Fabian Walter

SINGS: The SIRTF Nearby Galaxies Survey

The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging and spectroscopic survey of 75 nearby galaxies. Its primary goal is to characterize the infrared emission of galaxies and their principal infrared-emitting components, across a broad range of galaxy properties and star formation environments. SINGS will provide new insights into the physical processes connecting star formation to the interstellar medium properties of galaxies and provide a vital foundation for understanding infrared observations of the distant universe and ultraluminous and active galaxies. The galaxy sample and observing strategy have been designed to maximize the scientific and archival value of the data set for the SIRTF user community at large. The SIRTF images and spectra will be supplemented by a comprehensive multiwavelength library of ancillary and complementary observations, including radio continuum, H i, CO, submillimeter, BVRIJHK ,H a ,P aa, ultraviolet, and X-ray data. This paper describes the main astrophysical issues to be addressed by SINGS, the galaxy sample and the observing strategy, and the SIRTF and other ancillary data products.

The intense starburst HDF 850.1 in a galaxy overdensity at z ≈ 5.2 in the Hubble Deep Field

The Hubble Deep Field provides one of the deepest multiwavelength views of the distant Universe and has led to the detection of thousands of galaxies seen throughout cosmic time. An early map of the Hubble Deep Field at a wavelength of 850 micrometres, which is sensitive to dust emission powered by star formation, revealed the brightest source in the field, dubbed HDF 850.1 (ref. 2). For more than a decade, and despite significant efforts, no counterpart was found at shorter wavelengths, and it was not possible to determine its redshift, size or mass. Here we report a redshift of z = 5.183 for HDF 850.1, from a millimetre-wave molecular line scan. This places HDF 850.1 in a galaxy overdensity at z ≈ 5.2, corresponding to a cosmic age of only 1.1 billion years after the Big Bang. This redshift is significantly higher than earlier estimates and higher than those of most of the hundreds of submillimetre-bright galaxies identified so far. The source has a star-formation rate of 850 solar masses per year and is spatially resolved on scales of 5 kiloparsecs, with an implied dynamical mass of about 1.3 × 1011 solar masses, a significant fraction of which is present in the form of molecular gas. Despite our accurate determination of redshift and position, a counterpart emitting starlight remains elusive.

Mid-infrared IRS spectroscopy of NGC 7331: A first look at the Spitzer Infrared Nearby Galaxies Survey (SINGS) legacy

The nearby spiral galaxy NGC 7331 was spectrally mapped from 5-38um using all modules of Spitzers IRS spectrograph. A strong new dust emission feature, presumed due to PAHs, was discovered at 17.1um. The features intensity is nearly half that of the ubiquitous 11.3um band. The 7-14um spectral maps revealed significant variation in the 7.7 and 11.3um PAH features between the stellar ring and nucleus. Weak [OIV] 25.9um line emission was found to be centrally concentrated in the nucleus, with an observed strength over 10% of the combined neon line flux, indicating an AGN or unusually active massive star photo-ionization. Two [SIII] lines fix the characteristic electron density in the HII regions at n_e < ~200 cm^-3. Three detected H_2 rotational lines, tracing warm molecular gas, together with the observed IR continuum, are difficult to match with standard PDR models. Either additional PDR heating or shocks are required to simultaneously match lines and continuum.The nearby spiral galaxy NGC 7331 was spectrally mapped from 5 to 38 μm using all modules of Spitzers Infrared Spectrograph (IRS). A strong new dust emission feature, presumed due to polycyclic aromatic hydrocarbons, was discovered at 17.1 μm. The features intensity is nearly half that of the ubiquitous 11.3 μm band. The 7-14 μm spectral maps revealed significant variation in the 7.7 and 11.3 μm PAH features between the stellar ring and nucleus. Weak [O IV] 25.9 μm line emission was found to be centrally concentrated in the nucleus, with an observed strength of over 10% of the combined neon line flux, indicating an AGN or unusually active massive star photoionization. Two [S III] lines fix the characteristic electron density in the H II regions at ne 200 cm-3. Three detected H2 rotational lines, tracing warm molecular gas, together with the observed IR continuum, are difficult to match with standard photodissociation region (PDR) models. Either additional PDR heating or shocks are required to simultaneously match lines and continuum.

Spitzer Infrared Nearby Galaxies Survey (SINGS) imaging of NGC 7331: A panchromatic view of a ringed galaxy

Well-resolved infrared observations of nearby galaxies are of fundamental importance to the study of the processes that affect galactic evolution. In this paper we report on the first imaging results from the Spitzer Infrared Nearby Galaxies Survey (SINGS) using observations of the Sb galaxy NGC 7331. We present images of NGC 7331 over a large range of wavelengths that allow us to compare the distributions of gas, stars, and dust in unprecedented detail. As an example of the types of information that the full SINGS will provide, we use three methods to determine that the interstellar medium mass in the ring of NGC 7331 is ~5 × 109 M⊙. We also present the first images showing emission from small hot (~1000 K) dust grains, but we show that these dust grains contribute only a small fraction of the integrated 4.5 μm emission from NGC 7331.

Chandra Observations of Expanding Shells in the Dwarf Starburst Galaxy NGC 3077

Deep Chandra observations (53 ks, ACIS-S3) of NGC 3077, a starburst dwarf galaxy in the M81 triplet, resolve the X-ray emission from several supershells. The emission is brightest in the cavities defined by expanding shells detected previously in Hα emission. Thermal emission models fitted to the data imply temperatures ranging from ~1.3 to 4.9 × 106 K and indicate that the strongest absorption is coincident with the densest clouds traced by CO emission. The fitted emission measures give pressures of P/k ≈ (105-106)ξ-0.5f K cm-3 (where ξ is the metallicity of the hot gas in solar units and fv is the volume filling factor). Despite these high pressures, the radial density profile of the hot gas is not as steep as that expected in a freely expanding wind (e.g., as seen in the neighboring starburst galaxy M82), implying that the hot gas is still confined by the Hα shells. The chaotic dynamical state of NGC 3077 undermines reliable estimates of the escape velocity. The more relevant quantity for the ultimate fate of the outflow is probably the gas density in the rich intragroup medium. Based on the H I distribution of NGC 3077 and a connected tidal tail we argue that the wind has the potential to leave the gravitational well of NGC 3077 to the north but not to the south. The total 0.3-6.0 keV X-ray luminosity is ~(2-5) × 1039 ergs s-1 (depending on the selected thermal plasma model). Most (~85%) of the X-ray luminosity in NGC 3077 comes from the hot interstellar gas; the remainder comes from six X-ray point sources. In spite of previous claims to the contrary, we do not find X-ray emission originating from the prominent tidal tail near NGC 3077.

Millimeter Observations of GRB 030329: Continued Evidence for a Two-Component Jet

We present the results of a dedicated campaign on the afterglow of GRB 030329 with the millimeter interferometers of the Owens Valley Radio Observatory (OVRO) and the Berkeley-Illinois-Maryland Association (BIMA), and with the MAMBO-2 bolometer array on the IRAM 30 m telescope. These observations allow us to trace the full evolution of the afterglow of GRB 030329 at frequencies of 100 and 250 GHz for the first time. The millimeter light curves exhibit two main features: a bright, constant flux density portion and a steep power-law decline. The absence of bright, short-lived millimeter emission is used to show that the GRB central engine was not actively injecting energy well after the burst. The millimeter data support a model, advocated by Berger et al., of a two-component jetlike outflow in which a narrow-angle jet is responsible for the high-energy emission and early optical afterglow, and a wide-angle jet carrying most of the energy is powering the radio and late optical afterglow emission.

The Star Formation Law at Sub-Kiloparsec Resolution

In a sample of 9 spiral galaxies, we show that inside ∼ 0.4 r25, where the gas in spirals is mostly molecular, a single power law with a slope N = 1.0± 0.2 relates the star formation rate surface density,ΣSFR, to the gas surface density, Σgas, pixel-by-pixel at 500 pc resolution. This implies that gas forms stars with constant efficiency in this regime. The data in this regime match up well with disk-averaged measurements of ΣSFR and Σgas. We find a clear saturation in ΣHI at ∼ 12.5 M pc−2. All gas in excess of this surface density is molecular.

The Violent Interstellar Medium of Dwarf Irregular Galaxies

High-resolution observations of nearby dwarf irregular galaxies (in particular, IC 2574 and DDO 47) in the 21 cm line of neutral hydrogen (H I) obtained with the Very Large Array (VLA) reveal that their neutral interstellar medium (ISM) is dominated by holes and shells, most of which are expanding (the ““ violent II ISM). These features range in size from about 100 (a limit set by the size of the beam) to about 1000 pc, dominating the appearance of the H I surface brightness maps and inNuencing the velocity Ðelds. Current star formation, as traced by Ha emission, is predominantly found along the rims of the larger H I holes, suggesting propagating star formation. On linear scales of B100 pc, star formation occurs if the H I surface density reaches values higher than 1021 cm~2. The scale height of the H I layer in the dwarf irregular galaxies under study is found to be of order 400 pc, considerably thicker than that in massive disk galaxies. This is due to a lower gravitational potential (for the same observed one-dimensional velocity dispersion of B7 km s~1). This pu†ed-up disk implies a rather low H I volume density (0.15 cm~3). This, combined with the reduced gravitational pull, solid-body rotation throughout the disk and the absence of density waves, explains why the diameter distribution in dwarf galaxies extends to substantially larger values than in spiral galaxies. The radial expansion velocities of the H I holes (8E12 km s~1), the indicative ages [(10E60)] 106 yr], and the energy requirements for their formation (1050E1053 ergs) can be understood in terms of the combined e†ects of stellar winds and multiple supernova explosions of the most massive stars formed during a recent phase of active star formation. A comparison with other galaxies spanning a wide range of masses (M31, M33, and Holmberg II) shows that, somewhat surprisingly, the energies needed to create these structures are the same for all types of galaxies, at least to Ðrst order. The overall statistical properties of the H I holes and shells in galaxies show clear trends with Hubble type (or rather mass), such as in their diameter distribution, expansion velocities, and ages. The origin of the H I holes and shells is still not fully settled. In the standard picture, young star-forming regions and their aftermaths (strong stellar winds and supernova explosions) are held responsible for their formation. This picture, however, is not without its critics, and other mechanisms such as the infall of high-velocity clouds, turbulent motions, or even gamma-ray bursters have recently been proposed. A promising region that sheds light on the processes which lie at origin of the H I holes is the recently discovered supergiant shell in IC 2574 (IC 2574-SGS). Here, a multiwavelength study (optical broad and narrow band, H I, radio continuum, and X-ray) provides evidence that a young stellar cluster is driving the expansion of this particular H I shell. IC 2574-SGS measures roughly 1000] 500 pc in size and is expanding at about 25 km s~1. The H I data suggest an age of about 1.4] 107 yr ; the energy input must have been of order 2.6^ 1] 1053 ergs. Massive starforming regions, as traced by Ha emission, are situated predominantly on the rim of this H I shell. This supports the view that the accumulated H I on the rim has reached densities which are high enough for secondary star formation to commence. VLA radio continuum observations at j\ 6 cm show that these star-forming regions are the main sources of radio continuum emission in this galaxy. This emission is mainly thermal in origin. Soft X-ray emission coming from inside the H I shell is believed to be generated by an X-rayE emitting plasma keV)\ 1.6^ 0.5] 1038 ergs [L X(0.1E2.4 s~1]. The X-ray data are compatible with emission coming from a Raymond and Smith plasma at a temperature of about log T [K]\ 6.8 and a density of 0.03 cm~3. Deep R-band observations reveal the presence of an interior stellar cluster in IC 2574-SGS. It is believed to be the powering source for the formation and expansion of the H I shell as well as for the heating of the X-ray gas. These observations support the standard picture that at least some H I holes that are found in the ISM of galaxies are created as a result of massive star formation and the subsequent rapid evolution of the most massive stars.

Molecular Gas in Nearby Dwarf Galaxies: Single Dish and Interferometric Results

During the past two years we have undertaken a systematic millimeter-wave survey of molecular gas in a sample of 150 nearby, northern dwarf galaxies with IRAS emission. We have called this survey MIDGET (Millimeter Interferometry of Dwarf Galaxies). MIDGET has two parts: a single-dish search for CO done at the UASO Kitt Peak 12m telescope (Leroy et al., in preparation), and an interferometric follow-up of the galaxies with strong CO emission carried out at the Berkeley-Illinois-Maryland Array (BIMA). The single-dish survey targeted the centers of northern galaxies that are nearby (VLSR ≤ 1000 km s-1), of small mass (HI linewidths W20 ≤ 200 km s1), compact in size, and detected by IRAS. The single-dish observations were very successful, finding 41 new CO emitters and more than doubling the number of previously known CO sources within the defined sample. The typical 1 σ sensitivity attained was ∼ 0.2 K km s_1. Carbon monoxide emission was detected in galaxies with 12 + log([O/H]) ∼ 7.9-8.5, a metallicity regime similar to that of the Magellanic Clouds. We are using these data to address several questions related to molecular cloud and star formation in these objects. This paper concentrates on the issue of how CO traces molecular gas as a function of heavy element abundance Z.