Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Opacity of Spiral Galaxies from Counts of Distant Background Galaxies
228
0
0.0
(
0
)
Authors
B. W. Holwerda
Ask ChatGPT about the research
No Arabic abstract
We have applied the Synthetic Field Method on a sample of ~20 nearby galaxies in order to determine the opacity of their disks. We present preliminary results on the radial dependence of cold dust absorption for 3 examples. The spirals NGC4535 and NGC4725 show significant absorption at a half-light radius. UGC2302, a LSB galaxy, shows much less opacity.
rate research
Read More
Dust emission in the far-infrared (FIR) characterizes the temperature and quantity of interstellar dust in a spiral disk. The three Spitzer/MIPS bands are well suited to measuring the gradient in temperature and the total optical depth in the disk of a spiral galaxy. Another way to estimate the quantity of dust in a spiral disk is the Synthetic Field Method (SFM, Gonzalez et al. 1998), which uses the number of distant field galaxies seen through the disk of the nearby spiral. The optical depth estimated from this method can be compared to the values derived from the FIR emission. Since the two techniques depend on different assumptions regarding the dust geometry and emissivity, this comparison between the optical depth profiles can potentially shed light on the structure and quantity of the ISM in spiral disks, especially any colder components. The dust responsible for the opacity from distant galaxy counts appears to be predominantly cold (T < 20 K.). The differences between the radial absorption profiles can be explained by spiral arms in the SFM measurements. Taken over the same aperture, galaxy counts show higher extinction values than the FIR derived ones. The implications for dust geometry can hopefully be explored with a more rigorous estimate of dust mass from the FIR fluxes.
Dust extinction can be determined from the number of distant field galaxies seen through a spiral disk. To calibrate this number for the crowding and confusion introduced by the foreground image, Gonzalez et al.(1998) and Holwerda et al. (2005) developed the ``Synthetic Field Method (SFM), which analyses synthetic fields constructed by adding various deep exposures of unobstructed background fields to the candidate foreground galaxy field. The advantage of the SFM is that it gives the average opacity for area of galaxy disk without assumptions about either the distribution of absorbers or of the disk starlight. However it is limited by low statistics of the surviving field galaxies, hence the need to combine a larger sample of fields. This paper presents the first results for a sample of 32 deep HST/WFPC2 archival fields of 29 spirals. The radial profiles of average dust extinction in spiral galaxies based on calibrated counts of distant field galaxies is presented here, both for individual galaxies as well as for composites from our sample. The effects of inclination, spiral arms and Hubble type on the radial extinction profile are discussed. (Abbreviated)
We present first results from our project to examine the internal kinematics of disk galaxies in 7 rich clusters with 0.3<=z<0.6. Spatially resolved MOS spectra have been obtained with FORS at the VLT. We concentrate here on the clusters MS1008.1-1224 at z=0.30 and Cl0413-6559 (F1557.19TC) at z=0.51. Out of 22 cluster members, 12 galaxies exhibit a rotation curve of the universal form rising in the inner region and passing over into a flat part. The other members have intrinsically peculiar kinematics. The 12 cluster galaxies for which a maximum rotation velocity could be derived are distributed in the Tully-Fisher diagram very similar to field galaxies from the FORS Deep Field with corresponding redshifts. The same is true for 6 galaxies observed in the cluster fields that turned out not to be members. In particular, these cluster spirals do not show any significant luminosity evolution as might be expected from certain clusterspecific phenomena. Contrary to that, the other half of the cluster sample with disturbed kinematics also shows a higher degree of structural assymetries on average indicating ongoing or recent interaction processes.
(abridged) A detailed comparison is performed of the LFs compiled at infrared, radio and optical wavelengths and converted into XLFs using available relations with the XLF directly estimated in the 0.5--2 keV energy band from X-ray surveys (Norman et al). We find that the XLF from the local sample of IRAS galaxies (Takeuchi et al) provides a good representation of all available data samples; pure luminosity evolution of the form (1+z)^eta, with eta< ~3, is favoured over pure density. The local X-ray luminosity density is also well defined. We discuss different estimates of the galaxies LogN-LogS, selected from the Chandra Deep Fields with different selection criteria: these have similar slopes, but normalisations scattered within a factor ~2, of the same order of the Poissonian error on the counts. We compare the observed LogN-LogS with the counts predicted by integrating our reference z=0 XLF. By using number counts alone, it is not possible to discriminate between density and luminosity evolution; however, the evolution of galaxies must be stopped in both cases at z~1-2. The contribution of galaxies to the X-ray background is found to be in the range 6%--12%. Making use of cosmic star formation models, we find that the X-ray LogN-LogS might be not compatible with very large star formation rates at z ~ 3 as suggested by sub-mm observations in Blain et al. 1999. As to the content of current and, possibly, future X-ray surveys, we determine the fraction of galaxies around the current flux limit: (30+-12 %). At fainter fluxes the fraction of galaxies will probably rise, and overcome the counts from AGN at fluxes < ~10^{-17} erg/s/cm^2.
We introduce our project on galaxy evolution in the environment of rich clusters aiming at disentangling the importance of specific interaction and galaxy transformation processes from the hierarchical evolution of galaxies in the field. Emphasis is laid on the examination of the internal kinematics of disk galaxies through spatially resolved MOS spectroscopy with FORS at the VLT. First results are presented for the clusters MS1008.1-1224 (z=0.30), Cl0303+1706 (z=0.42), and Cl0413-6559 (F1557.19TC) (z=0.51). Out of 30 cluster members with emission-lines, 13 galaxies exhibit a rotation curve of the universal form rising in the inner region and passing over into a flat part. The other members have either intrinsically peculiar kinematics (4), or too strong geometric distortions (9) or too low S/N (4 galaxies) for a reliable classification of their velocity profiles. The 13 cluster galaxies for which a maximum rotation velocity could be derived are distributed in the Tully--Fisher diagram very similar to field galaxies from the FORS Deep Field that have corresponding redshifts and do not show any significant luminosity evolution with respect to local samples. The same is true for seven galaxies observed in the cluster fields that turned out not to be members. The mass-to-light ratios of the 13 TF cluster spirals cover the same range as the distant field population indicating that their stellar populations were not dramatically changed by possible clusterspecific interaction phenomena. The cluster members with distorted kinematics may be subject to interaction processes but it is impossible to determine whether these processes also lead to changes in the overall luminosity of their stellar populations.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
