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Register a new userTadpole Galaxies in the Hubble Ultra Deep Field
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Authors
Bruce G. Elmegreen
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Tadpole galaxies have a head-tail shape with a large clump of star formation at the head and a diffuse tail or streak of stars off to one side. We measured the head and tail masses, ages, surface brightnesses, and sizes for 66 tadpoles in the Hubble Ultra Deep Field (UDF), and we looked at the distribution of neighbor densities and tadpole orientations with respect to neighbors. The heads have masses of 10^7-10^8 Msun and photometric ages of ~0.1 Gyr for z~2. The tails have slightly larger masses than the heads, and comparable or slightly older ages. The most obvious interpretation of tadpoles as young merger remnants is difficult to verify. They have no enhanced proximity to other resolved galaxies as a class, and the heads, typically less than 0.2 kpc in diameter, usually have no obvious double-core structure. Another possibility is ram pressure interaction between a gas-rich galaxy and a diffuse cosmological flow. Ram pressure can trigger star formation on one side of a galaxy disk, giving the tadpole shape when viewed edge-on. Ram pressure can also strip away gas from a galaxy and put it into a tail, which then forms new stars and gravitationally drags along old stars with it. Such an effect might have been observed already in the Virgo cluster. Another possibility is that tadpoles are edge-on disks with large, off-center clumps. Analogous lop-sided star formation in UDF clump clusters are shown.
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We take advantage of the Hubble Ultra Deep Field (UDF) data to study the restframe optical and ultra violet (UV) morphologies of the novel population of Distant Red Galaxies (DRGs). Six galaxies with J-Ks > 2.3 are found to Ks=21.5, five of which have photometric redshifts z_phot > 2, corresponding to a surface density of 0.9/arcmin^2. The surface brightness distributions of the z_phot > 2 galaxies are better represented by exponential disks than R^{1/4}-laws. Two of the z_phot > 2 galaxies are extended, while three have compact morphologies. The restframe optical morphology of the z_phot > 2 galaxies is quite different from the restframe UV morphology: all the galaxies have red central components which dominate in the NICMOS H_{160}-band images, and distinct off-center blue features which show up in (and often dominate) the ACS images. The mean measured effective radius of the z_phot > 2 galaxies is <r_e> =1.9+/-1.4 kpc, similar (within the errors) to the mean size of LBGs at similar redshifts. All the DRGs are resolved in the ACS images, while four are resolved in the NICMOS images. Two of the z_phot > 2 galaxies are bright X-ray sources and hence host AGN. The diverse restframe optical and UV morphological properties of DRGs derived here suggest that they have complex stellar populations, consisting of both evolved populations that dominate the mass and the restframe optical light, and younger populations, which show up as patches of star formation in the restframe UV light; in many ways resembling the properties of normal local galaxies. This interpretation is supported by fits to the broadband SEDs, which for all five z_phot > 2 are best represented by models with extended star formation histories and substantial amounts of dust.
We present the 2012 Hubble Ultra Deep Field campaign (UDF12), a large 128-orbit Cycle 19 HST program aimed at extending previous WFC3/IR observations of the UDF by quadrupling the exposure time in the F105W filter, imaging in an additional F140W filter, and extending the F160W exposure time by 50%. The principal scientific goal of this project is to determine whether galaxies reionized the universe; our observations are designed to provide a robust determination of the star formation density at $z$$,gtrsim,$8, improve measurements of the ultraviolet continuum slope at $z$$,sim,7,-,$8, facilitate the construction of new samples of $z$$,sim,9,-,$10 candidates, and enable the detection of sources up to $z$$,sim,$12. For this project we committed to combining these and other WFC3/IR imaging observations of the UDF area into a single homogeneous dataset, to provide the deepest near-infrared observations of the sky currently achievable. In this paper we present the observational overview of the project, motivated by its scientific goals, and describe the procedures used in reducing the data as well as the final products that are produced. We have used the most up up-to-date methods for calibrating and combining the images, in particular paying attention to correcting several instrumental effects. We release the full combined mosaics, comprising a single, unified set of mosaics of the UDF, providing the deepest near-infrared blank-field view of the universe obtained to date, reaching magnitudes as deep as AB$,sim,$30 in the near-infrared, and yielding a legacy dataset on this field of lasting scientific value to the community.
Tadpole galaxies have a giant star-forming region at the end of an elongated intensity distribution. Here we use SDSS data to determine the ages, masses, and surface densities of the heads and tails in 14 local tadpoles selected from the Kiso and Michigan surveys of UV-bright galaxies, and we compare them to tadpoles previously studied in the Hubble Ultra Deep Field. The young stellar mass in the head scales linearly with restframe galaxy luminosity, ranging from ~10^5 M_solar at galaxy absolute magnitude U=-13 mag to 10^9 M_solar at U=-20 mag. The corresponding head surface density increases from several M_solar pc^{-2} locally to 10-100 M_solar pc^{-2} at high redshift, and the star formation rate per unit area in the head increases from ~0.01 M_solar yr^{-1} kpc^{-2} locally to ~1 M_solar yr^{-1} kpc^{-2} at high z. These local values are normal for star-forming regions, and the increases with redshift are consistent with other cosmological star formation rates, most likely reflecting an increase in gas abundance. The tails in the local sample look like bulge-free galaxy disks. Their photometric ages decrease from several Gyr to several hundred Myr with increasing z, and their surface densities are more constant than the surface densities of the heads. The far outer intensity profiles in the local sample are symmetric and exponential. We suggest that most local tadpoles are bulge-free galaxy disks with lopsided star formation, perhaps from environmental effects such as ram pressure or disk impacts, or from a Jeans length comparable to half the disk size.
We have searched for star-forming galaxies at z~7 by applying the Lyman-break technique to newly-released 1.1micron Y-band images from WFC3 on HST. By comparing these images of the Hubble Ultra Deep Field with the ACS z-band (0.85micron), we identify objects with red colours, (z-Y)_AB>1.3), consistent with the Ly-alpha forest absorption at z~6.7-8.8. We identify 12 of these z-drops down to a limiting magnitude Y_AB<28.5 (equivalent to a star formation rate of 1.3M_sun/yr at z=7.1), which are undetected in the other ACS filters. We use the WFC3 J-band image to eliminate contaminant low mass Galactic stars, which typically have redder colours than z~7 galaxies. One of our z-drops is a probably a T-dwarf star. The z~7 z-drops have much bluer spectral slopes than Lyman-break galaxies at lower redshift. Our brightest z-drop is not present in the NICMOS J-band image of the same field taken 5 years before, and is a possible transient object. From the 10 remaining z~7 candidates we determine a lower limit on the star formation rate density of 0.0017M_sun/yr/Mpc^3 for a Salpeter initial mass function, which rises to 0.0025-0.0034M_sun/yr/Mpc^3 after correction for luminosity bias. The star formation rate density is a factor of ~10 less than that at z=3-4, and is about half the value at z~6. While based on a single deep field, our results suggest that this star formation rate density would produce insufficient Lyman continuum photons to reionize the Universe unless the escape fraction of these photons is extremely high (f_esc>0.5), and the clumping factor of the Universe is low. Even then, we need to invoke a large contribution from galaxies below our detection limit. The apparent shortfall in ionizing photons might be alleviated if stellar populations at high redshift are low metallicity or have a top-heavy IMF.
Non-resonant FeII* 2365, 2396, 2612, 2626 emission can potentially trace galactic winds in emission and provide useful constraints to wind models. From the 3x3 mosaic of the Hubble Ultra Deep Field (UDF) obtained with the VLT/MUSE integral field spectrograph, we identify a statistical sample of 40 FeII* emitters and 50 MgII 2796, 2803 emitters from a sample of 271 [OII] 3726, 3729 emitters with reliable redshifts from z = 0.85 - 1.5 down to 2E-18 (3 sigma) ergs/s/cm^2 (for [OII]), covering the stellar mass range 10^8 - 10^11 Msun. The FeII* and MgII emitters follow the galaxy main sequence, but with a clear dichotomy. Galaxies with masses below 10^9 Msun and star formation rates (SFRs) of <1 Msun/year have MgII emission without accompanying FeII* emission, whereas galaxies with masses above 10^10 Msun and SFRs >10 Msun/year have FeII* emission without accompanying MgII emission. Between these two regimes, galaxies have both MgII and FeII* emission, typically with MgII P-Cygni profiles. Indeed, the MgII profile shows a progression along the main sequence from pure emission to P-Cygni profiles to strong absorption, due to resonant trapping. Combining the deep MUSE data with HST ancillary information, we find that galaxies with pure MgII emission profiles have lower star formation rate surface densities than those with either MgII P-Cygni profiles or FeII* emission. These spectral signatures produced through continuum scattering and fluorescence, MgII P-Cygni profiles and FeII* emission, are better candidates for tracing galactic outflows than pure MgII emission, which may originate from HII regions. We compare the absorption and emission rest-frame equivalent widths for pairs of FeII transitions to predictions from outflow models and find that the observations consistently have less total re-emission than absorption, suggesting either dust extinction or non-isotropic outflow geometries.
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