Metals in the Neutral Interstellar Medium of Starburst Galaxies (original) (raw)
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Metals in the Neutral Interstellar Medium of Dwarf Star-Forming Galaxies
The determination of the metal abundances in the neutral interstellar medium (ISM) of dwarf star-forming galaxies is a key step in understanding their physical and chemical evolution. This type of investigation has been possible in the last 5 years thanks to FUSE. We will give a flavor of the issues involved by presenting the work that we are performing in this astrophysical field.
Ultraviolet Line Spectra of Metal-poor Star-forming Galaxies
Astrophysical Journal, 2001
We present synthetic ultraviolet spectra of metal-poor star-forming galaxies which were calculated with the Starburst99 package. A new spectral library was generated from HST observations of O stars in the Large and Small Magellanic Clouds. The corresponding mean metallicity of the synthetic spectra is approximately 1/4\Zs. The spectra have a resolution of 1\AA and cover the spectral range 1200-1600\AA. A set of model spectra was calculated for a standard initial mass function and star-formation history and is compared to synthetic spectra at solar metallicity. We find that the spectral lines are generally weaker at lower metallicity, as expected from the lower elemental abundances. Stellar-wind lines, however, show a more complex behavior: the metallicity dependence of the ionization balance can be important in trace ions, like N$^{4+}$ and Si$^{3+}$. Therefore the strength of \nv and \siiv does not scale monotonically with metallicity. We compare our new models to ultraviolet spectra of NGC 5253 and \cB, two star-forming galaxies with 1/4 solar metallicity at low and high redshift, respectively. The new library provides significantly better fits to the observations than earlier models using the \Zs library. We discuss the potential of utilizing stellar photospheric and wind lines to estimate the chemical composition of star-forming galaxies. The new metal-poor synthetic spectra are available via the Starburst99 website.
The Astrophysical Journal
This is the first in a series of three papers describing a project with the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure abundances of the neutral interstellar medium (ISM) in a sample of 9 nearby star-forming galaxies. The goal is to assess the (in)homogeneities of the multiphase ISM in galaxies where the bulk of metals can be hidden in the neutral phase, yet the metallicity is inferred from the ionized gas in the H ii regions. The sample, spanning a wide range in physical properties, is to date the best suited to investigate the metallicity behavior of the neutral gas at redshift z = 0. ISM absorption lines were detected against the far-ultraviolet spectra of the brightest star-forming region(s) within each galaxy. Here we report on the observations, data reduction, and analysis of these spectra. Column densities were measured by a multi-component line-profile fitting technique, and neutral-gas abundances were obtained for a wide range of elements. Several caveats were considered including line saturation, ionization corrections, and dust depletion. Ionization effects were quantified with 'ad-hoc' CLOUDY models reproducing the complex photoionization structure of the ionized and neutral gas surrounding the UV-bright sources. An 'average spectrum of a redshift z = 0 star-forming galaxy' was obtained from the average column densities of unsaturated profiles of neutral-gas species. This template can be used as a powerful tool for studies of the neutral ISM at both low and high redshift.
The Astrophysical Journal Supplement Series, 2009
We have analyzed FUSE (905-1187Å) spectra of a sample of 16 local starburst galaxies. These galaxies cover almost three orders of magnitude in starformation rates and over two orders of magnitude in stellar mass. Absorption features from the stars and interstellar medium are observed in all the spectra. The strongest interstellar absorption features are generally blue-shifted by ∼ 50 to 300 km s −1 , implying the almost ubiquitous presence of starburst-driven galactic winds in this sample. The outflow velocites increase with both the star formation rate and the star formation rate per unit stellar mass, consistent with a galactic wind driven by the population of massive stars. We find outflowing coronal-phase gas (T ∼ 10 5.5 K) detected via the O VI absorption-line in nearly every galaxy. The O VI absorption-line profile is optically-thin, is generally weak near the galaxy systemic velocity, and has a higher mean outflow velocity than seen in the lower ionization lines. The relationship between the line width and column density for the O VI absorbing gas is in good agreement with expectations for radiatively cooling and outflowing gas. Such gas will be created in the interaction of the hot out-rushing wind seen in X-ray emission and cool dense ambient material. O VI emission is not generally detected in our sample, suggesting that radiative cooling by the coronal gas is not dynamically significant in draining energy from galactic winds. We find that the measured outflow velocities in the HI and HII phases of the interstellar gas in a given galaxy increase with the strength (equivalent width) of the absorption feature and not with the a FUV luminosity derived from FUSE LiF 2A continuum flux at 1150Å. Values are corrected for MW extinction using the method of and the E(B-V) values given in this table.
The Astronomical Journal, 2011
We present 46 rest-frame ultraviolet (UV) spectra of 28 local starburst and star-forming galaxies which were observed with the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) at a spectral resolution of a few 100 km s 1. We compare the HST spectra with lower resolution International Ultraviolet Explorer (IUE) spectra of the same galaxies and find systematic differences: the bright star clusters targeted in HST's ~1 apertures provide about 15% of the starburst luminosity traced by IUE's 10 20 aperture; they are bluer and have stronger stellar-wind features suggesting that the HST apertures have preferentially been placed on the youngest areas of the burst. In contrast, lines arising from the interstellar medium (ISM) show similar equivalent widths in both the large and small aperture observations, suggesting similar ISM properties from larger to smaller scales. In order to quantify the UV spectral morphology of star-forming galaxies, we created a set of UV line indices similar to the standard optical Lick indices. We discuss the relation between the UV spectral morphology and the properties of the galaxy host. We present our atlas of FOS and GHRS spectra both in print and electronically. The data set is useful as a baseline for comparisons with observations of the rest-frame UV spectra of star-forming galaxies at high redshift.
The Astrophysical Journal, 2004
The 905 to 1180 Å spectral range of the Far Ultraviolet Spectroscopic Explorer (FUSE) includes numerous transitions of molecular hydrogen, making it possible to study H 2 in diffuse interstellar environments directly through absorption measurements. We have searched for H 2 absorption in five starburst galaxies: NGC 1705, NGC 3310, NGC 4214, M83 (NGC 5236), and NGC 5253. We tentatively detect weak absorption by H 2 in M83 and NGC 5253, and set upper limits on the H 2 column density in the other galaxies. Conservative upper limits on the mass of molecular gas detected with FUSE are many orders of magnitude lower than the H 2 mass inferred from CO emission measurements for the four galaxies in our sample in which CO has been detected. This indicates that almost all of the H 2 is in the form of clouds with N(H 2 ) 10 20 cm −2 that are opaque to far-UV light and therefore cannot be probed with far-UV absorption measurements. The far-UV continuum visible in the FUSE spectra passes between the dense clouds, which have a covering factor < 1. The complex observational biases related to varying extinction across the extended UV emission in the FUSE apertures prevent an unambiguous characterization of the diffuse H 2 in these starbursts. However, the evidence is suggestive that there is less H 2 in the diffuse interstellar medium between the dense clouds compared to similarly reddened sight lines in the Milky Way. This holds with the expectation that the destructive UV radiation field is stronger in starbursts. However, previous UV observations of these starbursts have shown that there is reddening caused by the diffuse interstellar medium. This suggests that while diffuse H 2 may be destroyed in the starburst, dust still exists.
Monthly Notices of the Royal Astronomical Society, 2016
In this work, we investigate the abundance and distribution of metals in the intergalactic medium (IGM) at z 2.8 through the analysis of an ultra-high signal-to-noise ratio UVES spectrum of the quasar HE0940-1050. In the C IV forest, our deep spectrum is sensitive at 3σ to lines with column density down to log N CIV 11.4 and in 60 per cent of the considered redshift range down to 11.1. In our sample, all H I lines with log N HI ≥ 14.8 show an associated C IV absorption. In the range 14.0 ≤ log N HI < 14.8, 43 per cent of H I lines has an associated C IV absorption. At log N HI < 14.0, the detection rates drop to <10 per cent, possibly due to our sensitivity limits and not to an actual variation of the gas abundance properties. In the range log N HI ≥ 14, we observe a fraction of H I lines with detected C IV a factor of 2 larger than the fraction of H I lines lying in the circumgalactic medium (CGM) of relatively bright Lyman-break galaxies hosted by dark matter haloes with M ∼ 10 12 M. The comparison of our results with the output of a grid of photoionization models and of two cosmological simulations implies that the volume filling factor of the IGM gas enriched to a metallicity log Z/Z − 3 should be of the order of ∼10-13 per cent. In conclusion, our results favour a scenario in which metals are found also outside the CGM of bright star-forming galaxies, possibly due to pollution by lower mass objects and/or to an early enrichment by the first sources.
The Detection of the Diffuse Interstellar Bands in Dusty Starburst Galaxies
The Astrophysical Journal, 2000
We report the detection of the Diffuse Interstellar Bands ("DIBs") in the optical spectra of seven far-infrared-selected starburst galaxies. The λ6283.9Å and λ 5780.5Å features are detected with equivalent widths of ∼ 0.4 to 1Å and 0.1 to 0.6Å respectively. In the two starbursts with the highest quality spectra (M82 and NGC2146), four other weaker DIBs at λ 5797.0Å, 6010.1Å, 6203.1 A, and 6613.6Å are detected with equivalent widths of ∼ 0.1Å. The region over which the DIBs can be detected ranges from ∼ 1 kpc in the less powerful starbursts, to several kpc in the more powerful ones. The gas producing the DIBs is more kinematically quiescent on-average than the gas producing the strongly-blueshifted NaIλλ5890,5896 absorption in the same starbursts. We show that the DIBs in these intense starbursts are remarkably similar to those in our Galaxy: the relative strengths of the features detected are similar, and the equivalent widths follow the same dependence as Galactic DIBs on E(B − V ) and NaI column density. While the ISM in starbursts is heated by a photon and cosmic ray bath that is ∼ 10 3 times more intense than in the diffuse ISM of the Milky Way, the gas densities and pressures are also correspondingly larger in starbursts. This "homology" may help explain the strikingly similar DIB properties.
Physical conditions of the interstellar medium in star-forming galaxies at z ∼ 1.5
Publications of the Astronomical Society of Japan, 2015
We present results from Subaru/FMOS near-infrared (NIR) spectroscopy of 118 star-forming galaxies at z ∼ 1.5 in the Subaru Deep Field. These galaxies are selected as [O II]λ3727 emitters at z ≈ 1.47 and 1.62 from narrow-band imaging. We detect Hα emission line in 115 galaxies, [O III]λ5007 emission line in 45 galaxies, and Hβ, [N II]λ6584, and [S II]λλ6716,6731 in 13, 16, and 6 galaxies, respectively. Including the [O II] emission line, we use the six strong nebular emission lines in the individual and composite rest-frame optical spectra to investigate physical conditions of the interstellar medium in star-forming galaxies at z ∼1.5. We find a tight correlation between Hα and [O II], which suggests that [O II] can be a good star formation rate (SFR) indicator for galaxies at z ∼ 1.5. The line ratios of Hα/[O II] are consistent with those of local galaxies. We also find that [O II] emitters have strong [O III] emission lines. The [O III]/[O II]
The Astrophysical Journal, 2005
We present new FUSE spectroscopy of the dwarf starburst galaxy NGC 625. These observations probe multiple phases of the interstellar medium, including the coronal, ionized, neutral and molecular gas. This nearby (D = 3.9 ± 0.2 Mpc) system shows a clear detection of outflowing coronal gas as traced by O VI λ 1032 A absorption. The centroid of the O VI profile is blueshifted with respect to the galaxy systemic velocity by ∼ 30 km s −1 , suggesting a low-velocity outflow. The implied O VI velocity extent is found to be 100 ± 20 km s −1 , which is fully consistent with the detected H i outflow velocity found in radio synthesis