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.Comment: 29 pages, 13 Postscript figures, accepted for publication in the Astrophysical Journa

A spectroscopic study of component C and the extended emission around I Zw 18

Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are used to investigate the stellar population and the evolutionary status of I Zw 18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18. Hydrogen H$\alpha$ and H$\beta$ emission lines are detected in the spectra of I Zw 18C, implying that ionizing massive stars are present. High signal-to-noise Keck II spectra of different regions in I Zw 18C reveal H$\gamma$, H$\delta$ and higher order hydrogen lines in absorption. Several techniques are used to constrain the age of the stellar population in I Zw 18C. Ages derived from two different methods, one based on the equivalent widths of the H$\alpha$, H$\beta$ emission lines and the other on H$\gamma$, H$\delta$ absorption lines are consistent with a 15 Myr instantaneous burst model. We find that a small extinction in the range $A_V$ = 0.20 -- 0.65 mag is needed to fit the observed spectral energy distribution of I Zw 18C with that model. In the case of constant star formation, all observed properties are consistent with stars forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available observational constraints for I Zw 18C, including those obtained from Hubble Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18 should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized gas emission around I Zw 18. H$\alpha$ emission is detected as far as 30" from it. To a B surface brightness limit of ~ 27 mag arcsec$^{-2}$ we find no observational evidence for extended stellar emission in the outermost regions, at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the Astrophysical Journa

Luminous Blue Variable Stars In The Two Extremely Metal-Deficient Blue Compact Dwarf Galaxies DDO 68 and PHL 293B

We present photometric and spectroscopic observations of two luminous blue variable (LBV) stars in two extremely metal-deficient blue compact dwarf (BCD) galaxies, DDO 68 with 12+logO/H = 7.15 and PHL 293B with 12+logO/H = 7.72. These two BCDs are the lowest-metallicity galaxies where LBV stars have been detected, allowing to study the LBV phenomenon in the extremely low metallicity regime, and shedding light of the evolution of the first generation of massive stars born from primordial gas. We find that the strong outburst of the LBV star in DDO 68 occurred sometime between February 2007 and January 2008. We have compared the properties of the broad line emission in low-metallicity LBVs with those in higher metallicity LBVs. We find that, for the LBV star in DDO 68, broad emission with a P Cygni profile is seen in both H and He I emission lines. On the other hand, for the LBV star in PHL 293B, P Cygni profiles are detected only in H lines. For both LBVs, no heavy element emission line such as Fe II was detected. The Halpha luminosities of LBV stars in both galaxies are comparable to the one obtained for the LBV star in NGC 2363 (Mrk 71) which has a higher metallicity 12+logO/H = 7.89. On the other hand, the terminal velocities of the stellar winds in both low-metallicity LBVs are high, ~800 km/s, a factor of ~4 higher than the terminal velocities of the winds in high-metallicity LBVs. This suggests that stellar winds at low metallicity are driven by a different mechanism than the one operating in high-metallicity winds.Comment: 26 pages, 5 figures, accepted for publication in the Astrophysical Journa

HST observations of the cometary blue compact dwarf galaxy UGC 4483: a relatively young galaxy?

We present V and I photometry of the resolved stars in the cometary blue compact dwarf galaxy UGC 4483 using Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) images. The resulting I vs. (V-I) color-magnitude diagram (CMD) reaches limiting magnitudes V = 27.5 mag and I = 26.5 mag for photometric errors less than 0.2 mag. It reveals not only a young stellar population of blue main-sequence stars and blue and red supergiants, but also an older evolved population of red giant and asymptotic giant branch stars. The measured magnitude I = 23.65 +/- 0.10 mag of the red giant branch tip results in a distance modulus (m-M) = 27.63 +/- 0.12, corresponding to a distance of 3.4 +/- 0.2 Mpc. The youngest stars are associated with the bright H II region at the northern tip of the galaxy. The population of older stars is found throughout the low-surface-brightness body of the galaxy and is considerably more spread out than the young stellar population, suggesting stellar diffusion. The most striking characteristics of the CMD of UGC 4483 are the very blue colors of the red giant stars and the high luminosity of the asymptotic giant branch stars. Both of these characteristics are consistent with either: 1) a very low metallicity ([Fe/H] = -2.4 like the most metal-deficient globular clusters) and an old age of 10 Gyr, or 2) a higher metallicity ([Fe/H] = -1.4 as derived from the ionized gas emission lines) and a relatively young age of the oldest stellar population in UGC 4483, not exceeding ~ 2 Gyr. Thus our data do not exclude the possibility that UGC 4483 is a relatively young galaxy having formed its first stars only ~ 2 Gyr ago.Comment: 37 pages, 15 PS figures, to appear in Ap

An imaging and spectroscopic study of the very metal-deficient blue compact dwarf galaxy Tol 1214--277

We present a spectrophotometric study based on VLT/FORS I observations of one of the most metal-deficient blue compact dwarf (BCD) galaxies known, Tol 1214-277 (Z ~ Zsun/25). The data show that roughly half of the total luminosity of the BCD originates from a bright and compact starburst region located at the northeastern tip of a faint dwarf galaxy with cometary appearance. The starburst has ignited less than 4 Myr ago and its emission is powered by several thousands O7V stars and ~ 170 late-type nitrogen Wolf-Rayet stars located within a compact region with < 500 pc in diameter. For the first time in a BCD, a relatively strong [Fe V] 4227 emission line is seen which together with intense He II 4686 emission indicates the presence of a very hard radiation field in Tol 1214-277. We argue that this extraordinarily hard radiation originates from both Wolf--Rayet stars and radiative shocks in the starburst region. The structural properties of the low-surface-brightness (LSB) component underlying the starburst have been investigated by means of surface photometry down to 28 B mag/sq.arcsec. We find that, for a surface brightness level fainter than ~ 24.5 B mag/sq.arcsec, an exponential fitting law provides an adequate approximation to its radial intensity distribution. The broad-band colors in the outskirts of the LSB component of Tol 1214-277 are nearly constant and are consistent with an age below one Gyr. This conclusion is supported by the comparison of the observed spectral energy distribution (SED) of the LSB host with theoretical SEDs.Comment: 17 pages, 11 Postscript figures, uses emulateapj.sty, to appear in Astronomical Journa

The Embedded Super Star Cluster of SBS0335-052

We analyze the infrared (6-100 micron) spectral energy distribution of the blue compact dwarf and metal-poor (Z=Z_solar/41) galaxy SBS0335-052. With the help of DUSTY (Ivezic et al. 1999), a program that solves the radiation transfer equations in a spherical environment, we evaluate that the infrared (IR) emission of SBS0335-052 is produced by an embedded super-star cluster (SSC) hidden under 10^5 M_solar of dust, causing 30 mag of visual extinction. This implies that one cannot detect any stellar emission from the 2x10^6 M_solar stellar cluster even at near-infrared (NIR) wavelengths. The derived grain size distribution departs markedly from the widely accepted size distribution inferred for dust in our galaxy (the so-called MRN distribution, Mathis et al. 1977), but resembles what is seen around AGNs, namely an absence of PAH and smaller grains, and grains that grow to larger sizes (around 1 micron). The fact that a significant amount of dust is present in such a low-metallicity galaxy, hiding from UV and optical view most of the star formation activity in the galaxy, and that the dust size distribution cannot be reproduced by a standard galactic law, should be borne in mind when interpreting the spectrum of primeval galaxies.Comment: 32 pages, 3 figures,accepted for publication in A

New Light on the Stellar Populations in IZw18: Deep Near-Infrared Imaging

We present deep JHK images of IZw18, the most metal-deficient Blue Compact Dwarf galaxy known, and analyze them in conjunction with archival HST/WFPC2 optical images. To investigate the stellar populations, we have divided the main body of IZw18 into eight regions, and fit the optical, near-infrared (NIR), and hybrid optical-NIR colors of these and the C component with evolutionary synthesis models. The composite best fit is obtained for an age for evolved stellar populations of <= 200Myr; fits with an older age of <= 500Myr are less likely but possible. Our data show no evidence for stellar populations in IZw18 older than this, although as much as 22% of the stellar mass in older stars (4% in J light) could remain undetected. The colors of the young and intermediate-age stellar populations are significantly affected by widespread and inhomogeneously distributed ionized gas and dust. Ionized gas emission is important in every region examined except the NW star cluster. Extinction is significant in both the NW and SE clusters. Red H-K, B-H, and V-K colors are not reliable indicators of old stellar populations because ionized gas emission is also red in these colors. V-I, on the other hand, reliably separates stars from gas because the former are red (V-I >= 0.4) while the latter is blue (V-I ~ -0.4).Comment: 29 pages, 9 figures, accepted for publication in ApJ (May, 2003

Proenkephalin A 119-159 (Penkid) Is an Early Biomarker of Septic Acute Kidney Injury: The Kidney in Sepsis and Septic Shock (Kid-SSS) Study

Introduction: Sepsis is the leading cause of acute kidney injury (AKI) in critically ill patients. The Kidney in Sepsis and Septic Shock (Kid-SSS) study evaluated the value of proenkephalin A 119-159 (penkid)—a sensitive biomarker of glomerular function, drawn within 24 hours upon intensive care unit (ICU) admission and analyzed using a chemiluminescence immunoassay—for kidney events in sepsis and septic shock. Methods: The Kid-SSS study was a substudy of Adrenomedullin and Outcome in Severe Sepsis and Septic Shock (AdrenOSS) (NCT02393781), a prospective, observational, multinational study including 583 patients admitted to the intensive care unit with sepsis or septic shock and a validation cohort of 525 patients from the French and euRopean Outcome reGistry in Intensive Care Units (FROG-ICU) study. The primary endpoint was major adverse kidney events (MAKEs) at day 7, composite of death, renal replacement therapy, and persistent renal dysfunction. The secondary endpoints included AKI, transient AKI, worsening renal function (WRF), and 28-day mortality. Results: Median age was 66 years (interquartile range 55–75), and 28-day mortality was 22% (95% confidence interval [CI] 19%−25%). Of the patients, 293 (50.3%) were in shock upon ICU admission. Penkid was significantly elevated in patients with MAKEs, persistent AKI, and WRF (median = 65 [IQR = 45–106] vs. 179 [114–242]; 53 [39–70] vs. 133 [79–196] pmol/l; and 70 [47–121] vs. 174 [93–242] pmol/l, all P &lt; 0.0001), also after adjustment for confounding factors (adjusted odds ratio = 3.3 [95% CI = 1.8–6.0], 3.9 [95% CI = 2.1–7.2], and 3.4 [95% CI = 1.9–6.2], all P &lt; 0.0001). Penkid increase preceded elevation of serum creatinine with WRF and was low in renal recovery. Conclusion: Admission penkid concentration was associated with MAKEs, AKI, and WRF in a timely manner in septic patients

High resolution measurements of carbon monoxide along a late Holocene Greenland ice core: evidence for in situ production

We present high-resolution measurements of carbon monoxide (CO) concentrations from a shallow ice core of the North Greenland Eemian Ice Drilling project (NEEM-2011-S1). An optical-feedback cavity-enhanced absorption spectrometer (OF-CEAS) coupled to a continuous melter system performed continuous, online analysis during a four-week measurement campaign. This analytical setup generated stable measurements of CO concentrations with an external precision of 7.8 ppbv (1σ), based on repeated analyses of equivalent ice core sections. However, this first application of this measurement technique suffered from a poorly constrained procedural blank of 48 ± 25 ppbv and poor accuracy because an absolute calibration was not possible. The NEEM-2011-S1 CO record spans 1800 yr and the long-term trends within the most recent section of this record (i.e., post 1700 AD) resemble the existing discrete CO measurements from the Eurocore ice core. However, the CO concentration is highly variable (75–1327 ppbv range) throughout the ice core with high frequency (annual scale), high amplitude spikes characterizing the record. These CO signals are too abrupt and rapid to reflect atmospheric variability and their prevalence largely prevents interpretation of the record in terms of atmospheric CO variation. The abrupt CO spikes are likely the result of in situ production occurring within the ice itself, although the unlikely possibility of CO production driven by non-photolytic, fast kinetic processes within the continuous melter system cannot be excluded. We observe that 68% of the CO spikes are observed in ice layers enriched with pyrogenic aerosols. Such aerosols, originating from boreal biomass burning emissions, contain organic compounds, which may be oxidized or photodissociated to produce CO within the ice. However, the NEEM-2011-S1 record displays an increase of ~0.05 ppbv yr⁻¹ in baseline CO level prior to 1700 AD (129 m depth) and the concentration remains elevated, even for ice layers depleted in dissolved organic carbon (DOC). Thus, the processes driving the likely in situ production of CO within the NEEM ice may involve multiple, complex chemical pathways not all related to past fire history and require further investigation

Helium abundance in the most metal-deficient blue compact galaxies: I Zw 18 and SBS 0335-052

We present high-quality spectroscopic observations of the two most-metal deficient blue compact galaxies known, I Zw 18 and SBS 0335-052 to determine the helium abundance. The underlying stellar absorption strongly influences the observed intensities of He I emission lines in the brightest NW component of I Zw 18, and hence this component should not be used for primordial He abundance determination. The effect of underlying stellar absorption, though present, is much smaller in the SE component. Assuming all systematic uncertainties are negligible, the He mass fraction derived in this component is Y = 0.243+/-0.007. The high signal-to-noise ratio spectrum (> 100 in the continuum) of SBS 0335-052 allows us to measure the helium mass fraction with a precision better than 2% -- 5% in nine different regions along the slit. Assuming all systematic uncertainties are negligible, the weighted mean He mass fraction in SBS 0335-052 is Y = 0.2437+/-0.0014 when the three He I 4471, 5876 and 6678 emission lines are used, and is 0.2463+/-0.0015 when the He I 4471 emission line is excluded. The weighted mean helium mass fraction in the two most metal-deficient BCGs I Zw 18 and SBS 0335-052, Y=0.2462+/-0.0015, after correction for the stellar He production results in a primordial He mass fraction Yp = 0.2452+/-0.0015. The derived Yp leads to a baryon-to-photon ratio of (4.7+/-1.0) 10^{-10}, consistent with the values derived from the primordial D and 7Li abundances, and supporting the standard big bang nucleosynthesis theory. For the most consistent set of primordial D, 4He, and 7Li abundances we derive an equivalent number of light neutrino species 3.0+/-0.3 (95% C.L.).Comment: 28 pages, 10 figures. To appear in Ap