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Register a new userSpectroscopy and Stellar Populations of Star-forming Galaxies at z~3 in the Hubble Deep Field - South
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We present results of VLT/FORS2 spectroscopy of galaxies at z~3 in the Hubble Deep Field-South (HDF-S). A sample of galaxies was drawn from the photo-z catalogue based on the HST/WFPC2 optical images and the deep near-infrared images obtained with VLT/ISAAC as a part of the FIRES project. We selected galaxies with photometric redshift between 2.5 and 4. Most of the selected galaxies are bright in rest-frame UV wavelengths and satisfy color selection criteria of Lyman break galaxies (LBGs) at z~3. The number of target galaxies with I(AB)<25.0 was 15. We identified new 5 firm and 2 probable redshifts in addition to confirmations of previously known 6 galaxies at z~3. We found 6 among these 13 galaxies lie at a quite narrow redshift range at z = 2.80+-0.01. We examined stellar populations of the galaxies with spectroscopic redshifts through comparisons of their optical and near-IR photometry data with template spectra generated by a population synthesis code. The ages from the onset of star formation for these star-forming galaxies with I<=25.0 are typically 50-200 Myr, and their stellar masses are between (0.5-5) x 10^10 M_sun, consistent with previous studies. We also compared these SED fitting results with those for distant red galaxies (DRGs) at z>2 discovered by FIRES. DRGs have larger stellar masses, larger dust attenuation than our UV-luminous LBG sample, and their star formation rates are often comparable to LBGs. These trends suggest that majority of DRGs are indeed the most massive systems at the redshift and are still in the active star-forming phase. Unless the number density of DRGs is much smaller than LBGs, estimates based on UV selected sample could miss substantial part of stellar mass density at z~3.
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We report the discovery of a luminous z=5.78 star-forming galaxy in the Chandra Deep Field South. This galaxy was selected as an `i-drop from the GOODS public survey imaging with HST/ACS (object 3 in Stanway, Bunker & McMahon 2003, astro-ph/0302212). The large colour of (i-z)_AB=1.6 indicated a spectral break consistent with the Lyman-alpha forest absorption short-ward of Lyman-alpha at z~6. The galaxy is very compact (marginally resolved with ACS with a half-light radius of 0.08arcsec, so r_hl<0.5kpc/h_70). We have obtained a deep (5.5-hour) spectrum of this z(AB)=24.7 galaxy with the DEIMOS optical spectrograph on Keck, and here we report the discovery of a single emission line centred on 8245Ang detected at 20sigma with a flux of f~2E-17 ergs/s/cm^2. The line is clearly resolved with detectable structure at our resolution of better than 55km/s, and the only plausible interpretation consistent with the ACS photometry is that we are seeing Lyman-alpha emission from a z=5.78 galaxy. This is the highest redshift galaxy to be discovered and studied using HST data. The velocity width Delta(v)_FWHM=260km/s and rest-frame equivalent width (W=20Ang) indicate that this line is most probably powered by star formation, as an AGN would typically have larger values. The starburst interpretation is supported by our non-detection of the high-ionization NV1240Ang emission line, and the absence of this source from the deep Chandra X-ray images. The star formation rate inferred from the rest-frame UV continuum is 34M_sun/yr/h^2_70 (Omega_M=0.3, Omega_Lambda=0.7). This is the most luminous starburst known at z>5. Our spectroscopic redshift for this object confirms the validity of the i-drop technique of Stanway, Bunker & McMahon (2003) to select star-forming galaxies at z~6.
We present the results of a new study of the relationship between infrared excess (IRX), UV spectral slope (beta) and stellar mass at redshifts 2<z<3, based on a deep Atacama Large Millimeter Array (ALMA) 1.3-mm continuum mosaic of the Hubble Ultra Deep Field (HUDF). Excluding the most heavily-obscured sources, we use a stacking analysis to show that z~2.5 star-forming galaxies in the mass range 9.25 <= log(M/Msun) <= 10.75 are fully consistent with the IRX-beta relation expected for a relatively grey attenuation curve, similar to the commonly adopted Calzetti law. Based on a large, mass complete, sample of 2 <= z <= 3 star-forming galaxies drawn from multiple surveys, we proceed to derive a new empirical relationship between beta and stellar mass, making it possible to predict UV attenuation (A_1600) and IRX as a function of stellar mass, for any assumed attenuation law. Once again, we find that z~2.5 star-forming galaxies follow A_1600-mass and IRX-mass relations consistent with a relatively grey attenuation law, and find no compelling evidence that star-forming galaxies at this epoch follow a reddening law as steep as the Small Magellanic Cloud (SMC) extinction curve. In fact, we use a simple simulation to demonstrate that previous determinations of the IRX-beta relation may have been biased toward low values of IRX at red values of beta, mimicking the signature expected for an SMC-like dust law. We show that this provides a plausible mechanism for reconciling apparently contradictory results in the literature and that, based on typical measurement uncertainties, stellar mass provides a cleaner prediction of UV attenuation than beta. Although the situation at lower stellar masses remains uncertain, we conclude that for 2<z<3 star-forming galaxies with log(M/Msun) >= 9.75, both the IRX-beta and IRX-mass relations are well described by a Calzetti-like attenuation law.
We present deep 3.6 - 8 micron imaging of the Hubble Deep Field South with IRAC on the Spitzer Space Telescope. We study Distant Red Galaxies (DRGs) at z>2 selected by Js - Ks > 2.3 and compare them to a sample of Lyman Break Galaxies (LBGs) at z=2-3. The observed UV-to-8 micron spectral energy distributions are fit with stellar population models to constrain star formation histories and derive stellar masses. We find that 70% of the DRGs are best described by dust-reddened star forming models and 30% are very well fit with old and ``dead models. Using only the I - Ks and Ks - 4.5 micron colors we can effectively separate the two groups. The dead systems are among the most massive at z~2.5 (mean stellar mass <M*> = 0.8 x 10^11 Msun) and likely formed most of their stellar mass at z>5. To a limit of 0.5 x 10^11 Msun their number density is ~10 x lower than that of local early-type galaxies. Furthermore, we use the IRAC photometry to derive rest-frame near-infrared J, H, and K fluxes. The DRGs and LBGs together show a large variation (a factor of 6) in the rest-frame K-band mass-to-light ratios (M/L_K), implying that even a Spitzer 8 micron-selected sample would be very different from a mass-selected sample. The average M/L_K of the DRGs is about three times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/L_K ratios and ages of the two samples appear to correlate with derived stellar mass, with the most massive galaxies being the oldest and having the highest mass-to-light ratios, similar as found in the low-redshift universe.
Several UV and near-infrared color selection methods have identified galaxies at z = 1-3. Since each method suffers from selection biases, we have applied three leading techniques (Lyman break, BX/BM, and BzK selection) simultaneously in the Subaru Deep Field. This field has reliable ({Delta}z/(1 + z) = 0.02--0.09) photometric redshifts for ~53,000 galaxies from 20 bands (1500{AA}--2.2{mu}m). The BzK, LBG, and BX/BM samples suffer contamination from z<1 interlopers of 6%, 8%, and 20%, respectively. Around the redshifts where it is most sensitive (z~1.9 for star-forming BzK, z~1.8 for z~2 LBGs, z~1.6 for BM, and z~2.3 for BX), each technique finds 60-80% of the census of the three methods. In addition, each of the color techniques shares 75-96% of its galaxies with another method, which is consistent with previous studies that adopt identical criteria on magnitudes and colors. Combining the three samples gives a comprehensive census that includes ~90% of z-phot = 1-3 galaxies, using standard magnitude limits similar to previous studies. In fact, we find that among z = 1-2.5 galaxies in the color selection census, 81-90% of them can be selected by just combining the BzK selection with one of the UV techniques (z~2 LBG or BX and BM). The average galaxy stellar mass, reddening and SFRs all decrease systematically from the sBzK population to the LBGs, and to the BX/BMs. The combined color selections yield a total cosmic SFR density of 0.18 $pm$ 0.03 M_sun yr^{-1} Mpc^{-3} for K_AB <= 24. We find that 65% of the star formation is in galaxies with E(B-V) > 0.25 mag, even though they are only one-fourth of the census by number.
We present an analysis of the X-ray emission from a large sample of ultraviolet (UV) selected, star forming galaxies with 0.74<z<1.32 in the Hubble Deep Field North (HDF-N) region. By excluding all sources with significant detected X-ray emission in the 2 Ms Chandra observation we are able to examine the properties of galaxies for which the emission in both UV and X-ray is expected to be predominantly due to star formation. Stacking the X-ray flux from 216 galaxies in the soft and hard bands produces significant detections. The derived mean 2-10 keV rest-frame luminosity is 2.97+/-0.26x10^(40) erg/s, corresponding to an X-ray derived star formation rate (SFR) of 6.0+/-0.6 Msolar/yr. Comparing the X-ray value with the mean UV derived SFR, uncorrected for attenuation, we find that the average UV attenuation correction factor is ~3. By binning the galaxy sample according to UV magnitude and colour, correlations between UV and X-ray emission are also examined. We find a strong positive correlation between X-ray emission and rest-frame UV emission. A correlation between the ratio of X-ray-to-UV emission and UV colour is also seen, such that L(X)/L(UV) increases for redder galaxies. Given that X-ray emission offers a view of star formation regions that is relatively unaffected by extinction, results such as these can be used to evaluate the effects of dust on the UV emission from high-z galaxies. For instance we derive a relationship for estimating UV attenuation corrections as a function of colour excess. The observed relation is inconsistent with the Calzetti et al. (2000) reddening law which over predicts the range in UV attenuation corrections by a factor of ~100 for the UV selected z~1 galaxies in this sample (abridged).
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