arXiv subjects

B. Mobasher

Publications and source records attributed to B. Mobasher.

148 records · Page 9Linked to original sources

arXiv1998

Star Formation Rates in Faint Radio Galaxies

The decimetric radio continuum luminosity of a star-forming galaxy appears to be directly proportional to the rate of formation of supernovae in the galaxy. Since decimetric radiation does not suffer significant extinction and is not directive, radio luminosities may thus provide a particularly straightforward way to determine the current rate of star formation. Using a sample of over 700 local galaxies we confirm the utility of the radio luminosity as a measure of star formation rate by showing concordance with the rates predicted by U-band, H-alpha, and far-infrared luminosites. We also show that there are systematic discrepancies between these various indicators, suggesting that the H-alpha luminosity may underestimate the star formation rate by approximately an order of magnitude when the star formation rate is more than 20 solar mass per year. We use this calibration and the measured radio luminosities of sub-mJy radio sources to infer the star formation rate in approximately 60 star-forming galaxies at moderate (z = 0.1) redshifts, both as the actual rate and as the fraction of the existing mass of stars in the galaxy. For some of these objects the inferred current rate of star formation could increase the stellar mass in the galaxy by approximately 10% over an interval of about 30 Myr.

astro-ph↗

arXiv1998

A blind test of photometric redshift prediction

Results of a blind test of photometric redshift predictions against spectroscopic galaxy redshifts obtained in the Hubble Deep Field with the Keck Telescope are presented. The best photometric redshift schemes predict spectroscopic redshifts with a redshift accuracy of |Delta-z|<0.1 for more than 68 percent of sources and with |Delta-z|<0.3 for 100 percent, when single-feature spectroscopic redshifts are removed from consideration. This test shows that photometric redshift schemes work well at least when the photometric data are of high quality and when the sources are at moderate redshifts.

astro-ph↗

arXiv1998

The Near-Infrared Number Counts and Luminosity Functions of Local Galaxies

This study presents a wide-field near-infrared (K-band) survey in two fields; SA 68 and Lynx 2. The survey covers an area of 0.6 deg.$^2$, complete to K=16.5. A total of 867 galaxies are detected in this survey of which 175 have available redshifts. The near-infrared number counts to K=16.5 mag. are estimated from the complete photometric survey and are found to be in close agreement with other available studies. The sample is corrected for incompleteness in redshift space, using selection function in the form of a Fermi-Dirac distribution. This is then used to estimate the local near-infrared luminosity function of galaxies. A Schechter fit to the infrared data gives: M$^\ast_K = -25.1 \pm 0.3$, $α= -1.3\pm 0.2$ and $ϕ^\ast =(1.5\pm 0.5)\times 10^{-3}$ Mpc$^{-3}$ (for H$_0=50$ Km/sec/Mpc and q$_0=0.5$). When reduced to $α=-1$, this agrees with other available estimates of the local IRLF. We find a steeper slope for the faint-end of the infrared luminosity function when compared to previous studies. This is interpreted as due to the presence of a population of faint but evolved (metal rich) galaxies in the local Universe. However, it is not from the same population as the faint blue galaxies found in the optical surveys. The characteristic magnitude ($M^\ast_K$) of the local IRLF indicates that the bright red galaxies ($M_K\sim -27$ mag.) have a space density of $\le 5\times 10^{-5}$ Mpc$^{-3}$ and hence, are not likely to be local objects.

astro-ph↗

arXiv1996

The nature of the faint galaxies in the Hubble Deep Field

We present a study of the galaxies found in the Hubble Deep Field. A high proportion of HDF galaxies are undergoing a strong episode of star formation, as evidenced by their very blue colours. A wide range of morphological types is found, with a high proportion of peculiar and merger morphologies. Fitting the multiband spectra with redshifted SEDs of galaxy types E to HII, we predict the spectral types and redshifts of galaxies detected in the HDF. We find a median redshift of 1.6, with $68\%$ having $z > 1$ and $31\%$ with $z >2$. The I-band absolute magnitude distributions as a function of galaxy types show a plausible trend of decreasing luminosity towards later types. The derived I-band luminosity function agrees well with that from the Canada-France survey (Lilly et al 1996) for $z < 1$ and shows strong luminosity evolution at $M_I < -21$ for $1 < z < 3$, comparable to the rate seen in quasars and starburst galaxies. We have predicted infrared and submillimetre fluxes assuming most of the galaxies are undergoing a strong starburst. Several planned space-borne and ground-based deep surveys are capable of detecting interesting numbers of HDF galaxies.

astro-ph↗