Sabrina Stierwalt

THE ARECIBO LEGACY FAST ALFA SURVEY. I. SCIENCE GOALS, SURVEY DESIGN, AND STRATEGY

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map ~7000 deg2 of the high Galactic latitude sky visible from Arecibo, providing a H I line spectral database covering the redshift range between -1600 and 18,000 km s-1 with ~5 km s-1 resolution. Exploiting Arecibos large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H I mass function in the local universe and will provide a census of H I in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide-area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local Supercluster and nearby superclusters, allow measurement of the H I diameter function, and enable a first wide-area blind search for local H I tidal features, H I absorbers at z < 0.06, and OH megamasers in the redshift range 0.16 < z < 0.25. Although completion of the survey will require some 5 years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique that samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large-scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic H I line sources out to z ~ 0.06, including several hundred with H I masses M < 107.5 M⊙.

The Arecibo Legacy Fast ALFA Survey: the α.40 H I source catalog, its characteristics and their impact on the derivation of the H I mass function

We present a current catalog of 21 cm H I line sources extracted from the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey over ~2800 deg^2 of sky: the α.40 catalog. Covering 40% of the final survey area, the α.40 catalog contains 15,855 sources in the regions 07^h30^m < R.A. < 16^h30^m, +04° < decl. <+16°, and +24° < decl. <+28° and 22^h < R.A. < 03^h, +14° < decl. <+16°, and +24° < decl. < + 32°. Of those, 15,041 are certainly extragalactic, yielding a source density of 5.3 galaxies per deg^2, a factor of 29 improvement over the catalog extracted from the H I Parkes All-Sky Survey. In addition to the source centroid positions, H I line flux densities, recessional velocities, and line widths, the catalog includes the coordinates of the most probable optical counterpart of each H I line detection, and a separate compilation provides a cross-match to identifications given in the photometric and spectroscopic catalogs associated with the Sloan Digital Sky Survey Data Release 7. Fewer than 2% of the extragalactic H I line sources cannot be identified with a feasible optical counterpart; some of those may be rare OH megamasers at 0.16 < z < 0.25. A detailed analysis is presented of the completeness, width-dependent sensitivity function and bias inherent of the α.40 catalog. The impact of survey selection, distance errors, current volume coverage, and local large-scale structure on the derivation of the H I mass function is assessed. While α.40 does not yet provide a completely representative sampling of cosmological volume, derivations of the H I mass function using future data releases from ALFALFA will further improve both statistical and systematic uncertainties.

The Arecibo Legacy Fast ALFA Survey: X. The HI Mass Function and Omega_HI From the 40% ALFALFA Survey

The Arecibo Legacy Fast ALFA (ALFALFA) survey has completed source extraction for 40% of its total sky area, resulting in the largest sample of H I-selected galaxies to date. We measure the H I mass function from a sample of 10,119 galaxies with 6.2 < log(M_(HI)/M_☉) < 11.0 and with well-described mass errors that accurately reflect our knowledge of low-mass systems. We characterize the survey sensitivity and its dependence on profile velocity width, the effect of large-scale structure, and the impact of radio frequency interference in order to calculate the H I mass function with both the 1/V_(max) and 2DSWML methods. We also assess a flux-limited sample to test the robustness of the methods applied to the full sample. These measurements are in excellent agreement with one another; the derived Schechter function parameters are φ* (h^(3)_(70) Mpc^(–3) dex^(–1)) = 4.8 ± 0.3 × 10^(–3), log (M*/M_☉) + 2 log h_(70) = 9.96 ± 0.02, and α = –1.33 ± 0.02. We find Ω_(HI) = 4.3 ± 0.3 ×10^(–4) h^(–1)_(70), 16% larger than the 2005 HIPASS result, and our Schechter function fit extrapolated to log (M_(HI)/M_☉) = 11.0 predicts an order of magnitude more galaxies than HIPASS. The larger values of Ω_(HI) and of M* imply an upward adjustment for estimates of the detection rate of future large-scale H I line surveys with, e.g., the Square Kilometer Array. A comparison with simulated galaxies from the Millennium Run and a treatment of photoheating as a method of baryon removal from H I-selected halos indicate that the disagreement between dark matter mass functions and baryonic mass functions may soon be resolved.

Inefficient star formation in extremely metal poor galaxies

The first galaxies contain stars born out of gas with few or no ‘metals’ (that is, elements heavier than helium). The lack of metals is expected to inhibit efficient gas cooling and star formation, but this effect has yet to be observed in galaxies with an oxygen abundance (relative to hydrogen) below a tenth of that of the Sun. Extremely metal poor nearby galaxies may be our best local laboratories for studying in detail the conditions that prevailed in low metallicity galaxies at early epochs. Carbon monoxide emission is unreliable as a tracer of gas at low metallicities, and while dust has been used to trace gas in low-metallicity galaxies, low spatial resolution in the far-infrared has typically led to large uncertainties. Here we report spatially resolved infrared observations of two galaxies with oxygen abundances below ten per cent of the solar value, and show that stars formed very inefficiently in seven star-forming clumps in these galaxies. The efficiencies are less than a tenth of those found in normal, metal rich galaxies today, suggesting that star formation may have been very inefficient in the early Universe.

Explaining the [C II]157.7 μm Deficit in Luminous Infrared Galaxies : First Results from a Herschel/PACS Study of the GOALS Sample

We present the first results of a survey of the [C II] 157.7 μm emission line in 241 luminous infrared galaxies (LIRGs) comprising the Great Observatories All-sky Survey (GOALS) sample, obtained with the PACS instrument on board the Herschel Space Observatory. The [C II] luminosities, L_([C II]), of the LIRGs in GOALS range from ∼ 10^7 to 2×10^9 L_⊙. We find that LIRGs show a tight correlation of [C II]/FIR with far-IR flux density ratios, with a strong negative trend spanning from ∼ 10^(−2) to 10^(−4), as the average temperature of dust increases. We find correlations between the [C II]/FIR ratio and the strength of the 9.7 μm silicate absorption feature as well as with the luminosity surface density of the mid-IR emitting region (∑_(MIR)), suggesting that warmer, more compact starbursts have substantially smaller [C II]/FIR ratios. Pure star-forming LIRGs have a mean [C II]/FIR∼ 4 × 10^(−3), while galaxies with low 6.2 μm PAH equivalent widths (EWs), indicative of the presence of active galactic nuclei (AGN), span the full range in [C II]/FIR. However, we show that even when only pure star-forming galaxies are considered, the [C II]/FIR ratio still drops by an order of magnitude, from 10^(−2) to 10^(−3), with ∑_(MIR) and ∑_(IR), implying that the [C II] 157.7 μm luminosity is not a good indicator of the star formation rate (SFR) for most LIRGs, for it does not scale linearly with the warm dust emission most likely associated to the youngest stars. Moreover, even in LIRGs in which we detect an AGN in the mid-IR, the majority (2/3) of galaxies show [C II]/FIR≥ 10^(−3) typical of high 6.2 μm PAH EW sources, suggesting that most AGNs do not contribute significantly to the far-IR emission. We provide an empirical relation between the [C II]/FIR and the specific SFR (SSFR) for star-forming LIRGs. Finally, we present predictions for the starburst size based on the observed [C II] and far-IR luminosities which should be useful for comparing with results from future surveys of high-redshift galaxies with ALMA and CCAT.

MID-INFRARED SPECTRAL DIAGNOSTICS OF LUMINOUS INFRARED GALAXIES

We present a statistical analysis of 248 luminous infrared galaxies (LIRGs) which comprise the Great Observatories All-sky LIRG Survey (GOALS) observed with the Infrared Spectrograph (IRS) on-board Spitzer in the rest-frame wavelength range between 5 and 38 µm. The GOALS sample enables a direct measurement of the relative contributions of star-formation and active galactic nuclei (AGN) to the total infrared (IR) emission from a large, statistically complete sample of LIRGs in the local Universe.Several diagnostics effective at isolating the AGN contribution to the Mid-infrared (MIR) emission using [NeV], [OIV] and [NeII] gas emission lines, the 6.2 µm PAH equivalent width (EQW) and the shape of the MIR continuum are compared. The [NeV] line which indicates the presence of an AGN is detected in 22% of all LIRGs. The 6.2 µm PAH EQW, [NeV]/L_(IR), [NeV]/[NeII] and [OIV]/[NeII] ratios, and the ratios of 6.2 µm PAH flux to the integrated continuum flux between 5.3 and 5.8 µm suggest values of around 10% for the fractional AGN contribution to the total IR luminosity of LIRGs. The median of these estimates suggests that for local LIRGs the fractional AGN contribution to the total IR luminosity is ~12%. AGN dominated LIRGs have higher global and nuclear IR luminosities, warmer MIR colors and are interacting more than starburst (SB) dominated LIRGs. However there are no obvious linear correlations between these properties, suggesting that none of these properties alone can determine the activity and evolution of an individual LIRG. A study of the IRAC colors of LIRGs confirms that methods of finding AGN on the basis of their MIR colors are effective at choosing AGN but 50% to 40% of AGN dominated LIRGs are not selected as such with these methods.

Gas, Stars, and Star Formation in ALFALFA Dwarf Galaxies

We examine the global properties of the stellar and H I components of 229 low H_I mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with H_I masses <10^(7.7) M_☉ and H_I line widths <80 km s^(–1). Sloan Digital Sky Survey (SDSS) data are combined with photometric properties derived from Galaxy Evolution Explorer to derive stellar masses (M_*) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs), and estimates of their SFRs and M_* obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M_* ≲ 10^8 M_☉ is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo Cluster. The imposition of an upper H_I mass limit yields the selection of a sample with lower gas fractions for their M_* than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have H_I depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation (SF) laws is consistent with the general assumptions that gas-rich galaxies have lower SF efficiencies than do optically selected populations and that H_I disks are more extended than stellar ones.

THE NUCLEAR STRUCTURE IN NEARBY LUMINOUS INFRARED GALAXIES: HUBBLE SPACE TELESCOPE NICMOS IMAGING OF THE GOALS SAMPLE

We present results of Hubble Space Telescope NICMOS H-band imaging of 73 of most luminous (i.e., log[L_IR/L_0]>11.4) Infrared Galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey (GOALS). This dataset combines multi-wavelength imaging and spectroscopic data from space (Spitzer, HST, GALEX, and Chandra) and ground-based telescopes. In this paper we use the high-resolution near-infrared data to recover nuclear structure that is obscured by dust at optical wavelengths and measure the evolution in this structure along the merger sequence. A large fraction of all galaxies in our sample possess double nuclei (~63%) or show evidence for triple nuclei (~6%). Half of these double nuclei are not visible in the HST B-band images due to dust obscuration. The majority of interacting LIRGs have remaining merger timescales of 0.3 to 1.3 Gyrs, based on the projected nuclear separations and the mass ratio of nuclei. We find that the bulge luminosity surface density increases significantly along the merger sequence (primarily due to a decrease of the bulge radius), while the bulge luminosity shows a small increase towards late merger stages. No significant increase of the bulge Sersic index is found. LIRGs that show no interaction features have on average a significantly larger bulge luminosity, suggesting that non merging LIRGs have larger bulge masses than merging LIRGs. This may be related to the flux limited nature of the sample and the fact that mergers can significantly boost the IR luminosity of otherwise low luminosity galaxies. We find that the projected nuclear separation is significantly smaller for ULIRGs (median value of 1.2 kpc) than for LIRGs (mean value of 6.7 kpc), suggesting that the LIRG phase appears earlier in mergers than the ULIRG phase.

The Spatial Extent of (U)LIRGs in the Mid-infrared. I. The Continuum Emission

We present an analysis of the extended mid-infrared (MIR) emission of the Great Observatories All-Sky LIRG Survey sample based on 5–15μm low-resolution spectra obtained with the Infrared Spectrograph on Spitzer. We calculate the fraction of extended emission (FEE) as a function of wavelength for the galaxies in the sample, FEE_λ, defined as the fraction of the emission which originates outside of the unresolved component of a source at a given distance. We find that the FEE_λ varies from one galaxy to another, but we can identify three general types of FEE_λ: one where FEE_λ is constant, one where features due to emission lines and polycyclic aromatic hydrocarbons appear more extended than the continuum, and a third which is characteristic of sources with deep silicate absorption at 9.7 μm. More than 30% of the galaxies have a median FEE_λ larger than 0.5, implying that at least half of their MIR emission is extended. Luminous Infrared Galaxies (LIRGs) display a wide range of FEE in their warm dust continuum (0 ≲ FEE_(13.2μm) ≲ 0.85). The large values of FEE_(13.2μm) that we find in many LIRGs suggest that the extended component of their MIR continuum emission originates in scales up to 10 kpc and may contribute as much as the nuclear region to their total MIR luminosity. The mean size of the LIRG cores at 13.2 μm is 2.6 kpc. However, once the IR luminosity of the systems reaches the threshold of L_(IR) ~ 10^(11.8) L_⊙, slightly below the regime of Ultra-luminous Infrared Galaxies (ULIRGs), all sources become clearly more compact, with FEE_(13.2μm) ≲ 0.2, and their cores are unresolved. Our estimated upper limit for the core size of ULIRGs is less than 1.5 kpc. Furthermore, our analysis indicates that the compactness of systems with L_(IR) ≳ 10^(11.25) L_⊙ strongly increases in those classified as mergers in their final stage of interaction. The FEE_(13.2μm) is also related to the contribution of an active galactic nucleus (AGN) to the MIR emission. Galaxies which are more AGN dominated are less extended, independently of their L_(IR). We finally find that the extent of the MIR continuum emission is correlated with the far-IR IRAS log(f_(60μm)/f_(100μm)) color. This enables us to place a lower limit to the area in a galaxy from where the cold dust emission may originate, a prediction which can be tested soon with the Herschel Space Telescope.

The Hi content of early-type galaxies from the ALFALFA survey II. The case of low density environments

Aims. We present the analysis of the Hi content of a sample of early-type galaxies (ETGs) in low-density environments (LDEs) using the data set provided by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We compare their properties to the sample in the Virgo cluster that we studied in a previous paper (di Serego Alighieri et al. 2007, A&A, 474, 851, Paper I). Our aim is both to investigate how the cool interstellar medium (ISM) of these systems depends on the galaxy mass and the environment and to relate the properties of the neutral hydrogen to the warm phases of the ISM. Methods. We have selected a sample of 62 nearby ETGs (V −17). In both cases it is 10 times higher than that of the Virgo cluster. The presence of gas can be related to a recent star formation activity: 60% of all ETGs with Hi have optical emission line ratios typical of star-forming galaxies and blue colours suggesting the presence of young stellar populations, especially in the dwarf subsample. Conclusions. We show that the Hi detection rate of ETGs depends both on the environment and mass. The fraction of early-type systems with neutral hydrogen is higher in more massive objects when compared to early-type dwarfs. The ETGs in LDEs seem to have more heterogeneous properties than their Virgo cluster counterparts, since they are able to retain a cold interstellar gas component and to support star formation activity even at recent epochs.