The Local Group L-band Survey: Probing Cold Atomic Gas in IC 10 with Neutral Hydrogen Absorption

We have studied the small-scale distribution of atomic hydrogen (H i) using 21 cm absorption spectra against multiple-component background radio continuum sources from the 21-SPONGE survey and the Millennium Arecibo Absorption-Line Survey. We have found >5σ optical depth variations at a level of ∼0.03–0.5 between 13 out of 14 adjacent sightlines separated by a few arcseconds to a few arcminutes, suggesting the presence of neutral structures on spatial scales from a few to thousands of au (which we refer to as tiny-scale atomic structure, TSAS). The optical depth variations are strongest in directions where the H i column density and the fraction of H i in the cold neutral medium (CNM) are highest, which tend to be at low Galactic latitudes. By measuring changes in the properties of Gaussian components fitted to the absorption spectra, we find that changes in both the peak optical depth and the linewidth of TSAS absorption features contribute to the observed optical depth variations, while changes in the central velocity do not appear to strongly impact the observed variations. Both thermal and turbulent motions contribute appreciably to the linewidths, but the turbulence does not appear strong enough to confine overpressured TSAS. In a majority of cases, the TSAS column densities are sufficiently high that these structures can radiatively cool fast enough to maintain thermal equilibrium with their surroundings, even if they are overpressured. We also find that a majority of TSAS is associated with the CNM. For TSAS in the direction of the Taurus molecular cloud and the local Leo cold cloud, we estimate densities over an order of magnitude higher than typical CNM densities.

Cold neutral hydrogen (H i) is a crucial precursor for molecular gas formation and can be studied via H i absorption. This study investigates H i absorption in low column density regions of the Small and Large Magellanic Clouds (SMC and LMC) using the Galactic-ASKAP H i (GASKAP-H i) survey, conducted by the Australian Square Kilometer Array Pathfinder (ASKAP). We select 10 SMC directions in the outer regions and 18 LMC directions, with four in the outskirts and 14 within the main disk. Using the radiative transfer method, we decompose the emission and absorption spectra into individual cold neutral medium (CNM) and warm neutral medium (WNM) components. In the SMC, we find H i peak optical depths of 0.09–1.16, spin temperatures of ∼20–50 K, and CNM fractions of 1%–11%. In the LMC, optical depths range from 0.03–3.55, spin temperatures from ∼10–100 K, and CNM fractions from 1%–100%. The SMC’s low CNM fractions likely result from its low metallicity and large LOS depth. Additionally, the SMC’s outskirts show lower CNM fractions than the main body, potentially due to increased CNM evaporation influenced by the hot Magellanic Corona. Shell motions dominate the kinematics of the majority of CNM clouds in this study and likely supply cold H i to the Magellanic Stream. In the LMC, high CNM fraction clouds are found near supergiant shells, where thermal instability induced by stellar feedback promotes WNM-to-CNM transition. Although no carbon monoxide has been detected, enhanced dust shielding in these areas helps maintain the cold H i.

We present new observations of [C ii] 2 P 3/2 → 2 P 1/2 fine structure line emission from an isolated molecular cloud using the upGREAT instrument on board SOFIA. These data are analyzed together with archival CO J=1–0 and H i 21 cm emission spectra to investigate the role of converging atomic gas flows in the formation of molecular clouds. Bright [C ii] emission is detected throughout the mapped area that likely originates from photodissociation regions excited by UV radiation fields produced by newborn stars within the cloud. Upon spatial averaging of the [C ii] spectra, we identify weak [C ii] emission within velocity intervals where the H i 21 cm line is brightest; these are blueshifted relative to velocities of the CO and bright [C ii] emission by 4 km s−1. The brightness temperatures, velocity dispersions, and alignment with H i 21 cm velocities connect this [C ii] emission component to the cold, neutral atomic gas of the interstellar medium, known as the cold, neutral medium (CNM). We propose that this CNM feature is an accretion flow onto the farside of the existing molecular cloud. The mass infall rate is 3.2 × 10−4 M ⊙ yr−1. There is no direct evidence of a comparable redshifted component in the [C ii] or H i 21 cm spectral lines that would indicate the presence of a converging flow.

We present 21-cm absorption measurements towards 12 radio continuum sources with previously identified thermally-unstable warm neutral medium (WNM). These observations were obtained with the Expanded Very Large Array (EVLA) and were complemented with the HI emission spectra obtained with the Arecibo Observatory. Out of 12 sources, HI absorption was detected along 5 lines of sight (seven new absorption features in total), resulting in a detection rate of ~42%. While our observations are sensitive to the WNM with a spin temperature T_s<3000 K, we detected only two wide absorption lines with T_s=400-900 K. These temperatures lie above the range allowed for the cold neutral medium (CNM) by the thermal equilbrium models and signify the thermally unstable WNM. Several absorption features have an optical depth of only a few x10^{-3}. While this is close or lower than what is theoretically expected for the CNM, we show that these weak lines are important for constraining the fraction of the thermally unstable WNM. Our observations demonstrate that, for the first time, high bandpass stability can be achieved with the VLA, allowing detection of absorption lines with a peak optical depth of ~10^{-3}.

Intermediate-mass black holes (IMBHs, $10^{3\!-\!6} \, {\rm M_\odot }$), are typically found at the centre of dwarf galaxies and might be wandering, thus far undetected, in the Milky Way (MW). We use model spectra for advection-dominated accretion flows to compute the typical fluxes, in a range of frequencies spanning from radio to X-rays, emitted by a putative population of $10^5 \, {\rm M_\odot }$ IMBHs wandering in five realistic volume-weighted MW environments. We predict that $\sim 27{{\ \rm per\ cent}}$ of the wandering IMBHs can be detected in the X-ray with Chandra, $\sim 37{{\ \rm per\ cent}}$ in the near-infrared with the Roman Space Telescope, $\sim 49{{\ \rm per\ cent}}$ in the sub-mm with CMB-S4, and $\sim 57{{\ \rm per\ cent}}$ in the radio with ngVLA. We find that the brightest fluxes are emitted by IMBHs passing through molecular clouds or cold neutral medium, where they are always detectable. We propose criteria to facilitate the selection of candidates in multiwavelength surveys. Specifically, we compute the X-ray to optical ratio (αox) and the optical to sub-mm ratio, as a function of the accretion rate of the IMBH. We show that at low rates the sub-mm emission of IMBHs is significantly higher than the optical, UV, and X-ray emission. Finally, we place upper limits on the number N• of these objects in the MW: N• &amp;lt; 2000 and N• &amp;lt; 100, based on our detectability expectations and current lack of detections in molecular clouds and cold neutral medium, respectively. These predictions will guide future searches of IMBHs in the MW, which will be instrumental to understanding their demographics and evolution.

Optical depth variations in the Galactic neutral interstellar medium (ISM) with spatial scales from hundreds to thousands of astronomical units have been observed through H i absorption against pulsars and continuum sources, while extremely small structures with spatial scales of tens of astronomical units remain largely unexplored. The nature and formation of such tiny-scale atomic structures (TSAS) need to be better understood. We report a tentative detection of TSAS with a signal-to-noise ratio of 3.2 toward PSR B1557−50 in the second epoch of two Parkes sessions just 0.36 yr apart, which are the closest-spaced spectral observations toward this pulsar. One absorption component showing marginal variations has been identified. Based on the pulsar’s proper motion of 14 mas yr−1 and the component’s kinematic distance of 3.3 kpc, the corresponding characteristic spatial scale is 17 au, which is among the smallest sizes of known TSAS. Assuming a similar line-of-sight (LOS) depth, the tentative TSAS cloud detected here is overdense and overpressured relative to the cold neutral medium (CNM), and can radiatively cool fast enough to be in thermal equilibrium with the ambient environment. We find that turbulence is not sufficient to confine the overpressured TSAS. We explore the LOS elongation that would be required for the tentative TSAS to be at the canonical CNM pressure, and find that it is ∼5000—much larger than filaments observed in the ISM. We see some evidence of line width and temperature variations in the CNM components observed at the two epochs, as predicted by models of TSAS-like cloud formation colliding warm neutral medium flows.

We present integral field spectroscopy of the interacting galaxy system NGC 2207/IC 2163 obtained with the imaging Fourier Transform Spectrometer SITELLE. Approximately, 1000 H ii region complexes are detected in both galaxies and analysed using their strong optical emission lines. Their properties were studied via BPT (Baldwin–Phillips–Terlevich) diagrams and their luminosity function. We conducted a detailed study of the distribution of oxygen abundance across the system using a series of strong-line O/H indicators and calibrations. Both galaxies exhibit negative galactocentric abundance gradients with a slope $\sim$–0.015 dex kpc$^{-1}$. There are marginal signs of discontinuities in the O/H gradients with some indicators while no significant azimuthal variations are seen. A shallower slope in the H ii region luminosity function between the arm and inter-arm regions in IC 2163 is observed, supporting previous conclusion that the star formation process in this galaxy eyelids has been altered during the interaction. The kinematics of the ionised gas reveal disturbed velocity fields, active galactic nucleus (AGN)-like features in the nucleus of NGC 2207, and elevated velocity dispersion in turbulent or feedback-driven regions. To interpret these findings, we modelled the collision using the numerical algorithm GCD+. The simulation reproduces key features of the system and demonstrates how close passages drive enhanced star formation and localized chemical enrichment. Finally, two dwarf galaxies in the field are found to have very similar systemic velocities as their larger counterparts, and could well play a minor role in the global interaction based on their morphology and position.

We investigate the nature of molecular gas emission from z ∼ 6 quasars via the commonly observed tracer of H2, carbon monoxide (CO). We achieve this by combining non-LTE radiative transfer calculations with merger-driven models of z ∼ 6 quasar formation that arise naturally in Λ cold dark matter structure formation simulations. Motivated by observational constraints, we consider four representative z ∼ 6 quasars formed in the halo mass range ~1012-1013 M☉ from different merging histories. Our main results are as follows. We find that, owing to massive starbursts and funneling of dense gas into the nuclear regions of merging galaxies, the CO is highly excited during both the hierarchical buildup of the host galaxy and the quasar phase, and the CO flux density peaks between J = 5 and 8. The CO morphology of z ∼ 6 quasars often exhibits multiple CO emission peaks which arise from molecular gas concentrations which have not yet fully coalesced. Both of these results are found to be consistent with the sole CO detection at z ∼ 6, in quasar J1148+5251. Quasars which form at z ∼ 6 display a large range of sight line-dependent line widths. The sight line dependencies are such that the narrowest line widths are when the rotating molecular gas associated with the quasar is viewed face-on (when the LB is largest) and broadest when the quasar is seen edge-on (and the LB is lowest). Thus, we find that for all models selection effects exist such that quasars selected for optical luminosity are preferentially seen to be face-on which may result in CO detections of optically luminous quasars at z ∼ 6 having line widths narrower than the median. The mean sight line-averaged line width is found to be reflective of the circular velocity of the host halo and thus scales with halo mass. For example, the mean line width for the ~1012 M☉ halo is σ ∼ 300 km s−1, while the median for the ~1013 M☉ quasar host is σ ∼ 650 km s−1. Depending on the host halo mass, approximately 2%-10% of sight lines in our modeled quasars are found to have narrow line widths compatible with observations of J1148+5251. When considering the aforementioned selection effects, these percentages increase to 10%-25% for quasars selected for optical luminosity. When accounting for both temporal evolution of CO line widths in galaxies, as well as the redshift evolution of halo circular velocities, these models can self-consistently account for the observed line widths of both submillimeter galaxies and quasars at z ∼ 2. Finally, we find that the dynamical mass derived from the mean sight line-averaged line widths provide a good estimate of the total mass and allow for a massive molecular reservoir, supermassive black hole, and stellar bulge, consistent with the local MBH-Mbul relation.

We present XMM-Newton and Chandra observations of two low-metallicity cometary blue compact dwarf (BCD) galaxies, Mrk 59 and Mrk 71. The first BCD, Mrk 59, contains two ultraluminous X-ray (ULX) sources, IXO 72 and IXO 73, both associated with bright massive stars and H II complexes, as well as one fainter extended source associated with a massive H II complex at the head of the cometary structure. The low-metallicity of Mrk 59 appears to be responsible for the presence of the two ULXs. IXO 72 has varied little over the last 10 yr, while IXO 73 has demonstrated a variability factor of ~4 over the same period. The second BCD, Mrk 71, contains two faint X-ray point sources and two faint extended sources. One point source is likely a background AGN, while the other appears to be coincident with a very luminous star and a compact H II region at the "head" of the cometary structure. The two faint extended sources are also associated with massive H II complexes. Although both BCDs have the same metallicity, the three sources in Mrk 71 have X-ray luminosities ~1-2 orders of magnitude fainter than those in Mrk 59. The age of the starburst may play a role.

The spectral energy distributions (SEDs) of low-mass low-metallicity (dwarf) galaxies are a challenging piece of the puzzle of galaxy formation in the near Universe. These SEDs show some particular features in the submillimeter to far-infrared wavelength range compared to normal, larger, and metal-richer galaxies that cannot be explained by the current models. These can be summarized as: a broadening of the IR peak, which implies a warmer dust component; an excess of emission in the submm ($\sim$500 $\mu$m), that causes a flattening of the submm/FIR slope; and a very low intensity of PAH emission features. With the aim of explaining these emission patterns, the SEDs of a sample of 27 simulated dwarf galaxies were calculated using the GRASIL-3D radiation transfer code. This code has the particularity that it separately treats the radiative transfer through dust grains within molecular clouds and within the cirrus, the dense and diffuse components of the gas phase, respectively. The simulated galaxies have stellar masses ranging from 10$^6$-10$^9$ M$_\odot$, and were obtained from a single simulation run within a Local Group environment with initial conditions from the CLUES project. We report a study of the IRAS, Spitzer and Herschel bands luminosities, and of the SFRs, dust, and gas (HI and H$_2$) mass contents. We find a satisfactory agreement with observational data, with GRASIL-3D naturally reproducing the specific spectral features mentioned above. We conclude that the GRASIL-3D two-component dust model gives a physical interpretation of the emission of dwarf galaxies: molecular clouds and cirrus represent the warm and cold dust components, respectively, needed to reproduce observational data.

Molecular line emissions are commonly used to trace the distribution and properties of molecular Interstellar Medium. However, the emissions are heavily blended on the Galactic disk toward the inner Galaxy because of the relatively large line widths and the velocity overlaps of spiral arms. Structure identification methods based on voxel connectivity in Position-Position-Velocity (PPV) data cubes often produce unrealistically large structures, which is the “over-linking” problem. Therefore, identifying molecular cloud structures in these directions is not trivial. We propose a new method based on Gaussian decomposition and graph theory to solve the over-linking problem, named InterStellar Medium Gaussian Component Clustering (ISMGCC). Using the Milky Way Imaging Scroll Painting (MWISP) 13CO(1–0) data in the range of 13.°5 ≤ l ≤ 14.°5, ∣b∣ ≤ 0.°5, and −100 ≤ V lsr ≤ +200 km s−1, our method identified three hundred molecular gas structures with at least 16 pixels. These structures contain 92% of the total flux in the raw data cube and show single-peaked line profiles on more than 93% of their pixels. The ISMGCC method could distinguish gas structures in crowded regions and retain most of the flux without global data clipping or assumptions on the structure geometry, meanwhile, allowing multiple Gaussian components for complicated line profiles.

We present the results of a WIYN integral field unit study of the kinematics of the ionized gas in IC 10, a dwarf irregular starburst galaxy in the Local Group. Though the velocity field of the ionized gas closely matches that of the H I, there are several kinematically interesting features in the galaxy. The diffuse ionized gas in the galaxy exhibits larger Hα line widths than the bright complexes. In one case this is due to an infusion of energy into the gas associated with the radio superbubble discovered by Yang & Skillman. We find that the amount of energy in this region is consistent with their hypothesis that the region contains 10 supernova remnants. We also detect a high-velocity (70 km s-1) expanding shell in the ionized gas, which is likely driven by three confirmed Wolf-Rayet stars that are located within the shell. Extrapolating from Hunter's initial mass function, we find that the central starburst region contains approximately equal energy contributions from stellar winds and supernovae (SNe), suggesting that SNe are just beginning to play a significant role in shaping the kinematics of the ionized gas. However, all of this energy cannot be easily accounted for in the kinematics of the gas. We detect an energetic flow of gas (3 × 1052 ergs), which we believe originates from the starburst region. We also detect a high-velocity (70 km s-1) feature not coincident with any structure in our Hα image. This feature, along with the flow and shell, can account for the energy produced by stellar wind and SNe. The flow resembles one discovered by Wilcots & Thurow in NGC 4214; together they suggest that the porosity of the interstellar medium contributes significantly to the high velocity of some portion of the ionized gas in irregular galaxies.

Studying the galaxies responsible for reionization is often conducted through local reionization-era analogs; however, many of these local analogs are too massive to be representative of the low-mass star-forming galaxies that are thought to play a dominant role in reionization. The local, low-mass dwarf starburst galaxy Pox 186 is one such system with physical conditions representative of a reionization-era starburst galaxy. We present deep ultraviolet (UV) spectroscopy of Pox 186 to study its stellar population and ionization conditions and to compare these conditions to other local starburst galaxies. The new Cosmic Origins Spectrograph data are combined with archival observations to cover ∼1150–2000 Å and allow for an assessment of Pox 186's stellar population, the relative enrichment of C and O, and the escape of ionizing photons. We detect significant Lyα and low-ionization state absorption features, indicative of previously undetected neutral gas in Pox 186. The C/O relative abundance, log(C/O) = −0.62 ± 0.02, is consistent with other low-metallicity dwarf galaxies and suggests a comparable star formation history in these systems. We compare UV line ratios in Pox 186 to those of dwarf galaxies and photoionization models, and we find excellent agreement for the ratios utilizing the intense C iii], O iii], and double-peaked C iv lines. However, the UV and optical He ii emission is faint and distinguishes Pox 186 from other local starburst dwarf galaxies. We explore mechanisms that could produce faint He ii, which have implications for the low-mass reionization-era galaxies that may have similar ionization conditions.

This paper discusses results of multi-wavelength studies of low-mass galaxies with outflows. As a case study the extreme low-metallicity dwarf galaxy I Zw 18 is presented. The results from HST imaging, long-slit echelle spectroscopy and X-ray imaging are discussed in the context of chemical enrichment of dwarf galaxies. If outflows work primarily as a temporary repository of metals and not as a loss of metals into the intergalactic medium, we show that galactic winds can explain the relatively low N/O scatter seen in nearby low-metallicity dwarf galaxies as well as the apparent increase of N/O scatter observed in damped Lyα systems with metallicities near that of I Zw 18and below.

view Abstract Citations (46) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Integrated magnItudes and mean colors of DDO dwarf galaxies in the UBV system. II. Distances, luminosities, and HI properties. de Vaucouleurs, G. ; de Vaucouleurs, A. ; Buta, R. Abstract An analysis of the properties of the DDO magellanic irregular "dwarf" galaxies for which U, B, V photometry was reported in Paper I leads to the following conclusions: (1) The mean effective surface brightness (m'e)0 varies with zenith distance as expected from the dependence on sec z of atmospheric extinction and sky brightness. (2) The mean surface brightness is independent of distance modulus (25 < μ0 < 33). (3) The mean absolute magnitude M0T is brighter than average (and the mean effective linear diameter larger) south of +10° declination (dwarfs fainter than -15 are missing) and fainter near the zenith at Palomar. There is a curious near absence of DDO objects in the zone +20° < δ < + 30°. The range of absolute magnitudes (-19 < M0T < -12) is largest north of +60° declination. (4) The mean corrected effective color index (U - V )0e is independent of absolute magnitude at M 0T < -15, but fainter systems tend to be bluer. (5) The mean absolute magnitude M0T is correlated with distance modulus at μ0 > 28; we confirm that the DDO sample is not restricted to dwarf systems and includes galaxies as bright as M0T <∼ -18 at μ0 > 30.(6) We confirm also that the luminosity class L and luminosity index Λ are not valid indicators of absolute magnitude for late-type systems beyond μ0 ≃ (Δ = 4 Mpc). (7) Effective linear diameter and absolute magnitude are highly correlated (i.e., mean surface brightness is not a distance indicator). (8) Apparent magnitude and 21-cm line flux are loosely correlated, but there is a large range of HI index (hydrogen/luminosity ratio) among the DDO objects at a given Hubble type (Sd to Im). (9) The relative excess or deficiency in hydrogen/luminosity ratio is uncorrelated with mean effective surface brightness (except for selection effects). (10) The hydrogen/luminosity ratio residuals (at a given type) are loosely correlated with color residuals: DDO objects richer than average in neutral hydrogen tend to be bluer in (U - V )0e (11) The necessity of correcting H I linewidths for velocity dispersion is once again demonstrated by the conspicuous departure from linearity of the Tully-Fisher relation based on raw linewidths. Correction for turbulent motions restores the linearity of the relation. For want of reliable inclination estimates for individual objects a statistical correction depending on the log axis ratio log R is applied. (12) The derived T - F relation for the DDO sample of late-type systems is in near perfect agreement in slope and zero point with the relation adopted by Bottinelli et al. for normal spiral galaxies of earlier types. The distance moduli derived from the 21-cm linewidths are in good statistical agreement with moduli derived from group membership and redshift but with a larger scatter. (13) The supergalactic anisotropy of the velocity-distance relation is in evidence in the DDO sample, confirming the departures from the ideal Hubble law indicated by earlier studies of brighter spiral galaxies. Publication: The Astronomical Journal Pub Date: June 1983 DOI: 10.1086/113365 Bibcode: 1983AJ.....88..764D Keywords: Dwarf Galaxies; H Lines; Magellanic Clouds; Ubv Spectra; Brightness Distribution; Centimeter Waves; Color; Distance; Hubble Diagram; Luminosity; Magnitude; Radial Velocity; Spectral Line Width; Spiral Galaxies; Astrophysics full text sources ADS | data products SIMBAD (243) NED (236)