Normal Disk Galaxies Research Articles

STELLAR TIDAL STREAMS IN SPIRAL GALAXIES OF THE LOCAL VOLUME: A PILOT SURVEY WITH MODEST APERTURE TELESCOPES

Within the hierarchical framework for galaxy formation, minor merging and tidal interactions are expected to shape all large galaxies to the present day. As a consequence, most seemingly normal disk galaxies should be surrounded by spatially extended stellar "tidal features" of low surface brightness. As part of a pilot survey for such interaction signatures, we have carried out ultra deep, wide field imaging of eight isolated spiral galaxies in the Local Volume, with data taken at small (D = 0.1–0.5 m) robotic telescopes that provide exquisite surface brightness sensitivity (μlim(V) ∼ 28.5 mag arcsec−2). This initial observational effort has led to the discovery of six previously undetected extensive (to ∼30 kpc) stellar structures in the halos surrounding these galaxies, likely debris from tidally disrupted satellites. In addition, we confirm and clarify several enormous stellar over-densities previously reported in the literature, but never before interpreted as tidal streams. Even this pilot sample of galaxies exhibits strikingly diverse morphological characteristics of these extended stellar features: great circle-like features that resemble the Sagittarius stream surrounding the Milky Way, remote shells and giant clouds of presumed tidal debris far beyond the main stellar body, as well as jet-like features emerging from galactic disks. Together with presumed remains of already disrupted companions, our observations also capture surviving satellites caught in the act of tidal disruption. A qualitative comparison with available simulations set in a ΛCold Dark Matter cosmology (that model the stellar halo as the result of satellite disruption evolution) shows that the extraordinary variety of stellar morphologies detected in this pilot survey matches that seen in those simulations. The common existence of these tidal features around "normal" disk galaxies and the morphological match to the simulations constitutes new evidence that these theoretical models also apply to a large number of other Milky Way-mass disk galaxies in the Local Volume.

A study of the gas-star formation relation over cosmic time★

We use the first systematic data sets of CO molecular line emission in z~1-3 normal star forming galaxies for a comparison of the dependence of galaxy-averaged star formation rates on molecular gas masses at low and high redshifts, and in different galactic environments. Although the current high-z samples are still small and biased toward the luminous and massive tail of the actively star-forming 'main-sequence', a fairly clear picture is emerging. Independent of whether galaxy integrated quantities or surface densities are considered, low- and high-z SFG galaxy populations appear to follow similar molecular gas-star formation relations with slopes 1.1 to 1.2. The gas-depletion time scale in these SFGs grows from 0.5 Gyrs at z~2 to 1.5 Gyrs at z~0. Because star formation depletion times are significantly smaller than the Hubble time at all redshifts sampled, star formation rates and gas fractions are set by the balance between gas accretion from the halo and stellar feedback. In contrast, very luminous gas rich major mergers at both low-z and high-z produce on average 4 to10 times more far-infrared luminosity per unit gas mass. Only some fraction of this difference can be explained by uncertainties in gas-mass or luminosity estimators; much of it must be intrinsic. The most likely interpretation is that the star formation relation is driven by global dynamical effects. For a given mass, the more compact merger systems produce stars more rapidly because their gas clouds are more compressed with shorter dynamical times, so that they churn more quickly through the available gas reservoir than the typical normal disk galaxies. When the dependence on galactic dynamical time scale is explicitly included, disk galaxies and mergers appear to follow similar gas to star-formation relations. The mergers may be forming stars at slightly higher efficiencies than the disks.

Photometry and Kinematics of Two Minor Merger Candidate Galaxies

We observationally investigate the properties of disk galaxies undergoing minor merger or capture events. In this context, the properties of two double-nucleus candidate galaxies, ESO 381-IG 23 and MCG -3-35-14, are analyzed. Both are disk-dominated Sc-Scd galaxies that show a bright knot superposed on their disk body. The size and surface brightness of these knots are in both cases comparable to those of the galaxy nucleus, which has led previous work to classify these galaxies as double-nucleus galaxies. We present results from observations made with the SOAR 4.1 m, CASLEO 2.15 m, and Bosque Alegre 1.54 m telescopes. We determined the values for the apparent and absolute magnitudes of the nuclei and the bright regions and analyzed the surface brightness profiles and colors. We also study the kinematics of the galaxies via their rotation curves. Analytical mass models were fitted under the constraints of both kinematic and photometric observational data. It is found that ESO 381-IG 23 has an absolute magnitude MB = -19.59 and mass = (3.0 ± 0.2) × 1010 ⊙, and its nuclear spectrum shows strong emission lines typical of starbursts. MCG -3-35-14 has MB = -19.97 and = (9.6 ± 0.5) × 1010 ⊙. Both galaxies are morphologically normal disk galaxies. They have a bulge-to-disk ratio of ~0.1 and show no significant signatures of dynamical perturbation in their rotation curves. The secondary nuclei candidates are found to be giant H II regions, rather than nuclei of captured companions. They have masses of (2.2 ± 0.2) × 106 ⊙ (ESO 381-IG 23) and (4.1 ± 0.2) × 106 ⊙ (MCG -3-35-14), and ages of 6.6 ± 0.1 and 8.0 ± 1.0 Myr, respectively.

A Bar Fuels a Supermassive Black Hole?: Host Galaxies of Narrow‐Line Seyfert 1 Galaxies

We present optical images of nearby 50 narrow-line Seyfert 1 galaxies (NLS1s) which cover all the NLS1s at z<0.0666 and $\delta \ge -25^{\circ}$ known at the time of 2001. Among the 50 NLS1s, 40 images are newly obtained by our observations and 10 images are taken from archive data. Motivated by the hypothesis that NLS1s are in an early phase of a super-massive black hole (BH) evolution, we present a study of NLS1 host galaxy morphology to examine trigger mechanism(s) of active galactic nuclei (AGNs) by seeing the early phase of AGN. With these images, we made morphological classification by eye inspection and by quantitative method, and found a high bar frequency of the NLS1s in the optical band; the bar frequency is $85 \pm 7% among disk galaxies ($64- 71$% in total sample) which is more frequent than that (40-70%) of broad-line Seyfert 1 galaxies (BLS1s) and normal disk galaxies, though the significance is marginal. Our results confirm the claim by Crenshaw et al. (2003) with a similar analysis for 19 NLS1s. The frequency is comparable to that of HII/starburst galaxies. We also examined the bar frequency against width of the broad H$\beta$ emission line, Eddington ratio, and black hole mass, but no clear trend is seen. Possible implications such as an evolutionary sequence from NLS1s to BLS1 are discussed briefly.

Hot Gaseous Halos of Nearby Disk Galaxies

I review studies of the hot gaseous medium in and around nearby normal disk galaxies, including the Milky Way. This medium represents a reservoir of materials required for lasting star formation, a depository of galactic feedback (e.g., stellar mass loss and supernovae), and an interface between the interstellar and intergalactic media. Important progress has been made recently with the detection of X-ray absorption lines in the spectra of X-ray binaries and AGNs. The X-ray absorption line spectroscopy, together with existing X-ray emission and far-UV O iv absorption measurements now allows for the first time to characterize the global spatial, thermal, and chemical properties of hot gas in the Galaxy. The results are generally consistent with those inferred from X-ray imaging of nearby edge-on galaxies similar to the Milky Way. Observed diffuse X-ray emitting/absorbing gas does not extend significantly more than ~10 kpc away from galactic disks/bulges, except in nuclear starburst or very massive galaxies. The X-ray cooling rate of this gas is generally far less than the expected supernova mechanical energy input alone. So the bulk of the energy is “missing”. On the other hand, evidence for a large-scale (≲ 102 kpc) hot gaseous halo around the Milky Way to explain various high-velocity clouds is mounting. The theoretical argument for ongoing accretion of intergalactic gas onto disk galaxies is also compelling. I discuss possible solutions that reconcile these facts. In particular, large-scale hot gaseous halos appear to be low in metallicity, hence X-ray emission. The metal enrichment in the intergalactic medium may be substantially non-uniform; fast-cooling clumps of relatively high metallicity may have largely dropped out and may partly account for high-velocity clouds. In addition, ongoing galactic mechanical energy feedback is likely important in balancing the cooling of the halos and may be strong enough to produce galactic winds in bulge-dominated galaxies.

The Dust Content and Temperature Distribution of Galaxies in The Local Universe

AbstractWe measured the SCUBA fluxes of a sample of Seyfert 1, Seyfert 2, starburst and normal disc galaxies, taken from the Extended 12 Micron Galaxy Sample. From these data, we determine the entire dust content and temperature distribution of galaxies of different classes through a Planckian inversion technique.

Structure Formation in Anisotropic Disks

The majority of normal disk galaxies are characterized by non-axisymmetric structures like spirals or bars. These structural elements have been widely discussed in the literature as a result of gravitational instabilities which are connected to growing density waves or global instabilities of disks. A first insight into the properties of galactic discs was provided by linear stability analysis. However, a disadvantage of linear stability analysis remained its restriction to small perturbations, both in amplitude and wavelength. Thus, numerical simulations, especially hydrodynamical and stellar-hydrodynamical simulations became a primary tool for the analysis of galactic evolution.

Effects of dust scattering albedo and 2175-A bump on ultraviolet colours of normal disc galaxies

We discuss dust properties in the interstellar medium (ISM) of nearby normal galaxies, by comparing observations in the ultraviolet (UV) with simulations by a radiative transfer model. The observed UV colours of nearby galaxies show a reddening relative to their expected intrinsic colours. Some authors argued that the Milky Way dust cannot reproduce the reddening because of the prominent 2175 \AA absorption bump. Other authors proposed a reduction mechanism of the bump strength in an {\it attenuation law} derived from the ratio of the observed intensity to the intrinsic one through an age-selective attenuation (i.e., young stars are more attenuated selectively). We newly find that the wavelength dependence of the scattering albedo also has a strong effect on the UV colour; an albedo decreasing toward shorter wavelengths (except for the absorption bump range) produces a significant UV reddening. After comparing the observed UV colours of nearby normal galaxies with those expected from radiative transfer simulations assumed several dust models, we find two sorts of dust suitable for these galaxies: (1) dust with a bump and a smaller albedo for a shorter wavelength (except for the bump range), and (2) dust without any bump but with an almost constant albedo. If very small carbonaceous grains responsible for the common unidentified infrared emission band are also the bump carrier, the former dust is favorable. Finally, we derive mean attenuation laws of various dust models as a function of the UV attenuation, and derive some relations between the UV attenuation and observable/theoretical quantities.

The SAURON project - VII. Integral-field absorption and emission-line kinematics of 24 spiral galaxy bulges

We present observations of the stellar and gas kinematics for a representative sample of 24 Sa galaxies obtained with our custom-built integral-field spectrograph SAURON operating on the William Herschel Telescope. The data have been homogeneously reduced and analysed by means of a dedicated pipeline. All resulting data cubes were spatially binned to a minimum mean signal-to-noise ratio of 60 per spatial and spectral resolution element. Our maps typically cover the bulge-dominated region. We find a significant fraction of kinematically decoupled components (12/24), many of them displaying central velocity dispersion minima. They are mostly aligned and co-rotating with the main body of the galaxies, and are usually associated with dust discs and rings detected in unsharp-masked images. Almost all the galaxies in the sample (22/24) contain significant amounts of ionized gas which, in general, is accompanied by the presence of dust. The kinematics of the ionized gas are consistent with circular rotation in a disc co-rotating with respect to the stars. The distribution of mean misalignments between the stellar and gaseous angular momenta in the sample suggests that the gas has an internal origin. The [O-III]/H beta ratio is usually very low, indicative of current star formation, and shows various morphologies (ring-like structures, alignments with dust lanes or amorphous shapes). The star formation rates (SFRs) in the sample are comparable with that of normal disc galaxies. Low gas velocity dispersion values appear to be linked to regions of intense star formation activity. We interpret this result as stars being formed from dynamically cold gas in those regions. In the case of NGC 5953, the data suggest that we are witnessing the formation of a kinematically decoupled component from cold gas being acquired during the ongoing interaction with NGC 5954.

Spatially resolved spectroscopy of the E+A galaxies in thez= 0.32cluster AC 114

We present spatially resolved intermediate resolution spectroscopy of a sample of twelve E+A galaxies in the z=0.32 rich galaxy cluster AC 114, obtained with the FLAMES multi-integral field unit system on the European Southern Observatory's VLT. Previous integrated spectroscopy of all these galaxies by Couch & Sharples (1987) had shown them to have strong Balmer line absorption and an absence of [OII 3727] emission -- the defining characteristics of the``E+A'' spectral signature, indicative of an abrupt halt to a recent episode of quite vigorous star formation. We have used our spectral data to determine the radial variation in the strength of Hdelta absorption in these galaxies and hence map out the distribution of this recently formed stellar population. Such information provides important clues as to what physical event might have been responsible for this quite dramatic change in star formation activity in these galaxies' recent past. We find a diversity of behaviour amongst these galaxies in terms of the radial variation in Hdelta absorption: Four galaxies show little Hdelta absorption across their entire extent; it would appear they were misidentified as E+A galaxies in the earlier integrated spectroscopic studies. The remainder show strong Hdelta absorption, with a gradient that is either negative (Hdelta equivalent width decreasing with radius), flat, or positive. By comparing with numerical simulations we suggest that the first of these different types of radial behaviour provides evidence for a merger/interaction origin, whereas the latter two types of behaviour are more consistent with the truncation of star formation in normal disk galaxies. It would seem therefore that more than one physical mechanism is responsible for E+A formation in the same environment.

Attenuation law of normal disc galaxies with clumpy distributions of stars and dust

We investigate the attenuation law seen through an interstellar medium (ISM) with clumpy spatial distributions of stars and dust. The clumpiness of the dust distribution is introduced by a multiphase ISM model. We solve a set of radiative transfer equations with multiple anisotropic scatterings through the clumpy ISM in a one-dimensional plane-parallel geometry by using the mega-grain approximation, in which dusty clumps are regarded as very large particles (i.e. mega-grains). The clumpiness of the stellar distribution is introduced by the youngest stars embedded in the clumps. We assume a smooth spatial distribution for older stars. The youngest stars are surrounded by denser dusty gas and suffer stronger attenuation than diffuse older stars (i.e. age-selective attenuation). The apparent attenuation law is a composite of the attenuation laws for the clumpy younger stars and for the diffuse older stars with a luminosity weight. In general, the stellar population dominating the luminosity changes from older stars to younger stars as the wavelength decreases. This makes the attenuation law steep; the composite attenuation rapidly increases from small attenuation for older stars at a long wavelength to large attenuation for younger stars at a short wavelength. The resultant attenuation law of normal disc galaxies is expected to be much steeper than that of starburst galaxies observed by Calzetti et al. Finally, the Calzetti attenuation law is regarded as a special case with a large density in our framework.

Attenuation law of normal disc galaxies with clumpy distributions of stars and dust

Attenuation law of normal disc galaxies with clumpy distributions of stars and dust

Chain Galaxies in the Tadpole Advanced Camera for Surveys Field

Colors and magnitudes were determined for 69 chain galaxies, 58 other linear structures, 32 normal edge-on galaxies, and all of their large star formation clumps in the Hubble Space Telescope Advanced Camera for Surveys field of the Tadpole galaxy. Redshifts of 0.5-2 are inferred from comparisons with published color-evolution models. The linear galaxies have no red nuclear bulges like the normal disk galaxies in our field, but the star formation clumps in each have about the same colors and magnitudes. Light profiles along the linear galaxies tend to be flat, unlike the exponential profiles of normal galaxies. Although the most extreme of the linear objects look like beaded filaments, they are all probably edge-on disks that will evolve to late Hubble type galaxies. The lack of an exponential profile is the result of either a dust scale height that is comparable to the stellar scale height or an intrinsically irregular structure. Examples of galaxies that could be face-on versions of linear galaxies are shown. They have an irregular clumpy structure with no central bulge and with clump colors and magnitudes that are comparable to those in the linear galaxies. Radiative transfer solutions to the magnitudes and surface brightnesses of inclined dusty galaxies suggest that edge-on disks should become more prominent near the detection limit for surface brightness. The surface brightness distribution of the edge-on galaxies in this field confirm this selection effect. The star formation regions are much more massive than in modern galaxies, averaging up to ~109 M☉ for kiloparsec scales.

Discovery of Face-on Counterparts of Chain Galaxies in the Tadpole Advanced Camera for Surveys Field

The background field of the Tadpole galaxy image taken with the Hubble Space Telescope Advanced Camera for Surveys contains 87 clump clusters that are probably low-inclination versions of the 69 chain galaxies that we found in a previous paper. This conclusion follows from the similarity in colors and apparent magnitudes of the brightest clumps in the clump clusters and chain galaxies and from the similarity between the distribution of axial ratios for the combined sample and the distribution for normal disk galaxies. These distributions are consistent with chains and clump clusters being disks viewed at random angles with intrinsic axial ratios of 0.1-0.2 or less. The disks appear to be gas-rich galaxies at z = 1-2 that are forming an early generation of stars in a small number of very large star complexes.

Galaxies on the Blue Edge

We have successfully constructed a catalog of H I–rich galaxies selected from the Minnesota Automated Plate Scanner catalog of the Palomar Observatory Sky Survey (POSS I*), based solely on optical criteria. We identify H I–rich candidates by selecting the bluest galaxies at a given apparent magnitude, those galaxies on the blue edge of POSS I color-magnitude parameter space. Subsequent 21 cm observations on the upgraded Arecibo 305 m dish detected over 50% of the observed candidates. The detected galaxies are H I–rich, with H I masses comparable to normal high surface brightness disk galaxies, and they have gas mass-to-light ratios ranging from 0.1 to 4.8 (in solar units). Comparison of our candidate galaxies with known low surface brightness galaxies (hereafter LSBs) shows that they exhibit similar optical and H I properties to that population. We also show that previously identified LSBs, including several LSBs with red B-V colors, preferentially occupy the blue edge of POSS I color-magnitude parameter space. Their presence on the blue edge appears to be a selection effect due to differing plate limits in the two POSS I bandpasses. This suggests the POSS I is a good filter for separating galaxies on the higher surface brightness end of the LSB population from the general population of galaxies in the night sky.

The relationship between star formation rates and mid-infrared emission in galactic disks

The H and mid-infrared mean disk surface brightnesses are compared in a sample of nearby spirals observed by ISOCAM. This shows that, in spiral disks, dust emission at 7 and 15m provides a reasonable star formation tracer. The fact that the 15 to 7m flux ratio is nearly constant in various global exciting conditions indicates a common origin, namely the aromatic infrared band carriers, and implies that at these wavelengths, dust emission from the disks of normal galaxies is dominated by photodissociation regions and not by HII regions themselves. We use this newly-found correlation between the mid-infrared and the H line to investigate the nature of the link between the far-infrared (60 and 100m) and H. Although the separation of the central regions from the disk is impossible to achieve in the far-infrared, we show that a circumnuclear contribution to the dust emission, having no equivalent counterpart in H, is most likely responsible for the well-known non- linearity between far-infrared and H fluxes in spiral galaxies. We derive a calibration of 7 and 15m fluxes in terms of star formation rates from a primary calibration of H in the literature, and also outline the applicability limits of the proposed conversion, which should not be blindly extrapolated to objects whose nature is unknown.

Deep-Imaging Observations of a Candidate of an Absorbed QSO at z = 0.653, AX J131831+3341

The results of deep-imaging observations of a candidate of an absorbed QSO at z = 0.653, AX J131831+3341, are presented. AX J131831+3341 was found during the course of optical follow-up observations of the ASCA Large Sky Survey, and has an X-ray luminosity of 1045 erg s−1 (2–10 keV), which corresponds to those of QSOs. Its optical spectrum shows no significant broad Hβ emission line, suggesting that the object is an absorbed QSO. Deep R and V band images reveal the presence of a point-like nucleus and an asymmetric extended component. The nuclear component has a blue color, and the optical magnitude is much fainter than that expected from the observed X-ray flux for typical type-1 AGNs. These photometric properties and the presence of broad MgII 2800 Å emission can be explained simultaneously if the observed nuclear light is dominated by scattered nuclear light, though there is a possibility that the nuclear component is a slightly absorbed nucleus if its intrinsic X-ray to optical flux ratio is the largest among X-ray selected AGNs. The size of the extended component, which is thought to be the host galaxy of the QSO, is larger than those of normal disk galaxies at z = 0–0.75, and the absolute magnitude is similar to those of the brightest host galaxies of QSOs at redshifts smaller than 0.5. The V – R and R – I colors of the component are consistent with a 1 Gyr-old stellar population model without absorption.

The star formation histories of Low Surface Brightness galaxies

AbstractNear-infrared images of a sample of red, blue and giant low surface brightness disk galaxies (LSBGs) were combined with optical data with the aim of constraining their star formation histories. Most LSBGs have strong colour gradients consistent with mean stellar age gradients. We find that LSBGs have a large range of ages and metallicities, spanning those observed in normal disk galaxies. In particular, red and blue LSBGs have very different star formation histories and represent two independent routes to low B band surface brightness. Blue LSBGs are well described by models with low, roughly constant star formation rates, whereas red LSBGs are better described by a ‘faded disk’ scenario.

Evidence for Rotation in the Galaxy atz= 3.15 Responsible for a Damped Lyman‐Alpha Absorption System in the Spectrum of Q2233+1310

Proof of the existence of a significant population of normal disk galaxies at redshift z > 2 would have profound implications for theories of structure formation and evolution. We present evidence based on Keck HIRES spectra that the damped Lyα absorber at z = 3.15 toward the quasar Q2233+1310 may well be such an example. Djorgovski et al. have recently detected the Lyα emission from the absorber, which we assume is at the systemic redshift of the absorbing galaxy. By examining the profiles of the metal absorption lines arising from the absorbing galaxy in relation to its systemic redshift, we find strong kinematical evidence for rotation. Therefore the absorber is likely to be a disk galaxy. The inferred circular velocity for the galaxy is greater than or equal to 200 km s-1. With a separation of 17 kpc (q0 = 0.1, H0 = 75) between the galaxy and the quasar sight line, the implied dynamic mass for the galaxy is greater than or equal to 1.6 × 1011 M☉. The metallicity of the galaxy is found to be [Fe/H] = -1.4, typical of damped Lyα galaxies at such redshifts. However, in another damped Lyα absorber at z = 2.81 toward Q0528-2505, no kinematical evidence for galactic rotation is evident. In the latter case, the damped Lyα absorber occurs near the background quasar in redshift, so its properties may be influenced by the background quasar. These represent the only two cases at present for which the technique used here may be applied. Future applications of the same technique to a large sample of damped Lyα galaxies may allow us to determine if a significant population of disk galaxies already existed only a few billion years after the big bang.

Identification of a galaxy responsible for a high-redshift Lyman-α absorption system

DAMPED Lyman-α systems are high-column-density intergalactic clouds of hydrogen, the existence of which is inferred from absorption lines appearing in the emission spectra of distant quasars. The galaxies believed to be responsible for these absorption systems have been suggested as possible progenitors of the normal disk galaxies observed in the local Universe. Indeed, Lyman-α systems appear to contain a substantial fraction of the baryons known to exist in galaxies today. Here we report the optical detection of a galaxy (designated DLA2233 + 131) associated with a known damped Lyman-α absorption system at a redshift of z = 3.150. The properties of this galaxy correspond closely to those expected of a young disk galaxy in the early stages of formation, and show no evidence for an active nucleus. This finding gives strong support to the idea that damped Lyman-α systems represent a population of young galaxies at high red-shifts.