1,040 research outputs found

    HII Galaxies as deep cosmological probes

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    We re-investigated the use of the Hubble diagram to measure the cosmological constant (Λ\Lambda) and the mass density of the Universe (ΩM\Omega_M). We find an important focusing effect in Λ\Lambda for redshifts about 3. This effect implies that the apparent magnitude of a standard candle at redshifts z=2-3 has almost no dependence on Λ\Lambda for ΩM>0.2\Omega_M>0.2. This means that ΩM\Omega_M can be measured independently of ΩΛ\Omega_{\Lambda} by targeting the redshift range according to an estimate of the value of ΩM\Omega_M. We explore the evidence in support of the suggestion that extreme starburst galaxies also known as HII galaxies can be used as distance estimators over a wide range of redshifts and reaching very high values. We have compiled literature data of HII galaxies up to z3z\sim 3 and found a good correlation between their luminosity and velocity dispersion measured from their strong emission lines, thus confirming the correlation already known to exist for HII galaxies in the nearby Universe. Several systematic effects such as age, extinction, kinematics, and metallicity are discussed as well as the effects of different cosmologies.Comment: To appear in MNRAS, 7 pages, 6 figure

    An Active Instance-based Machine Learning method for Stellar Population Studies

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    We have developed a method for fast and accurate stellar population parameters determination in order to apply it to high resolution galaxy spectra. The method is based on an optimization technique that combines active learning with an instance-based machine learning algorithm. We tested the method with the retrieval of the star-formation history and dust content in "synthetic" galaxies with a wide range of S/N ratios. The "synthetic" galaxies where constructed using two different grids of high resolution theoretical population synthesis models. The results of our controlled experiment shows that our method can estimate with good speed and accuracy the parameters of the stellar populations that make up the galaxy even for very low S/N input. For a spectrum with S/N=5 the typical average deviation between the input and fitted spectrum is less than 10**{-5}. Additional improvements are achieved using prior knowledge.Comment: 14 pages, 25 figures, accepted by Monthly Notice

    A photometric comprehensive study of circumnuclear star forming rings: the sample

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    We present photometry in U, B, V, R and I continuum bands and in Hα\alpha and Hβ\beta emission lines for a sample of 336 circumnuclear star forming regions (CNSFR) located in early type spiral galaxies with different levels of activity in their nuclei. They are nearby galaxies, with distances less than 100 Mpc, 60\% of which are considered as interacting objects. This survey of 20 nuclear rings aims to provide insight into their star formation properties as age, stellar population and star formation rate. Extinction corrected Hα\alpha luminosities range from 1.3×10381.3\times 10^{38} to 4×1041ergs14\times 10^{41} erg s^{-1}, with most of the regions showing values between 39.5 logL(Hα)\leq log L(H\alpha) \leq 40, which implies masses for the ionizing clusters higher than 2×105M2\times 10^{5} M_\odot . Hα\alpha and Hβ\beta images have allowed us to obtain an accurate measure of extinction. We have found an average value of AV_V = 1.85 magnitudes. (U-B) colour follows a two maximum distribution around (U-B) \simeq -0.7, and -0.3; (R-I) also presents a bimodal behaviour, with maximum values of 0.6 and 0.9. Reddest (U-B) and (R-I) regions appear in non-interacting galaxies. Reddest (R-I) regions lie in strongly barred galaxies. For a significant number of HII regions the observed colours and equivalent widths are not well reproduced by single burst evolutionary theoretical models.Comment: Accepted for publication in MNRAS. Accepted 15 May 2015. Received, 21 April 201

    Stellar indices and kinematics in Seyfert 1 nuclei

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    We present spectra of 6 type 1 Seyfert galaxies, 2 Seyfert 2, a starburst galaxy and a compact narrow line radiogalaxy, taken in two spectral ranges centered around the near--IR CaII triplet (CaT) (at ~8600 Angstroms), and the Mgb stellar feature at 5180 Angstroms. We measured the equivalent width (EWs) of these features and the Fe52 and Fe53 spectral indices. We found that the strength of the CaT in type 1 Seyfert galaxies with prominent central point sources, is larger than what would be expected from the observed strength of the blue indices. This could be explained by the presence of red supergiants in the nuclei of Seyfert 1 galaxies. On the other hand, the blue indices of these galaxies could also be diluted by the strong FeII multiplets that can be seen in their spectra. We have also measured the stellar and gas velocity dispersions of the galaxies in the sample. The stellar velocity dispersions were measured using both, the Mgb and CaT stellar features. The velocity dispersion of the gas in the narrow line region (NLR) was measured using the strong emission lines [OIII] 5007, 4959 and [SIII] 9069. We compare the gas and star velocity dispersions and find that both magnitudes are correlated in Seyfert galaxies. Most of the Seyfert 1 we observe have stellar velocity dispersion somehow greater than that of the gas in the NLR.Comment: To appear in MNRAS, 18 pages, 9 figure

    Properties of the ionized gas of circumnuclear star-forming regions in early type spirals

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    (Abbr.) A study of cicumnuclear star-forming regions (CNSFRs) in several early type spirals has been made in order to investigate their main properties: stellar and gas kinematics, dynamical masses, ionising stellar masses, chemical abundances and other properties of the ionised gas. Both high resolution (R \sim 20000) and moderate resolution (R ~ 5000) have been used. In some cases these regions, about 100 to 150 pc in size, are seen to be composed of several individual star clusters with sizes between 1.5 and 4.9 pc estimated from Hubble Space Telescope (HST) images. Stellar and gas velocity dispersions are found to differ by about 20 to 30 km/s with the Hβ\beta emission lines being narrower than both the stellar lines and the [OIII] λ\lambda 5007 \AA lines. The twice ionized oxygen, on the other hand, shows velocity dispersions comparable to those shown by stars. We have applied the virial theorem to estimate dynamical masses of the clusters, assuming that systems are gravitationally bounded and spherically symmetric, and using previously measured sizes. The measured values of the stellar velocity dispersions yield dynamical masses of the order of 107^7 to 108^8 solar masses for the whole CNSFRs. ...Comment: 7 pages, 8 figures, proceeding of the meeting "Young massive star clusters - Initial conditions and environments", Granada, Spain, 200