arXiv subjects

Y. I. Izotov

Publications and source records attributed to Y. I. Izotov.

At least 91 records · Page 5Linked to original sources

arXiv2007

Broad Line Emission in Low-Metallicity Blue Compact Dwarf Galaxies: Evidence for Stellar Wind, Supernova and Possible AGN Activity

We present spectra of a large sample of low-metallicity blue compact dwarf galaxies which exhibit broad components in their strong emission lines, mainly in Hbeta, [O III]4959, 5007 and Halpha. Twenty-three spectra have been obtained with the MMT, 14 of which show broad emission. The remaining 21 spectra with broad emission have been selected from the Data Release 5 of the Sloan Digital Sky Survey. The most plausible origin of broad line emission is the evolution of massive stars and their interaction with the circumstellar and interstellar medium. The broad emission with the lowest H$α$ luminosities (10^36 - 10^39 erg/s) is likely produced in circumstellar envelopes around hot Ofp/WN9 and/or LBV stars. The broad emission with the highest Halpha luminosities (10^40 - 10^42 erg/s) probably arises from type IIp or type IIn supernovae (SNe). It can also come from active galactic nuclei (AGN) containing intermediate-mass black holes, although we find no strong evidence for hard non-thermal radiation in our sample galaxies. The oxygen abundance in the host galaxies with SN candidates is low and varies in the range 12 + log O/H = 7.36 - 8.31. However, type IIn SN / AGN candidates are found only in galaxies with 12 + log O/H < 7.99. Spectroscopic monitoring of these type IIn SN / AGN candidates over a time scale of several years is necessary to distinguish between the two possibilities.

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arXiv2007

MMT observations of new extremely metal-poor emission-line galaxies in the Sloan Digital Sky Survey

We present 6.5-meter MMT spectrophotometry of 20 H II regions in 13 extremely metal-poor emission-line galaxies selected from the Data Release 5 of the Sloan Digital Sky Survey to have [O III] 4959/Hbeta < 1 and [N II] 6583/Hbeta < 0.05. The electron temperature-sensitive emission line [O III] 4363 is detected in 13 H II regions allowing a direct abundance determination. The oxygen abundance in the remaining H II regions is derived using a semi-empirical method. The oxygen abundance of the galaxies in our sample ranges from 12+logO/H ~ 7.1 to ~ 7.8, with 10 H II regions having an oxygen abundance lower than 7.5. The lowest oxygen abundances, 12+logO/H = 7.14+/-0.03 and 7.13+/-0.07, are found in two H II regions of the blue compact dwarf galaxy SDSSJ0956+2849=DDO 68, making it the second most-metal deficient emission-line galaxy known, after SBS 0335-052W.

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arXiv2007

The primordial abundance of 4He: a self-consistent empirical analysis of systematic effects in a large sample of low-metallicity HII regions

We determine the primordial helium mass fraction Yp using 93 spectra of 86 low-metallicity extragalactic HII regions. This sample constitutes the largest and most homogeneous high-quality data sets in existence for the determination of Yp. For comparison and to improve the statistics in our investigation of systematic effects affecting the Yp determination, we have also considered a sample of 271 low-metallicity HII regions selected from the DR5 of the SDSS. Although this larger sample shows more scatter, it gives results that are consistent at the 2sigma level with our original sample. We have considered known systematic effects which may affect the 4He abundance determination. They include different sets of HeI line emissivities and reddening laws, collisional and fluorescent enhancements of HeI recombination lines, underlying HeI stellar absorption lines, collisional excitation of hydrogen lines, temperature and ionization structure of the HII region, and deviation of HeI and H emission line intensities from case B. However, the most likely value of Yp depends on the adopted set of HeI line emissivities. Using Monte Carlo methods to solve simultaneously the above systematic effects we find a primordial helium mass fraction Yp = 0.2472+/-0.0012 when using the HeI emissivities from Benjamin et al. (1999, 2002) and 0.2516+/-0.0011 when using those from Porter et al. (2005). The first value agrees well with the value given by SBBN theory, while the value obtained with likely more accurate emissivities of Porter et al. (2005) is higher at the 2sigma level. This latter value, if confirmed, would imply slight deviations from SBBN.

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arXiv2007

Balmer jump temperature determination in a large sample of low-metallicity H II regions

Aims. Continuing the systematic determination of the electron temperature of H II regions using the Balmer and/or Paschen discontinuities by Guseva et al. (2006) we focus here on 3.6m ESO telescope observations of a large new sample of 69 H II regions in 45 blue compact dwarf (BCD) galaxies. This data set spans a wide range in metallicity (Zsun/60 11000 K) H II regions the temperature of the O2+ zone, determined from doubly ionised oxygen forbidden lines, does not differ statistically from the temperature of the H+ zone. Thus, we confirm and strengthen the finding by Guseva et al. (2006). We emphasize that due to a number of modelling assumptions and the observational uncertainties for individual objects, only a large, homogeneous sample, as the one used here, can enable a conclusive study of the relation between Te(H+) and Te(O III).

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arXiv2006

The Spitzer View of Low-Metallicity Star Formation: I. Haro 3

We present Spitzer observations of the blue compact dwarf galaxy (BCD) Haro 3, with an oxygen abundance of 12+log(O/H)=8.32. These data are part of a larger study of star formation and dust in low-metallicity environments.The IRS spectrum of Haro 3 shows strong narrow Polycyclic Aromatic Hydrocarbon (PAH) emission, with high equivalent widths. Gaseous nebular fine-structure lines are also seen. Despite the absence of optical high-excitation lines, a faint high-ionization [O IV] line at 25.89 micron indicates the presence of radiation as hard as 54.9 eV. A CLOUDY model suggests that the MIR lines originate in two regions: a low-extinction optically-emitting region, and an optically invisible one with much higher extinction. The morphology of Haro 3 changes with wavelength. IRAC 4.5 micron traces extended stellar photospheric emission from the body of the galaxy and hot dust continuum coming mainly from star-forming regions; 8 micron probes extended PAH emission coming mainly from the general ISM; MIPS 24 and 70 micron images map compact small-grain warm dust emission associated with active star formation, and 160 micron reflects cooler extended dust associated with older stellar populations. We have derived the optical-to-radio spectral energy distribution (SED) of the brightest star-forming region A in Haro 3. The best-fit DUSTY model of the SED gives a total luminosity of 2.8e9 Lsun and a mass of 2.8e6 Msun for the ionizing clusters. We infer an extinction A(V)<3, intermediate between the optical A(V)~0.5 and the radio A(V)~8, consistent with the picture that longer wavelength observations probe more deeply into star-forming regions.

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arXiv2006

New southern blue compact dwarf galaxies in the 2dF Galaxy Redshift Survey

Aiming to find new extremely metal-deficient star-forming galaxies we extracted from the Two-Degree Field Galaxy Redshift Survey (2dFGRS) 100K Data Release 14 emission-line galaxies with relatively strong [OIII] 4363 emission. Spectroscopic and photometric studies of this sample and, in addition, of 7 Tololo and 2 UM galaxies were performed on the basis of observations with the ESO 3.6m telescope. All sample galaxies qualify with respect to their photometric and spectroscopic properties as blue compact dwarf (BCD) galaxies. Additionally, they show a good overlap with a comparison sample of 100 well-studied emission-line galaxies on the 12+log(O/H) vs. log(Ne/O), log(Ar/O) and log(Fe/O) planes. From the analysis of the 2dFGRS subsample we report the discovery of two new extremely metal-deficient BCDs with an oxygen abundance 12+log(O/H) < 7.6 and of another seven galaxies with 12+log(O/H) < 7.8. Furthermore, we confirm previous oxygen abundance determinations for the BCDs Tol 1304-353, Tol 2146-391, UM 559 and UM 570 to be 12+log(O/H) < 7.8.

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arXiv2006

Oxygen abundance variations in the system of the two blue compact dwarf galaxies SBS 0335-052E and SBS 0335-052W

We present 3.6m ESO telescope spectroscopic observations of the system of the two blue compact dwarf galaxies SBS 0335-052W and SBS 0335-052E. The oxygen abundance in SBS 0335-052W is 12 + log O/H = 7.13 +/- 0.08, confirming that this galaxy is the most metal-deficient emission-line galaxy known. We find that the oxygen abundance in SBS 0335-052E varies from region to region in the range from 7.20 to 7.31, suggesting the presence of an abundance gradient over a spatial scale of 1 kpc. Signatures of early carbon-type Wolf-Rayet stars are detected in cluster 3 of SBS 0335-052E, corresponding to the emission of three to eighteen WC4 stars, depending on the adopted luminosity of a single WC4 star in the CIV 4658 emission line.

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arXiv2006

Two extremely metal-poor emission-line galaxies in the Sloan Digital Sky Survey

We present spectroscopic observations with the 3.6m ESO telescope of two emission-line galaxies, J2104-0035 and J0113+0052, selected from the Data Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). From our data we determine the oxygen abundance of these systems to be respectively 12+logO/H = 7.26+/-0.03 and 7.17+/-0.09, making them the two most metal-deficient galaxies found thus far in the SDSS and placing them among the five most metal-deficient emission-line galaxies ever discovered. Their oxygen abundances are close to those of the two most metal-deficient emission-line galaxies known, SBS0335-052W with 12+logO/H = 7.12+/-0.03 and I Zw 18 with 12+logO/H = 7.17+/-0.01.

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arXiv2006

Balmer and Paschen jump temperature determinations in low-metallicity emission-line galaxies

We have used the Balmer and Paschen jumps to determine the temperatures of the H+ zones of a total sample of 47 H II regions. The Balmer jump was used on MMT spectrophotometric data of 22 low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies and of one H II region in the spiral galaxy M101. The Paschen jump was used on spectra of 24 H II emission-line galaxies selected from the Data Release 3 of the Sloan Digital Sky Survey (SDSS). To derive the temperatures, we have used a Monte Carlo technique varying the electron temperature in the H+ zone, the extinction of the ionized gas and that of the stellar population, the relative contribution of the ionized gas to the total emission and the star formation history to fit the spectral energy distribution of the galaxies. For the MMT spectra, the fit was done in the wavelength range of 3200 - 5200A which includes the Balmer discontinuity, and for the SDSS spectra, in the wavelength range of 3900 - 9200A which includes the Paschen discontinuity. We find for our sample of H II regions that the temperatures of the O+2 zones determined from the nebular to auroral line intensity ratio of doubly ionized oxygen [O III] (4959+5007)/4363 do not differ, in a statistical sense, from the temperatures of the H+ zones determined from fitting the Balmer and Paschen jumps and the SEDs. We cannot rule out small temperature differences of the order of 3-5%.

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arXiv2005

The chemical composition of metal-poor emission-line galaxies in the Data Release 3 of the Sloan Digital Sky Survey

We have re-evaluated empirical expressions for the abundance determination of N, O, Ne, S, Cl, Ar and Fe taking into account the latest atomic data and constructing an appropriate grid of photoionization models with state-of-the art model atmospheres. Using these expressions we have derived heavy element abundances in the $\sim$ 310 emission-line galaxies from the Data Release 3 of the Sloan Digital Sky Survey (SDSS)with an observed Hbeta flux F(Hbeta)> 1E-14 erg s^{-1} cm^{-2} and for which the [O III] 4363 emission line was detected at least at a 2sigma level, allowing abundance determination by direct methods. The oxygen abundance 12 + log O/H of the SDSS galaxies lies in the range from ~ 7.1 (Zsun/30) to 8.5 (0.7 Zsun). The SDSS sample is merged with a sample of 109 blue compact dwarf (BCD) galaxies with high quality spectra, which contains extremely low-metallicity objects. We use the merged sample to study the abundance patterns of low-metallicity emission-line galaxies. We find that extremely metal-poor galaxies (12 + log O/H < 7.6, i.e. Z < Zsun/12) are rare in the SDSS sample. The alpha element-to-oxygen abundance ratios do not show any significant trends with oxygen abundance, in agreement with previous studies, except for a slight increase of Ne/O with increasing metallicity, which we interpret as due to a moderate depletion of O onto grains in the most metal-rich galaxies. The Fe/O abundance ratio is smaller than the solar value, by up to 1 dex at the high metallicity end. (abridged)

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arXiv2005

High-Ionization Emission in Metal-Deficient Blue Compact Dwarf Galaxies

Primordial stars are expected to be very massive and hot, producing copious amounts of hard ionizing radiation. The best place to study hard ionizing radiation in the local universe is in very metal-deficient Blue Compact Dwarf (BCD) galaxies. We have carried out a MMT spectroscopic search for [Ne V] 3426 (ionization potential of 7.1 Ryd), [Fe V] 4227 (ionization potential of 4 Ryd) and He II 4686 (ionization potential of 4 Ryd) emission in a sample of 18 BCDs. We have added data from previous work and from the Data Release 3 of the Sloan Digital Sky Survey. In total, we have assembled a BCD high-ionization sample with [Ne V] emission in 4 galaxies, [Fe V] emission in 15 galaxies and He II emission in 465 galaxies. With this large sample, we have reached the following conclusions. There is a general trend of higher [Ne V], [Fe V] and He II emission at lower metallicities. However metallicity is not the only factor which controls the hardness of the radiation. High-mass X-ray binaries and main-sequence stars are probably excluded as the main sources of the very hard ionizing radiation responsible for [Ne V] emission. The most likely source of [Ne V] emission is probably fast radiative shocks moving with velocities > 450 km/s through a dense interstellar medium with an electron number density of several hundreds per cm^-3 and associated with supernova explosions of the most massive stars. These have masses of ~ 50 - 100 Msun and are formed in very compact super-star clusters. The softer ionizing radiation required for He II emission is likely associated with less massive evolved stars and/or radiative shocks moving through a less dense interstellar medium.

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arXiv2005

The Hamburg/SAO survey for emission-line galaxies. VI. The sixth list of 126 galaxies

We present the sixth list with results of the Hamburg/SAO Survey for Emission-Line Galaxies. The final list resulted from follow-up spectroscopy conducted with the 4.5m MMT telescope in 1996, and with 2.2m CAHA and 6m SAO telescopes in 2000 to 2003. The data of this snap-shot spectroscopy survey confirmed 134 emission-line objects out of 182 observed candidates and allowed their quantitative spectral classification and redshift determination. We classify 73 emission-line objects as definite or probable blue compact or HII galaxies (BCG), 8 as QSOs, 4 as Seyfert 1 and 2 galaxies. 30 low-excitation objects were classified as definite or probable starburst nuclei (SBN), 3 as dwarf amorphous nuclei starburst galaxies (DANS) and 2 as LINERs. Due to the low signal-to-noise ratio we could not classify 14 ELGs (NON). For another 9 galaxies we did not detect any significant emission lines. For 98 emission-line galaxies, the redshifts and/or line intensities are determined for the first time. For the remaining 28 previously-known ELGs we give either improved data the line intensities or some independent measurements. The detection rate of ELGs is ~70%. This paper completes the classification of strong-lined ELGs found in the zone of the Hamburg/SAO survey. Together with previously known BCG/HII galaxies in this zone, this sample of ~500 objects is the largest to date in a well bound region.

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arXiv2005

SBS 0335-052W: The Lowest-Metallicity Star-Forming Galaxy Known

We present 4-meter Kitt Peak telescope and 6.5-meter MMT spectrophotometry of the extremely low-metallicity galaxy SBS 0335-052W, the western companion of the blue compact dwarf galaxy SBS 0335-052E. These observations have been combined with published 10-meter Keck data to derive for the brightest region of SBS 0335-052W an oxygen abundance 12+logO/H=7.12+/-0.03. This makes SBS 0335-052W the lowest metallicity star-forming galaxy known in the local universe. Using a Monte Carlo technique, we fit the spectral energy distribution of SBS 0335-052W to derive the age of the oldest stars contributing to its optical light. We find that star formation in SBS 0335-052W began less than 500 Myr ago, making it a likely nearby young dwarf galaxy.

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arXiv2005

A HST study of the stellar populations in the cometary dwarf irregular galaxy NGC 2366

We present V and I photometry of the resolved stars in the cometary dwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram reaches down to I~26.0 mag. It reveals not only a young population of blue main-sequence stars (age <30 Myr) but also an intermediate-age population of blue and red supergiants (20 Myr 100 Myr) and red giant branch (RGB) stars (age >1 Gyr). The measured magnitude I=23.65+/-0.10 mag of the RGB tip results in a distance modulus m-M=27.67+/-0.10, which corresponds to a distance of 3.42+/-0.15 Mpc, in agreement with previous distance determinations. The youngest stars are associated with the bright complex of HII regions NGC 2363=Mrk 71 in the southwest extremity of the galaxy. As a consequence of the diffusion and relaxation processes of stellar ensembles, the older the stellar population is, the smoother and more extended is its spatial distribution. An underlying population of older stars is found throughout the body of NGC 2366. The most notable feature of this older population is the presence of numerous relatively bright AGB stars. The number ratio of AGB to RGB stars and the average absolute brightness of AGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403, indicating a younger age of the AGB stars in NGC 2366. In addition to the present burst of age <100 Myr, there has been strong star formation activity in the past of NGC 2366, from ~100 Myr to <3 Gyr ago.

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arXiv2004

Abundances in the H I envelope of the extremely low-metallicity Blue Compact Dwarf Galaxy SBS 0335-052 from Far Ultraviolet Spectroscopic Explorer observations

We present FUSE spectroscopy of SBS 0335-052, the second most metal-deficient blue compact dwarf (BCD) galaxy known (log O/H = -4.70). In addition to the H I Lyman series, we detect C II, N I, N II, O I, Si II, Ar I and Fe II absorption lines, mainly arising from the extended H I envelope in which SBS 0335-052 is embedded. No H2 absorption lines are seen. The absence of diffuse H2 implies that the warm H2 detected through infrared emission must be very clumpy and associated with the star-forming regions.The clumps should be denser than ~ 1000 cm^-3 and hotter than ~ 1000 K and account for > 5% of the total H I mass. Although SBS 0335-052 is a probable young galaxy, its neutral gas is not pristine. The metallicity of its neutral gas is similar to that of its ionized gas and is equal log O/H ~ -5. This metallicity is comparable to those found in the H I envelopes of four other BCDs with ionized gas metallicities spanning the wide range from log O/H = -4.8 to log O/H = -3.8, and in Ly-alpha absorbers, fueling the speculation that there may have been previous enrichment of the primordial neutral gas to a common metallicity level of log O/H ~ -5, possibly by Population III stars.

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arXiv2004

Deep VLT spectroscopy of the blue compact dwarf galaxies Tol 1214-277 and Tol 65

We present VLT spectroscopic observations with different spectral resolutions and different slit orientations of the two metal-deficient blue compact dwarf (BCD) galaxies Tol 1214-277 and Tol 65. The oxygen abundances in the brightest HII regions of Tol 1214-277 and Tol 65 are found to be 12+log O/H = 7.55+/-0.01 and 7.54+/-0.01, or Zsun/24. The nitrogen-to-oxygen abundance ratios in the two galaxies are log N/O =-1.64+/-0.03 and -1.60+/-0.02 and lie in the narrow range found for other most metal-deficient BCDs. The helium mass fraction derived in several HII regions in both galaxies is consistent with a high primordial helium mass fraction, Yp ~ 0.244. We confirm the detection of the high-ionization forbidden emission line [FeV]4227 in the spectrum of Tol 1214-277. Additionally, weak [NeIV]4725, [FeVI]5146, 5177, and [FeVII]5721, 6087 emission lines are detected in the high-resolution spectrum of Tol 1214-277. The detection of these lines implies the presence of hard radiation with photon energy in the range ~ 4-8 Ryd. Emission lines are detected in the spectra of eight galaxies in the fields of Tol 1214-277 and Tol 65. Seven of these galaxies are background objects, while one galaxy has a redshift close to that of Tol 1214-277. Situated at a projected distance of ~ 14.5 kpc from Tol 1214-277, this galaxy is probably a companion of the BCD.

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arXiv2004

Pox 186: an ultracompact galaxy with dominant ionized gas emission

We present a ground-based optical spectroscopic and HST U, V, I photometric study of the blue compact dwarf (BCD) galaxy Pox 186. It is found that the emission of the low-surface brightness (LSB) component in Pox 186 at radii < 3'' (< 270 pc in linear scale) is mainly gaseous in origin. We detect Halpha emission out to radii as large as 6''. At radii > 3'' the light of the LSB component is contaminated by the emission of background galaxies complicating the study of the outermost regions. The surface brightness distribution in the LSB component can be approximated by an exponential law with a scale length alpha < 120 pc. This places Pox 186 among the most compact dwarf galaxies known. The derived alpha is likely to be an upper limit to the scale length of the LSB component because of the strong contribution of the gaseous emission. The oxygen abundance in the bright HII region derived from the 4.5m Multiple Mirror Telescope (MMT) and 3.6m ESO telescope spectra are 12+log(O/H)= 7.76+/-0.02 and 7.74+/-0.01 (~ Zsun/15), respectively, in accordance with previous determinations. The helium mass fractions found in this region are Y=0.248+/-0.009 (MMT) and Y=0.248+/-0.004 (3.6m) suggesting a high primordial helium abundance.

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arXiv2004

Deep Hubble Space Telescope/ACS Observations of I Zw 18: a Young Galaxy in Formation

We present V and I photometry of the resolved stars in the most metal-deficient blue compact dwarf galaxy known, I Zw 18 (Zsun/50), using Hubble Space Telescope/Advanced Camera for Surveys (ACS) images, the deepest ones ever obtained for this galaxy. The resulting I vs. V-I color-magnitude diagram (CMD) reaches limiting magnitudes V=I=29 mag. It reveals a young stellar population of blue main-sequence (MS) stars (age <30 Myr) and blue and red supergiants (10 Myr<age<100 Myr), but also an older evolved population of asymptotic giant branch (AGB) stars (100 Myr<age<500 Myr). We derive a distance to I Zw 18 in the range 12.6 Mpc - 15 Mpc from the brightness of its AGB stars, with preferred values in the higher range. The red giant branch (RGB) stars are conspicuous by their absence, although, for a distance of I Zw 18 <15 Mpc, our imaging data go ~ 1-2 mag below the tip of the RGB. Thus, the most evolved stars in the galaxy are not older than 500 Myr and I Zw 18 is a bona fide young galaxy. Several star formation episodes can be inferred from the CMDs of the main body and the C component. There have been respectively three and two episodes in these two parts, separated by periods of ~ 100-200 Myr. In the main body, the younger MS and massive post-MS stars are distributed over a larger area than the older AGB stars, suggesting that I Zw 18 is still forming from the inside out. In the C component, different star formation episodes are spatially distinct, with stellar population ages decreasing from the northwest to the southeast, also suggesting the ongoing build-up of a young galaxy.

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