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Register a new userThe Influence of Environment on the Star Formation Rates of Galaxies
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We have used a sample of 15749 galaxies taken from the Las Campanas Redshift Survey to investigate the effects of environment on the rate of star formation (SFR) in galaxies. The size and homogeneity of this data set allows us to sample, for the first time, the entire range of galactic environment, from the voids to the clusters, in a uniform manner, thus, we could decouple the local galaxy density from the membership in associations. This decoupling is very crucial for constraining the physical processes responsible for the environmental dependencies of SFR. On the other hand, the use of an automatically-measured concentration index (C), rather than Hubble type, allows us to cleanly separate the morphological component from the SFR vs. environment relationship. We find that cluster galaxies exhibit lower SFR for the same C than field galaxies, while a further division of clusters by `richness reveals a new possible excitation of `starbursts in poor clusters. Meanwhile, a more general environmental investigation reveals that the SFR of a given C shows a continuous correlation with the local density. Interestingly, this trend is also observed both inside and outside of clusters, implying that physical processes responsible for this correlation might not be intrinsic to the cluster environment. On the other hand, galaxies with differing levels of SFR appear to respond differently to the local density. Low levels of SFR are more sensitive to environment inside than outside of clusters. In contrast, high levels of SFR, identified as ``starbursts, are as sensitive to local density in the field as in clusters. We conclude that at least two separate processes are responsible for the environmental sensitivity of the SFR.
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The influence of the environment on gas surface density and star formation efficiency of cluster spiral galaxies is investigated. We extend previous work on radial profiles by a pixel-to pixel analysis looking for asymmetries due to environmental interactions. The star formation rate is derived from GALEX UV and Spitzer total infrared data. As in field galaxies, the star formation rate for most Virgo galaxies is approximately proportional to the molecular gas mass. Except for NGC 4438, the cluster environment does not affect the star formation efficiency with respect to the molecular gas. Gas truncation is not associated with major changes in the total gas surface density distribution of the inner disk of Virgo spiral galaxies. In three galaxies, possible increases in the molecular fraction and the star formation efficiency with respect to the total gas, of factors of 1.5 to 2, are observed on the windward side of the galactic disk. A significant increase of the star formation efficiency with respect to the molecular gas content on the windward side of ram pressure-stripped galaxies is not observed. The ram-pressure stripped extraplanar gas of 3 highly inclined spiral galaxies shows a depressed star formation efficiency with respect to the total gas, and one of them (NGC 4438) shows a depressed rate even with respect to the molecular gas. The interpretation is that stripped gas loses the gravitational confinement and associated pressure of the galactic disk, and the gas flow is diverging, so the gas density decreases and the star formation rate drops. However, the stripped extraplanar gas in one highly inclined galaxy (NGC 4569) shows a normal star formation efficiency with respect to the total gas. We propose this galaxy is different because it is observed long after peak pressure, and its extraplanar gas is now in a converging flow as it resettles back into the disk.
A comparison of star formation properties as a function of environment is made from the spectra of identically selected cluster and field galaxies in the CNOC 1 redshift survey of over 2000 galaxies in the fields of fifteen X-ray luminous clusters at 0.18<z<0.55. The ratio of bulge luminosity to total galaxy luminosity (B/T) is computed for galaxies in this sample, and this measure of morphology is compared with the galaxy star formation rate as determined from the [OII]3727 emission line. The mean star formation rate of cluster galaxies brighter than M_r= -17.5 + 5 log h is found to vary from 0.17 +- 0.02 h^{-2} M_sun/yr at R200 (1.5-2 Mpc/h) to zero in the cluster center, and is always less than the mean star formation rate of field galaxies, which is 0.39 +- 0.01 h^{-2}M_sun/yr. It is demonstrated that this significant difference is not due exclusively to the difference in morphological type, as parameterized by the B/T value, by correcting for the B/T-radius relation. The distribution of [OII] equivalent widths among cluster galaxies is skewed toward lower values relative to the distribution for field galaxies of comparable physical size, B/T and redshift, with a statistical significance of more than 99%. The cluster environment affects not only the morphological mix of the galaxy population, but also suppresses the star formation rate within those galaxies, relative to morphologically similar galaxies in the field.
Galaxy mergers and interactions are mechanisms which could drive the formation of bars. Therefore, we could expect that the fraction of barred galaxies increases with the local density. Here we show the first results of an extensive search for barred galaxies in different environments. We conclude that the bar fraction on bright (L>L*) field, Virgo, and Coma cluster galaxies is compatible. These results point towards an scenario where the formation and/or evolution of bars depend mostly on internal galaxy processes rather than external ones.
We present evolutionary synthesis models of starbursts on top of old stellar populations to investigate in detailed time evolution the relation between Ha luminosity and star formation rate (SFR). The models show that several effects have an impact on the ratio between L(Ha) and SFR. Metallicity different from solar abundance, a time delay between star formation and maximum Ha-luminosity, and a varying stellar initial mass function give rise to strong variations in the ratio of Ha luminosity to SFR and can cause large errors in the determination of the SFR when employing well-known calibrations. When studying star-bursting dwarf galaxies, and sub-galactic fragments at high redshift, which show SFR fluctuating on short timescales, these effects can add up to errors of two orders of magnitude compared with the calibrations. To accurately determine the true current SFR additional data in combination with models for the spectral energy distribution are needed.
We examine the star formation rates (SFRs) of galaxies in a redshift slice encompassing the z=0.834 cluster RX J0152.7-1357. We used a low-dispersion prism in the Inamori Magellan Areal Camera and Spectrograph (IMACS) to identify galaxies with z<23.3 AB mag in diverse environments around the cluster out to projected distances of ~8 Mpc from the cluster center. We utilize a mass-limited sample (M>2x10^{10} M_sun) of 330 galaxies that were imaged by Spitzer MIPS at 24 micron to derive SFRs and study the dependence of specific SFR (SSFR) on stellar mass and environment. We find that the SFR and SSFR show a strong decrease with increasing local density, similar to the relation at z~0. Our result contrasts with other work at z~1 that find the SFR-density trend to reverse for luminosity-limited samples. These other results appear to be driven by star-formation in lower mass systems (M~10^{10} M_sun). Our results imply that the processes that shut down star-formation are present in groups and other dense regions in the field. Our data also suggest that the lower SFRs of galaxies in higher density environments may reflect a change in the ratio of star-forming to non-star-forming galaxies, rather than a change in SFRs. As a consequence, the SFRs of star-forming galaxies, in environments ranging from small groups to clusters, appear to be similar and largely unaffected by the local processes that truncate star-formation at z~0.8.
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