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Register a new userThe current star formation rate of K+A galaxies
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Danielle Nielsen
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We derive the stacked 1.4 GHz flux from FIRST (Faint Images of the Radio Sky at Twenty Centimeters) survey for 811 K+A galaxies selected from the SDSS DR7. For these objects we find a mean flux density of $56pm 9$ $mu$Jy. A similar stack of radio-quiet white dwarfs yields an upper limit of 43 $mu$Jy at a 5$sigma$ significance to the flux in blank regions of the sky. This implies an average star formation rate of 1.6 $pm$ 0.3 M$_{odot}$ year$^{-1}$ for K+A galaxies. However the majority of the signal comes from $sim$4% of K+A fields that have aperture fluxes above the $5sigma$ noise level of the FIRST survey. A stack of the remaining galaxies shows little residual flux consistent with an upper limit on star formation of 1.3 M$_{odot}$ year$^{-1}$. Even for a subset of 456 `young (spectral ages $<$ 250 Myr) K+A galaxies we find that the stacked 1.4 GHz flux is consistent with no current star formation. Our data suggest that the original starburst has been terminated in the majority of K+A galaxies, but that this may represent part of a duty cycle where a fraction of these galaxies may be active at a given moment with dusty starbursts and AGNs being present.
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I. Y. Izotova Astronomical Observatory ofn Taras Shevchenko National University of Kyiv
, Kyiv
2018
We use the data for the Hbeta emission-line, far-ultraviolet (FUV) and mid-infrared 22 micron continuum luminosities to estimate star formation rates <SFR> averaged over the galaxy lifetime for a sample of about 14000 bursting compact star-forming galaxies (CSFGs) selected from the Data Release 12 (DR12) of the Sloan Digital Sky Survey (SDSS). The average coefficient linking <SFR> and the star formation rate SFR_0 derived from the Hbeta luminosity at zero starburst age is found to be 0.04. We compare <SFR>s with some commonly used SFRs which are derived adopting a continuous star formation during a period of ~100 Myr, and find that the latter ones are 2-3 times higher. It is shown that the relations between SFRs derived using a geometric mean of two star-formation indicators in the UV and IR ranges and reduced to zero starburst age have considerably lower dispersion compared to those with single star-formation indicators. We suggest that our relations for <SFR> determination are more appropriate for CSFGs because they take into account a proper temporal evolution of their luminosities. On the other hand, we show that commonly used SFR relations can be applied for approximate estimation within a factor of ~2 of the <SFR> averaged over the lifetime of the bursting compact galaxy.
We present the results of an effort to identify and study a sample of the likely progenitors of elliptical (E) and lenticular (S0) K+A galaxies. To achieve this, we have searched a sample ~11,000 nearby (m(r)<16) early-type galaxies selected by morphology from the Sloan Digital Sky Survey (SDSS) Main spectroscopic sample for actively star-forming E and S0 galaxies. Using emission line ratios and visual inspection of SDSS g-band images, we have identified 335 galaxies from the SDSS Fourth Data Release (DR4) as actively star-forming E and S0 galaxies. These galaxies make up about 3% of the total early-type sample and less than 1% of all Main galaxies with m(r)<16. We also identified a sample of ~400 K+A galaxies from DR4 with m(r)<16; more than half of these are E and S0 galaxies. We find that star-forming early-type galaxies and K+A galaxies have similar mass distributions; they are on average less massive than typical early-type galaxies but more massive than the average star-forming galaxy. Both of these types of galaxies are found in higher fractions among all galaxies in lower density environments. The fractions of star-forming E and S0 galaxies and E and S0 K+A galaxies depend on environment in nearly the same way. Model spectra fit to the stellar continua of the star-forming E and S0 galaxies showed that their properties are consistent with star formation episodes of <1 Gyr in duration. The modelling results imply that on average, the star formation episodes will increase the stellar masses by about 4%. There is also evidence that the star-forming regions within these galaxies are rotationally supported.
We use the James Clerk Maxwell Telescopes SCUBA-2 camera to image a 400 arcmin^2 area surrounding the GOODS-N field. The 850 micron rms noise ranges from a value of 0.49 mJy in the central region to 3.5 mJy at the outside edge. From these data, we construct an 850 micron source catalog to 2 mJy containing 49 sources detected above the 4-sigma level. We use an ultradeep (11.5 uJy at 5-sigma) 1.4 GHz image obtained with the Karl G. Jansky Very Large Array together with observations made with the Submillimeter Array to identify counterparts to the submillimeter galaxies. For most cases of multiple radio counterparts, we can identify the correct counterpart from new and existing Submillimeter Array data. We have spectroscopic redshifts for 62% of the radio sources in the 9 arcmin radius highest sensitivity region (556/894) and 67% of the radio sources in the GOODS-N region (367/543). We supplement these with a modest number of additional photometric redshifts in the GOODS-N region (30). We measure millimetric redshifts from the radio to submillimeter flux ratios for the unidentified submillimeter sample, assuming an Arp 220 spectral energy distribution. We find a radio flux dependent K-z relation for the radio sources, which we use to estimate redshifts for the remaining radio sources. We determine the star formation rates (SFRs) of the submillimeter sources based on their radio powers and their submillimeter and find that they agree well. The radio data are deep enough to detect star-forming galaxies with SFRs >2000 solar masses per year to z~6. We find galaxies with SFRs up to ~6,000 solar masses per year over the redshift range z=1.5-6, but we see evidence for a turn-down in the SFR distribution function above 2000 solar masses per year.
We explore how the estimated star formation rate (SFR) of a sample of isolated, massive dusty star-forming galaxies at early cosmic epochs ($1.5 < z < 3.5$) changes when their ultraviolet (UV) to near-infrared (NIR) spectral energy distribution is extended to longer wavelengths by adding far-infrared/sub-millimeter data to trace the reprocessed radiation from dust heated by young massive stars. We use large-area surveys with multi-wavelength datasets that include DECam UV-to-optical, VICS82 NIR, Spitzer-IRAC NIR, and Herschel-SPIRE far-infrared/sub-millimeter data. We find that the inclusion of far-infrared/sub-millimeter data leads to SFRs that span $sim$100-3500 $M_{odot} yr^{-1}$ and are higher than the extinction-corrected UV-based SFR by an average factor of $sim$3.5, and by a factor of over 10 in many individual galaxies. Our study demonstrates the importance of far-IR/sub-millimeter data for deriving accurate SFRs in massive dusty galaxies at early epochs, and underscores the need for next-generation far-IR/sub-millimeter facilities with high sensitivity, field of view, and angular resolution.
We study the spatially resolved Radio Continuum-Star Formation Rate (RC-SFR) relation using state-of-the-art star-formation (SF) tracers in a sample of 17 THINGS galaxies. We use hybrid Sigma_SFR maps (GALEX FUV plus Spitzer 24 mu), RC maps at 22/18 cm from the WSRT SINGS survey, and H-alpha maps to correct for thermal RC emission. We compare azimuthally averaged radial profiles of the RC and FUV/MIR-based Sigma_SFR maps and study pixel-by-pixel correlations at fixed linear scales of 1.2 and 0.7 kpc. The ratio of the integrated SFRs from the RC emission to that of the FUV/MIR-based SF tracers is R_int = 0.78 +/- 0.38, consistent with Condons relation. We find a tight correlation between the radial profiles of the radio and FUV/MIR-based Sigma_SFR for the entire extent of the disk. The ratio R of the azimuthally averaged radio to FUV/MIR-based Sigma_SFR agrees with the integrated ratio with only small quasi-random fluctuations as function of radius. Pixel-by-pixel plots show a tight correlation in log-log diagrams of radio to FUV/MIR-based Sigma_SFR, with a typical standard deviation of a factor of two. Averaged over our sample we find (Sigma_SFR)_RC ~ (Sigma_SFR)_hyb^{0.63+/-0.25} implying that data points with high Sigma_SFR are relatively radio dim, whereas the reverse is true for low Sigma_SFR. We interpret this as a result of spectral ageing of CRe, which is supported by the radio spectral index: data points dominated by young CRe are relatively radio dim, those dominated by old CRe are relatively radio bright. The ratio of radio to FUV/MIR-based integrated SFR is independent of global galaxy parameters, suggesting that we can use RC emission as a universal SF tracer for galaxies, if we restrict ourselves to global or azimuthally averaged measurements. A magnetic field-SFR relation, B ~ SFR_hyb^{0.30+/-0.02}, holding both globally and locally, can explain our results. (abridged)
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