Extragalactic HII Regions Research Articles

The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale

(Abridged) We use high S/N spectra of 20 HII regions in the giant spiral galaxy M101 to derive electron temperatures for the HII regions and robust metal abundances over radii R = 0.19-1.25 Ro (6-41 kpc). We compare the consistency of electron temperatures measured from the [O III]4363, [N II]5755, [S III]6312, and [O II]7325 auroral lines. Temperatures from [O III], [S III], and [N II] are correlated with relative offsets that are consistent with expectations from nebular photoionization models. However, the temperatures derived from the [O II]7325 line show a large scatter and are nearly uncorrelated with temperatures derived from other ions. Our derived oxygen abundances O/H are well fitted by an exponential distribution over six disk scale lengths, from approximately 1.3 solar in the center to 1/15 solar in the outermost region studied (for solar 12 + log (O/H)=8.7). We measure significant radial gradients in N/O and He/H abundance ratios, but relatively constant S/O and Ar/O. Our abundances are systematically lower by 0.2-0.5 dex than those derived from the most widely used strong-line empirical abundance indicators. We suspect that most of the disagreement with the strong-line abundances arises from uncertainties in the nebular models that are used to calibrate the empirical scale, and that strong-line abundances derived for HII regions and emission-line galaxies are as much as a factor of two higher than the actual oxygen abundances. However other explanations, such as the effects of temperature fluctuations on the auroral line based abundances cannot be completely ruled out. These results point to the need for direct abundance determinations of a larger sample of extragalactic HII regions, especially for objects more metal-rich than solar.

Optical spectroscopy and the UV luminosity function of galaxies in the Abell 1367, Coma and Virgo clusters

Optical spectroscopy of 93 galaxies, 60 projected in the direction of Abell 1367, 21 onto the Coma cluster and 12 on Virgo, is reported. The targets were selected either because they were detected in previous H\alpha, UV or r' surveys. The present observations bring to 100% the redshift completeness of H\alpha selected galaxies in the Coma region and to 75% in Abell 1367. All observed galaxies except one show H\alpha emission and belong to the clusters. This confirms previous determinations of the H\alpha luminosity function of the two clusters that were based on the assumption that all H\alpha detected galaxies were cluster members. Using the newly obtained data we re-determine the UV luminosity function of Coma and we compute for the first time the UV luminosity function of A1367. Their faint end slopes remain uncertain (-2.00 < \alpha < -1.35) due to insufficient knowledge of the background counts. If 90% of the UV selected galaxies without redshift will be found in the background (as our survey indicates), the slope of UV luminosity function will be \alpha ~ -1.35, in agreement with the UV luminosity function of the field (Sullivan et al. 2000) and with the H$\alpha$ luminosity functions of the two clusters (Iglesias-Paramo et al. 2002). We discover a point like H\alpha source in the Virgo cluster, associated with the giant galaxy VCC873, possibly an extragalactic HII region similar to the one recently observed in Virgo by Gerhard et al. (2002).

On biases in the predictions of stellar population synthesis models

Sampling fluctuations in stellar populations give rise to dispersions in observables when a small number of sources contribute effectively to the observables. This is the case for a variety of linear functions of the spectral energy distribution (SED) in small stellar systems, such as galactic and extragalactic HII regions, dwarf galaxies or stellar clusters. In this paper we show that sampling fluctuations also introduce systematic biases and multi-modality in non-linear functions of the SED, such as luminosity ratios, magnitudes and colours. In some cases, the distribution functions of rational and logarithmic quantities are bimodal, hence complicating considerably the interpretation of these quantities in terms of age or evolutionary stages. These biases can be only assessed by Monte Carlo simulations. We find that biases are usually negligible when the effective number of stars, Neff, which contribute to a given observable is larger than 10. Bimodal distributions may appear when Neff is between 10 and 0.1. Predictions from any model of stellar population synthesis become extremely unreliable for small Neff values, providing an operational limit to the applicability of such models for the interpretation of integrated properties of stellar systems. In terms of stellar masses, assuming a Salpeter Initial Mass Function in the range 0.08 -- 120 Mo, Neff=10 corresponds to about 10**5 Mo (although the exact value depends on the age and the observable). This bias may account, at least in part, for claimed variations in the properties of the stellar initial mass function in small systems, and arises from the discrete nature of small stellar populations.

Optical Spectroscopy of Metal‐rich HiiRegions and Circumnuclear Hot Spots in M83 and NGC 3351

We present optical spectroscopy of a sample of metal-rich extragalactic HII regions in the spiral galaxies M83, NGC 3351 and NGC 6384, including a number of circumnuclear HII regions (hot spots). Different age estimators (equivalent width of the Hbeta emission line W(Hb), Balmer line absorption profiles, UV spectra) provide consistently young ages (4-6 Myr) for the hot spots. We have detected the W-R bump feature at 4650 A in five of the objects, and in fewer cases possibly the WC features at 5696 A and 5808 A. Six additional objects showing W-R features are drawn from our previous work on extragalactic HII regions. From the measured luminosity of the W-R blue bump we estimate a small number of WN stars (from 1-2 to about 30). By assuming instantaneous bursts of star formation the ages derived from W(Hb) (3-6 Myr) and the strength of the W-R bump are in agreement with the predictions of evolutionary stellar population models, provided we account for stochastic effects on the IMF. From a comparison with published photoionization models based on synthetic cluster spectral energy distributions we find some evidence for an overestimation of the number of He ionizing photons in the model fluxes. In general, however, the massive star diagnostics are in agreement with the predictions of recent evolutionary models calculated with a Salpeter IMF and a high mass limit. We find no compelling evidence for a depletion of massive stars (M > 40-50 solar masses) in the IMF of metal-rich clusters, contrary to our previous conclusions based on older evolutionary models.

Clusters in Extragalactic HII Regions and their Modelling

This review deals with properties of very young (≤ 10–20 Myr) stellar clusters which are in the nebular phase and are embedded in photoionized regions (classical extragalactic HII regions (RHIIs) or starburst galaxies). Based on the analysis of the integrated light of these clusters at the UV and optical wavelengths, different techniques are discussed that allow to estimate in consistent the stellar content and the evolutionary state of the ionizing clusters. Figures illustrating this contribution can be found in: http://www.iaa.csic.es/∼rosa/

Star Clusters in Giant Extragalactic HII Regions

In this paper we study the relationship between the star formation efficiency and luminosity of Hα emission, Lyman continuum radiation, and Hβ emission on 35 giant extragalactic HII regions in seven galaxies. Using the observational results we obtain the relationship, and find that the star formation efficiency is correlation with Halpha luminocity, and Lyman continuum luminosity, and Hβ lumonosity, respectively. Key words: external galaxy—giant HII region—star cluster

Systematic uncertainties in the determination of the primordial $^\mathsf{4}$He abundance

The primordial helium abundance Y_p is commonly inferred from abundance determinations in low-metallicity extragalactic HII-regions. Such determinations may be subject to systematic uncertainties that are investigated here. Particular attention is paid to two effects: icf-corrections for ``imperfect'' ionization structure leading to significant amounts of (unobservable) neutral helium or hydrogen and ``tcf''-corrections due to non-uniform temperature. Model HII-regions with a large number of parameters are constructed and it is shown that required corrections are almost exclusively functions of two physical parameters: the number of helium- to hydrogen- ionizing photons in the illuminating continuum Q(He0)/Q(H0), and the ratio of width to radius delta r_S/r_S of the Stromgren sphere. For clouds of sufficient He-ionizing photons Q(He0)/Q(H0) >~ 0.15 and non-negligible width of the Stromgren sphere a significant overestimate of helium abundances may result. Such clouds show radiation softness parameters in the range -0.4 <~ log(eta) <~ 0.3 coincident with the range of eta in observed HII-regions. Existing data of HII-regions indeed seem to display a correlation which is consistent with a typical ~ 2-4% overestimate of helium abundances due to these effects. In case such an interpretation prevails, and in the absence of other compensating effects, a significant downward revision of Y_p may result. It is argued that caution should be exercised regarding the validity of commonly quoted error bars on Y_p.

Density Structure of the Giant Hii Region NGC 2363

We perform a detailed measurement of the electron density along two slit position angles in the bright, low metallicity extragalactic Hii region NGC 2363. A comparison of the density structures obtained with the two independent diagnostics given by [Aiv]4711/4740 and [Sii]6716/6731, show that they present both different absolute values and different radial dependencies, with the [Aiv] densities reaching up to 1000 cm−3. We explore the implications for the computation of the He abundance.

New catalogue of Wolf-Rayet galaxies and high-excitation extra-galactic Hii regions

We present a new compilation of Wolf-Rayet (WR) galaxies and extra-galactic Hii regions showing broad He ii 4686 emission drawn from the literature. Relevant information on the presence of other broad emis- sion lines (N iii 4640, C iv 5808 and others) from WR stars of WN and WC subtypes, and other existing broad nebular lines is provided. In total we include 139 known WR galaxies. Among these, 57 objects show both broad He ii 4686 and C iv 5808 features. In addition to the broad (stellar) He ii 4686 emission, a nebular He ii component is well es- tablished (suspected) in 44 (54) objects. We nd 19 extra-galatic Hii regions without WR detections showing nebular He ii 4686 emission. The present sample can be used for a variety of studies on massive stars, interactions of massive stars with the ISM, stellar populations, starburst galaxies etc. The data is accessible electronically and will be updated periodically.

Supersonic gas motion in giant extragalactic Hii regions

Supersonic gas motion in giant extragalactic Hii regions

Kinematical Study of Old HII Regions and Optical Counterpart to the DRAO Canadian Galactic Plane Survey

AbstractThis project aims to tackle a few unresolved problems related to the interstellar medium (ISM) by acting as an optical counterpart to the Canadian Galactic Plane Survey (CGPS). We have three main objectives: (1) the study of large/old HII regions; (2) observations of targets-of-opportunity that may be found by the CGPS; (3) comparison of the kinematics of extragalactic (M33) and galactic HII regions. (1) Old HII regions having large spatial extents will be observed to establish their kinematical structure when almost no molecular material is left to produce photodissociated flows. Does turbulence play a role in these objects as for younger nebulae? (2) Ionised nebulae observed by the CGPS that are peculiar, either morphologically or by special association with neutral hydrogen, will also be observed. (3) The kinematical behaviour of old HII regions in M33 will be compared with that of galactic HII regions. With both data sets in hand we will check if HII regions, like supernovae, dump energy into the neutral ISM. The instrument used as well as some very preliminary data are presented.

Temperature Fluctuations and Abundances in HiiGalaxies

There is evidence for temperature fluctuations in Planetary Nebulae and in Galactic HII regions. If such fluctuations occur in the low-metallicity, extragalactic HII regions used to probe the primordial helium abundance, the derived 4He mass fraction, Y_P, could be systematically different from the true primordial value. For cooler, mainly high-metallicity HII regions the derived helium abundance may be nearly unchanged but the oxygen abundance could have been seriously underestimated. For hotter, mainly low-metallicity HII regions the oxygen abundance is likely accurate but the helium abundance could be underestimated. The net effect is to tilt the Y vs. Z relation, making it flatter and resulting in a higher inferred Y_P. Although this effect could be large, there are no data which allow us to estimate the size of the temperature fluctuations for the extragalactic HII regions. Therefore, we have explored this effect via Monte Carlos in which the abundances derived from a fiducial data set are modified by \Delta-T chosen from a distribution with 0 < \Delta-T < \Delta-T_max where \Delta-T_max is varied from 500K to 4000K. It is interesting that although this effect shifts the locations of the HII regions in Y vs. O/H plane, it does not introduce any significant additional dispersion.

Embedded Clusters in Giant Extragalactic HII Regions.III.Extinction and Star Formation

A study of star formation is carried out on 35 giant extragalactic HII regions (GEHRs) in seven galaxies using optical photometric data in BVR broad bands and in the emission line of H alpha. Interstellar extinction, metallicity and nebular contributions to the broad bands are estimated using spectroscopic data on these objects. Dimensionless diagrams involving B-V and V-R colors and the flux ratio of Balmer line to B band continuum are used to study star formation. The cluster colors indicate reduced extinction towards stellar continuum compared to the values derived from Balmer lines for the ionized gas. The frequency of detection of classical young (t < 3 Myr) regions with only one burst of star formation is found to be low as compared to young regions with an accompanying population rich in red supergiants from a previous burst (t ~ 10 Myr). Reduced extinction towards cluster stars, destruction of ionizing photons and the existence of older population, often spatially unresolvable from the younger population, all conspire to make the observed Balmer line equivalent widths low in a majority of the GEHRs. A scenario of star formation is suggested which explains many of the observed properties of GEHRs, including the core-halo structure, reduced extinction for the radiation from stars as compared to that from the nebular gas, non-detection of young single burst regions and the co-existence of two populations of different ages.

ON THE RELATIONSHIP BETWEEN STAR FORMATION AND ACTIVITY IN GALAXIES

ON THE RELATIONSHIP BETWEEN STAR FORMATION AND ACTIVITY IN GALAXIES

Embedded Clusters in Giant Extragalactic H II Regions. II. Evolutionary Population Synthesis Model

A stellar population synthesis model, suitable for comparison with Giant Extragalactic HII Regions (GEHRs), is constructed incorporating the recent developments in modelling stellar evolution by Maeder and co-workers and stellar atmospheres by Kurucz. A number of quantities suitable for comparison with broad band data of GEHRs in visible and near infrared parts of the spectrum are synthesized in addition to the hydrogen and helium ionizing photon production rates at solar metallicities, for three scenarios of star formation --- (i) Instantaneous burst (IB) (ii) Continuous star formation (CSF) and (iii) Two bursts of star formation, with the older burst rich in red supergiants. For IB case, evolution of colors shows three distinct phases --- an initial steady blue phase, followed by a red bump (5--15~Myr) and another steady phase with colors intermediate to the earlier two phases. CSF colors asymptotically reach peak values at $\sim 10$~Myr, never reaching the reddest IB colors. Ionizing photon production rate falls off by an order of magnitude in 6~Myr for IB, where as it almost remains constant for CSF model. Two-burst models with burst separations $\sim 10$~Myr have properties of both IB and CSF, simultaneously producing the red IB colors and high ionizing photon rate, making such regions easily distinguishable using optical observations. Flat IMFs result in bluest colors when the massive stars are on the main sequence and reddest colors during the red supergiant phase of the evolving massive stars. Errors on the computed quantities due to the statistical uncertainties inherent in the process of star formation become negligible for cluster masses in excess of $10^5$\,\msun.

On the abundance of primordial helium

We have used recent observations of helium-4, nitrogen and oxygen from some four dozen, low metallicity, extra-galactic HII regions to define mean $N$ versus $O$, $^4He$ versus $N$ and $^4He$ versus $O$ relations which are extrapolated to zero metallicity to determine the primordial $^4He$ mass fraction $Y_P$. The data and various subsets of the data, selected on the basis of nitrogen and oxygen, are all consistent with $Y_P = 0.232 \pm 0.003$. For the 2$\sigma$ (statistical) upper bound we find $Y_P^{2\sigma} \le 0.238$. Estimating a 2\% systematic uncertainty $(\sigma _{syst} = \pm 0.005)$ leads to a maximum upper bound to the primordial helium mass fraction: $Y_P^{MAX} = Y_P^{2\sigma} + \sigma_{syst} \le 0.243$. We compare these upper bounds to $Y_P$ with recent calculations of the predicted yield from big bang nucleosynthesis to derive upper bounds to the nucleon-to-photon ratio $\eta$ ($\eta_{10} \equiv 10^{10}\eta$) and the number of equivalent light (\lsim 10 MeV) neutrino species. For $Y_P \le 0.238$ ($0.243$), we find $\eta_{10} \le 2.5 (3.9)$ and $N_\nu \leq 2.7 (3.1)$. If indeed $Y_P \le 0.238$, then BBN predicts enhanced production of deuterium and helium-3 which may be in conflict with the primordial abundances inferred from model dependent (chemical evolution) extrapolations of solar system and interstellar observations. Better chemical evolution models and more data - especially $D$-absorption in the QSO Ly-$\alpha$ clouds - will be crucial to resolve this potential crisis for BBN. The larger upper bound, $Y_P \leq 0.243$ is completely

TAURUS 2 Observations of Giant Extragalactic HII Regions

AbstractThe use of TAURUS 2, a Fabry-Perot imaging spectrograph, for the study of the kinematics and the dynamics of giant extragalactic HII regions in nearby galaxies is discussed. The technical aspects of the instrument, together with the procedures adopted for data analysis and reduction, and some of the first scientific results of our programme are also presented.

Supersonic turbulence in giant extragalactic HII regions

I discuss in this paper the more likely physical mechanisms that could provide the energy input for the supersonic motions observed in giant extragalactic HII regions, together with preliminary results of an ongoing observational program that aims to study, with with high spatial and spectral resolution, the kinematics of the ionized gas of the regions.

Spectrophotometry of giant extragalactic regions of ionized hydrogen in dwarf galaxies of M81 galaxy group

Abstract The results of spectrophotometric study of 9 giant extragalactic HII regions in M81 group are presented. The physical conditions and chemical composition of gas in studied HII regions are determined. The observed differences between heavy element abundances determined from data in optical and far infrared + radio ranges are shown to be due to the radiation in radio and far infrared ranges, generated in different galaxy regions.

Comparison of turbulence in HII regions and molecular clouds

Both the HII Regions and the Molecular Clouds show broadening of their emission lines beyond that expected from thermal motion and this is ascribed to turbulence. Turbulence in molecular clouds generally agrees with a model where the velocity of motion is determined by the Alfv én velocity.Turbulence in Galactic HII Regions and Giant Extragalactic HII Regions can also be studied by the width of the emission lines. The magnitude of the turbulent velocities in these regions are characteristically about 10 km/s. There is a general increase in turbulent velocity with the size of the HII Region, and this relation is close to but different from the one third power dependence expected from the most naive application of Kolmogorov theory. When a detailed study is conducted of each Galactic HII Region by means of the structure function, one finds that there is not agreement with Kolmogorov theory.The Size-Turbulent versus Velocity relation for Galactic HII Regions differs slightly from the better defined velocity relation for Giant Extragalactic HII Regions. This difference is probably due to the fact that the larger extragalactic objects are probably complexes of multiple individual HII Regions. There is no evidence that broadening of extragalactic HII Regions is due to motion about a common center of mass.