velocities), despite the HI extending ~ 16% further to the North than the South. The HI projected within the optical disk must have been perturbed within the HI relaxation time (~ 10^8 yr) which implies that this can not have been caused by any of the three nearest companions, as their distance (~ 0.5Mpc) is too large. Neither HI-rich companions nor tidal tails were found within our field of view and velocity range covered. Our kinematical data suggest that the inner part harbors an oval distortion whereas the outer regions show signs of a modest warp. The mild asymmetry in the HI global profile thus actually masks stronger asymmetries in the two-dimensional distributions of gas and star forming regions in this galaxy. Since the galaxy is isolated, this must predominantly be due to processes related to its formation and secular evolution.

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arXiv2011

A Molecular Star Formation Law in the Atomic Gas Dominated Regime in Nearby Galaxies

We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using atomic hydrogen (HI) data, mostly from THINGS, we predict the local mean CO velocity from the mean HI velocity. By renormalizing the CO velocity axis so that zero corresponds to the local mean HI velocity we are able to stack spectra coherently over large regions as function of radius. This enables us to measure CO intensities with high significance as low as Ico = 0.3 K km/s (H2_SD = 1 Msun/pc2), an improvement of about one order of magnitude over previous studies. We detect CO out to radii Rgal = R25 and find the CO radial profile to follow a uniform exponential decline with scale length of 0.2 R25. Comparing our sensitive CO profiles to matched profiles of HI, Halpha, FUV, and IR emission at 24um and 70um, we observe a tight, roughly linear relation between CO and IR intensity that does not show any notable break between regions that are dominated by molecular (H2) gas (H2_SD > HI_SD) and those dominated by atomic gas (H2_SD < HI_SD). We use combinations of FUV+24um and Halpha+24um to estimate the recent star formation rate (SFR) surface density, SFR_SD, and find approximately linear relations between SFR_SD and H2_SD. We interpret this as evidence for stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H2 ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relations between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between HI and H2 depends strongly on the total gas surface density and radius. It must also depend on additional parameters.

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arXiv2011

Dark and luminous matter in THINGS dwarf galaxies

We present mass models for the dark matter component of seven dwarf galaxies taken from "The HI Nearby Galaxy Survey" (THINGS) and compare these with those from numerical Lambda Cold Dark Matter (LCDM) simulations. The THINGS high-resolution data significantly reduce observational uncertainties and thus allow us to derive accurate dark matter distributions in these systems. We here use the bulk velocity fields when deriving the rotation curves of the galaxies. Compared to other types of velocity fields, the bulk velocity field minimizes the effect of small-scale random motions more effectively and traces the underlying kinematics of a galaxy more properly. The "Spitzer Infrared Nearby Galaxies Survey" (SINGS) 3.6 micron and ancillary optical data are used for separating the baryons from their total matter content in the galaxies. The sample dwarf galaxies are found to be dark matter dominated over most radii. We find discrepancies between the derived dark matter distributions of the galaxies and those of LCDM simulations, even after corrections for non-circular motions have been applied. The observed solid body-like rotation curves of the galaxies rise too slowly to reflect the cusp-like dark matter distribution in CDM halos. Instead, they are better described by core-like models such as pseudo-isothermal halo models dominated by a central constant-density core. The mean value of the logarithmic inner slopes of the mass density profiles is alpha = -0.29 +- 0.07. They are significantly different from the steep slope of ~ -1.0 inferred from previous dark-matter-only simulations, and are more consistent with shallower slopes found in recent LCDM simulations of dwarf galaxies in which the effects of baryonic feedback processes are included.

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arXiv2011

The central slope of dark matter cores in dwarf galaxies: Simulations vs. THINGS

We make a direct comparison of the derived dark matter (DM) distributions between hydrodynamical simulations of dwarf galaxies assuming a LCDM cosmology and the observed dwarf galaxies sample from the THINGS survey in terms of (1) the rotation curve shape and (2) the logarithmic inner density slope alpha of mass density profiles. The simulations, which include the effect of baryonic feedback processes, such as gas cooling, star formation, cosmic UV background heating and most importantly physically motivated gas outflows driven by supernovae (SNe), form bulgeless galaxies with DM cores. We show that the stellar and baryonic mass is similar to that inferred from photometric and kinematic methods for galaxies of similar circular velocity. Analyzing the simulations in exactly the same way as the observational sample allows us to address directly the so-called "cusp/core" problem in the LCDM model. We show that the rotation curves of the simulated dwarf galaxies rise less steeply than CDM rotation curves and are consistent with those of the THINGS dwarf galaxies. The mean value of the logarithmic inner density slopes alpha of the simulated galaxies' dark matter density profiles is ~ -0.4 +- 0.1, which shows good agreement with α= -0.29 +- 0.07 of the THINGS dwarf galaxies. The effect of non-circular motions is not significant enough to affect the results. This confirms that the baryonic feedback processes included in the simulations are efficiently able to make the initial cusps with α~ -1.0 to -1.5 predicted by dark-matter-only simulations shallower, and induce DM halos with a central mass distribution similar to that observed in nearby dwarf galaxies.

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arXiv2011

Theoretical determination of HI vertical scale heights in the dwarf galaxies: DDO 154, HoII, IC2574 & NGC2366

In this paper, we model dwarf galaxies as a two-component system of gravitationally coupled stars and atomic hydrogen gas in the external force field of a pseudo-isothermal dark matter halo, and numerically obtain the radial distribution of {H\,{\sc i}} vertical scale heights. This is done for a group of four dwarf galaxies (DDO\,154, Ho\,II, IC\,2574 and NGC\,2366) for which most necessary input parameters are available from observations. The formulation of the equations takes into account the rising rotation curves generally observed in dwarf galaxies. The inclusion of self-gravity of the gas into the model at par with that of the stars results in scale heights that are smaller than what was obtained by previous authors. This is important as the gas scale height is often used for deriving other physical quantities. The inclusion of gas self-gravity is particularly relevant in the case of dwarf galaxies where the gas cannot be considered a minor perturbation to the mass distribution of the stars. We find that three out of four galaxies studied show a flaring of their {H\,{\sc i}} disks with increasing radius, by a factor of a few within several disk scale lengths. The fourth galaxy has a thick {H\,{\sc i}} disk throughout. This arises as a result of the gas velocity dispersion remaining constant or decreasing only slightly while the disk mass distribution declines exponentially as a function of radius.

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arXiv2011

NGC 2207/IC 2163: A Grazing Encounter with Large Scale Shocks

Radio continuum, Spitzer infrared, optical and XMM-Newton X-ray and UVM2 observations are used to study large-scale shock fronts, young star complexes, and the galactic nuclei in the interacting galaxies NGC 2207/IC 2163. There are two types of large-scale shock fronts in this galaxy pair. The shock front along the rim of the ocular oval in IC 2163 has produced vigorous star formation in a dusty environment. In the outer part of the companion side of NGC 2207, a large-scale front attributed to disk or halo scraping is particularly bright in the radio continuum but not in any tracers of recent star formation or in X-rays. This radio continuum front may be mainly in the halo on the back side of NGC 2207 between the two galaxies. Values of the flux density ratio S(8 um)/S(6 cm) of kpc-sized, Spitzer IRAC star-forming clumps in NGC 2207/IC 2163 are compared with those of giant H II regions in M81. We find evidence that in 2001 a radio supernova was present in the core of feature i, a mini-starburst on an outer arm of NGC 2207. X-ray emission is detected from the NGC 2207 nucleus and from nine discrete sources, one of which corresponds to SN 1999ec, and another may be a radio supernova or a background quasar. The X-ray luminosity and X-ray spectrum of the NGC 2207 nucleus suggests it is a highly absorbed, low luminosity AGN.

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arXiv2010

Dwarf-Galaxy Cosmology

Dwarf galaxies provide opportunities for drawing inferences about the processes in the early universe by observing our "cosmological backyard"-the Local Group and its vicinity. This special issue of the open-access journal Advances in Astronomy is a snapshot of the current state of the art of dwarf-galaxy cosmology.

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arXiv2009

Molecular gas in Tidal Dwarf Galaxies: Exploring the conditions for star formation

Tidal Dwarf Galaxies (TDGs), produced from material expelled in galactic interactions, are well--suited to test the laws of star formation (SF) due to their simple structure, high metallicity -- making CO a reliable tracer of the molecular gas content -- and recent SF. Here, we study the conditions for the onset of SF and for the rate at which SF proceeds once above a threshold in a small sample of TDGs. We use data for the gas (atomic and molecular) surface density and SF rate per area to test the laws of SF found for spiral and dwarf galaxies in this more extreme environment. We find in general a good agreement with the Schmidt law found for the total gas and for the molecular gas but note that higher resolution CO observations are necessary to clarify some possible discrepancies. We find, down to a scale of $\sim$1 kpc, in general a good agreement between the peaks of SF and of the molecular gas, but also find in some objects surprisingly large quantities of molecular gas at places where no SF is occuring. A high column density of molecular gas is therefore not a sufficient condition for the onset of SF. We find that the kinematical properties of the gas are also relevant: in two objects our observations showed that SF only occured in regions with a narrow line width.

astro-ph.CO↗

arXiv2008

Triggered Star Formation and the Creation of the Supergiant HI Shell in IC 2574

Based on deep imaging from the Advanced Camera for Surveys aboard the Hubble Space Telescope, we present new evidence that stellar feedback created a ~ 1 kpc supergiant HI shell (SGS) and triggered star formation (SF) around its rim in the M81 Group dwarf irregular galaxy IC 2574. Using photometry of the resolved stars from the HST images, we measure the star formation history of a region including the SGS, focusing on the past 500 Myr, and employ the unique properties of blue helium burning stars to create a movie of SF in the SGS. We find two significant episodes of SF inside the SGS from 200 - 300 Myr and ~ 25 Myr ago. Comparing the timing of the SF events to the dynamic age of the SGS and the energetics from the HI and SF, we find compelling evidence that stellar feedback is responsible for creating the SGS and triggering secondary SF around its rim.

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arXiv2008

The Star Formation Efficiency in Nearby Galaxies: Measuring Where Gas Forms Stars Effectively

We measure the star formation efficiency (SFE), the star formation rate per unit gas, in 23 nearby galaxies and compare it to expectations from proposed star formation laws and thresholds. We use HI maps from THINGS and derive H2 maps from HERACLES and BIMA SONG CO. We estimate the star formation rate by combining GALEX FUV maps and SINGS 24 micron maps, infer stellar surface density profiles from SINGS 3.6 micron data, and use kinematics from THINGS. We measure the SFE as a function of: the free-fall and orbital timescales; midplane gas pressure; stability of the gas disk to collapse (including the effects of stars); the ability of perturbations to grow despite shear; and the ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly constant at 5.25 +/- 2.5 x 10^(-10) yr^(-1) (equivalent to an H2 depletion time of 1.9x10^9 yr) as a function of all of these variables at our 800 pc resolution. Where the ISM is mostly HI, on the other hand, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2-0.25 r_25. We interpret this decline as a strong dependence of GMC formation on environment. The ratio of H2 to HI appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H2-dominated to HI-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe - phase balance in the HI, H2 formation and destruction, and stellar feedback - governs the formation of GMCs from HI.

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arXiv2008

The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales

(Abridged) We present a comprehensive analysis of the relationship between star formation rate surface density (SFR SD) and gas surface density (gas SD) at sub-kpc resolution in a sample of 18 nearby galaxies. We use high resolution HI data from THINGS, CO data from HERACLES and BIMA SONG, 24 micron data from the Spitzer Space Telescope, and UV data from GALEX. We target 7 spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H2-dominated centers of spiral galaxies, their HI-dominated outskirts and the HI-rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N=1.0+-0.2 relates the SFR SD and the H2 SD across our sample of spiral galaxies, i.e., that H2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ~2*10^9 yrs. We interpret the linear relation and constant depletion time as evidence that stars are forming in GMCs with approximately uniform properties and that the H2 SD may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas SD and SFR SD varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between the HI SD and the SFR SD. As a result, the star formation efficiency (SFE = SFR SD / gas SD) varies strongly across our sample and within individual galaxies. We show that in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. Another general feature of our sample is a sharp saturation of the HI SD at ~9 M_sol/pc^2 in both the spiral and dwarf galaxies.

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arXiv2008

Are the kinematics of DLAs in agreement with their arising in the gas disks of galaxies?

We demonstrate in this paper that the velocity widths of the neutral gas in Damped Ly Alpha (DLA) systems are inconsistent with these systems originating in gas disks of galaxies similar to those seen in the local Universe. We examine the gas kinematics of local galaxies using the high quality HI 21-cm data from the HI Nearby Galaxies Survey (THINGS) and make a comparison with the velocity profiles measured in the low-ionization metal lines observed in DLAs at high redshifts. The median velocity width of z=0 HI gas above the DLA column density limit of N=2x10^20 cm-2 is approximately 30 km/s, whereas the typical value in DLAs is a factor of two higher. We argue that the gas kinematics at higher redshifts are increasingly influenced by gas that is not participating in ordered rotation in cold disks, but is more likely associated with tidal gas related to galaxy interactions or processes such as superwinds and outflows. An analysis of the HI in the local interacting star-burst galaxy M82 shows that the velocity widths in this galaxy are indeed similar to what is seen in DLAs.

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arXiv2008

High-resolution dark matter density profiles of THINGS dwarf galaxies: Correcting for non-circular motions

We present a new method to remove the impact of random and small-scale non-circular motions from HI velocity fields in galaxies in order to better constrain the dark matter properties for these objects. This method extracts the circularly rotating velocity components from the HI data cube and condenses them into a so-called bulk velocity field. We derive high-resolution rotation curves of IC 2574 and NGC 2366 based on bulk velocity fields derived from The HI Nearby Galaxy Survey (THINGS) obtained at the VLA. The bulk velocity field rotation curves are significantly less affected by non-circular motions and constrain the dark matter distribution in our galaxies, allowing us to address the discrepancy between the inferred and predicted dark matter distribution in galaxies (the "cusp/core" problem). Spitzer Infrared Nearby Galaxies Survey (SINGS) 3.6 micron data as well as ancillary optical information, are used to separate the baryons from the total matter content. Using stellar population synthesis models, assuming various sets of metallicity and star formation histories, we compute stellar mass-to-light ratios for the 3.6 and 4.5 micron bands. Using our predicted value for the 3.6 micron stellar mass-to-light ratio, we find that the observed dark matter distributions of IC 2574 and NGC 2366 are inconsistent with the cuspy dark matter halo predicted by LCDM models, even after corrections for non-circular motions. This result also holds for other assumptions about the stellar mass-to-light ratio. The distribution of dark matter within our sample galaxies is best described by models with a kpc-sized constant-density core.

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arXiv2007

VCC 2062: an old Tidal Dwarf Galaxy in the Virgo Cluster?

Numerical simulations predict the existence of old Tidal Dwarf Galaxies (TDGs) that would have survived several Gyr after the collision lying at their origin. Such survivors, which would by now have become independent relaxed galaxies, would be ideal laboratories, if nearby enough, to tackle a number of topical issues, including the distribution of Dark Matter in and around galaxies. However finding old dwarf galaxies with a confirmed tidal origin is an observational challenge. A dwarf galaxy in the Virgo Cluster, VCC 2062, exhibits several unusual properties that are typical of a galaxy made out of recycled material. We discuss whether it may indeed be a TDG. We analysed multi-wavelength observations of VCC 2062, including an IRAM CO map, an optical spectrum of its HII regions, GALEX ultraviolet and archival broad-band and narrow-band optical images as well as a VLA HI datacube, originally obtained as part of the VIVA project. VCC 2062 appears to be the optical, low surface brightness counterpart of a kinematically detached, rotating condensation that formed within an HI tail apparently physically linked to the disturbed galaxy NGC 4694. In contrast to its faint optical luminosity, VCC 2062 is characterised by strong CO emission and a high oxygen abundance more typical of spiral disks. Its dynamical mass however, is that of a dwarf galaxy. VCC 2062 was most likely formed within a pre-enriched gaseous structure expelled from a larger galaxy as a result of a tidal interaction. The natural provider for the gaseous tail is NGC 4694 or rather a former companion which subsequently has been accreted by the massive galaxy. According to that scenario, VCC 2062 has been formed by a past tidal encounter. Since its parent galaxies have most probably already totally merged, it qualifies as an old TDG.

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arXiv2007

Free-Floating HI Clouds in the M 81 Group

Recent VLA observations pointed at dwarf spheroidal (dSph) galaxies in the M 81 group reveal a hitherto hidden population of extremely low mass (~1e5 Msol) HI clouds with no obvious optical counterparts. We have searched 10 fields in the M81 group totalling 2.2 square degree, both targeting known dwarf spheroidal galaxies and blank fields around the central triplet. Our observations show that the new population of low-mass HI clouds appears to be confined to a region toward the South-East of the central triplet (at distances of ~100 kpc from M 81). Possible explanations for these free-floating HI clouds are that they are related to the dSphs found to the South-East of M 81, that they belong to the galaxies of the M 81 triplet (equivalent to HVCs), that they are of primordial nature and provide fresh, unenriched material falling into the M 81 group, or that they are tidal debris from the 3-body interaction involving M 81-M 82-NGC 3077. Based on circumstantial evidence, we currently favour the latter explanation.

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