HI Line Research Articles

Studies of Star-forming Complexes in the Galaxies NGC 628, NGC 2976, and NGC 3351

We analyze parameters of the interstellar matter emission in star-forming complexes in the high metallicity galaxies NGC~628, NGC~2976, and NGC~3351, which have different morphological types. The relation between H$\alpha$ emission and emission in CO and HI lines is considered along with the relation between H$\alpha$ emission and dust emission in the infrared range (IR). The fluxes and surface brightnesses in the UV and IR correlate well with H$\alpha$ emission. The HI emission also correlates well with H$\alpha$, while the correlation between the CO and H$\alpha$ emission is much less prominent. The ratio of the fluxes at 8 and 24 $\mu$m decreases with increasing H$\alpha$ flux. This may be due to changes in the properties of the dust ensemble (a decrease in the mass fraction of polycyclic aromatic hydrocarbons) or to changes in excitation conditions. Analysis of the kinematics of the CO lines shows that the CO flux grows with increasing velocity scatter $\Delta V$ when $\Delta V\lesssim70$~km/s. Preliminary evidence for the existence of star-forming complexes with higher values of $\Delta V$ is presented, and the increase in the velocity scatter is accompanied by a decrease in the CO line luminosity of the complex.

On the uncertainties of results derived from H i spectral line stacking experiments

We present the results of a set of mock experiments aimed at quantifying the accuracy of results derived from HI spectral line stacking experiments. We focus on the effects of spatial and spectral aperture sizes and redshift uncertainties on co-added HI spectra, and by implication on the usefulness of results from HI line spectral stacking experiments. Using large spatial apertures to extract constituent galaxy spectra yields co-added spectra with high levels of contamination and with relatively low signal-to-noise ratios. These properties are also affected by the size of the spectral aperture as well as HI redshift uncertainties of galaxies. When redshift uncertainties are high, S/N decreases while the contamination level remains roughly constant. Using small spectral apertures in the presence of large HI redshift uncertainties can yield significant decreases in S/N without the expected decrease in amount of contaminant flux. Our simulations show that a co-added spectrum rarely yields an accurate measure of the total HI mass of a galaxy sample. Total mass is generally over/under-estimated for large/small spatial apertures, regardless of spectral aperture size. Our findings strongly suggest that any co-added HI galaxy spectrum needs to be fully modelled in the ways presented in this paper in order to apply accurate corrections for flux contamination and derive realistic uncertainties in total HI galaxy mass. Failing to do so will result in unreliable inferences of galaxy and cosmological parameters.

Spectrum of the Yellow Symbiotic Star LT Delphini before, during, and after the 2017 Outburst

LT Del is a yellow symbiotic system that consists of a bright K3-type giant and a hot subdwarf with a temperature of about 10$^5$ K. We present the results of our spectroscopic observations of LT Del over the period 2010-2018. In 2017 the star experienced a second low-amplitude ($\Delta V\sim0.^{m}7$) outburst in the history of its studies. The emission spectrum of the star represented in the optical range by hydrogen, neutral and ionized helium lines underwent significant changes in the outburst. The fluxes in the HI and HeI emission lines increased by a factor of 5-6, the HeII $\lambda$4686 line grew by a factor of 10. According to our estimates, in the 2017 outburst, the temperature of the exciting star rose to $T_{\text{hot}}\sim$130~000 K, while during the first 1994 outburst the change in temperature was insignificant. This suggests cool and hot outbursts of LT Del by analogy with similar events of another yellow symbiotic star, AG Dra.

Line Intensity Mapping with [C ii] and CO(1-0) as Probes of Primordial Non-Gaussianity

Constraints on primordial non-Gaussianity (PNG) will shed light on the origin of primordial fluctuations and the physics of the early universe. The intensity mapping technique is a promising probe of structure formation on large scales; at high redshifts, it can provide complementary information to other cosmological probes. We explore the potential of future wide-field [CII] and CO intensity mapping surveys in constraining PNG of the local shape, which induces a distinct, scale-dependent correction to the line bias. We explore the parameter space of CO and [CII] survey designs that can achieve the nominal target of $\sigma(f_{\rm NL}^{\rm loc}) =1$ and further calculate what constraints such surveys can place on PNG of the equilateral, orthogonal, and quasi-single field shapes. We further test the dependence of these constraints on various model assumptions; namely the halo mass function, modeling of line bias, and correlation between mass and [CII]/CO luminosity. We find that the ability of CO and [CII] intensity mapping surveys to constrain $f_{\rm NL}^{\rm loc}$ relies heavily on the spectral (redshift) coverage, requiring at least a coverage of an octave in frequency, to produce significant results, with the optimized surveys generally covering between $z\sim[2-8]$. As this redshift window partially overlaps with 21-cm EoR experiments like the Hydrogen Epoch of Reionization Array (HERA), we additionally explore the prospects for a hypothetical set of surveys to place constraints on $\sigma(f_{\rm NL}^{\rm loc})$ utilizing multi-tracer analysis between the lines of HI, CO, and [CII].

Redshift space distortion of 21 cm line at 1 < $z$ < 5 with cosmological hydrodynamic simulations

We measure the scale dependence and redshift dependence of 21 cm line emitted from the neutral hydrogen gas at redshift 1<z<5 using full cosmological hydrodynamic simulations by taking the ratios between the power spectra of HI-dark matter cross correlation and dark matter auto-correlation. The neutral hydrogen distribution is computed in full cosmological hydrodynamic simulations including star formation and supernova feedback under a uniform ultra-violet background radiation. We find a significant scale dependence of HI bias at z>3 on scales of k>1h/Mpc, but it is roughly constant at lower redshift z<3. The redshift evolution of HI bias is relatively slow compared to that of QSOs at similar redshift range. We also measure a redshift space distortion (RSD) of HI gas to explore the properties of HI clustering. Fitting to a widely applied theoretical prediction, we find that the constant bias is consistent with that measured directly from the real-space power spectra, and the velocity dispersion is marginally consistent with the linear perturbation prediction. Finally we compare the results obtained from our simulation and the Illustris simulation, and conclude that the detailed astrophysical effects do not affect the scale dependence of HI bias very much, which implies that the cosmological analysis using 21 cm line of HI will be robust against the uncertainties arising from small-scale astrophysical processes such as star formation and supernova feedback.

The scale of homogeneity in the local Universe with the ALFALFA catalogue

We use the scaled counts in spherical caps \U0001d4a9(<θ) and the fractal correlation dimension \U0001d49f2(θ) procedures to search for a transition scale to homogeneity in the local universe as given by the ALFALFA catalogue (a sample of extragalactic HI line sources, in the redshift range 0 < z < 0.06). Our analyses, in the 2-dimensional sky projected data, show a transition to homogeneity at θH = 16.49̂ ± 0.29̂, in remarkable accordance with the angular scale expected from simulations, a result that strengthens the validity of the cosmological principle in the local universe. We test the robustness of our results by analysing the data sample using three versions of the \U0001d4a9(<θ) estimator, which show a well agreement between them. These statistical estimators were validated using mock realizations generated assuming a fractal distribution of points, successfully recovering the input information. In addition, we perform further analyses showing that our approach is also able to indicate the presence of under- and over-densities in the data. Finally, we verify the influence of the sample size to the θH estimates by using segment Cox processes of different projected areas, confirming the suitability of the ALFALFA surveyed area for the current analyses.

Low Surface Brightness Galaxy catalogue selected from the α.40-SDSS DR7 Survey and Tully–Fisher relation

We present a catalogue of an HI-selected sample of 1129 low surface brightness galaxies (LSBGs) searched from the alpha.40-SDSS DR7 survey. This sample, consisting of various types of galaxies in terms of luminosity and morphology, has extended the parameter space covered by the existing LSBG samples. Based on a subsample of 173 LSBGs which are selected from our entire LSBG sample to have the 2-horn shapes of the HI line profiles, minor-to-major axial ratios (b/a) less than 0.6 and signal-to-noise ratio (S/N) of HI detection greater than 6.5, we investigated the Tully-Fisher relation (TFr) of LSBGs in the optical B, g and r bands and near-infrared J, H and K bands as well. In optical bands, the LSBG subsample follows the fundamental TFr which was previously defined for normal spiral galaxies. In NIR bands, the TFrs for our LSBG subsample are slightly different from the TFrs for the normal bright galaxies. This might be due to the internal extinction issue. Furthermore, the mass-to-light ratio (M/L),disk scale length (h) and mass surface density (sigma) for our LSBG subsample were deduced from the optical TFr results. Compared with High Surface Brightness Galaxies(HSBGs), our LSBGs have higher M/L, larger h and lower sigma than HSBGs.

Bivariate luminosity-HI mass distribution function of galaxies based on the NIBLES survey

We present a new optical luminosity-HImass bivariate luminosity function (BLF) based on HIline observations from the Nançay Interstellar Baryons Legacy Extragalactic Survey (NIBLES). NIBLES sources lie within the local universe (900 ≤ cz ≤ 12 000 km s−1) and were chosen from SDSS DR5 such that the optical luminosity function was sampled as uniformly as possible. The HImass function (HIMF) derived from our raw-data BLF, which is based on HIdetections only, is consistent with the HIMFs derived from other optically selected surveys in that the low-mass slope is flatter than those derived from blind HIsurveys. However, spanning the entire luminosity range of NIBLES, we identify a highly consistent distribution of the HIgas mass to luminosity ratio (gas-to-light ratio) with a predictable progression in the meanMHI/Lrratio as a function ofLr. This consistency allows us to construct plausible gas-to-light ratio distributions for very low-luminosity bins which lie outside the NIBLES sample. We also identify a ∼10% decrease in detection fraction for galaxies fainter than log(Lr) = 9.25, consistent with the expected decrease due to distance and sensitivity effects. Accounting for these trends, we reconstruct plausible gas-to-light distributions spanning luminosity bins down to log(Lr) = 5.25, thus producing a corrected BLF. This corrected BLF is in good qualitative agreement with optical luminosity-HImass distributions from the ALFALFA survey and is able to accurately reproduce blind survey HIMFs, lending credibility that this two dimensional optical luminosity-HImass distribution is an accurate representation of the volume density distribution of galaxies in the local universe. We also note that our agreement with HIMFs from other surveys is dependent on accounting for all systematic differences such as selection method, Hubble constant and HIflux scale.

Evolution of Intergalactic Gas in the Neighborhood of Dwarf Galaxies and Its Manifestations in the HI 21 cm Line

Low-mass galaxies are known to have played the crucial role in the hydrogen reionization in the Universe. In this paper we investigate the contribution of soft x-ray radiation (E ~ 0.1–1 keV) from dwarf galaxies to hydrogen ionization during the initial reionization stages. The only possible sources of this radiation in the process of star formation in dwarf galaxies during the epochs preceding the hydrogen reionization epoch are hot intermediate-mass stars (M ~ 5–8 M⊙) that entered the asymptotic giant branch (AGB) stage and massive x-ray binaries. We analyze the evolution of the intergalactic gas in the neighborhood of a dwarf galaxy with a total mass of 6 × 108M⊙ formed at the redshift of z ~ 15 and having constant star-formation rate of 0.01–0.1 M⊙ yr−1 over a starburst with a duration of up to 100 Myr. We show that the radiation from AGB stars heats intergalactic gas to above 100 K and ensures its ionization xe ≳ 0.03 within about 4–10 kpc from the galaxy in the case of a star-formation rate of star formation 0.03–0.1 M⊙ yr−1, and that after the end of the starburst this region remains quasi-stationary over the following 200–300 Myr, i.e., until z ~ 7.5. Formation of x-ray binaries form in dwarf galaxies at z ~ 15 results in a 2–3 and 5–6 times greater size of the ionized and heated region compared to the case where ionization is produced by AGB stars exclusively, if computed with the “x-ray luminosity–star-formation rate” dependence (LX ~ fXSFR) factor fX = 0.1 and fX ~ 1, respectively. For fX ≲ 0.03 the effect of x-ray binaries is smaller that that of AGB star population. Lyα emission, heating, and ionization of the intergalactic gas in the neighborhood of dwarf galaxies result in the excitation of the 21 cm HI line. We found that during the period of the starburst end at z ~11.5–12.5 the brightness temperature in the neighborhood of galaxies is 15–25 mK and the region where the brightness temperature remains close to its maximum has a size of about 12–30 kpc. Hence the epoch of the starburst end is most favorable for 21 cm HI line observations of dwarf galaxies, because at that time the size of the region of maximum brightness temperature is the greatest over the entire evolution of the dwarf galaxy. In the case of the sizes corresponding to almost 0.’1 for z ~ 12 regions with maximum emission can be detected with the Square Kilometre Array, which is currently under construction.

Line identification and photometric history of the hot post-AGB star Hen 3−1013 (IRAS 14331−6435)

We present a study of the high-resolution optical spectrum for the hot post-asymptotic giant branch (post-AGB) star, Hen 3-1013, identified as the optical counterpart of the infrared sourceIRAS 14331-6435. For the first time the detailed identifications of the observed absorption and emission features in the wavelength range 3700-9000 \AA\ is carried out. Absorption lines of HI, HeI, CI, NI, OI, NeI, CII, NII, OII, SiII, SII, ArII, FeII, MnII, CrII, TiII, CoII, NiII, SIII, FeIII and SIV were detected. From the absorption lines, we derived heliocentric radial velocities of $V_r=-29.6\pm0.4$ km/s. We have identified emission permitted lines of OI, NI, FeII, MgII, SiII and AlII. The forbidden lines of [NI], [FeII], [CrII] and [NiII] have been identified also. Analysis of [NiII] lines in the gaseous shell gives an estimate for the electron density $N_e\sim10^7$ cm$^{-3}$ and the expansion velocity of the nebula $V_{exp}=12$ km/s. The mean radial velocity as measured from emission features of the envelope is $V_r=-36.0\pm0.4$ km/s. The Balmer lines H$\alpha$, H$\beta$ and H$\gamma$ show P Cyg behaviour which indicate ongoing post-AGB mass-loss. Based on ASAS and ASAS-SN data, we have detected rapid photometric variability in Hen 3-1013 with an amplitude up to 0.2 mag in the V band. The star's low-resolution spectrum underwent no significant changes from 1994 to 2012. Based on archival data, we have traced the photometric history of the star over more than 100 years. No significant changes in the star brightness have been found.

The Nançay H i Zone of Avoidance survey of 2MASS bright galaxies

To complement the 2MASS Redshift Survey (2MRS) and the 2MASS Tully-Fisher survey (2MTF) a search for 21cm HI line emission of 2MASS bright galaxy candidates has been pursued along the dust-obscured plane of the Milky Way with the 100m Nancay Radio Telescope. For our sample selection we adopted an isophotal extinction-corrected K-band magnitude limit of $K_s^o = 11.25$mag, corresponding to the first 2MRS data release and 2MTF, for which the 2MASX completeness level remains fairly constant deep into the Zone of Avoidance (ZoA). About one thousand galaxies without prior redshift measurement accessible from Nancay (Dec > -40\deg) were observed to an rms noise level of ~3 mJy for the velocity range -250 to 10'600 km/s. This resulted in 220 clear and 12 marginal detections of the target sample. Only few detections have redshifts above 8000 km/s due to recurring radio frequency interference (RFI). A further 29 detections and 6 marginals have their origin in non-target galaxies in the telescope beam. The newly detected galaxies are on average considerably more \HI-rich (mostly $10^9 - 10^{10}$M$_\odot$) compared to systematic (blind) HI surveys. The HI detections reveal various new filaments crossing the mostly uncharted northern ZoA (e.g. at $\ell \sim 90\deg, 130\deg, 160\deg$), whilst consolidating galaxy agglomerations in Monoceros and Puppis ($\ell \sim 220\deg, 240\deg$). Considerably new insight has been gained about the extent of the Perseus-Pisces Supercluster through the confirmation of a ridge ($\ell \sim 160\deg$) encompassing the 3C129 cluster that links Perseus-Pisces to Lynx, and the continuation of the second Perseus-Pisces arm ($\ell \sim 90\deg$) across the ZoA.

Dwarf Galaxies in the Local Volume

AbstractThe Updated Nearby Galaxy Catalog (=UNGC) of 1047 Local Volume (=LV) galaxies, situated within a distance of 11 Mpc, contains 870 dwarfs, i.e. 5/6 of the sample. Almost 40% of them have accurate distances measured with Hubble Space Telescope. Most of the LV dwarfs have been observed in HI and Hα emission lines, as well in far-ultraviolet with GALEX. We present basic properties of the LV dwarfs, their HI -mass content and star-formation rate in different local environments. We discuss a baryonic Tully-Fisher relation for the LV dwarfs, and apply it to determine TF -distances for several hundreds local galaxies. The accurate distances and radial velocities of the LV dwarfs are used by us to trace dark matter distribution within 11 Mpc. We discuss also does the Local Group may be treated as a typical or uncommon representative of the LV population.

The Arecibo Legacy Fast ALFA Survey: The ALFALFA Extragalactic H i Source Catalog

Abstract We present the catalog of ∼31,500 extragalactic H i line sources detected by the completed Arecibo Legacy Fast ALFA (ALFALFA) survey out to z &lt; 0.06, including both high signal-to-noise ratio (&gt;6.5) detections and ones of lower quality that coincide in both position and recessional velocity with galaxies of known redshift. We review the observing technique, data reduction pipeline, and catalog construction process, focusing on details of particular relevance to understanding the catalog’s compiled parameters. We further describe and make available the digital H i line spectra associated with the cataloged sources. In addition to the extragalactic H i line detections, we report nine confirmed OH megamasers (OHMs) and 10 OHM candidates at 0.16 &lt; z &lt; 0.22 whose OH line signals are redshifted into the ALFALFA frequency band. Because of complexities in data collection and processing associated with the use of a feed-horn array on a complex single-dish antenna in the terrestrial radio frequency interference environment, we also present a list of suggestions and caveats for consideration by users of the ALFALFA extragalactic catalog for future scientific investigations.

Kinematics of the atomic ISM in M33 on 80 pc scales

We present new L-band ($1\mbox{--}2$ GHz) observations of the nearby spiral galaxy M33 with 80 pc resolution obtained with the Karl G. Jansky Very Large Array. The HI observations, combined with HI measurements from the Green Bank Telescope, improve the spectral resolution and sensitivity ($2.8$ K rms noise in a $0.2$ km s$^{-1}$ channel) compared to previous observations. We find individual profiles are usually non-Gaussian, harbouring line wings, multiple components, and asymmetries. Given this spectral complexity, we quantify the motions in the atomic ISM through moment analysis of the spectra and fits to aligned, stacked profiles. The measured value of the HI line width depends strongly on the method used, with the velocity stacked profiles aligned to the peak velocity giving the minimum value of $\sigma = 7$ km s$^{-1}$ and all other methods giving higher values ($\sigma\sim10$ km s$^{-1}$). All measurements of the line width show a shallow radial trend, with $\sigma$ decreasing by $\sim2$ km s$^{-1}$ from $R_{\rm gal}=0$ to $R_{\rm gal}=8$ kpc. We consider a number of energy sources that might maintain the line width against turbulent dissipation, but no single source is adequate. We find excess emission relative to a Gaussian in the stacked profile line wings, ranging from 9% to 26% depending on how the spectra are aligned. By splitting the line wings into symmetric and asymmetric components, we find that the lagging rotational disk accounts for one-third of the line wing flux. We also find emission far from the rotation axis of the galaxy in multiple discrete HI clouds, including a filament with a projected length of $\sim8$ kpc.

The star formation law at GMC scales in M33, the Triangulum galaxy

We present a high spatial resolution study, on scales of $\sim$100pc, of the relationship between star-formation rate (SFR) and gas content within Local Group galaxy M33. Combining deep SCUBA-2 observations with archival GALEX, SDSS, WISE, Spitzer and submillimetre Herschel data, we are able to model the entire SED from UV to sub-mm wavelengths. We calculate the SFR on a pixel-by-pixel basis using the total infrared luminosity, and find a total SFR of $0.17 \pm 0.06\,\rm{M}_\odot$/yr, somewhat lower than our other two measures of SFR -- combined FUV and 24$\mu$m SFR ($0.25^{+0.10}_{-0.07}\,\rm{M}_\odot$/yr) and SED-fitting tool MAGPHYS ($0.33^{+0.05}_{-0.06}\,\rm{M}_\odot$/yr). We trace the total gas using a combination of the 21cm HI line for atomic hydrogen, and CO($\textit{J}$=2-1) data for molecular hydrogen. We have also traced the total gas using dust masses. We study the star-formation law in terms of molecular gas, total gas, and gas from dust. We perform an analysis of the star-formation law on a variety of pixel scales, from 25$^{\prime\prime}$ to 500$^{\prime\prime}$ (100pc to 2kpc). At kpc scales, we find that a linear Schmidt-type power law index is suitable for molecular gas, but the index appears to be much higher with total gas, and gas from dust. Whilst we find a strong scale dependence on the Schmidt index, the gas depletion timescale is invariant with pixel scale.

COS-Weak: probing the CGM using analogues of weak Mg ii absorbers at z &lt; 0.3

We present a sample of 34 weak metal line absorbers at $z< 0.3$ complied via the simultaneous detections ($3\sigma$) of the SiII$\lambda1260$ and CII$\lambda1334$ absorption lines, with $W_{r}$(SiII)$<0.2$ \AA\ and $W_{r}$(CII)$<0.3$ \AA, in archival HST/COS spectra. Our sample increases the number of known low-$z$ "weak absorbers" by a factor of $>5$. The column densities of HI and low-ionization metal lines obtained from Voigt profile fitting are used to build simple photoionization models using CLOUDY. The inferred densities and total hydrogen column densities are in the ranges of $-3.3 < \log n_{\rm H}/{\rm cm^{-3}} < -2.4$ and $16.0 < \log N_{\rm H}/{\rm cm^{-2}}<20.3$, respectively. The line of sight thicknesses of the absorbers have a wide range of $\sim$1 pc$-$50 kpc with a median value of $\sim$500 pc. The high-ionization OVI absorption, detected in 12/18 cases, always stems from a different gas-phase. Most importantly, 85% (50%) of these absorbers show a metallicity of $\rm [Si/H] > -1.0$ (0.0). The fraction of systems showing high metallicity (i.e., $\rm [Si/H]>-1.0$) in our sample is significantly higher than the HI-selected sample (Wotta et al. 2016) and the galaxy-selected sample (Prochaska et al. 2017) of absorbers probing the circum-galactic medium (CGM) at similar redshift. A search for galaxies has revealed a significant galaxy-overdensity around these weak absorbers compared to random places with a median impact parameter of 166 kpc to the nearest galaxy. Moreover, we find the presence of multiple galaxies in $\sim80$% of the cases, suggesting group environments. The observed $d\mathcal{N}/dz$ of $0.8\pm0.2$ indicates that such metal-enriched, compact, dense structures are ubiquitous in the halos of low-$z$ galaxies that are in groups. We suggest that these are transient structures that are related to outflows and/or stripping of metal-rich gas from galaxies.

H α and H β Raman scattering line profiles of the symbiotic star AG Pegasi

The H$\alpha$ and H$\beta$ line profiles of the symbiotic star AG Peg observed on 1998 September (phase $\phi$ = 10.24), display the top narrow double Gaussian and bottom broad (FWHM of 200 -- 400 kms$^{-1}$) components. The photo-ionization model indicated that the ionized zone, responsible for the hydrogen Balmer and Lyman lines, is radiation-bounded, with a hydrogen gas number density of $n_{\rm H} \sim 10^{9.85}$ cm$^{-3}$ and a gas temperature of T$_e$ = 12\,000 -- 15\,000~K. We carried out Monte Carlo simulations to fit the Raman scattering broad wings, assuming that the hydrogen Ly$\beta$ and Ly$\gamma$ lines emitted within the radiation-bounded HII zone around a white dwarf have the same double Gaussian line profile shape as the hydrogen Balmer lines. The simulation shows that the scattering HI zones are attached to (or located just outside of) the inner HII shells. The best fit to the observed broad HI line profiles indicates that the column density of the scattering neutral zone is $N_{\rm H} \simeq $ 3 -- 5 $\times$ 10$^{19}$ cm$^{-2}$. We examined whether the geometrical structure responsible for the observed H$\alpha$ and H$\beta$ line profiles is a bipolar conical shell structure, consisting of the radiation-bounded ionized zone and the outer material bounded neutral zone. The expanding bipolar structure might be two opposite regions of the common envelope or the outer shell of the Roche lobe around the hot white dwarf, formed through the mass inflows from the giant star and pushed out by the fast winds from the hot white dwarf.

On the Evolutionary State of High-Latitude Variable V534 Lyr

Based on the high resolution spectral monitoring conducted at the 6-m BTA telescope, we study the optical spectrum of the high-latitude variable V534 Lyr. Heliocentric radial velocities Vr corresponding to the positions of all metal absorption components, as well as the NaI D and H$\alpha$ lines were measured during all the observational dates. The analysis of the velocity field examining the lines of various nature revealed a low-amplitude variability of Vr based on the lines with a high excitation potential, which are formed in deep layers of the stellar atmosphere, and allowed to estimate the systemic velocity of Vsys$\approx-125$ km/s (V(lsr)$\approx-105$ km/s). The distance estimate of d$\approx$6 kpc for the star leads us to its absolute magnitude of Mv$\approx -5.3^{m}$, what corresponds to the MK spectral classification. The previously undetected for this star spectral phenomenon was revealed: at certain times a splitting of the profiles of low-excited absorptions is observed, reaching $\Delta$Vr=20$\div$50 km/s. A combination of the parameters: reduced metallicity [Met/H]$_{\odot}=-0.28$, increased nitrogen abundance [N/Fe]=+1.10, large spatial velocity, high luminosity, a strong variability of the emission-absorption profiles of HI lines, splitting of metal absorptions at different observation moments and the variability of the velocity field in the atmosphere allow us to consider V534 Lyr as a pulsating star in the instability band near the HB and belonging to the thick disk of our Galaxy.

A multi-wavelength analysis of the diffuse H ii region G25.8700+0.1350

We present a multiwavelength investigation of the HII region G25.8700+0.1350, located in the inner part of the Galaxy. In radio continuum emission, the region is seen as a bright arc-shaped structure. An analysis of the HI line suggests that G25.8700+0.1350 lies at a distance of 6.5 kpc. The ionized gas is bordered by a photodissociation region which is encircled by a molecular structure where four molecular clumps are detected. At infrared wavelengths, the region is also very conspicuous. Given the high level of visual absorption in the region, the exciting stars should be searched for in the infrared band. In this context, we found in the literature one Wolf-Rayet and one red supergiant which, together with 37 2MASS sources candidates to be O-type stars, could be related to the origin of G25.8700+0.1350. Finally, as expanding HII regions are hypothesized to trigger star formation, we used different infrared point source catalogues to search for young stellar object candidates (cYSOs). A total of 45 cYSOs were identified projected onto the molecular clouds.

HI content in galactic disks: The role of gravitational instability

We examine the dependence between hydrogen total mass $M_{HI}$ and rotation speed $V_{rot}$, optical size $D_{25}$ or disc radial scale $R_0$ for two samples of late-type galaxies: a) isolated galaxy (AMIGA sample), and b) the edge-on galaxies (flat galaxies of Karachentsev et al. 1999). Estimates of $M_{HI}$, given in the HYPERLEDA database for flat galaxies appear to be on average higher at $\sim $0.2 dex, than for isolated galaxies with similar $V_{rot}$ or $D_{25}$ values, most probably, due to the overvaluation of self-absorption in the HI line. We confirm that the hydrogen mass for both samples closely correlates with galactic disc integral specific angular momentum $J$, which is proportional to $V_{rot}D_{25}$ or $V_{rot}R_0$, with low surface brightness galaxies lie along a common $V_{rot}R_0$ sequence. This relationship can be explained, assuming that gas mass in the disc is regulated by marginal gravitational stability condition of gas layer. A comparison of the observed and theoretically expected dependences leads to a conclusion that either gravitational stability corresponds to higher values of Toomre parameter than is usually assumed, or the threshold stability condition for most galaxies took place only in the past, when gas mass in discs was 2-4 times higher than at present (with the exception of galaxies with abnormally high $HI$ content). The last condition requires that the gas accretion was not compensated by gas consumption during the evolution of most of galaxies.