Alan F. M. Moorwood

A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way

Many galaxies are thought to have supermassive black holes at their centres—more than a million times the mass of the Sun. Measurements of stellar velocities and the discovery of variable X-ray emission have provided strong evidence in favour of such a black hole at the centre of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass. Here we report ten years of high-resolution astrometric imaging that allows us to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericentre and apocentre passages, show that the star is on a bound, highly elliptical keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a pericentre distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 ± 1.5) × 106 solar masses (M[circdot]). The data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions.

The size evolution of galaxies since z~3: combining SDSS, GEMS, and FIRES

We present the evolution of the luminosity-size and stellar mass-size relations of luminous ( L-V greater than or similar to 3.4 x 10(10) h(70)(-2) L-circle dot) and massive ( M-* greater than or similar to 3 x 10(10) h(70)(-2) M-circle dot) galaxies in the last similar to 11 Gyr. We use very deep near-infrared images of the Hubble Deep Field-South and the MS 1054-03 field in the J(s), H, and K-s bands from FIRES to retrieve the sizes in the optical rest frame for galaxies with z > 1. We combine our results with those from GEMS at 0.2 < z < 1 and SDSS at z similar to 0.1 to achieve a comprehensive picture of the optical rest-frame size evolution from z = 0 to 3. Galaxies are differentiated according to their light concentration using the Sersic index n. For less concentrated objects, the galaxies at a given luminosity were typically similar to 3 +/- 0.5 ( +/- 2 sigma) times smaller at z similar to 2: 5 than those we see today. The stellar mass-size relation has evolved less: the mean size at a given stellar mass was similar to 2 +/- 0.5 times smaller at z similar to 2.5, evolving proportionally to ( 1 + z) - 0.40 +/- 0.06. Simple scaling relations between dark matter halos and baryons in a hierarchical cosmogony predict a stronger ( although consistent within the error bars) than observed evolution of the stellar mass-size relation. The observed luminosity-size evolution out to z similar to 2.5 matches well recent infall model predictions for Milky Way-type objects. For low-n galaxies, the evolution of the stellar mass-size relation would follow naturally if the individual galaxies grow inside out. For highly concentrated objects, the situation is as follows: at a given luminosity, these galaxies were similar to 2.7 +/- 1.1 times smaller at z similar to 2.5 ( or, put differently, were typically similar to 2.2 +/- 0.7 mag brighter at a given size than they are today), and at a given stellar mass the size has evolved proportionally to ( 1 + z)(-0.45 +/- 0.10).

THE REST-FRAME OPTICAL LUMINOSITY DENSITY, COLOR, AND STELLAR MASS DENSITY OF THE UNIVERSE FROM z = 0 TO z = 3

We present the evolution of the rest-frame optical luminosity density j, the integrated rest-frame optical color, and the stellar mass density, ?*, for a sample of Ks band-selected galaxies in the Hubble Deep Field-South (HDF-S). We derived j in the rest-frame U, B, and V bands and found that j increases by a factor of 1.9 ? 0.4, 2.9 ? 0.6, and 4.9 ? 1.0 in the V, B, and U rest-frame bands, respectively, between redshifts of 0.1 and 3.2. We derived the luminosity-weighted mean cosmic (U-B)rest and (B-V)rest colors as a function of redshift. The colors bluen almost monotonically with increasing redshift; at z = 0.1, the (U-B)rest and (B-V)rest colors are 0.16 and 0.75, respectively, while at z = 2.8 they are -0.39 and 0.29, respectively. We derived the luminosity-weighted mean M/L, using the correlation between (U-V)rest and log M/L that exists for a range in smooth star formation histories (SFHs) and moderate extinctions. We have shown that the mean of individual M/L estimates can overpredict the true value by ~70%, while our method overpredicts the true value by only ~35%. We find that the universe at z ~ 3 had ~10 times lower stellar mass density than it does today in galaxies with L > 1.4 ? 1010 h L?. Half of the stellar mass of the universe was formed by z ~ 1-1.5. The rate of increase in ?* with decreasing redshift is similar to but above that for independent estimates from the HDF-N, but it is slightly less than that predicted by the integral of the SFR(z) curve.

What Do We Learn from IRAC Observations of Galaxies at 2 < z < 3.5?*

We analyze very deep HST, VLT, and Spitzer photometry of galaxies at 2 2 galaxies. The estimated distributions of these properties do not change significantly when IRAC data are added to the UBVIJHK photometry. However, for individual galaxies the addition of IRAC can improve the constraints on the stellar populations, especially for red galaxies: uncertainties in stellar mass decrease by a factor of 2.7 for red [(U − V)rest > 1] galaxies, but only by a factor of 1.3 for blue [(U − V)rest 2. The most massive galaxies at high redshift have red rest-frame U - V colors compared to lower mass galaxies, even when allowing for complex star formation histories.

The Nature and Evolution of Ultraluminous Infrared Galaxies:A Mid-Infrared Spectroscopic Survey

We report the first results of a low-resolution mid-infrared spectroscopic survey of an unbiased, far-infrared-selected sample of 60 ultraluminous infrared galaxies (ULIRGs) (LIR

IRAC Mid-Infrared Imaging of the Hubble Deep Field-South: Star Formation Histories and Stellar Masses of Red Galaxies at z > 2

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ISOCAM-CVF 5-12 Micron Spectroscopy of Ultraluminous Infrared Galaxies*

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CRIRES: A High Resolution Infrared Spectrograph for ESO’s VLT

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Hα Spectroscopy of Galaxies at z > 2: Kinematics and Star Formation*

--> > 10 -->12 L?) using ISOPHOT-S on board the Infrared Space Observatory (ISO). We use the ratio of the 7.7 ?m polycyclic aromatic hydrocarbon (PAH) emission feature to the local continuum as a discriminator between starburst and active galactic nucleus (AGN) activity. About 80% of all ULIRGs are found to be predominantly powered by star formation, but the fraction of AGN-powered objects increases with luminosity. Whereas only about 15% of ULIRGs at luminosities below 2 ? 10 -->12 L? are AGN powered, this fraction increases to about half at higher luminosity. Observed ratios of the PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. This can be plausibly explained by the higher extinction and/or different physical conditions in the interstellar medium of ULIRGs. The PAH feature-to-continuum ratio is anticorrelated with the ratio of feature-free 5.9 ?m continuum to the IRAS 60 ?m continuum, confirming suggestions that strong mid-infrared continuum is a prime AGN signature. The location of starburst-dominated ULIRGs in such a diagram is consistent with previous ISO-Short Wavelength Spectrograph spectroscopy, which implies significant extinction even in the mid-infrared. We have searched for indications that ULIRGs that are advanced mergers might be more AGN-like, as postulated by the classical evolutionary scenario. No such trend has been found among those objects for which near-infrared images are available to assess their likely merger status.

NAOS-CONICA first on sky results in a variety of observing modes

We present deep 3.6–8 mm imaging of the Hubble Deep Field–South with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We study distant red galaxies (DRGs) at z 1 2 selected by JsKs 1 2.3 and compare them with a sample of Lyman break galaxies (LBGs) at z p 2–3. The observed UV–to–8 mm spectral energy distributions are fitted 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 fitted with old and “dead” models. Using only the IKs and Ks4.5 mm colors, we can effectively separate the two groups. The dead systems are among the most massive at z ∼ 2.5 (mean stellar mass AM * S p 0.8 #10 11 M,) and likely formed most of their stellar mass at z 1 5. To a limit of 0.5 #10 11 M,, their number density is ∼10 times 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/LK), implying that even a Spitzer 8 mm–selected sample would be very different from a mass-selected sample. The average M/LK of the DRGs is about 3 times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/LK values 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 to what is found in the low-redshift universe. Subject headings: galaxies: evolution — galaxies: high-redshift — infrared: galaxies