LMC-II

From Tauwiki

1 Survey of OVI in the LMC
2 FUSE data and its analysis
3 Steps in calculating OVI column density and equivalent width
4 Fitting OVI profiles
5 Column Density of O VI
6 Kinematics

[edit] Survey of OVI in the LMC

OVI represents a link between warm gas (few thousands of Kelvin) and hot gas (~10⁶ Kelvin) in the interstellar medium of galaxies. Observations of OVI provides information about hot gas (Savage & Lehner 06). Such hot gas cools rapidly and any emission from OVI is the most efficient cooling agent. OVI, therefore, is a direct tracer of transition layers of gas, i.e., gas at the interface of hot and warm temperatures. Interest in OVI observations is very old but these observations have been redefined by the launch of FUSE telescope. FUSE covers the wavelength range of 1000-1200 Ang. that is most suitable for OVI doublet observations (1032 Ang and 1037 Ang). With S/N of ~ 20,000, FUSE provides the best data for OVI absorption in the local interstellar medium and interstellar medium of external galaxies.

Large Magellanic Cloud (LMC) is a nearby galaxy with a high rate of star formation. LMC observations has little foreground contamination and the inclination angle of LMC is suitable for studying the complete galaxy. To understand better the properties of hot ISM gas in a nearby galaxy, we have studied OVI absorption lines in LMC for 78 different locations.

[edit] FUSE data and its analysis

FUSE satellite has done observations for about a decade. FUSE has obtained about 600 spectra of LMC targets. We checked the suitability of these spectra and downloaded the spectra for 78 different locations in LMC. The latest version of CALFUSE pipeline (ver. -) has been used to reduce the raw FUSE data.

[edit] Steps in calculating OVI column density and equivalent width

OVI data downloaded for 78 different locations of LMC. All the spectra is normalized using IRAF routine "continuum". For normalizing the spectra, "Legendre" polynomials of the order of 5 in most of the cases (few spectra needed higher order polynomial fits because of the presence of lines close to OVI absorption) were fitted to the continuum to obtain the normalized spectra. SPLOT in IRAF was used to view the raw as well as continuum fitted data. Below are the steps to calculate the column density of OVI from the normalized spectra. Note that the spectra contains OVI from the Milky Way and LMC. The Milky Way contribution is visible in most of the spectra but has to be subtracted manually wherever it is not clear.

1. Convert from frequency space to velocity space.

2. Obtain optical depth profiles from the absorption spectrum for each case (example shown below).

3. Integrate the optical depth profiles (the integration has to be done only for LMC velocities so one has to be careful in choosing the velocity limits of integration for cases where LMC OVI profiles are indistinguishable from MW OVI).

4. Calculating the column density. The integrated apparent column density is equivalent to the actual column density if the line is not saturated and optical depth is less than 1. This approach is known as apparent optical depth method (Savage & Sembach 1991).

Specfit has been used for fitting gaussian(s) to OVI lines (1031.9 Ang). Input requires line centre, equivalent width and FWHM (km/s). These parameters have been obtained using "splot" in IRAF. This fit gives the equivalent width of the profile (one has to be careful in deciding the wavelength limits of integration to obtain the equivalent width for LMC OVI).

[edit] Fitting OVI profiles

Fitting gaussians to OVI lines in the spectra of SN 1987A (Z0080103)

Original FUSE spectra

FUSE spectra (normalized) of SN 1987A

Model fitting using SPECFIT (G. Kriss 1994, PASP Conf. Series, Vol. 61, p. 437) in IRAF. The program does the fitting by iterations (Chisquare = 4.217083 for 76 points and 8 freely varying parameters).