Matthew D. Lehnert

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.

SINFONI integral field spectroscopy of z ~ 2 UV-selected galaxies: rotation curves and dynamical evolution

We present ~0.5 resolution near-infrared integral field spectroscopy of the Hα line emission of 14 z ~ 2 UV-selected BM/BX galaxies, obtained with SINFONI at the ESO Very Large Telescope. The average Hα half-light radius is r_(1/2)≈ 4 h^(-1)_(70) kpc, and line emission is detected over ≳20 h^(-1)_(70)kpc in several sources. In nine galaxies, we detect spatially resolved velocity gradients, from 40 to 410 km s^(-1) over ~10 h^(-1)_(70) kpc. The kinematics of the larger systems are generally consistent with orbital motions. Four galaxies are well described by rotating clumpy disks, and we extracted rotation curves out to radii ≳10 h^(-1)_(70) kpc. One or two galaxies exhibit signatures more consistent with mergers. Analyzing all 14 galaxies in the framework of rotating disks, we infer mean inclination- and beam-corrected maximum circular velocities of v_c ~ 180 ± 90 km s^(-1) and dynamical masses from ~0.5 to 25 × 10^(10) h^(-1)_(70) M_☉ within r_(1/2). The specific angular momenta of our BM/BX galaxies are similar to those of local late-type galaxies. Moreover, the specific angular momenta of their baryons are comparable to those of their dark matter halos. Extrapolating from the average vc at 10 himg1.gif kpc, the virial mass of the typical halo of a galaxy in our sample is 10^(11.7±0.5) h^(-1)_(70) M_☉. Kinematic modeling of the three best cases implies a ratio of v_c to local velocity dispersion v_(c)/σ ~ 2-4 and, accordingly, a large geometric thickness. We argue that this suggests a mass accretion (alternatively, gas exhaustion) timescale of ~500 Myr. We also argue that if our BM/BX galaxies were initially gas-rich, their clumpy disks would subsequently lose their angular momentum and form compact bulges on a timescale of ~1 Gyr.

The rapid formation of a large rotating disk galaxy three billion years after the Big Bang.

Observations and theoretical simulations have established a framework for galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the centres of collapsing dark-matter haloes; mergers of haloes and galaxies then led to the hierarchical build-up of galaxy mass. It remains unclear, however, over what timescales galaxies were assembled and when and how bulges and disks—the primary components of present-day galaxies—were formed. It is also puzzling that the most massive galaxies were more abundant and were forming stars more rapidly at early epochs than expected from models. Here we report high-angular-resolution observations of a representative luminous star-forming galaxy when the Universe was only 20% of its current age. A large and massive rotating protodisk is channelling gas towards a growing central stellar bulge hosting an accreting massive black hole. The high surface densities of gas, the high rate of star formation and the moderately young stellar ages suggest rapid assembly, fragmentation and conversion to stars of an initially very gas-rich protodisk, with no obvious evidence for a major merger.

The Panchromatic starburst intensity limit at low and high redshift

The integrated bolometric effective surface brightness S_e distributions of starbursts are investigated for samples observed in 1. the rest frame ultraviolet (UV), 2. the far-infrared and H-alpha, and 3. 21cm radio continuum emission. For the UV sample we exploit a tight empirical relationship between UV reddening and extinction to recover the bolometric flux. Parameterizing the S_e upper limit by the 90th percentile of the distribution, we find a mean S_{e,90} = 2.0e11 L_{sun}/kpc^2 for the three samples, with a factor of three difference between the samples. This is consistent with what is expected from the calibration uncertainties alone. We find little variation in S_{e,90} with effective radii for R_e ~ 0.1 - 10 kpc, and little evolution out to redshifts z ~ 3. The lack of a strong dependence of S_{e,90} on wavelength, and its consistency with the pressure measured in strong galactic winds, argue that it corresponds to a global star formation intensity limit (dotSigma_{e,90} ~ 45 M_{sun}/kpc^2/yr) rather than being an opacity effect. There are several important implications of these results: 1. There is a robust physical mechanism limiting starburst intensity. We note that starbursts have S_e consistent with the expectations of gravitational instability models applied to the solid body rotation portion of galaxies. 2. Elliptical galaxies and spiral bulges can plausibly be built with maximum intensity bursts, while normal spiral disks can not. 3. The UV extinction of high-z galaxies is significant, implying that star formation in the early universe is moderately obscured. After correcting for extinction, the observed metal production rate at z ~ 3 agrees well with independent estimates made for the epoch of elliptical galaxy formation.

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

The Adaptive Optics NIR Instrument NAOS-CONICA has been commissioned at the VLT (UT4) between November 2001 and March 2002. After summarizing the observational capabilities of this multimode instrument in combination with the powerful AO-system, we will present first on sky results of the instrumental performance for several non-direct imaging modes: High spatial resolution slit-spectroscopy in the optical and thermal NIR region has been tested. For compact sources below 2 arcsec extension, Wollaston prism polarimetry is used. For larger objects the linear polarization pattern can be analyzed by wire grids down to the diffraction limit. Coronographic masks are applied to optimize imaging and polarimetric capabilities. The cryogenic Fabry-Perot Interferometer in combination with an 8m-telescope AO-system is shown to be a powerful tool for imaging spectroscopy (3D-scans).

Lyman-break galaxies at z ~ 5 – I. First significant stellar mass assembly in galaxies that are not simply z ~ 3 LBGs at higher redshift

We determine the ensemble properties of z ∼ 5 Lyman-break galaxies (LBGs) selected as V-band dropouts to i AB < 26.3 in the Chandra Deep Field-South using their rest-frame UV-to-visible spectral energy distributions. By matching the selection and performing the same analysis that has been used for z ∼ 3 samples, we show clear differences in the ensemble properties of two samples of LBGs which are separated by 1 Gyr in look-back time. We find that z ∼ 5 LBGs are typically much younger (<100Myr) and have lower stellar masses (∼10 9 M ⊙ ) than their z ∼ 3 counterparts (which are typically ∼ few x 10 10 M ⊙ and ∼320 Myr old). The difference in mass is significant even when considering the presence of an older, underlying population in both samples. Such young and moderately massive systems dominate the luminous z ∼ 5 LBG population (≥70 per cent), whereas they comprise ≤30 per cent of LBG samples at z ∼ 3. This result, which we demonstrate is robust under all reasonable modelling assumptions, shows a clear change in the properties of the luminous LBGs between z ∼ 5 and z ∼ 3. These young and moderately massive z ∼ 5 LBGs appear to be experiencing their first (few) generations of large-scale star formation and are accumulating their first significant stellar mass. Their dominance in luminous LBG samples suggests that z ∼ 5 witnesses a period of wide-spread, recent galaxy formation. As such, z ∼ 5 LBGs are the likely progenitors of the spheroidal components of present-day massive galaxies. This is supported by their high stellar mass surface densities, and is consistent with their core phase-space densities, as well as the ages of stars in the bulge of our Galaxy and other massive systems. With implied formation redshifts of z ∼ 6-7, these luminous z ∼ 5 LBGs could have only contributed to the UV photon budget at the end of reionization. However, their high star formation rates per unit area suggest these systems host outflows or winds that enrich the intragalactic and intergalactic media with metals, as has been established for z ∼ 3 LBGs. Their estimated young ages are consistent with inefficient metal-mixing on galaxy-wide scales. Therefore these galaxies may contain a significant fraction of Population III stars as proposed for z ∼ 3 LBGs by Jiminez & Haimann.

Luminous Lyman Break Galaxies at z > 5 and the Source of Reionization

We have discovered six galaxies with spectroscopically confirmed redshifts of 4.8 1.5 in the field; this photometric cut is designed to select galaxies at z > 4.8. The line fluxes range between 0.2 and 2.5 × 10-17 ergs cm-2 s-1, indicating luminosities of around 1042-1043 ergs s-1 for Lyα, and their high emission line equivalent widths suggest very young ages (108 yr). A further line-emitting object with no detectable continuum was serendipitously detected by spectroscopy. If this line is Lyα, then it is from a source at z = 6.6, making this the most distant galaxy known. However, the redshift cannot be considered secure as it is based on a single line. No broad emission line objects (quasars) were detected. The 13 sources at IAB -21.

Molecular Gas in the Lensed Lyman Break Galaxy cB58

We have used the IRAM Plateau de Bure Interferometer to map CO (3-2) emission from the gravitationally lensed Lyman break galaxy MS 1512-cB58. This is the first detection of a molecular emission line in any Lyman break system; its integrated intensity implies a total molecular gas mass of (6.6) × 109 h M☉, while its width implies a dynamical mass of (1.0) × 1010 i h M☉ (for a flat ΩΛ = 0.7 cosmology). These estimates are in excellent concordance with nearly all parameters of the system measured at other wavelengths and yield a consistent picture of past and future star formation with no obvious discrepancies requiring explanation by differential lensing. In particular, we find that the age and remaining lifetime of the current episode of star formation are likely to be similar, the surface densities of star formation and molecular gas mass are related by a Schmidt law, and the fraction of baryonic mass already converted into stars is sufficient to account for the observed enrichment of the interstellar medium to 0.4 Z☉. Barring substantial gas inflow or a major merger, the stars forming in the current episode will have mass and coevality at z = 0 similar to those of a spiral bulge. Assuming that cB58 is a typical Lyman break galaxy apart from its magnification, its global parameters suggest that the prescriptions for star formation used in some semianalytic models of galaxy evolution require moderate revision, although the general prediction that gas mass fraction should increase with redshift is validated. The length of cB58s star formation episode relative to the time elapsed over the redshift range 2.5 ≤ z ≤ 3.5 strongly argues against scenarios in which observed LBGs cohabit their halos with a large number of similar but dormant systems whose starbursts have faded or not yet begun. As a useful empirical result, we find that the observed line/continuum ratio for cB58 is similar to those of high-redshift systems with quite different dust luminosities and nuclear activity levels. Finally, we report the detection of a second source close to the position of the cD elliptical in the z = 0.37 lensing cluster, which may be nonthermal continuum emission from the cD or CO line emission from a hitherto unknown background galaxy at z ~ 1.48 or ~2.73.

New spectroscopic redshifts from the CDFS and a test of the cosmological relevance of the GOODS-South field

(Abbrev.) This paper prepares a series of papers analysing the Intermediate MAss Galaxy Evolution Sequence (IMAGES) up to z=1. Intermediate mass galaxies (MJ <=-20.3) are selected from the Chandra Deep Field South (CDFS) for which we identify a serious lack of spectroscopically determined redshifts..... We have spectroscopically identified 691 objects including 580 gal., 7 QSOs, and 104 stars. This study provides 531 new redshifts in the CDFS. It confirms the presence of several large scale structures in the CDFS. To test the impact of these structures in the GOODS-South field, we ... compare the evolution of rest-frame U, B, V and K galaxy luminosity densities to that derived from the CFRS. The CDFS field shows a significant excess of luminosity densities in the z=0.5-0.75 range, which increases with the wavelength, reaching up to 0.5 dex at 2.1 um. Stellar mass and specific star formation evolutions might be significantly affected by the presence of the peculiar large scale structures at z= 0.668 and at z= 0.735, that contain a significant excess of evolved, massive galaxies when compared to other fields. This leads to a clear warning to results based on the CDFS/GOODS South fields, especially those related to the evolution of red luminosity densities, i.e. stellar mass density and specific star formation rate. Photometric redshift techniques, when applied to that field, are producing quantities which are apparently less affected by cosmic variance (0.25 dex at 2.1 um), however at the cost of the difficulty in disentangling between evolutionary and cosmic variance effects.

Ionized gas in the halos of edge-on, starburst galaxies: Data and results

We present narrowband H-alpha and broadband R images, as well as long-slit spectra oriented along the minor and major axes of a sample of about 50 edge-on (a/b greater than or equal to 2), infrared-warm (S(sub 60 microns)/S(sub 100 microns) greater than 0.04), infrared-bright S(sub 60 microns) greater than or equal to 5.4 Jy galaxies. The infrared luminosity of the sample ranges over 10(exp 10) - 10(exp 12) solar luminosity. The spatially resolved spectroscopy includes the measurement of velocity relative to the nuclear velocity, full width at half-maximum, total integrated flux in the profile (for those spectra taken under photometric conditions) for the lines (N II) lambda lambda 6548, 6583, (O I) lambda 6300, H-alpha, and (S II) lambda lambda 6716, 6713 and line ratios as a function of slit position along both the major and minor axes. The resolution of the spectra are between about 3 and 5 A. The spectroscopic data are presented for 5 bins along each axis -- a nuclear bin that is a sum of the CCD rows that cover the half-light diameter centered on the nucleus of the galaxy, two near-nuclear bins which are sums of the CCD rows that cover from one to two half-light radii on each side of the nucleus, and two off-nuclear bins which are sums of the rows at nuclear distances greater than two half-light radii on each side of the nucleus. Additionally, we present recession velocities, nuclear line asymmetries, rotation speeds, minor axis velocity shears, H-alpha luminosities, R-band absolute magnitudes, minor axis H-alpha `excess and effective radii of the galaxies in h-alpha and the R continuum. We defer discussion of the properties of the emission-line gas and their correlation with the infrared properties of this sample of galaxies to a later paper and limit ourselves to a presentation of the data and analysis.