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Register a new userA New Distance to The Antennae Galaxies (NGC 4038/39) Based on the Type Ia Supernova 2007sr
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Francois Schweizer
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Traditionally, the distance to NGC 4038/39 has been derived from the systemic recession velocity, yielding about 20 Mpc for H_0 = 72 km/s/Mpc. Recently, this widely adopted distance has been challenged based on photometry of the presumed tip of the red giant branch (TRGB), which seems to yield a shorter distance of 13.3+-1.0 Mpc and, with it, nearly 1 mag lower luminosities and smaller radii for objects in this prototypical merger. Here we present a new distance estimate based on observations of the Type Ia supernova (SN) 2007sr in the southern tail, made at Las Campanas Observatory as part of the Carnegie Supernova Project. The resulting distance of D(SN Ia) = 22.3+-2.8 Mpc [(m-M)_0 = 31.74+-0.27 mag] is in good agreement with a refined distance estimate based on the recession velocity and the large-scale flow model developed by Tonry and collaborators, D(flow) = 22.5+-2.8 Mpc. We point out three serious problems that a short distance of 13.3 Mpc would entail, and trace the claimed short distance to a likely misidentification of the TRGB. Reanalyzing Hubble Space Telescope (HST) data in the Archive with an improved method, we find a TRGB fainter by 0.9 mag and derive from it a preliminary new TRGB distance of D(TRGB) = 20.0+-1.6 Mpc. Finally, assessing our three distance estimates we recommend using a conservative, rounded value of D = 22+-3 Mpc as the best currently available distance to The Antennae.
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We study the relationship between dense gas and star formation in the Antennae galaxies by comparing ALMA observations of dense gas tracers (HCN, HCO$^+$, and HNC $mathrm{J}=1-0$) to the total infrared luminosity ($mathrm{L_{TIR}}$) calculated using data from the textit{Herschel} Space Observatory and the textit{Spitzer} Space Telescope. We compare the luminosities of our SFR and gas tracers using aperture photometry and employing two methods for defining apertures. We taper the ALMA dataset to match the resolution of our $mathrm{L_{TIR}}$ maps and present new detections of dense gas emission from complexes in the overlap and western arm regions. Using OVRO CO $mathrm{J}=1-0$ data, we compare with the total molecular gas content, $mathrm{M(H_2)_{tot}}$, and calculate star formation efficiencies and dense gas mass fractions for these different regions. We derive HCN, HCO$^+$ and HNC upper limits for apertures where emission was not significantly detected, as we expect emission from dense gas should be present in most star-forming regions. The Antennae extends the linear $mathrm{L_{TIR}-L_{HCN}}$ relationship found in previous studies. The $mathrm{L_{TIR}-L_{HCN}}$ ratio varies by up to a factor of $sim$10 across different regions of the Antennae implying variations in the star formation efficiency of dense gas, with the nuclei, NGC 4038 and NGC 4039, showing the lowest SFE$_mathrm{dense}$ (0.44 and 0.70 $times10^{-8}$ yr$^{-1}$). The nuclei also exhibit the highest dense gas fractions ($sim 9.1%$ and $sim7.9%$).
(abridged) We report here a factor of 5.7 higher total CO flux in Arp~244 (the ``Antennae galaxies) than that previously accepted in the literature (thus a total molecular gas mass of 1.5x10$^{10}$ Msun), based on our fully sampled CO(1-0) observations at the NRAO 12m telescope. Our observations show that the molecular gas peaks predominately in the disk-disk overlap region between the nuclei, similar to the far-infrared (FIR) and mid-infrared (MIR) emission. The bulk of the molecular gas is forming into stars with a normal star formation efficiency (SFE) L_{IR}/M(H_2) approx 4.2 Lsun/Msun, same as that of giant molecular clouds in the Galactic disk. Additional supportive evidence is the extremely low fraction of the dense molecular gas in Arp~244, revealed by our detections of the HCN(1-0) emission. We estimate the local SFE indicated by the ratio map of the radio continuum to CO(1-0) emission. Remarkably, the local SFE stays roughly same over the bulk of the molecular gas distribution. Only some localized regions show the highest radio-to-CO ratios that we have identified as the sites of the most intense starbursts with SFE >~ 20 Lsun/Msun. These starburst regions are confined exclusively in the dusty patches seen in the HST images near the CO and FIR peaks where presumably the violent starbursts are heavily obscured. Nevertheless, recent large-scale star formation is going on throughout the system, yet the measured level is more suggestive of a moderate starburst (SFE >~ 10 Lsun/Msun) or a weak to normal star formation (SFE ~ 4 Lsun/Msun). The overall starburst from the bulk of the molecular gas is yet to be initiated as most of the gas further condenses into kpc scale in the final coalescence.
We present deep, photometrically calibrated BVRJHK images of the nearby interacting galaxy pair NGC 4038/39 (``The Antennae). Color maps of the images are derived, and those using the B, V, and K-bands are analyzed with techniques developed for examining the colors of stars. From these data we derive pixel-by-pixel maps of the distributions of stellar populations and dust extinction for the galaxies. Analysis of the stellar population map reveals two distinct episodes of recent star formation: one currently in progress and a second that occurred ~600 Myr ago. A roughly 15 Gyr-old population is found which traces the old disks of the galaxies and the bulge of NGC 4038. The models used successfully reproduce the locations of clusters, and the ages we derive are consistent with those found from previous Hubble Space Telescope observations of individual star clusters. We also find 5 luminous ``super star clusters in our K-band images that do not appear in the B or V-band images. These clusters are located in the overlap region between the two galaxies, and are hidden by dust with visual extinctions of A_V ~> 3 mag. The techniques we describe in this paper should be generally applicable to the study of stellar populations in galaxies for which detailed spatial resolution with Hubble is not possible.
The ACS and NICMOS have been used to obtain new HST images of NGC 4038/4039 (The Antennae). These new observations allow us to better differentiate compact star clusters from individual stars, based on both size and color. We use this ability to extend the cluster luminosity function by approximately two magnitudes over our previous WFPC2 results, and find that it continues as a single power law, dN/dL propto L^alpha with alpha=-2.13+/-0.07, down to the observational limit of Mv~-7. Similarly, the mass function is a single power law dN/dM propto M^beta with beta=-2.10+/-0.20 for clusters with ages t<3x10^8 yr, corresponding to lower mass limits that range from 10^4 to 10^5 Msun, depending on the age range of the subsample. Hence the power law indices for the luminosity and mass functions are essentially the same. The luminosity function for intermediate-age clusters (i.e., ~100-300 Myr old objects found in the loops, tails, and outer areas) shows no bend or turnover down to Mv~-6, consistent with relaxation-driven cluster disruption models which predict the turnover should not be observed until Mv~-4. An analysis of individual ~0.5-kpc sized areas over diverse environments shows good agreement between values of alpha and beta, similar to the results for the total population of clusters in the system. Several of the areas studied show evidence for age gradients, with somewhat older clusters appearing to have triggered the formation of younger clusters. The area around Knot B is a particularly interesting example, with an ~10-50 Myr old cluster of estimated mass ~10^6 Msun having apparently triggered the formation of several younger, more massive (up to 5x10^6 Msun) clusters along a dust lane.
We present the snapshot distance method (SDM), a modern incarnation of a proposed technique for estimating the distance to a Type Ia supernova (SN Ia) from minimal observations. Our method, which has become possible owing to recent work in the application of deep learning to SN Ia spectra (we use the deepSIP package), allows us to estimate the distance to an SN Ia from a single optical spectrum and epoch of $2+$ passband photometry -- one nights worth of observations (though contemporaneity is not a requirement). Using a compilation of well-observed SNe Ia, we generate snapshot distances across a wide range of spectral and photometric phases, light-curve shapes, photometric passband combinations, and spectrum signal-to-noise ratios. By comparing these estimates to the corresponding distances derived from fitting all available photometry for each object, we demonstrate that our method is robust to the relative temporal sampling of the provided spectroscopic and photometric information, and to a broad range of light-curve shapes that lie within the domain of standard width-luminosity relations. Indeed, the median residual (and asymmetric scatter) between SDM distances derived from two-passband photometry and conventional light-curve-derived distances that utilise all available photometry is $0.013_{-0.143}^{+0.154}$ mag. Moreover, we find that the time of maximum brightness and light-curve shape (both of which are spectroscopically derived in our method) are only minimally responsible for the observed scatter. In a companion paper, we apply the SDM to a large number of sparsely observed SNe Ia as part of a cosmological study.
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